22. The Respiratory System

Anatomy, Ventilation, Gas Exchange, and Transport

Learning Objectives

22.1 Functional Anatomy: A Tube That Becomes a Membrane

The respiratory system solves one problem: the atmosphere contains oxygen and your mitochondria need it, but your mitochondria are up to a metre away from the atmosphere and diffusion over that distance would take years. The solution has two halves. Bulk flow moves air rapidly to within a fraction of a millimetre of blood; diffusion takes over for the last stretch, where it is fast because the distance is tiny. Every anatomical feature in this chapter serves one of those two halves.

That split is so fundamental that the system is formally divided along it.

• The conducting zone is plumbing. Nose, pharynx, larynx, trachea, bronchi, and bronchioles down to and including the terminal bronchioles. No gas exchange happens here — these walls are too thick and have no capillary bed pressed against them. The conducting zone cleans, warms, humidifies, and delivers.
• The respiratory zone is membrane. It begins at the respiratory bronchioles — named for the fact that alveoli begin to bud directly off their walls — and continues through alveolar ducts to alveolar sacs and their alveoli. This is the only place in the body where a gas and the blood are separated by less than a micrometre.

The nose and nasal cavity

Air enters through the external nares into the nasal cavity, which is far more than a hole. Three shelves of bone — the superior, middle, and inferior nasal conchae — project from each lateral wall, curling downward and creating turbulent meatuses beneath them. Turbulence is normally an engineering problem; here it is the point. Swirling air is thrown repeatedly against mucus-covered walls, and the nasal cavity uses that contact to do three jobs before the air is 7 cm into your head:

• Warming. A rich venous plexus beneath the mucosa brings the air to near body temperature. This plexus is also why nosebleeds are common and why cold air makes your nose run.
• Humidifying. Air arrives at the trachea near 100% relative humidity, regardless of the weather. This matters more than it sounds: the alveolar membrane must stay wet, and drying it would be fatal.
• Filtering. Coarse hairs (vibrissae) trap large particles at the entrance; the mucus blanket traps everything down to about 5 µm.

The olfactory epithelium occupies the roof of the cavity (Chapter 15); the paranasal sinuses — frontal, sphenoid, ethmoid, maxillary — drain into it, lighten the skull, and resonate the voice.

The pharynx, in three parts

The pharynx is a shared corridor, and the design compromise it represents is one of the best examples in the body of evolution inheