25. Body Temperature Regulation

Thermal Physiology, Heat Balance, and the Limits of Human Tolerance

Learning Objectives

25.1 The Problem: Why 37 °C, and What Temperature Does to Biology

Start with the strangeness of the fact. You are a bag of water at 37 °C standing in air that is almost never 37 °C. Over an ordinary year you will encounter ambient temperatures spanning sixty degrees. Your core will span less than one.

Nothing else in your physiology is held that tightly. Blood glucose swings threefold after a meal and nobody calls it a crisis. Arterial pressure varies by 40 mm Hg between sleep and sprinting. Core temperature moves 0.5 °C and you notice.

The reason is that temperature is not a physiological variable in the way glucose is. Glucose is a substrate: the body cares about how much there is. Temperature is a rate constant. It does not participate in reactions; it sets the speed of all of them at once, and it sets the stability of every protein and membrane doing the reacting. Change temperature and you have not changed one thing. You have changed everything simultaneously, in different proportions.

Q₁₀: temperature is a throttle on every reaction at once

The temperature sensitivity of a biological rate is expressed as Q₁₀ — the factor by which the rate changes for a 10 °C rise:

Q₁₀ = (rate at T + 10) ÷ (rate at T)

For most enzyme-catalysed reactions in the physiological range, Q₁₀ falls between 2 and 3. A 10 °C rise roughly doubles to triples the rate; a 10 °C fall roughly halves to thirds it. Over the small excursions the body actually experiences, this scales to about 10–13% per degree Celsius, and that single number does an enormous amount of clinical work:

• A febrile patient at 39 °C has a whole-body metabolic rate and oxygen consumption roughly 20–25% above their afebrile baseline. In a patient with fixed cardiac output or limited respiratory reserve — Amara, for instance, with an ejection fraction of 48% — that is not a trivial tax.
• A brain cooled to 33 °C consumes roughly 25–30% less oxygen than at 37 °C, which is the entire physiological argument for therapeutic temperature management after cardiac arrest.
• A hypothermic patient at 28 °C has a cerebral metabolic rate around 40–45% of normal, which is why people are resuscitated successfully after an hour of cold-water submersion and never after four minutes of warm-water submersion.

Here is the difficulty Q₁₀ creates. If every reaction had the same Q₁₀, cooling would simply run the whole body in slow motion, coherently. But they do not. A membrane pump, a diffusion step, a binding equilibrium, and an enzymatic step have different temperature coefficients. Diffusion has a Q₁₀ near 1.3; ion pumping near 3. So cooling does not slow the body down uniformly — it decouples processes that are normally matched. Sodium leaks in at close to its warm rate while the Na⁺/K⁺-ATPase that should pump it back out has slowed threefold. The resting membrane potential drifts. That is why cold cells depolarize, and it is one reason hypothermic myocardium becomes electrically unstable.

Above 40 °C, the problem becomes structural

Cooling slows chemistry. Heating destroys it, and by a different mechanism.

Protein denaturation. A folded protein is held in shape by an accumulation of individually weak interactions — hydrogen bonds, van der Waals contacts, the hydrophobic effect (Chapter 2). The free-energy margin separating the folded from the unfolded state is small, typically 20–60 kJ/mol, equivalent to a handful of hydrogen bonds. Above roughly 40–41 °C, thermal energy begins to overwhelm that margin for the least stable proteins in the human proteome, and the fraction of unfolded protein climbs steeply — not linearly, but as a cooperative transition, because unfolding one domain destabilizes the next.

The cell's answer is the heat shock response: a family of chaperones (HSP70, HSP90, HSP27) transcribed within minutes of thermal stress under the control of heat shock factor 1, which hold partially unfolded proteins in a refoldable state. The heat shock response is inducible and it is protective, and it is one of the mechanisms of heat acclimatization (§25.9). It is also blunted in older adults, which matters for Adwoa.

Threshold values worth carrying:

Membrane fluidity. The phospholipid bilayer is a two-dimensional fluid, and its viscosity is exquisitely temperature-dependent. Cool it and the acyl chains order, the membrane approaches a gel state, embedded transporters and channels lose the conformational freedom they need to cycle, and receptor–ligand kinetics slow disproportionately. Heat it and the membrane becomes too fluid: it leaks, ion gradients bleed away, and integral proteins are no longer held in the geometry their function requires. Organisms that live across a temperature range solve this by homeoviscous adaptation — changing the saturation and length of their membrane fatty acids. A human, holding core temperature constant, does not need to. That is part of what constancy buys.

The window is narrow, and it is asymmetric

Put the numbers together and a picture emerges that surprises most students.

   COLD                                                              HEAT
   ◄──────────────────────────── 37 °C ────────────────────────────►
   13.7      20      25      28      32      35   37   40  42  44
    │         │       │       │       │       │    │    │   │   │
    │         │       │       │       │       │    │    │   │   └ death, minutes
    │         │       │       │       │       │    │    │   └ death, ~1 hour
    │         │       │       │       │       │    │    └ upper physiological limit
    │         │       │       │       │       └ shivering maximal
    │         │       │       │       └ shivering stops; consciousness fails
    │         │       │       └ ventricular fibrillation risk high
    │         │       └ asystole
    │         └ documented survival with full recovery
    └ lowest documented adult survival (accidental hypothermia, 1999)

   Tolerated downward excursion: ~23 °C     Tolerated upward excursion: ~5 °C

Humans survive being 23 degrees too cold and die from being five degrees too hot. The asymmetry is not an accident of evolution; it follows directly from the two mechanisms above.

• Cooling is reversible and self-protecting. It slows every enzyme, including the enzymes of the destructive cascades, and it lowers oxygen demand faster than it lowers oxygen delivery. A cold tissue is a tissue that has been switched partly off. Switch it back on carefully and the structure is still there.
• Heating is irreversible and self-amplifying. It raises oxygen demand by 10–13% per degree at exactly the moment when cardiac output is being diverted to skin and plasma volume is falling. It denatures proteins, and a denatured protein does not refold when you cool it down. Worse, heat injures the gut barrier, allowing endotoxin translocation, which triggers a systemic inflammatory response that continues after the temperature is normal (§25.8).

That last point is the single most important idea in this chapter's clinical half. Heat injury is a dose, not a threshold. What determines outcome in heat stroke is the area under the time–temperature curve — how high, multiplied by how long. It is why speed of cooling predicts survival more powerfully than peak temperature does, and it is a large part of the answer to Case File Question 1.

Core and shell: a variable, not an anatomy

Textbook diagrams draw a body with a red centre and a blue rim and label them core and shell. That is correct as far as it goes, and it conceals the most useful fact about the arrangement: the boundary moves. Core and shell are not organs. They are a state the circulation puts the body into, and the state changes within minutes.

• The core is the tissue whose temperature is defended: brain, thoracic and abdominal viscera, and — variably — deep skeletal muscle. It comprises roughly 65–70% of body mass in a warm environment.
• The shell is tissue permitted to vary: skin, subcutaneous fat, and superficial muscle. In a warm environment it is a few millimetres thick. In the cold it expands to include the entire thickness of the arms and legs, and can fall to within a few degrees of ambient at the fingertips while the core has not moved at all.

The shell is therefore a variable insulator, and the circulation is its thermostat. Fully vasoconstricted shell has roughly two to three times the insulating value of fully vasodilated shell — and in a lean person, subcutaneous fat contributes about half of the total tissue insulation, which is a large part of why body composition predicts cold tolerance in swimmers better than fitness does.

 ═══ THE CORE–SHELL BOUNDARY IS A VARIABLE, NOT AN ANATOMY ═══════════════════

  PANEL A — COLD (air 5 °C, resting, light clothing)
  Shell expands to swallow the limbs. Core shrinks to trunk + head.

              ( 36.9 )  head: always core (brain is never sacrificed)
             ╭─────────╮
        31 ─►│░░░░░░░░░│◄─ 31       ░░  20–30 °C   outer shell
        34 ─►│▒▒▒▒▒▒▒▒▒│◄─ 34       ▒▒  30–35 °C   inner shell
             │▓▓▓ 37 ▓▓│            ▓▓  >36.5 °C   CORE
             │▓▓▓▓▓▓▓▓▓│
             ╰──┬───┬──╯
        ░░░░░░░░│   │░░░░░░░░       ARM PROFILE, shoulder → fingertip
        27 ░░░░ │   │ ░░░░ 27         shoulder ....... 36.4 °C
        24 ░░   │   │   ░░ 24         mid-upper arm .. 34.0
        20 ░    │   │    ░ 20         elbow .......... 32.0
                                      wrist .......... 27.0
                                      fingertip ...... 18–22 °C
                                      GRADIENT ACROSS ONE LIMB: ~18 °C

  PANEL B — WARM (air 35 °C, resting)
  Shell collapses to a few millimetres. Core extends nearly to the skin.

              ( 37.1 )
             ╭─────────╮
        36 ─►│▒▒▒▒▒▒▒▒▒│◄─ 36       shell is now only the epidermis +
             │▓▓▓▓▓▓▓▓▓│            superficial dermis
             │▓▓▓ 37 ▓▓│
             │▓▓▓▓▓▓▓▓▓│
             ╰──┬───┬──╯
        ▒▒▒▒▒▒▒▒│   │▒▒▒▒▒▒▒▒       ARM PROFILE, shoulder → fingertip
        36 ▒▒▒▒ │   │ ▒▒▒▒ 36         shoulder ....... 36.9 °C
        36 ▒▒   │   │   ▒▒ 36         elbow .......... 36.5
        35 ▒    │   │    ▒ 35         wrist .......... 36.0
                                      fingertip ...... 35.5 °C
                                      GRADIENT ACROSS ONE LIMB: ~1.4 °C

 ═══ WHAT CHANGED ════════════════════════════════════════════════════════════
   Not the core temperature (36.9 vs 37.1 — essentially identical).
   What changed is the SIZE OF THE CORE and the STEEPNESS OF THE GRADIENT.

   COLD:  core ≈ 50% of body mass · shell insulation HIGH · skin flow ~20 mL/min
   WARM:  core ≈ 90% of body mass · shell insulation LOW  · skin flow 6–8 L/min

   The controlled variable is core temperature.
   The controlling action is WHERE THE BOUNDARY SITS.

Three consequences follow immediately, and they organize much of the rest of this chapter.

First, "core temperature" is a slight fiction even inside the core. The liver, generating heat continuously, runs about 0.3–0.5 °C warmer than arterial blood. Working muscle during hard exercise can reach 40–41 °C while the core is at 38.5. The brain runs a few tenths above arterial blood. When we say "core temperature," we mean the temperature of central arterial blood, because that is the medium that couples the core together and the medium the hypothalamus samples.

Second, skin temperature and core temperature are separate signals carrying separate information, and the control system uses both. Skin temperature is the early warning: it changes within seconds of an environmental change, long before the core has moved. Core temperature is the error signal: it says whether the defence has failed. A control system using only core temperature would always be too late. A control system using only skin temperature would react to a cold doorknob as though it were a snowstorm. The hypothalamus weights them, roughly 4:1 in favour of core for autonomic responses and much more heavily toward skin for behavioural ones — which is precisely why you reach for a jumper before your core has changed by a hundredth of a degree.

Third, thermal comfort is a report on the shell, not the core. Amara's cold hands at 03:00 and Nia's blazing discomfort at 40.1 °C are both readouts of skin temperature and skin wettedness. Adwoa's absent sense of heat is a failure of that same shell-level reporting. We will return to this repeatedly: comfort and safety are different variables, and they can move in opposite directions (§25.10).

25.2 Measuring Temperature: What Each Site Actually Tells You

This section is unusually practical, and it deserves to be, because temperature is the vital sign most often measured badly and most often over-interpreted. Two rules govern everything that follows.

Rule 1 — A thermometer measures the temperature of the thing it is touching. Not the core. The thing it is touching. Whether that thing tracks the core is a separate, site-specific, condition-dependent question.

Rule 2 — Always record the site. "Temp 38.4" is an incomplete measurement. "Temp 38.4, oral" and "Temp 38.4, rectal" describe two different patients.

Site-specific reference ranges are tabulated in Appendix B · Reference values.

The reference standards

The temperature of pulmonary artery blood, measured by the thermistor on a pulmonary artery catheter, is the accepted physiological reference for core temperature: it is mixed venous blood returning from the whole body, sampled inside the heart. It is also invasive, increasingly rare, and unavailable for the questions most clinicians actually ask.

The practical reference is the distal oesophagus. A flexible thermistor probe placed in the lower third of the oesophagus — roughly 38–42 cm from the nares in an adult, or about one-quarter of standing height below the larynx — sits directly behind the left atrium, separated from central blood by a few millimetres of tissue. It agrees with pulmonary artery temperature to within about 0.2 °C, and its response time is the shortest of any practical site: it follows a change in core temperature within 1–2 minutes. That combination — small offset, short lag — is what makes it the standard in anaesthesia, in intensive care, and in any situation where the temperature is moving.

The two failure modes of oesophageal measurement are worth knowing because they are both avoidable. Placed too high, the probe reads the temperature of ventilator gases through the trachea and under-reads badly. Placed in an awake patient, it is intolerable. It is therefore essentially a technique for the sedated or anaesthetized.

The clinical sites, compared

 ═══ TEMPERATURE MEASUREMENT SITES ═══════════════════════════════════════════

 SITE            WHAT IT PHYSICALLY MEASURES        OFFSET*    LAG      USE
 ─────────────────────────────────────────────────────────────────────────────
 Pulmonary       Mixed venous blood in the           0.0     seconds   reference
 artery          right heart                                           (invasive)

 Oesophageal     Tissue behind the left atrium      −0.1     1–2 min   BEST
 (lower 1/3)     ≈ central arterial blood            to                practical
                                                    +0.1                core

 Rectal          Deep pelvic tissue + blood         +0.2     10–30 min steady
 (10–15 cm)      returning from the legs             to +0.5           state,
                 ▲ warmed by leg muscle in exercise                    heat
                 ▼ cooled by cold legs in hypothermia                  illness

 Bladder         Urine + bladder wall               +0.1     5–20 min  ICU, if
 (Foley probe)   ACCURACY DEPENDS ON URINE FLOW      to +0.4           anuric:
                 — near-useless if oliguric                            unreliable

 Ingestible      Gastrointestinal wall              −0.1     10–20 min field
 telemetry pill  (position migrates over hours)      to +0.3           research,
                 must ingest 6–12 h before use                         athletes

 Tympanic        TRUE contact probe on the drum:    −0.2     <2 min    research
 membrane        hypothalamic/carotid temperature     to 0.0

 Infrared aural  A blend of drum + ear canal        −1.0 to  1–3 min   SCREENING
 ("tympanic")    wall + whatever the beam hits       +0.5             ONLY
                 Highly technique-dependent         (WIDE)

 Temporal        Skin over the temporal artery,     −1.5 to  seconds   SCREENING
 artery scanner  with an arterial heat-balance       +0.5             ONLY
                 ALGORITHM applied. Defeated by     (WIDE)
                 sweat, vasoconstriction, ambient

 Oral            Sublingual pocket at the base of   −0.3     3–5 min   ambulatory
 (posterior      the tongue, near the lingual        to −0.6           adults,
 sublingual)     artery. Defeated by drinks (15–30                     cooperative
                 min), mouth-breathing, tachypnoea

 Axillary        Skin of a closed axilla            −0.5     5–10 min  neonatal
                                                     to −1.5           screening
                 Poorest agreement of any site      (WIDE)

 Zero-heat-flux  Forehead sensor that abolishes     ±0.3     3–5 min   OR/ICU
 (double sensor) local heat loss so skin ≈ deep                        non-invasive
                 tissue under an insulated patch

 Skin            Skin. Only skin.                   −4 to    seconds   thermal
 (any site)      Never a core measurement           −20                comfort
 ─────────────────────────────────────────────────────────────────────────────
 * Offset = typical steady-state difference from oesophageal/PA temperature in a
   thermally stable adult. All offsets widen — often dramatically — when the
   patient is heating or cooling rapidly, or is vasoconstricted.

 ═══ THE ONE RULE THAT PREVENTS MOST ERRORS ══════════════════════════════════
   OFFSET is a constant you can correct for.
   LAG is not. A lagging site is WRONG IN A DIRECTION THAT DEPENDS ON WHICH WAY
   THE PATIENT IS MOVING:
        heating up  → rectal UNDER-reads the true core
        cooling down → rectal OVER-reads the true core
   Which is why rectal is right for DIAGNOSING heat stroke and misleading for
   deciding when to STOP cooling it.

Choosing a site by the question you are asking

"Does this person have a fever?" In a cooperative, thermally stable adult, oral is adequate: apply the offset (add about 0.4 °C) or, better, use the site-specific threshold. Infrared aural and temporal artery devices are acceptable as screening tools and unacceptable as the basis for a decision, because their limits of agreement with core routinely span 1.5 °C or more — wide enough to convert a fever of 38.5 into a normal 37.0.

"Is this collapsed athlete in heat stroke?" Rectal, and only rectal. This is not fastidiousness. In a hyperthermic, vasodilated, sweating runner, every peripheral site under- reads, often by 2 °C or more, and the entire diagnosis of exertional heat stroke turns on whether the core is above 40 °C. Field studies comparing sites in collapsed runners have found oral, aural, temporal, and axillary readings so poorly correlated with rectal that they are formally recommended against. Nia's 40.1 °C is a rectal measurement, which is why it can be trusted.

"When do I stop cooling?" Here rectal lag becomes a hazard in the opposite direction. During aggressive immersion cooling, deep pelvic tissue empties its heat slowly, so the rectal probe keeps reading high after the core has already come down. Standard practice is therefore to stop active cooling at 38.5–39.0 °C rectal, not 37, and accept the continued drift — otherwise you overshoot into iatrogenic hypothermia.

"Is this person hypothermic, and how cold?" See the sidebar below; this is where ordinary equipment fails outright.

25.3 Heat Balance: The Equation That Governs Everything Else

Every thermal event in this chapter — fever, heat stroke, hypothermia, a marathon, an incubator — is a rearrangement of one equation. Learn it once and the rest becomes bookkeeping.

S = M − W ± R ± C ± K − E

where, all in watts (joules per second):

Two features deserve immediate attention. M is always positive — you cannot switch off metabolism, so a living body is always adding heat and heat balance is always a problem of disposal, never of supply. And E has only one sign: evaporation of water from a body surface always removes heat, never adds it, because condensation onto skin warmer than the dew point does not occur. That asymmetry is why evaporation becomes the last line of defence when everything else has reversed.

Converting storage to temperature

Storage in watts becomes temperature through the specific heat capacity of human tissue, which is about 3.47 kJ per kilogram per degree Celsius (0.83 kcal/kg/°C) — lower than water's 4.18 because the body is only 60% water and protein, fat, and mineral have lower heat capacities. (For conversions between watts, kilojoules, kilocalories, and METs, see Appendix H · Units and Conversions.)

ΔT_core = (S × t) ÷ (specific heat × body mass)

For a 70 kg adult, the whole body absorbs 243 kJ per degree Celsius (58 kcal/°C). That number is worth memorizing, because it explains why thermal events happen on the timescales they do. A 100-watt heater — a resting human — with no heat loss at all would raise its own core temperature by 1 °C every 40 minutes. A 700-watt runner would do it every 6 minutes.

The production side, itemized

 ═══ THE HEAT BALANCE EQUATION ═══════════════════════════════════════════════

        S    =    M   −   W   ±   R   ±   C   ±   K   −   E
        │         │       │       │       │       │       │
     STORAGE   METABOL  EXTERNAL  RADIA-  CONVEC- CONDUC- EVAPOR-
     (what's   PRODUCT  WORK      TION    TION    TION    ATION
     left over)  ION

     S > 0 → core temperature RISES at S ÷ (3.47 kJ/kg/°C × mass) °C/s
     S = 0 → thermal steady state
     S < 0 → core temperature FALLS

 ─── PRODUCTION SIDE (M) ─────────────────────────────────────────────────────
   70 kg adult                      RESTING          RUNNING (10 METs)
   ┌──────────────────────────┬────────────────┬──────────────────────────┐
   │ Basal metabolic rate     │  80 W   (80%)  │   80 W    (11%)          │
   │ Thermic effect of food   │  10 W   (10%)  │   10 W     (1%)          │
   │ Activity                 │  10 W   (10%)  │  640 W    (88%)  ◄───────┤
   │ (shivering, if cold)     │ +200–400 W     │    —                     │
   │ (non-shivering, BAT)     │  +10–30 W      │    —                     │
   ├──────────────────────────┼────────────────┼──────────────────────────┤
   │ TOTAL M                  │ 100 W          │  900 W                   │
   │ minus external work W    │   0 W          │ −200 W  (22% efficient)  │
   │ ═ HEAT TO BE DISPOSED OF │ 100 W          │  700 W    ◄── 7× MORE    │
   └──────────────────────────┴────────────────┴──────────────────────────┘

 ─── LOSS SIDE (R, C, K, E) ──────────────────────────────────────────────────
                          REST, 21 °C          EXERCISE, 21 °C
                          still air, clothed   moving at 3 m/s
   RADIATION      R        60 W    (60%)        ~90 W    (13%)
   CONVECTION     C        15 W    (15%)       ~175 W    (25%)
   CONDUCTION     K         3 W     (3%)        ~14 W     (2%)
   EVAPORATION    E        22 W    (22%)       ~420 W    (60%)
      of which:  insensible 20 W                    30 W
                 sweat       2 W                   390 W
   ───────────────────────────────────────────────────────────────────
   TOTAL LOSS             100 W                 ~700 W
   STORAGE  S               0 W                    0 W  (in steady state)

 ─── WHAT CHANGED ────────────────────────────────────────────────────────────
   Heat to dispose of ......... ×7
   Radiation's SHARE .......... 60% → 13%   (absolute value barely rises;
                                             its share collapses)
   Evaporation's SHARE ........ 22% → 60%   (absolute value rises ×19)
   Sweat required ............. 3 mL/h → 620 mL/h

   THE RULE: as heat load rises, EVAPORATION TAKES OVER — and evaporation is
   the one avenue that depends on the environment's HUMIDITY rather than its
   TEMPERATURE. This is why a 24 °C, 80% humidity marathon is harder on a
   runner than a 30 °C, 30% humidity one.

The loss side: four avenues and no others

Heat leaves a body by exactly four physical routes. Chapter 5 (§5.7) introduced them at the level of the skin; here is the physics, and — more usefully — the conditions under which each one turns around and starts delivering heat into you.

 ═══ THE FOUR AVENUES OF HEAT EXCHANGE ═══════════════════════════════════════

 ① RADIATION                          R = h_r · A_r · (T_skin − T_surroundings)
   ))) ))) )))                        h_r ≈ 4.7 W/m²/°C for skin
  SKIN ──────► cooler wall            Depends on the FOURTH POWER of absolute
   ))) ))) )))                        temperature, but is near-linear over the
                                      small ranges bodies encounter.
   Emissivity of human skin ≈ 0.98 — essentially a perfect black body in the
   infrared, REGARDLESS OF SKIN PIGMENT. (Visible-light colour is irrelevant;
   thermal radiation is at 8–14 µm.)
   Needs NO contact, NO air movement. Works in a vacuum.
   ► REVERSES when surroundings are hotter than skin: sunlight, a fire, a
     radiant heater, a metal roof, a hot road surface, a car interior.
   REST: ~60% of loss.  EXERCISE: falls to ~10–15% of a much larger total.

 ② CONVECTION                         C = h_c · A · (T_skin − T_air)
    ↑ ↑ ↑                             h_c ≈ 8.3 × v^0.6 W/m²/°C  (v = air speed)
   SKIN                               still air ≈ 3;  at 3 m/s ≈ 17;
    ↑ ↑ ↑                             IN WATER ≈ 25–100 × the still-air value
   Air (or water) touching skin is warmed, becomes less dense, rises, and is
   replaced. Moving fluid strips the insulating BOUNDARY LAYER away.
   ► REVERSES when air is hotter than skin (above ~35 °C).
     A FAN IN 40 °C AIR IS A CONVECTION HEATER. This matters for Adwoa.
   REST: ~15%.  EXERCISE (moving through air): 20–30%.

 ③ CONDUCTION                         K = k · A · (T_skin − T_object) / d
   ▪▪▪▪▪▪                             k = thermal conductivity of the contact
   SKIN                               air 0.026 · fat 0.2 · muscle 0.5 ·
   ██████ cold floor                  water 0.6 · steel 50 W/m/°C
   Direct molecular transfer to a touching solid or liquid. Trivial at rest
   (small contact area); enormous lying on cold ground or immersed.
   Subcutaneous FAT is the body's insulation against this route.
   ► REVERSES on hot sand, hot pavement, a heating pad, a warmed mattress.
   REST: ~3%.  EXERCISE: ~2%.  IMMERSED: can be >90%.

 ④ EVAPORATION                        E = h_e · A · w · (P_skin − P_air)
   ↟ ↟ ↟                              driven by WATER VAPOUR PRESSURE, not by
   SKIN + airways                     temperature.  h_e ≈ 16.5 × h_c
   Latent heat of vaporization at skin temperature ≈ 2,426 kJ/kg
     = 0.58 kcal per gram = 580 kcal per LITRE evaporated
   ► NEVER REVERSES. Evaporation only removes heat.
   ► BUT IT CAN BE ABOLISHED: at 100% relative humidity at skin temperature,
     P_skin − P_air = 0 and E = 0 no matter how much sweat is produced.
   ► SWEAT THAT DRIPS REMOVES NO HEAT. Only the phase change cools.
   REST: ~22%.  EXERCISE: 60–85%.  HOT ENVIRONMENT: ~100%.

 ═══ THE CROSSOVER ═══════════════════════════════════════════════════════════
   T_ambient rises above T_skin (≈ 33–35 °C):
      RADIATION  ──► reverses, becomes GAIN
      CONVECTION ──► reverses, becomes GAIN
      CONDUCTION ──► reverses, becomes GAIN
      EVAPORATION ─► the ONLY remaining route out. 100% of heat loss.
   Add high humidity and evaporation fails too.
   HOT + HUMID + no evaporation = NO ROUTE OUT AT ALL. Core temperature then
   rises at M ÷ (3.47 × mass) °C per second, and nothing can stop it. This is
   the physical definition of an UNCOMPENSABLE heat load (§25.10).

Insensible loss: the part you cannot switch off

Two evaporative losses proceed continuously whether or not you sweat, and they are not regulated for temperature at all.

• Transepidermal water loss: roughly 300–400 mL/day diffusing through the stratum corneum. This is not sweat; it is water leaking through the lipid lamellae of the epidermal barrier (Chapter 5). It rises enormously when the barrier is damaged — a large burn, or the immature skin of a 24-week preterm infant, which can lose 100–200 mL/kg/day.
• Respiratory water loss: roughly 250–400 mL/day. Inspired air is warmed to 37 °C and saturated to 100% relative humidity in the airway; expired air carries that water away. Because the driving force is the humidity of inspired air, this loss rises in cold dry air — which is why your breath is visible and why polar travellers dehydrate.

Together, insensible losses total 600–900 mL/day and dissipate 350–500 kcal/day, which is 20–25% of basal heat production. Humans, unlike dogs, do not pant, so respiratory evaporation is a fixed background rather than a thermoregulatory effector.

A worked heat balance: Nia's marathon

Take the case file's numbers and run them through the equation. Nia weighs 57 kg with a VO₂max of 58.4 mL/kg/min, and runs the marathon at about 72% of VO₂max.

Step 1 — Metabolic rate. VO₂ = 0.72 × 58.4 × 57 = 2.40 L O₂/min. At approximately 20.9 kJ per litre of oxygen consumed, M = 2.40 × 20.9 = 50.1 kJ/min = 836 W.

Step 2 — External work. Running is about 22% efficient, so W ≈ 0.22 × 836 = 184 W of useful mechanical work.

Step 3 — Heat produced. M − W = 836 − 184 = 652 W. Round to 650 W — six and a half 100-watt bulbs, inside a 57 kg body.

Step 4 — What that would do unopposed. Her heat capacity is 57 × 3.47 = 198 kJ per °C. 650 W = 39 kJ/min, so ΔT = 39 ÷ 198 = 0.20 °C per minute — one degree every five minutes. Unopposed, she would be at 42 °C in twenty-five minutes and dead within the first 10 km.

Step 5 — What actually happened. She ran for 3 h 30 min (12,600 s), producing 650 × 12,600 = 8,190 kJ of heat. She stored (40.1 − 37.0) × 198 = 614 kJ. So she dissipated 7,576 kJ — 92.5% of everything she produced — and stored the remaining 7.5%. Her thermoregulatory system did not fail. It succeeded 92.5 times out of a hundred, and the 40.1 °C reading is the visible residue of the 7.5% it could not place.

Step 6 — What the dissipation cost. At 24 °C air, her skin at ~35 °C, and running at 12 km/h (3.3 m/s), the convective coefficient is h_c ≈ 8.3 × 3.3^0.6 ≈ 17 W/m²/°C. Over an effective 1.4 m² with an 11 °C gradient, convection removed roughly 260 W; radiation to 24 °C surroundings removed another 60 W. That leaves about 330 W for evaporation. At 2,426 kJ per kilogram:

330 J/s × 3,600 s/h ÷ 2,426,000 J/kg = 0.49 kg/h of sweat actually evaporated

Over 3.5 hours: 1.7 L evaporated. Because on an 80%-humidity day a large fraction of secreted sweat drips instead of evaporating, her secretion rate was probably 1.1–1.3 L/h, around 4 L total — roughly 7% of her body mass, which is why she needed 1.5 L during the race and still finished 2 kg lighter.

25.4 The Controller: Receptors, Hypothalamus, and the Two-Threshold Model

Chapter 1 asked you to force every control system into four boxes: variable, receptor, control centre, effector. Thermoregulation fills all four more completely than any other system in this book, and it also breaks the simple version of the model in an instructive way.

Peripheral thermoreceptors

Cutaneous thermoreception is carried by free nerve endings — no corpuscle, no capsule, no accessory structure. Two populations exist, and they are not symmetrical.

Three implications fall out of that table.

Cold is detected faster than warmth. More receptors, more superficial, on faster fibres. Touch something cold and you know instantly; touch something warm and there is a perceptible delay. This is a defensible design: the environment threatens humans with cold far more often than with heat, and behavioural escape from cold must be quick.

Both populations are strongly dynamic. A receptor's firing rate depends not only on temperature but on its rate of change. That is why stepping into a 32 °C pool feels distinctly cold for thirty seconds and then feels like nothing: the dynamic response adapts, and only the smaller static response remains. Thermal sensation is largely a derivative signal, which is why a body can be dangerously overheated without feeling any thermal change at all — nothing is changing.

There is a paradox worth knowing. Cold receptors have a second, small firing peak above 45 °C. Stimulate them there and the brain, reading a labelled line, reports cold. This is paradoxical cold — the reason a very hot shower can feel briefly icy, and part of the reason grabbing something extremely hot produces a confused, hard-to-localize sensation.

TRP channels: the molecular thermometers

The transduction machinery is a family of transient receptor potential (TRP) channels — non-selective cation channels in the free nerve ending whose open probability depends on temperature. Each is a molecular thermometer with a threshold.

This is why chilli peppers hurt. Capsaicin binds a pocket on TRPV1's intracellular side and lowers its thermal activation threshold from 43 °C down toward body temperature. The channel opens. The neuron fires. The action potential travelling up that axon is identical to the one it would fire if the tissue were genuinely at 45 °C, and the brain has no independent way to check. This is the labelled line principle from Chapter 15 in its purest form: the meaning of a signal is determined by which axon carries it, not by what opened the channel.

The consequences are not merely perceptual. Capsaicin on the skin produces real reflex sweating and real cutaneous vasodilation, because the reflex arc is downstream of the same afferent. And repeated high-dose capsaicin depletes the terminal of substance P and eventually defunctionalizes it — which is why an 8% capsaicin patch is a licensed treatment for neuropathic pain. A channel agonist that makes you sweat is, on longer exposure, an analgesic.

And this is why mint feels cold. Menthol binds TRPM8 and shifts its activation threshold upward, so the channel opens at skin temperatures where it normally would not. The cold fibres fire; you feel cool. Your skin temperature does not change by a hundredth of a degree. Hold that fact — a sensation of cooling entirely dissociated from actual cooling — because §25.6 and §25.10 will use it to make a point about athletes, and §25.8 will use its mirror image to make a point about Adwoa.

The afferent pathway splits in two

Signals from cutaneous thermoreceptors enter the dorsal horn and synapse in lamina I, then cross and ascend in the lateral spinothalamic tract (Chapters 12–13). At the brainstem the pathway divides, and the division is the single most illuminating piece of modern thermoregulatory neuroscience:

• The perceptual arm continues to the ventral posterolateral thalamus, then to posterior insular cortex. This produces conscious thermal sensation — "my hands are cold."
• The reflex arm branches to the lateral parabrachial nucleus (LPB) in the pons, which projects to the median preoptic nucleus (MnPO) of the hypothalamus. This drives autonomic thermoregulation — vasoconstriction, shivering, sweating — without any conscious involvement.

Two arms means the two can dissociate, and clinically they do. A patient can regulate temperature correctly while perceiving nothing (insular lesions), or perceive normally while failing to regulate. More relevantly for this chapter: the perceptual arm is what makes behavioural thermoregulation possible, and it is the arm that degrades first with age. If the insular readout is blunted, the autonomic arm may still be working — and the person will still be in mortal danger, because behaviour is the effector with the largest capacity and behaviour requires perception. That is the skeleton of the answer to Case File Question 2.

The preoptic area: the integrator

The preoptic area of the anterior hypothalamus (POA/AH), immediately rostral to the optic chiasm and adjacent to the third ventricle, is the control centre. Three things make it suited to the job.

1. It senses temperature directly. Roughly 30% of POA neurons are warm-sensitive, increasing their firing rate as local tissue temperature rises; about 5% are cold-sensitive; the rest are insensitive. Warm-sensitive neurons are the workhorses. Because the POA sits in a densely perfused region, it samples the temperature of arterial blood — which is to say, the core.
2. It receives the skin signal via the LPB → MnPO relay, and integrates it with the core signal. The weighting is not equal: for autonomic effectors, core temperature carries roughly four times the weight of mean skin temperature. For behavioural and perceptual responses the weighting reverses, and skin dominates.
3. It also receives core signals from outside the brain — thermosensitive neurons in the spinal cord, the abdominal viscera, and the walls of the great veins. Spinal thermosensitivity alone contributes a measurable fraction of the total drive.

The output logic is elegantly simple and counter-intuitive: warm-sensitive POA neurons tonically inhibit the heat-production and heat-conservation pathways. They are a brake, held on. Cool the POA and the brake releases — a disinhibition — and the downstream circuitry in the dorsomedial hypothalamus (DMH) and the rostral raphe pallidus (rRPa) of the medulla fires, driving sympathetic outflow to brown fat and cutaneous vessels and somatic drive to shivering. Warm the POA and the brake is applied harder, plus separate POA populations actively drive sweating and cutaneous vasodilation.

Why does this matter? Because fever works by inhibiting those warm-sensitive neurons (§25.7). PGE₂ does not push a set-point dial. It silences the brake.

There is no set point. There are two thresholds.

Chapter 1 introduced homeostasis with a thermostat metaphor: a stored target, an error signal, a corrective response. It is a good first model and thermoregulation is where it starts to strain.

A thermostat has one temperature. The human thermoregulatory system has two, and the gap between them is where most of physiology lives:

• The vasoconstriction/shivering threshold — the core temperature below which cold defence activates.
• The vasodilation/sweating threshold — the core temperature above which heat defence activates.

Between them lies the interthreshold zone (or "null zone"), in which neither autonomic effector is active and core temperature is allowed to drift. In a healthy awake adult this zone is astonishingly narrow — about 0.2–0.4 °C. That narrowness is why core temperature looks like a defended set point: the two thresholds are so close that a single number is a good approximation.

But they are separately adjustable, and separating them experimentally is what proves the model:

Sitting outside this, at the level of the environment rather than the body, is the thermoneutral zone (TNZ): the range of ambient temperatures over which a resting person maintains core temperature using vasomotor tone alone — no shivering, no sweating. For a nude, resting adult in still air, the TNZ is about 27–31 °C, which strikes most students as absurdly warm until they remember they have never once been nude, resting, and still. Add ordinary clothing (about 1 clo of insulation) and the zone slides down to roughly 20–22 °C — the temperature at which offices are set, which is not a coincidence.

 ═══ THE COMPLETE THERMOREGULATORY CONTROL LOOP ══════════════════════════════

  ┌─── RECEPTORS ──────────────────────────────────────────────────────────┐
  │ PERIPHERAL (skin)                    CENTRAL (core)                    │
  │  cold: TRPM8/TRPA1, Aδ, 10× more      POA warm-sensitive neurons (~30%)│
  │  warm: TRPV3/V4/V1, C fibres          spinal cord · viscera · great    │
  │  ► fast, dynamic, EARLY WARNING       veins ► the ERROR SIGNAL          │
  └────────────┬───────────────────────────────────┬───────────────────────┘
               │ lamina I → spinothalamic          │ (direct, local
               │                                   │  blood temperature)
        ┌──────┴────────┐                          │
        ▼               ▼                          │
  ┌───────────┐   ┌──────────────┐                 │
  │ VPL       │   │ LATERAL      │                 │
  │ thalamus  │   │ PARABRACHIAL │                 │
  │ → INSULA  │   │ NUCLEUS      │                 │
  │           │   └──────┬───────┘                 │
  │ CONSCIOUS │          ▼                         ▼
  │ SENSATION │   ┌──────────────────────────────────────┐
  │     │     │   │  MEDIAN PREOPTIC ─► PREOPTIC AREA /  │
  │     │     │──►│  ANTERIOR HYPOTHALAMUS               │
  │     ▼     │   │  integrates skin (×1) + core (×4)    │
  │ BEHAVIOUR │   │  warm-sensitive neurons TONICALLY    │
  │  (skin    │   │  INHIBIT the cold-defence pathway    │
  │  dominates)│  └───────┬──────────────────┬───────────┘
  └───────────┘           │                  │
                 COLD ◄───┘                  └───► HOT
                 (disinhibition)                   (active drive)
                          │                              │
                  ┌───────┴────────┐            ┌────────┴────────┐
                  ▼                ▼            ▼                 ▼
             DORSOMEDIAL      ROSTRAL RAPHE   POA →           POA → sympathetic
             HYPOTHALAMUS     PALLIDUS        medulla →       CHOLINERGIC
                  └──────┬─────────┘          withdrawal of   sudomotor (M3)
                         ▼                    vasoconstrictor      │
              ┌──────────────────────┐        tone + ACTIVE        ▼
              │ EFFECTORS — COLD     │        vasodilator     ┌──────────────┐
              │ 1 vasoconstriction α₁│             │          │ EFFECTORS —  │
              │ 2 behaviour          │             ▼          │ HOT          │
              │ 3 shivering (somatic)│      ┌──────────────┐  │ 1 vasodilat- │
              │ 4 NST: BAT, UCP1, β₃ │      │ skin flow    │  │   ation      │
              │ 5 thyroid (slow)     │      │ 0.25 → 8 L/m │  │ 2 behaviour  │
              └──────────┬───────────┘      └──────────────┘  │ 3 SWEATING   │
                         │                                    └──────┬───────┘
                         └──────────────► CORE TEMPERATURE ◄─────────┘
                                    (feedback closes the loop)

 ═══ THE TWO-THRESHOLD MODEL ═════════════════════════════════════════════════

   metabolic
   heat prod.  ▲                                              ▲ sweat rate
      5× BMR   │╲                                            ╱│  2 L/h
               │ ╲ SHIVERING                        SWEATING╱ │
               │  ╲                                        ╱  │
      1× BMR   │   ╲______________________________________╱   │  0
               └────┬──────────┬──────────────┬─────────┬─────► core T
                   35.0      36.7           37.1      38.0
                            ▲            ▲
                            │            │
                  COLD THRESHOLD    WARM THRESHOLD
                  (vasoconstrict,   (vasodilate,
                   then shiver)      then sweat)
                            └────┬───────┘
                        INTERTHRESHOLD ZONE
                        awake adult .......... 0.2–0.4 °C
                        under anaesthesia .... 2–4 °C
                        FEVER: both thresholds slide RIGHT together
                        CIRCADIAN: both slide together, 0.5–1.0 °C/24 h

   AMBIENT AXIS — THE THERMONEUTRAL ZONE (resting adult)
   ────────────────────────────────────────────────────────────────
   °C ambient:  10    15    20    25   [27 ── 31]   35    40    45
                └─ shivering ─┘  vaso-  TNZ: vaso-  └── sweating ──┘
                                 constrict  motor tone ONLY
   Clothed (1 clo): the whole scale shifts LEFT by ~7 °C → TNZ 20–22 °C
   Naked term newborn: TNZ is 32–34 °C — narrow and HIGH (§25.10)

25.5 Effector Responses to Cold

The effectors are recruited in a strict economic order: cheapest first. Vasomotor adjustment is instantaneous and costs nothing. Behaviour costs almost nothing and has the largest capacity. Shivering costs fuel at three to five times basal rate. Non-shivering thermogenesis costs lipid. Thyroid modulation costs days. The body spends in that order, and knowing the order lets you predict which response a given patient has already exhausted.

1 · Cutaneous vasoconstriction — free, instant, and worth 2–3× insulation

Sympathetic noradrenergic fibres to cutaneous arterioles and to the arteriovenous anastomoses of acral skin (§5.7) increase their firing, and skin blood flow falls from a thermoneutral 250 mL/min to as little as 20–50 mL/min. Two receptor populations do the work:

• α₁-adrenoceptors mediate the bulk of the response and are active at all skin temperatures.
• α₂C-adrenoceptors are the interesting ones. At warm temperatures they sit sequestered in the Golgi apparatus and are functionally silent. Cooling causes them to translocate to the plasma membrane, where they become responsive to the same noradrenaline that was already present. This is the molecular basis of local cold-induced vasoconstriction — a finger cooled in isolation constricts even without any change in central drive — and it is implicated in Raynaud phenomenon, where the translocation is exaggerated and a normal cold stimulus produces a pathological, painful, tri-colour vasospasm.

The functional gain is large: fully constricted shell has roughly two to three times the insulating value of dilated shell, and the effect is free. It is also the reason cold and shock look identical from the outside — the same effector, driven by two different control systems, which is the lesson Chapter 5 built around Amara's cool, pale, diaphoretic skin at 36.8 °C.

2 · Countercurrent heat exchange — recycling heat before it escapes

In the limbs, deep arteries run in intimate contact with paired deep veins (the venae comitantes). Warm arterial blood flowing distally transfers heat directly across the vessel wall into cool venous blood flowing proximally. The heat is returned to the core before it ever reaches the hand.

The system is switchable, and the switch is which veins carry the return:

• Cold: venous return shifts to the deep venae comitantes, adjacent to the arteries. Countercurrent exchange is maximal. Arterial blood arriving at the wrist can be 6–8 °C cooler than it was at the axilla. The hand is sacrificed to protect the core.
• Warm: venous return shifts to the superficial veins — the ones you can see on the back of a warm hand — which run far from the arteries. Countercurrent exchange is bypassed, warm blood reaches the surface, and heat is delivered to the skin for disposal.

The arrangement has a cost: the distal limb runs cold, and in extreme cold it would freeze. The body's partial answer is cold-induced vasodilation (CIVD), or the hunting reaction: when finger skin falls below about 10 °C, vasoconstriction periodically fails and the finger flushes warm for a few minutes, on a cycle of roughly 5–10 minutes. It is a compromise — trading a little core heat to keep the digits above freezing — and it is better developed in populations with generational cold exposure and in cold-acclimatized individuals such as fishers and mountaineers.

3 · Shivering thermogenesis — expensive, effective, and self-limiting

Shivering is involuntary, rhythmic, asynchronous contraction of antagonistic muscle pairs at 4–8 Hz. Because agonist and antagonist oppose one another, net external work is approximately zero — which means 100% of the chemical energy consumed becomes heat. Shivering is the only effector in the body whose inefficiency is the point.

The sequence is stereotyped and clinically useful:

1. Pre-shivering tone — a general rise in muscle tone that raises heat production 20–50% before any visible tremor.
2. Masseter and neck — the first visible shivering, which is why teeth chatter first.
3. Trunk, then proximal limbs, then distal.
4. Maximal shivering, recruiting most of the body's muscle mass.

Capacity: a sustained 2–3 × BMR, with brief peaks near 5 ×. For a 70 kg adult that is roughly 200–400 W of extra heat — very substantial, and roughly the output of the whole body during brisk walking.

Four limitations matter clinically:

• It burns carbohydrate. Shivering is fuelled disproportionately by muscle glycogen and blood glucose early on. A hypoglycaemic or glycogen-depleted person shivers poorly, which is why exhausted, underfed hikers become hypothermic at temperatures that would not trouble a fed one. Alcohol compounds this by suppressing gluconeogenesis and lowering the vasoconstriction threshold.
• It increases heat loss too. Shivering muscle needs blood, so it partially defeats the vasoconstriction that preceded it, and the movement stirs the insulating boundary layer of air. Net efficiency is therefore lower than the raw metabolic figure suggests — measured net gain is often only half of the metabolic increment.
• It costs oxygen. Shivering raises whole-body oxygen consumption two- to fivefold. In a patient with limited cardiac or coronary reserve, shivering is a stress test nobody ordered.
• It fails at about 30–32 °C core. Below that, the hypothalamic drive itself is suppressed and shivering stops. A hypothermic patient who is not shivering is not comfortable; they are past the point where they can rewarm themselves.

4 · Non-shivering thermogenesis: brown adipose tissue and UCP1

Mitochondria normally couple the proton gradient across the inner membrane to ATP synthesis. Uncoupling protein 1 (UCP1, thermogenin), expressed almost exclusively in brown adipocytes, provides a regulated proton channel that short-circuits that gradient. Protons flow back into the matrix without passing through ATP synthase, the free energy is released as heat, and the electron transport chain — relieved of its back-pressure — runs at maximal rate, consuming oxygen and burning fatty acids furiously.

The activation pathway is a clean adrenergic cascade:

POA disinhibition → rRPa → sympathetic preganglionic → noradrenaline on β₃-adrenoceptors → Gs → adenylyl cyclase → cAMP → PKA → hormone-sensitive lipase → free fatty acids → which both fuel UCP1 and directly activate it by displacing its inhibitory purine nucleotides

Fatty acids are simultaneously the substrate and the switch, which makes the response fast and self-reinforcing.

How much does it matter in adults? For a long time the textbook answer was "not at all after infancy." PET-CT imaging overturned that: metabolically active brown fat is detectable in the supraclavicular, paravertebral, mediastinal, and perirenal depots of most adults when they are cold, in quantities of roughly 50–100 g. Cold-activated brown fat raises adult metabolic rate by perhaps 5–15% — call it 10–30 W. That is real, it is more in cold-acclimatized people, and it is metabolically interesting (brown fat is a glucose and triglyceride sink, and its mass correlates inversely with BMI and with type 2 diabetes). It is not, however, a major thermal effector in an adult: 30 W against shivering's 300 W.

Beige (brite) adipocytes are UCP1-expressing cells that appear within white adipose depots after prolonged cold exposure or β₃-agonist treatment. They arise from a different precursor lineage than classical brown adipocytes — which develop from a Myf5-positive dermomyotomal lineage shared with skeletal muscle, a genuinely surprising fact that explains why brown fat is metabolically closer to muscle than to white fat.

5 · Thyroid hormone — the slow gain control

Cold exposure over days to weeks increases hypothalamic TRH, hence TSH, hence T4 and T3. Thyroid hormone raises basal heat production by several mechanisms (Chapter 16): increased Na⁺/K⁺-ATPase expression, increased mitochondrial biogenesis, increased futile cycling, and — critically for this chapter — increased β-adrenoceptor density, which makes every other adrenergic effector in the list more responsive. Thyroid hormone is permissive: it does not produce the cold response, it sets the gain on it.

The clinical read-out is unambiguous. Hypothyroid patients are famously cold-intolerant; severe untreated hypothyroidism presents as myxoedema coma, in which hypothermia is a defining feature and in which the patient characteristically does not shiver. Hyperthyroid patients are heat-intolerant, sweat excessively, and run warm.

6 · Behaviour — and why it is the most powerful effector you have

Students consistently underrate behaviour because it does not look like physiology. Consider the numbers.

Behaviour beats every autonomic effector combined, and it does so for free. It also has three properties none of the autonomic effectors share:

1. It is anticipatory. It is driven mainly by skin thermoreceptors and by thermal discomfort, so it fires before core temperature has moved at all. Every other effector is a response to an error; behaviour prevents the error.
2. It is unlimited in range. No autonomic effector can generate a 40 °C gradient. A heated building does it routinely.
3. It is why humans occupy every climate on Earth while remaining, physiologically, a tropical animal with essentially no fur, high sweat capacity, and feeble cold acclimatization (§25.9).

And behaviour has a specific, catastrophic failure mode, which is the entire mechanism of Case File Question 2. Behaviour requires an intact chain: perception → interpretation → decision → mobility → resources. Break any link and the most powerful effector in the body silently goes offline while the autonomic effectors, which are far weaker, are left to do the whole job. Adwoa's fan was two metres away and unplugged.

25.6 Effector Responses to Heat

Heat defence has only two autonomic effectors — move blood to the surface, and wet the surface — and both of them are borrowed from other systems. That borrowing is the source of every limitation in this section.

Cutaneous vasodilation: two mechanisms, not one

Skin blood flow rises from a thermoneutral 250 mL/min to 6–8 L/min under severe heat stress — up to 60% of cardiac output. Two distinct mechanisms produce it:

• Passive vasodilation (withdrawal of vasoconstrictor tone). Sympathetic noradrenergic firing falls, α₁ tone is released, and vessels dilate. This accounts for only about 10–20% of the maximal response, and it is all that is available in acral skin, which has no active vasodilator innervation.
• Active vasodilation. A separate population of sympathetic cholinergic fibres — the same fibres that innervate sweat glands — releases acetylcholine together with an as-yet unidentified co-transmitter (VIP and substance P are the leading candidates), acting through nitric oxide. This accounts for 80–90% of the rise in non-acral skin.

The dependency is the clinically important part: active vasodilation requires intact sudomotor nerves. Anything that destroys the sympathetic cholinergic supply — diabetic autonomic neuropathy, some spinal injuries, extensive scarring or graft (Chapter 5) — abolishes sweating and active vasodilation in the same territory, removing both heat effectors at once.

The eccrine sweat gland: why sweat is hypotonic

There are 2–4 million eccrine glands, and each is a simple coiled tubular gland with two functionally opposite halves.

The secretory coil sits in the deep dermis. Its clear cells secrete a near-isotonic precursor fluid resembling plasma ultrafiltrate: basolateral NKCC1 loads the cell with Na⁺, K⁺, and Cl⁻; Cl⁻ exits apically through CFTR and other channels; Na⁺ follows paracellularly down the electrical gradient; water follows osmotically. Myoepithelial cells wrapped around the coil contract to expel the fluid. The whole coil is driven by sympathetic postganglionic fibres releasing acetylcholine onto M3 muscarinic receptors — the great exception to sympathetic pharmacology (Chapter 14), and the reason anticholinergic drugs abolish sweating.

The duct does the opposite. Its cells reabsorb Na⁺ through ENaC and Cl⁻ through CFTR, and — critically — the duct is relatively water-impermeable, so salt leaves and water does not follow. The fluid that reaches the surface is therefore hypotonic: typically 20–60 mmol/L Na⁺ against plasma's 140.

Two predictions follow immediately, and both are correct:

1. Sweat sodium rises with sweat rate. Faster flow gives the duct less contact time, so a smaller fraction is reabsorbed. At low rates sweat can be as dilute as 10 mmol/L; at maximal rates it may exceed 70.
2. Aldosterone lowers sweat sodium, by upregulating ductal ENaC exactly as it does in the distal nephron (Chapter 26). This is the mechanism behind one of the most useful adaptations in heat acclimatization (§25.9), which can drive sweat sodium down to 5–20 mmol/L.

And a third, diagnostic: in cystic fibrosis, defective CFTR prevents ductal chloride reabsorption, so sweat chloride exceeds 60 mmol/L — the basis of the sweat chloride test, which detects a channel defect in the duct of a gland that has nothing to do with the lungs.

Maximal sweat rates run about 1 L/h unacclimatized and 2–3 L/h acclimatized (briefly), with sustainable rates near 1–1.5 L/h and daily totals of up to 10–12 L in extreme conditions.

The limits: humidity, and the competition for cardiac output

Sweat that does not evaporate does nothing. The evaporative capacity of an environment, E_max, is set by the water vapour pressure gradient between skin and air — not by air temperature. Two examples make the point sharper than any explanation:

Note that the hot dry day (35 °C, 20%) has more evaporative capacity than the mild humid day (24 °C, 80%), which is the single most counter-intuitive and most lethal fact in environmental physiology.

The second limit is haemodynamic. Skin needs up to 8 L/min; maximal cardiac output is fixed; working muscle wants everything it can get. Add the fact that cutaneous veins are highly compliant and pool 600–800 mL of blood when dilated, reducing central blood volume and therefore preload, and you have the mechanism of cardiovascular drift and of heat syncope (§25.8). Chapters 19 and 32 develop the circulatory arithmetic; the thermal consequence is simply that skin and muscle are drawing on one account.

25.7 Fever, Properly Explained

Fever is the most common abnormal temperature in medicine and the most widely misunderstood. The misunderstanding is always the same: people treat it as heat that has got in, when it is heat the body has deliberately generated toward a target it has deliberately raised.

From pathogen to hypothalamus

Exogenous pyrogens are molecules from outside the body that initiate the cascade — lipopolysaccharide from Gram-negative bacterial walls (the classic), lipoteichoic acid and peptidoglycan from Gram-positives, viral double-stranded RNA, and superantigens. They are not themselves the fever signal; they are recognized by pattern recognition receptors on macrophages and other innate immune cells (Chapter 21).

Those cells release endogenous pyrogens: IL-1β, IL-6, TNF-α, and interferons. These are the true fever signals, and they are the reason non-infectious conditions — trauma, infarction, malignancy, autoimmune disease, drug reactions — also cause fever. Amara's low-grade temperature in the 48 hours after her infarct (Chapter 21) was IL-6 from infarcted myocardium, not infection.

The signal must now cross into the brain, and cytokines are large hydrophilic proteins that cannot cross the blood–brain barrier. The solution is anatomical: the organum vasculosum of the lamina terminalis (OVLT), a circumventricular organ on the anterior wall of the third ventricle, immediately adjacent to the preoptic area, which lacks a blood–brain barrier. Its fenestrated capillaries let circulating cytokines reach the endothelial and perivascular cells lining them.

There the cascade converts from a protein signal to a lipid one:

cytokines at the OVLT → induction of COX-2 and microsomal PGE synthase-1 → synthesis of prostaglandin E₂ → PGE₂ diffuses the short distance to the median preoptic nucleus → binds EP3 receptors on GABAergic neurons → inhibits the warm-sensitive POA neurons

And now recall §25.4: warm-sensitive POA neurons are a tonic brake on cold defence. Inhibit them and the brake releases. DMH and rostral raphe pallidus fire; cutaneous vasoconstriction, shivering, and brown fat thermogenesis are recruited at once. The thresholds for both cold defence and heat defence slide upward together — which is the formal definition of a raised set point.

A parallel fast route exists via vagal afferents from hepatic and abdominal immune-sensing sites, which explains fevers that begin within minutes, before circulating cytokine levels have risen appreciably.

The three phases, predicted from the model

Once you accept that the set point has moved to, say, 39 °C, everything the patient does becomes obvious.

Phase 1 — Chill (rising). Actual core is 37 °C; target is 39 °C. Relative to the new target, the patient is cold, and every cold effector fires correctly: intense cutaneous vasoconstriction (pale, cold hands and feet, "goose flesh"), violent shivering (a rigor), and behavioural heat-seeking (curling up, piling on blankets). The patient feels freezing while their temperature climbs. Nothing is malfunctioning.

Phase 2 — Plateau. Core reaches 39 °C. The error is now zero, so the effectors quiet. Skin becomes warm and flushed; shivering stops; the patient feels hot but not cold. They are defending 39 °C as accurately as they defended 37 °C yesterday.

Phase 3 — Flush / defervescence. Pyrogen production falls, or an antipyretic blocks PGE₂ synthesis, and the set point drops back toward 37 °C. Now the patient at 39 °C is 2 °C too hot, and every heat effector fires: profuse cutaneous vasodilation and drenching sweat — the "crisis" of pre-antibiotic clinical description.

Fever versus hyperthermia

 ═══ TWO WAYS TO REACH 40 °C ═════════════════════════════════════════════════

 PANEL A — FEVER: THE SET POINT MOVED. REGULATION IS INTACT.

   core T                          SET POINT (both thresholds)
   °C  40 ┤                    ┌════════════════════┐
       39 ┤              ╭─────┤  NEW TARGET 39 °C  ├──────╮
       38 ┤          ╭───╯     └════════════════════┘      ╰──╮
       37 ┤══════════╯                                         ╰═════
          └──┬────────┬──────────┬──────────────┬─────────┬────────►
          pyrogen  CHILL       PLATEAU       antipyretic  FLUSH
                   shiver +    effectors     or pyrogen   vasodilate
                   vasoCON     quiet;        clears;      + SWEAT
                   feels COLD  feels hot     set point ▼

   MECHANISM: IL-1/IL-6/TNF → OVLT → COX-2 → PGE2 → EP3 on MnPO
              → inhibits warm-sensitive POA neurons → brake released
   CEILING:   self-limited, rarely >41 °C (endogenous cryogens: AVP, α-MSH)
   SKIN:      appropriate for the new set point (dry then wet)
   ANTIPYRETICS: WORK — they lower the set point

 PANEL B — HYPERTHERMIA: THE SET POINT IS NORMAL. REGULATION IS DEFEATED.

   core T                     SET POINT stays at 37 — and is IGNORED
   °C  43 ┤                                              ╭─── death
       42 ┤                                         ╭────╯
       41 ┤                                   ╭─────╯
       40 ┤                            ╭──────╯
       39 ┤                    ╭───────╯   ▲ all heat effectors
       38 ┤            ╭───────╯           │ MAXIMAL and INSUFFICIENT
       37 ┤════════════╯════════════════════════════════════════════
          └──┬─────────────────────────────────────────────────────►
          heat load begins (exogenous, endogenous, or effector failure)

   MECHANISM: heat GAIN exceeds maximal heat LOSS
              · environment overwhelming (classic heat stroke)
              · production overwhelming (exertion, MH, thyroid storm)
              · effectors blocked (anticholinergics, dehydration, age)
   CEILING:   NONE. Rises until the load stops or the patient dies.
   SKIN:      dry (effector failure) OR drenched (effectors maxed out)
   ANTIPYRETICS: USELESS — there is no raised set point to lower

 ═══ THE DISCRIMINATOR ═══════════════════════════════════════════════════════
   Ask: IS THE BODY DEFENDING THIS TEMPERATURE, OR FIGHTING IT?
   Defending it (shivering/vasoconstricting on the way up) ....... FEVER
   Fighting it  (sweating/vasodilating maximally on the way up) .. HYPERTHERMIA
   Temperature above 41.5 °C is almost never fever alone.

This is the same "which box moved?" analysis Chapter 1 introduced with its table of broken feedback loops. In fever, nothing is broken — the set point was moved on purpose and every component works. In hyperthermia, the set point is correct and the effector limb has been outmatched or disabled. Two conditions that look identical on a thermometer and require opposite treatments.

25.8 Thermal Illness: The Full Spectrum

The heat illnesses, in ascending severity

Heat cramps (exercise-associated muscle cramping). Painful, involuntary, often excruciating spasms in heavily worked muscle, typically calves, quadriceps, or abdominals, usually appearing late in or shortly after prolonged exertion. Two mechanisms compete, and the honest answer is that both operate:

• Electrolyte depletion: heavy sweating in a "salty sweater" (sweat Na⁺ 60–80 mmol/L) with replacement by plain water produces a whole-body sodium and chloride deficit; cramping athletes are disproportionately from this group, and salty fluids relieve them.
• Altered neuromuscular control: muscle fatigue increases spindle afferent drive and decreases inhibitory Golgi tendon organ feedback, disinhibiting the alpha motor neuron. This explains cramps that occur in cool conditions, in single muscles, and in the absence of any electrolyte abnormality.

Treatment follows from both: passive stretching (which loads the Golgi tendon organ and inhibits the motor neuron directly — it works within seconds, before any electrolyte could move) plus salted fluid. Core temperature is normal. This is not, strictly, a thermal illness.

Heat syncope. Transient loss of consciousness in the heat, usually on standing or on stopping exercise. Mechanism is purely orthostatic: massive cutaneous vasodilation plus venous pooling plus some volume depletion, minus the skeletal muscle pump once the person stops moving. Core temperature is normal or near-normal, which is the diagnostic key. Recovery on lying down is prompt and complete. A soldier fainting on parade and a runner collapsing two steps past the finish line are the same event.

Heat exhaustion. The inability to sustain cardiac output against the combined demands of skin and muscle. Core temperature is elevated but usually below 40 °C. Presentation: heavy sweating, weakness, dizziness, headache, nausea, tachycardia, hypotension, sometimes brief irritability or poor judgement — but the central nervous system works. The patient can answer questions, follow commands, and give a history. Two variants: water-depletion (thirsty, hypernatraemic, evolves over days) and salt-depletion (nauseated, hyponatraemic, evolves over days of sweating with water-only replacement). Treatment is rest, a cool environment, and oral or intravenous fluid.

Exertional heat stroke. Core temperature above 40 °C with central nervous system dysfunction — confusion, ataxia, aggression, seizure, or coma. That combination is the definition, and the CNS finding is what separates it from heat exhaustion.

The pathophysiology is worth stating because it explains the treatment. Direct thermal cytotoxicity is only the first insult. Splanchnic vasoconstriction (blood diverted to skin and muscle) produces gut ischaemia; the intestinal barrier leaks; endotoxin translocates into the portal circulation and triggers a systemic inflammatory response that is, physiologically, indistinguishable from sepsis. The consequences — disseminated intravascular coagulation, rhabdomyolysis, acute kidney injury, hepatic failure, acute respiratory distress syndrome — can progress after the temperature is normal.

Which yields the governing rule: outcome depends on the area under the time–temperature curve. Survival with rapid cooling — core below 40 °C within 30 minutes — approaches 100%. Beyond that window, mortality climbs steeply with every additional minute. Hence the doctrine:

Cool first, transport second.

Classic (non-exertional) heat stroke. The same end state reached by an entirely different route.

Two other hyperthermic states belong in the differential and are covered in the next sidebar: thyroid storm (a fever-like hyperthermia driven by extreme thyroid hormone excess) and the four drug-induced syndromes.

Hyperthermia from inside: four drug-induced syndromes

All four produce hyperthermia with a normal set point. All four are missed when the elevated temperature is read as fever. They are distinguished at the bedside by muscle tone and reflexes.

The mnemonic for anticholinergic toxicity — hot as a hare, dry as a bone, red as a beet, blind as a bat, mad as a hatter — is a list of blocked muscarinic effectors, and the "hot" and "dry" are the same lesion: M3 blockade at the eccrine sweat gland removes the only effector capable of dissipating heat above skin temperature. Keep that in mind when you reach Adwoa's medication list.

The cold injuries

Cold produces two categories of harm that are frequently confused: systemic cooling of the core (hypothermia) and local injury to tissue that may occur with a completely normal core temperature (frostbite, non-freezing cold injury).

 ═══ STAGES OF HYPOTHERMIA ═══════════════════════════════════════════════════

 CORE   STAGE            NEURO / BEHAVIOUR      CARDIOVASCULAR   ECG
 ─────────────────────────────────────────────────────────────────────────────
 37.0   normothermia     normal                 HR 60–100        normal
 ─────────────────────────────────────────────────────────────────────────────
 36–35  COLD STRESS      shivering begins;      HR ↑, BP ↑       normal;
        (not yet         fine motor loss        vasoconstriction shiver
        hypothermia)     "the fumbles"                           artefact
 ─────────────────────────────────────────────────────────────────────────────
 35–32  MILD  (HT I)     MAXIMAL SHIVERING;     HR ↑ then ↓      sinus tach
                         apathy, poor           BP normal        → sinus brady
                         judgement, amnesia     COLD DIURESIS    ± shiver
                         "stumbles, mumbles,    (↓ ADH + central artefact
                          fumbles, grumbles"     volume shift)
 ─────────────────────────────────────────────────────────────────────────────
 32–28  MODERATE (HT II) SHIVERING STOPS        bradycardia      ★ OSBORN (J)
                         (~30–32 °C)            hypotension      WAVES appear
                         consciousness ↓        ATRIAL FIB       PR, QRS, QT
                         PARADOXICAL            common           all PROLONG
                         UNDRESSING             CO falls ~50%
                         pupils dilate
 ─────────────────────────────────────────────────────────────────────────────
 28–24  SEVERE (HT III)  coma; areflexia;       myocardium       prominent J
                         no pain response;      HIGHLY           waves; brady
                         appears dead           IRRITABLE —      → junctional;
                         (cold protects the     ROUGH HANDLING   VF risk HIGH
                         brain: CMRO2 ~40%)     CAN CAUSE VF
 ─────────────────────────────────────────────────────────────────────────────
 < 24   PROFOUND (HT IV) no vital signs         asystole         asystole
                         PUPILS FIXED           (≈ 24 °C or      or VF
                         ► THIS IS NOT DEATH      below)
 ─────────────────────────────────────────────────────────────────────────────
 13.7   lowest documented adult survival with FULL neurological recovery

 ═══ THE OSBORN (J) WAVE ═════════════════════════════════════════════════════
        R                    A positive deflection at the J point —
        ╱▏                   the junction of QRS and ST segment.
       ╱ ▏  ╭─╮              · Appears below ~32 °C
      ╱  ▏ ╱   ╲___ T        · AMPLITUDE ∝ DEGREE OF HYPOTHERMIA
   ──╱   ▏╱  ▲              · Best seen in leads II, V5, V6
     P    S   └ J wave       · Not specific: also in hypercalcaemia,
              (camel hump)     SAH, Brugada, early repolarization

 ═══ AFTERDROP: WHY CORE TEMPERATURE KEEPS FALLING DURING REWARMING ══════════
   MECHANISM 1 — CONDUCTIVE (dominant, and pure physics)
     A cold shell sits against a warmer core. Heat flows down the gradient,
     from core to shell, and CONTINUES TO DO SO after external rewarming
     starts — because the gradient has not yet reversed.

     core 33 °C  ──heat──►  shell 20 °C  ──heat──►  warm blanket 40 °C
                 (still flowing OUT of the core)

   MECHANISM 2 — CONVECTIVE ("rescue collapse")
     Warming the LIMBS first vasodilates them. Blood that has been sitting in
     cold, ischaemic, acidotic, hyperkalaemic tissue is flushed back to the
     heart. Core temperature drops, pH drops, K+ rises, and a marginally
     stable myocardium fibrillates.

   THEREFORE:  ► rewarm the CORE, insulate the periphery
               ► handle the patient GENTLY (VF is mechanically triggerable)
               ► keep them HORIZONTAL during extrication from water

Why hypothermic cardiac arrest is resuscitated differently. Three physiological facts drive the whole approach.

1. Cold protects the brain. Cerebral metabolic rate falls 6–7% per degree, so at 20 °C the brain is running at roughly a fifth of normal demand. Neurological recovery has been documented after more than six hours of hypothermic cardiac arrest with continuous CPR and extracorporeal rewarming. Hence the aphorism: "nobody is dead until they are warm and dead."
2. The cold heart does not respond normally to drugs or shocks. Below 30 °C, defibrillation is usually ineffective — attempt up to three times, then withhold further shocks until the core is above 30 °C. Drug metabolism is slowed and receptors are less responsive, so adrenaline is withheld below 30 °C and given at doubled intervals between 30 and 35 °C. Giving standard doses to a cold patient simply accumulates them for a toxic bolus effect on rewarming.
3. The definitive treatment is rewarming, not the arrest algorithm. Extracorporeal life support (ECMO or cardiopulmonary bypass) rewarms at 4–10 °C/hour while providing circulation, and produces the best outcomes by a wide margin. Where it is unavailable: warmed humidified oxygen, warmed intravenous fluid, and body-cavity lavage.

One prognostic marker deserves mention because it answers the hardest question at the scene: serum potassium. A very high potassium (above roughly 12 mmol/L in avalanche burial) indicates that cell lysis — and therefore asphyxial death — preceded the cooling, and resuscitation is futile. Hypothermia that preceded the arrest carries a good prognosis; hypothermia that followed it does not.

Frostbite. Actual freezing of tissue, requiring skin temperature below about −0.5 °C. The injury has two phases. In the freezing phase, ice crystals form first in the extracellular space, drawing water osmotically out of cells and dehydrating them; with faster cooling, intracellular ice forms and shears membranes directly. In the reperfusion phase — which begins when you rewarm and is often the larger injury — endothelial damage, thromboxane and prostaglandin release, platelet aggregation, and progressive microvascular thrombosis extend the necrosis well beyond the frozen zone.

Management follows directly from that mechanism: rapid rewarming in circulating water at 37–39 °C; do not rub (mechanical damage to ice-laden tissue); do not rewarm if refreezing is possible, because a freeze–thaw–refreeze cycle is far more destructive than continued freezing; consider iloprost or thrombolysis within 24 hours for severe injury, aimed squarely at the microvascular thrombosis; and delay amputation for weeks to months ("frostbite in January, amputate in July") because the true line of demarcation takes that long to declare itself.

Non-freezing cold injury (trench foot, immersion foot). Prolonged exposure — hours to days — to wet cold above freezing, typically 0–15 °C. No ice forms. The injury is to peripheral nerves and microvasculature, from sustained vasoconstriction and ischaemia. The characteristic course is deceptive: the foot is initially numb, white, and painless; on rewarming it becomes hyperaemic, swollen, and intensely painful for weeks, and long-term cold hypersensitivity and neuropathic pain are common. It disabled more soldiers in the trenches of 1914–18 than frostbite ever has, and it is prevented by keeping feet dry and changing socks — a behavioural intervention, again.

Chilblains (pernio) sit at the mild end: itchy, red-purple, inflammatory papules on digits after repeated exposure to damp cold, resolving over weeks.

25.9 Circadian Thermoregulation and Acclimatization

The clock, and what it does to temperature

The suprachiasmatic nucleus (SCN) is a paired hypothalamic nucleus of roughly 20,000 neurons on each side, sitting directly above the optic chiasm and immediately adjacent to the preoptic area — which is not a coincidence.

Its rhythm is generated cell-autonomously by a transcription–translation feedback loop: CLOCK and BMAL1 dimerize and drive transcription of Period (Per1, Per2, Per3) and Cryptochrome (Cry1, Cry2); the PER and CRY proteins accumulate, dimerize, re-enter the nucleus, and inhibit CLOCK/BMAL1; they are then degraded and the cycle restarts. One turn takes approximately 24.2 hours in humans — slightly longer than a day, which is why our clock has a natural tendency to delay rather than advance, and why phase delays are easier to achieve than phase advances.

Entrainment to the 24-hour day is achieved almost entirely by light. Intrinsically photosensitive retinal ganglion cells (ipRGCs) containing the photopigment melanopsin, maximally sensitive around 480 nm (blue), project directly to the SCN through the retinohypothalamic tract. These cells are not part of image formation; a person with no conscious vision may still entrain normally if the ipRGC pathway is intact.

Light does not simply "reset" the clock. Its effect depends entirely on when it arrives, described by the phase response curve, whose pivot is the core temperature minimum (CTmin):

Light BEFORE the core temperature minimum → PHASE DELAY (clock pushed later) Light AFTER the core temperature minimum → PHASE ADVANCE (clock pulled earlier) Light in the middle of the biological day → little effect

Hold on to that rule. It is the whole of Case File Question 3.

The temperature rhythm itself has an amplitude of 0.5–1.0 °C, with a minimum around 04:00–06:00 (about two hours before habitual waking) and a maximum around 17:00–19:00. It is genuinely endogenous: it persists under "constant routine" conditions, with the subject awake, semi-recumbent, in dim light, eating identical hourly snacks. It is produced by the SCN shifting both thermoregulatory thresholds together (§25.4) — not by activity, and not by meals, though both add exogenous masking on top.

How it relates to sleep. The relationships are tight and are worth knowing because they are clinically actionable:

• Sleep onset occurs on the falling limb of the temperature curve, when core temperature is dropping fastest.
• That fall is achieved by distal vasodilation — hands and feet warm as heat is dumped from the periphery. The distal-to-proximal skin temperature gradient is the best single physiological predictor of how quickly a person will fall asleep.
• Melatonin, secreted by the pineal gland under SCN control beginning about two hours before habitual sleep (dim-light melatonin onset), causes that distal vasodilation. It is a heat-loss signal as much as a sleep signal.
• Hence the practical trick: a warm bath 1–2 hours before bed provokes a rebound of distal vasodilation and accelerates the core temperature fall, shortening sleep latency. The bath works by making you lose heat faster, not by warming you.
• The CTmin is also the nadir of alertness, reaction time, and body temperature simultaneously — which is why the small hours are when errors and single-vehicle crashes cluster.

 ═══ CORE TEMPERATURE OVER 24 HOURS ══════════════════════════════════════════

 °C
 37.6 ┤                                        ╭───●───╮        NORMAL (───)
      │                                    ╭───╯       ╰──╮     day worker
 37.4 ┤                                ╭───╯              ╰─╮
      │        ╭──╮                ╭───╯                    ╰╮
 37.2 ┤    ╭───╯  ╰──╮        ╭────╯                         ╰──╮
      │  ╭─╯         ╰────╮ ╭─╯                                 ╰──╮
 37.0 ┤╭─╯               ╰─╯                                       ╰─╮
      ││                                                             ╰
 36.8 ┼╯      ░░░░                                                    ╰──
      │      ░    ░░                                            ╭──
 36.6 ┤     ░       ░░                                       ╭──╯
      │    ░           ░░░                              ╭────╯
 36.4 ┤  ░░               ░░░░                    ╭─────╯
      │ ░                     ░░░░░         ╭─────╯
 36.2 ┤░                           ░░░░░░░░░╯  ← AMARA (░░░) inverted
      │                            ▲
 36.0 ┤                         CTmin 14:00
      └──┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────►
        00   02   04   06   08   10   12   14   16   18   20   22
              ▲                                    ▲
           NORMAL CTmin ~04:00                NORMAL peak ~17–19:00
           (○ = alertness nadir)              (● = peak)

   NORMAL:  min 04:00 · max 18:00 · amplitude 0.9 °C
   AMARA:   min 14:00 · max 02:00 · amplitude 0.9 °C  → PHASE SHIFT 10 h
            (a normal rhythm, correctly generated, in the wrong place)

 ═══ WHY IT DOES NOT RESET ON DAYS OFF ═══════════════════════════════════════
   PHASE RESPONSE CURVE — the effect of light depends on when it arrives

     phase   ADVANCE ▲                    ╭──────╮
     shift        +2h │                 ╭─╯      ╰─╮
                    0 ┼───────╮───────╭─╯          ╰─────────────
                      │        ╲     ╱ ▲
             DELAY −2h│         ╰───╯  │
                      └──┬────┬────┬───┼───┬────┬────┬───────────►
                         18   21   00 CTmin 06   09   12
                                       ▲
                              LIGHT BEFORE CTmin → DELAY
                              LIGHT AFTER  CTmin → ADVANCE

   AMARA'S CTmin IS AT 14:00. So on a day off:
     morning light 08:00 ....... falls BEFORE her CTmin → DELAYS her further
     midday light 12:00 ........ still BEFORE her CTmin → DELAYS her further
     afternoon light 16:00 ..... falls AFTER  her CTmin → advances (weakly)

   ► ORDINARY DAYLIGHT ON A DAY OFF ACTIVELY HOLDS HER PHASE WHERE IT IS.
   ► Re-entrainment runs at ~1 h/day (advance) to ~1.5 h/day (delay).
     A 10-hour correction needs 7–10 CONSECUTIVE correctly-timed days.
     Two days off cannot do it — and then she returns to nights.

Shift work, misalignment, and internal desynchronization

Only about 3% of permanent night workers achieve complete circadian adaptation to their schedule, and perhaps a quarter achieve substantial adaptation. The usual reason for failure is the phase response curve working against them: the night worker gets bright light on the morning commute home — which, for someone whose CTmin has already drifted into the early morning, falls after CTmin and produces a phase advance, pulling them back toward a day schedule, while their sleep occurs in a dim bedroom that supplies no correcting signal at all. The clock is dragged in one direction by the shift and back in the other by the commute, and settles nowhere.

Amara is in the small minority who did adapt — twenty years of predominantly night work has produced a stable, high-amplitude, fully inverted rhythm. And that adaptation is now the problem, because her social life still runs on days.

Three distinct phenomena must be separated to understand her situation.

1 · Phase shift. Her SCN has genuinely re-entrained to a schedule ten hours from the solar day. This is a real change in the pacemaker, not a temporary displacement.

2 · Masking. On days off, her behaviour changes — she is upright in the day, eats in the day, sleeps at night. Posture, activity, and meals all impose exogenous temperature variation on top of the endogenous rhythm. A casual measurement on a day off would therefore show a partly "normalized"-looking curve, which is why her true phase only became visible under continuous monitoring in the controlled setting of cardiac rehabilitation. Masking hides the phase; it does not move it.

3 · Internal desynchronization. The SCN is not the only clock. Nearly every tissue — liver, heart, kidney, pancreas, adipose, skeletal muscle — contains a peripheral oscillator running the same molecular loop. Peripheral clocks are entrained principally by feeding time, whereas the SCN is entrained by light. A shift worker eats at night and gets light at unusual hours, so the two entraining signals point in different directions and the clocks dissociate from one another. She is not simply "on the wrong schedule." Her hypothalamus, her liver, and her behaviour can each be in a different phase at the same moment.

The consequences of that dissociation are measurable and are not merely about sleepiness. Forced-desynchrony experiments, in which healthy volunteers are placed on a 28-hour day so that behaviour cycles across all circadian phases, show that misalignment alone — with diet, activity, and sleep duration controlled — raises postprandial glucose, reduces insulin sensitivity, inverts the cortisol rhythm, raises blood pressure, and lowers leptin. Which is to say: circadian misalignment is not a comorbidity of Amara's metabolic syndrome. It is one of its causes, and it has been running for twenty years alongside the insulin resistance of Chapter 16 and the metabolic syndrome of Chapter 24.

Heat acclimatization: the most impressive adaptation humans make

Heat acclimatization is fast, large, and reliable — which is rare in physiology.

The sequence is cardiovascular first, sudomotor second, which is worth knowing because the early cardiovascular gains give a false sense of readiness before the sweat adaptations exist. Adaptation is roughly 75–80% complete by day 8–10 and essentially complete by day 14. It decays from about a week after heat exposure stops and is largely gone in 3–4 weeks — which is why re-acclimatization is required after a break, and why an athlete who acclimatized in May is unprotected in a July heat wave if June was spent indoors.

Cold acclimatization: modest by comparison

Humans acclimatize to cold in three ways, none of them dramatic:

• Habituation (the commonest): repeated cold exposure blunts the responses — less shivering, less vasoconstriction, less subjective discomfort, and blunting of the cold shock response on immersion. Note that this is a reduction in defence, not an improvement in it. Its practical value is real for winter swimmers, because the cold shock response — a reflex gasp and hyperventilation on sudden immersion — is a leading cause of drowning.
• Metabolic acclimatization: increased non-shivering thermogenesis. Ten days of daily exposure to 15–17 °C measurably increases brown adipose tissue volume and activity on PET imaging and raises cold-induced thermogenesis.
• Insulative acclimatization: enhanced peripheral vasoconstriction with preserved core temperature, documented in habitual cold-water divers such as the Korean haenyeo.

The contrast with heat acclimatization is stark, and the explanation is behavioural. Humans are tropical animals: essentially hairless, with an enormous evaporative capacity and a modest capacity for heat generation. We never evolved strong physiological cold defence because clothing, shelter, and fire arrived first and removed the selection pressure. Behaviour did not merely supplement the autonomic effectors. It replaced the need to evolve better ones.

25.10 Advanced Topic · The Limits of Human Thermal Tolerance

Measuring the environment, not the air

Air temperature is a poor description of thermal stress because it ignores three of the four avenues in Figure 25.4. The standard index in occupational, military, and athletic settings is the wet-bulb globe temperature (WBGT), which combines three separate instruments:

Outdoors, with solar load: WBGT = 0.7 T_nwb + 0.2 T_g + 0.1 T_db Indoors or no solar load: WBGT = 0.7 T_nwb + 0.3 T_g

• T_nwb, the natural wet-bulb temperature, is read from a thermometer with a wetted wick exposed to the prevailing air movement. It integrates humidity and wind — the two things that determine evaporative capacity.
• T_g, the globe temperature, comes from a thermometer inside a black copper sphere and measures radiant load — sunshine, hot pavement, a metal roof.
• T_db is ordinary shaded air temperature.

Notice the weighting: air temperature carries 10% and the wet-bulb term carries 70%. The index is telling you, in its coefficients, that humidity is the dominant variable — the same conclusion §25.9 reached from vapour pressures.

The wet-bulb limit

Wet-bulb temperature (T_w) is the lowest temperature that can be reached by evaporating water into the air. It is a physical property of that air. It follows directly that:

If the wet-bulb temperature of the environment equals or exceeds skin temperature (~35 °C), no net heat can leave the body by any avenue. Radiation, convection, and conduction have reversed. Evaporation has been abolished. Metabolic heat has nowhere to go.

Core temperature must then rise at M ÷ (3.47 × mass) °C per second — for a resting adult, roughly 0.4 °C per hour, and considerably faster with any activity. From 37 °C, the 43–44 °C limit is a matter of a few hours, and no amount of water, fitness, acclimatization, shade, or behaviour changes it. Only removal from the environment does. This is the theoretical survivability threshold identified in climate-physiology modelling, and wet-bulb temperatures above 35 °C have now been recorded briefly in the Persian Gulf and the Indus Valley.

Two important refinements:

1. The practical limit is well below the theoretical one. Direct human experiments measuring the critical environmental limit — the point at which core temperature begins to rise uncontrollably in young healthy subjects performing minimal activity — find thresholds around 25–28 °C wet-bulb in humid conditions, not 35. The theoretical value assumes an unclothed, resting, fully hydrated young adult in the shade with perfect skin wettedness. Real humans do not meet those assumptions.
2. Older adults and people on the drugs listed in §25.8 reach their limit far sooner still. The 35 °C figure describes the limit for the healthiest possible human. Nobody in an actual heat wave is that person.

Thermal comfort is not thermal safety

These are two different variables, measured differently, and this chapter has repeatedly shown them separating:

Every dissociation in this chapter is an instance of that table. Menthol produces comfort without cooling. A breeze in saturated air produces comfort without evaporation. Nia was extremely uncomfortable and entirely safe. Adwoa was comfortable and dying. Because there is no receptor that reports heat storage, comfort is the only signal a person has — and it is a proxy that can be wrong in either direction.

The two ends of life

The neonate and the older adult fail thermally for opposite structural reasons, and comparing them makes the general principle visible.

The newborn's problem is physics — too much surface, too little insulation, too little muscle. The older adult's problem is control — the sensors, the effectors, and the behavioural chain all degrade. The newborn's deficit is fully correctable by an incubator, because you can solve a physics problem with an environment. The older adult's deficit is not correctable by any device, because you cannot install a warning system, which is why heat-wave mortality is addressed by social interventions — welfare checks, cooling centres, and someone knocking on the door.

Chapter Summary

§25.1 Temperature is not a substrate but a rate constant, setting the speed of every reaction (Q₁₀ ≈ 2–3, or 10–13% per °C) and the stability of every protein and membrane. Cooling is reversible and self-protecting; heating denatures proteins above 40–41 °C and is self-amplifying, which is why humans survive being 23 °C too cold and die from being 5 °C too hot. Core and shell are not anatomy but a circulatory state: the boundary moves from a few millimetres in the heat to the whole thickness of the limbs in the cold.

§25.2 A thermometer measures what it touches. The distal oesophagus is the best practical core site (offset ±0.1 °C, lag 1–2 min); rectal is the standard for heat illness but lags 10–30 minutes and therefore errs in whichever direction the patient is moving; infrared aural, temporal artery, and axillary devices are screening tools only. Ordinary thermometers bottom out at 34 °C and cannot stage hypothermia.

§25.3 S = M − W ± R ± C ± K − E. Body heat capacity is 3.47 kJ/kg/°C, so a 70 kg adult stores 243 kJ per degree. At rest, radiation carries 60% of loss and evaporation 22%; during exercise evaporation carries 60–85%. Nia produced 8,190 kJ of heat over her marathon, stored 614 kJ (7.5%), and dissipated the other 92.5%.

§25.4 Cold receptors outnumber warm ten to one, are more superficial, and run on faster fibres. TRPV1 (heat, capsaicin) and TRPM8 (cool, menthol) are molecular thermometers whose chemical agonists produce genuine thermal sensations by the labelled-line principle. The afferent pathway splits into a perceptual arm (thalamus → insula, driving behaviour) and a reflex arm (parabrachial → preoptic, driving autonomics). Warm-sensitive preoptic neurons tonically inhibit cold defence. There is no single set point: there are two thresholds separated by an interthreshold zone of 0.2–0.4 °C, which anaesthesia widens tenfold.

§25.5 Cold effectors are recruited cheapest-first: vasoconstriction (free, 2–3× insulation, α₁ and cold-recruited α₂C), countercurrent exchange in the limbs, behaviour, shivering (2–5× BMR, fails below 30–32 °C), brown-fat non-shivering thermogenesis through UCP1 (10–30 W in adults, doubling of heat production in neonates), and thyroid gain control. Behaviour is quantitatively the most powerful effector, is anticipatory, is unlimited in range — and requires perception, cognition, mobility, and resources.

§25.6 Cutaneous vasodilation is 80–90% active, driven by sympathetic cholinergic fibres shared with the sweat glands; skin flow rises from 250 mL/min to 6–8 L/min. The eccrine gland secretes an isotonic precursor in its coil and reabsorbs sodium through ENaC in its water-impermeable duct, making sweat hypotonic. Evaporation is limited by the vapour pressure gradient, not temperature, and skin competes with muscle for a fixed cardiac output.

§25.7 Pyrogens → IL-1/IL-6/TNF → the OVLT, which lacks a blood–brain barrier → COX-2 → PGE₂ → EP3 receptors → inhibition of warm-sensitive preoptic neurons → both thresholds rise together. Chill, plateau, and flush phases follow logically. Antipyretics lower the set point and cannot lower a normal one. Fever is regulated; hyperthermia is regulation defeated — and antipyretics do nothing for the latter.

§25.8 Heat cramps and heat syncope occur at normal core temperature; heat exhaustion is below 40 °C with an intact CNS; heat stroke is above 40 °C with CNS dysfunction, and outcome depends on the area under the time–temperature curve — cool first, transport second. Exertional and classic heat stroke differ in patient, tempo, skin, and cooling method. Malignant hyperthermia (RyR1, dantrolene), neuroleptic malignant syndrome, serotonin syndrome, and anticholinergic toxicity are distinguished by muscle tone and reflexes. Hypothermia is staged by shivering, consciousness, and the Osborn wave; afterdrop is mainly conductive; cold protects the brain, so hypothermic arrest is resuscitated with prolonged CPR, withheld drugs below 30 °C, and extracorporeal rewarming.

§25.9 The suprachiasmatic nucleus generates a 0.5–1.0 °C temperature rhythm by shifting both thresholds together, entrained by melanopsin-containing retinal ganglion cells. Light before the core temperature minimum delays the clock; light after it advances. Shift work produces phase shift, masking, and internal desynchronization between the light-entrained SCN and feeding-entrained peripheral clocks. Heat acclimatization is fast and large — plasma volume, then earlier and heavier sweating, then aldosterone-driven salt conservation, over 10–14 days. Cold acclimatization is feeble by comparison, because behaviour removed the selection pressure.

§25.10 WBGT weights the wet-bulb term at 70% because humidity dominates. Above a wet-bulb temperature of ~35 °C no heat can leave the body at all, though real-world limits are much lower. Thermal comfort and thermal safety are different variables, and every dissociation in this chapter — menthol, a breeze in saturated air, a 78-year-old in a heat wave — exploits the gap between them.

Case File 25 · Resolution

Question 1 — A core temperature of 40.1 °C was benign in Nia and would have been an emergency in Adwoa. What makes the same number mean two different things?

Because core temperature is a state variable, not a diagnosis. It tells you where the system currently is; it says nothing about how it got there, whether it is still regulating, or how long it has been there. Five things determine the meaning of any given number.

1 · Is the body defending this temperature or fighting it? This is the primary discriminator (Figure 25.6). Nia arrived at 40.1 °C with every heat effector at maximum: skin flushed, sweat running, skin blood flow near 8 L/min. Her regulation was intact and losing by a small margin — 7.5% of the total load. Adwoa arrived warm and dry, with no sweat at all: her effectors had failed or been blocked. A rising temperature with maximal effectors is a system at its limit; a rising temperature with absent effectors is a system that is no longer in the loop.

2 · Endogenous or environmental heat? Nia's heat came from inside and stopped the moment she stopped running; the load was self-terminating and self-paced. Adwoa's came from a 34 °C flat and did not stop, day or night, for four days.

3 · Duration — the area under the time–temperature curve. Nia was above 40 °C for a few minutes at the end of a race and back below 38.5 °C within thirty minutes of cooling. Adwoa had been storing heat for days. Thermal injury is a dose. The same peak temperature multiplied by a hundredfold longer exposure is a hundredfold larger insult.

4 · Central nervous system function — the clinical definition. Nia was fully lucid; she gave a coherent account of her race. That single finding excludes heat stroke and places her in the category of benign exertional hyperthermia, which is genuinely common: marathon finishers routinely record 39–40 °C and recover uneventfully. Adwoa was confused, and confusion in the setting of heat stress is heat stroke until proven otherwise. It was the neurological finding, not the thermometer, that made her an emergency.

5 · Reserve. Nia is 24, trained, acclimatized, euvolaemic, with a VO₂max of 58.4 and a maximal cardiac output that can supply skin and muscle simultaneously. Adwoa is 78, volume depleted, with reduced maximal cardiac output, blunted vasodilation, and a heat shock response that is itself attenuated with age. The same thermal dose is delivered to two systems with completely different capacities to absorb it.

One further point about the measurement itself. Adwoa's 38.9 °C was rectal on arrival — after extraction from the flat, after transport, after some passive cooling — and rectal temperature lags, so it under-reads a body that was hotter an hour earlier. Her true peak was almost certainly higher, and had a temporal-artery scanner been used instead it might have read 37.5 and reassured everyone. A number is only as meaningful as the site and the moment that produced it (§25.2).

Question 2 — Why does an aged thermoregulatory system fail silently?

Because the warning system and the defence system are the same system, and the warning half degrades first.

Work around the loop of Figure 25.5. The afferent limb goes first: cutaneous thermoreceptor density falls, conduction slows, and central integration coarsens, so an older adult needs roughly twice the temperature difference to detect a change. Add the fact from §25.1 that thermal sensation is largely a derivative signal — it reports change, and a flat that has been 34 °C for three days is not changing — and the perceptual signal that should have said "you are hot" was never generated.

The autonomic effectors then fail quietly: 25% fewer active sweat glands, 40–70% less output per gland, cutaneous vasodilation attenuated by 30–50%, maximal cardiac output reduced, and thirst blunted while total body water is already lower.

And the behavioural effector — the most powerful one she had, capable of removing the entire heat load by the act of plugging in a fan — never fired, because behaviour requires an intact chain of perception, interpretation, decision, mobility, and resources. Her mild cognitive change broke the middle of that chain; her blunted thermoreception broke the front of it; living alone removed the redundancy that another person would have supplied.

Her medications finished the job. If she takes an anticholinergic for urinary urgency, her sweat glands are pharmacologically silenced — M3 blockade removes the only effector that works above skin temperature. A diuretic depletes the volume needed to fill a dilated cutaneous bed. A beta-blocker limits the cardiac output required to perfuse the skin and blunts the tachycardia that would have warned the paramedics: her heart rate of 96 in a hyperthermic, hypotensive, dehydrated 78-year-old is not reassuring — it is a suppressed response where 120 would be expected.

So the answer is not that older adults ignore the heat, or are stoical, or are confused about what they feel. From the inside there was nothing to feel. The failure is silent because silence is its first symptom.

Question 3 — How can twenty years of night shift invert a hypothalamic rhythm, and why does it not reset on days off?

How it inverts. The SCN's ~24.2-hour molecular oscillation is entrained by light through melanopsin-containing retinal ganglion cells. Entrainment is not a preference for daytime; it is a mechanical response to the timing of light relative to the clock's own phase. Twenty years of predominantly nocturnal light exposure, daytime sleep in a dark room, and nocturnal eating supplied a consistent alternative zeitgeber pattern, and the pacemaker did what pacemakers do: it entrained to the pattern it was given. Amara's trace is not a damaged rhythm. Its amplitude is a healthy 0.9 °C and its shape is textbook. It is a correctly generated rhythm in the wrong place — CTmin at 14:00 rather than 04:00, a ten-hour phase shift.

Why two days off cannot undo it. Three reasons, and the first is the decisive one.

The phase response curve is working against her. Light shifts the clock in a direction that depends on when it arrives relative to her CTmin, not relative to the sun. Her CTmin is at 14:00. Light before that delays her; light after it advances her. On a day off, her brightest exposures — waking light at 08:00, midday light at 12:00 — all fall before her CTmin and therefore push her phase further from normal. Ordinary daylight is not a neutral influence on her days off. It is actively reinforcing the inversion.

Re-entrainment is slow. The clock moves roughly 1 hour per day for advances and 1.5 hours per day for delays. A ten-hour correction requires seven to ten consecutive days of correctly-timed zeitgebers. Two days is not a partial fix; it is barely a nudge, and the nudge is erased by the next run of nights.

What looks like re-adaptation is masking. On days off her behaviour changes — she is upright, active, and eating during daylight — and posture, activity, and meals impose exogenous temperature variation on top of the endogenous rhythm. A spot measurement would look half-normalized. Continuous monitoring in the controlled environment of cardiac rehabilitation stripped the masking away and revealed the true phase. Masking hides a rhythm; it does not move it.

And then a fourth phenomenon compounds all three: internal desynchronization. Peripheral clocks in liver, heart, adipose, and pancreas are entrained mainly by feeding time, while the SCN is entrained by light. Amara eats at night and gets light at unusual hours, so her central and peripheral clocks are pulled in different directions and dissociate. She is not merely shifted; her tissues are in different phases from one another.

Which explains both of her complaints, and neither is a thermoregulatory defect. Her temperature maximum falls at 02:00, in the middle of her shift, and the circadian rise in core temperature is achieved largely by distal cutaneous vasoconstriction — so at her peak, her hands and feet are genuinely at their coldest, in a ward held at 20–21 °C while she sits doing paperwork. She feels cold because her fingers are cold, exactly as designed. Her rehabilitation sessions fall at 14:00 — her circadian minimum, where both thermoregulatory thresholds are at their lowest. She therefore vasodilates and begins sweating at a lower core temperature and thus earlier in the session than she expects, and reports overheating immediately. Both symptoms are the correct output of a correctly working system whose clock is twelve hours out of step with when she is asked to do things.

Finally, the clinical significance. Circadian misalignment is not merely a sleep complaint. Controlled forced-desynchrony studies show that misalignment alone raises postprandial glucose, reduces insulin sensitivity, inverts the cortisol rhythm, and raises blood pressure. Twenty years of it sits upstream of the insulin resistance of Chapter 16, the metabolic syndrome of Chapter 24, and the hypertension of Chapter 19 — which is to say, upstream of the coronary disease that brought her to her own emergency department in Chapter 1.

Systems Integration Case File · Entry 25

Review

Level 1 · Recall

25.1 The rate of most enzymatic reactions in the body changes by approximately what percentage per degree Celsius?

a) 1–2%    b) 5%    c) 10–13%    d) 25–30%

25.2 Which measurement site combines the smallest offset from true core temperature with the shortest lag?

a) rectal    b) distal oesophageal    c) infrared aural    d) temporal artery

25.3 During hard exercise in a warm environment, which avenue carries the largest share of heat loss?

a) radiation    b) conduction    c) convection    d) evaporation

25.4 Menthol produces a sensation of coolness because it:

a) evaporates from the skin, removing latent heat b) causes cutaneous vasodilation c) shifts the activation threshold of TRPM8 so cold fibres fire at normal skin temperature d) inhibits warm-sensitive preoptic neurons

25.5 Sweat is hypotonic to plasma because:

a) the secretory coil produces a hypotonic primary secretion b) the duct reabsorbs sodium through ENaC while remaining relatively water-impermeable c) water is added to the sweat as it crosses the epidermis d) potassium is exchanged for sodium in the acrosyringium

25.6 The defining distinction between fever and hyperthermia is that in fever:

a) the temperature is lower b) the patient sweats c) the hypothalamic set point has been raised and is being defended d) the cause is infectious

25.7 Shivering ceases at approximately which core temperature?

a) 35 °C    b) 33 °C    c) 30–32 °C    d) 28 °C

25.8 Light exposure occurring before a person's core temperature minimum produces:

a) a phase advance    b) a phase delay    c) no effect    d) suppression of the rhythm's amplitude

Level 2 · Comprehension

25.9 Explain why humans tolerate a 23 °C fall in core temperature but only a 5 °C rise.

25.10 A patient is anaesthetized in a 21 °C operating theatre and becomes hypothermic within an hour, while the awake staff are comfortable. Explain, using the two-threshold model.

25.11 A wet-bulb temperature of 35 °C is described as the theoretical limit of human survivability. Explain why, and explain why the practical limit is lower.

Level 3 · Clinical Application

25.12 A 19-year-old collapses at kilometre 8 of a summer road race. He is combative and cannot say where he is. Skin is hot and drenched. Aural temperature reads 38.2 °C. A colleague suggests paracetamol and transport to hospital, 25 minutes away. Give your management and the physiological justification for each step.

25.13 A 68-year-old on haloperidol for three days is found rigid and febrile at 40.2 °C, with CK 12,000 U/L, reduced reflexes, and normal pupils. A colleague suggests serotonin syndrome. How would you distinguish the two, and what does the distinction change?

25.14 Amara asks whether she should worry about the coming heat wave forecast at 34 °C for four days. She takes metoprolol, an ACE inhibitor, a statin, and a thiazide-like diuretic; her ejection fraction is 48% with grade 2 diastolic dysfunction and her eGFR is 46 mL/min/1.73 m². Give her a mechanistic answer.

Level 4 · Integration and Synthesis

25.15 Nia is one year further on and pregnant (Chapter 28), training through a warm summer. Construct the physiological argument for why pregnancy narrows her thermal safety margin, using at least four systems and stating the direction of causation in each case.

25.16 Amara, now 47, is admitted with pneumonia and a temperature of 39.4 °C. She is shivering, her hands are cold, her heart rate is 118, and she is breathless. A junior colleague says "we don't treat fever, fever is protective." Argue the case both ways and give your decision with reasoning.

Concept Map to Complete

Copy onto blank paper and fill every bracket from memory first, then correct in a second colour. Definitions for any term you cannot place are in the Glossary.

                    S  =  [ __ ] − [ __ ] ± [ __ ] ± [ __ ] ± [ __ ] − [ __ ]
                                                │      │      │        │
   AT REST (%):                              [ __ ]  [ __ ] [ __ ]  [ __ ]
   IN EXERCISE (%):                          [ __ ]  [ __ ] [ __ ]  [ __ ]
   WHICH ONE NEVER REVERSES? [ ________ ]   WHICH CAN BE ABOLISHED? [ ______ ]

 ══════════════════════════════════════════════════════════════════════════════
   RECEPTORS                 INTEGRATOR                    EFFECTORS
   skin: [ ___ ] channel      [ ______ ] area /       COLD: 1 [ __________ ]
         for cool             [ ______ ] hypothal.          2 [ __________ ]
         [ ___ ] channel                                    3 [ __________ ]
         for heat/capsaicin   warm-sensitive neurons        4 [ ___ ]/UCP1
   core: [ ______ ] neurons   [ inhibit / excite ]    HOT:  1 [ __________ ]
                              the cold pathway              2 [ __________ ]
   afferent splits:                                         3 [ __________ ]
     perceptual → [ ______ ] → [ ______ ] cortex
     reflex     → [ ______ ] nucleus → [ ______ ] nucleus

   Weighting for AUTONOMIC responses: core × [ __ ] vs skin × [ __ ]
   Weighting for BEHAVIOURAL responses: [ ______ ] dominates
 ══════════════════════════════════════════════════════════════════════════════
   TWO THRESHOLDS, not one set point
     cold threshold [ ___ °C ]  ── interthreshold zone [ ___ °C ] ──
                                                      warm threshold [ ___ °C ]
     ANAESTHESIA widens it to [ ___ °C ]   FEVER moves [ one / both ]
 ══════════════════════════════════════════════════════════════════════════════
   FEVER: [ ___ ] pyrogen → [ ___ , ___ , ___ ] → [ ______ ] (no BBB)
          → COX-2 → [ ______ ] → [ ___ ] receptors → set point [ ↑ / ↓ ]
          antipyretics work because [ ____________________________ ]

   HYPERTHERMIA: set point [ ______ ], effectors [ ______ ]
          antipyretics work? [ yes / no ] because [ __________ ]
 ══════════════════════════════════════════════════════════════════════════════
   NIA 40.1 °C = benign because [ 5 reasons ]
   ADWOA 38.9 °C = emergency because [ 5 reasons ]
   AMARA CTmin at 14:00 does not reset because [ 3 reasons ]

Lab / Self-Exploration

1. Map your own core–shell boundary. With a cheap infrared thermometer, record skin temperature at shoulder, elbow, wrist, and fingertip. Do it in a warm room, then after five minutes with a bare arm at an open window on a cold day. Plot both profiles. You have reproduced Figure 25.1 on yourself; the gradient should widen dramatically in the cold while your oral temperature barely moves.
2. Demonstrate the vapour pressure limit. Wet the back of one hand and leave the other dry. Wave both. The wet hand feels dramatically cooler — that is 2,426 kJ/kg of latent heat. Now repeat in a steamy bathroom immediately after a hot shower. The effect largely disappears, because you have removed the vapour pressure gradient without changing the temperature at all.
3. Find your own paradoxical cold. Touch a metal surface at around 45–50 °C briefly (a mug of hot, not boiling, water). Many people report a momentary icy sensation before the heat registers — the high-temperature firing peak of cold receptors, read by the brain as cold because of the labelled line it arrives on.
4. Test TRPM8 and TRPV1 directly. Apply a menthol-containing balm to one forearm and capsaicin cream (a low-strength preparation) to a small patch on the other. Measure skin temperature at both sites with an infrared thermometer before and 10 minutes after. Record your sensation and your measurement separately. The dissociation is the point.
5. Track your own circadian rhythm. Take an oral temperature every two hours for a full waking day (avoiding food and drink for 20 minutes beforehand) and plot it. Most people find a 0.5–0.8 °C swing with a late-afternoon peak. If you work nights or have an irregular schedule, compare a work day against a day off — and note whether the curve has genuinely shifted or is merely being masked by your activity.
6. Measure your sweat rate. Weigh yourself nude before and after an hour of exercise, accounting for fluid drunk and urine passed. Each kilogram lost is roughly one litre of sweat. Then compute the heat that represents (litres × 580 kcal) and compare it with your estimated energy expenditure for the session. The two should be broadly similar, which is the heat balance equation closing on your own body.

Key Terms

Next: Chapter 26 · The Urinary System — where the water and sodium that this chapter spent on sweat are recovered, and where the cardiorenal loop that has been building since Chapter 18 finally closes.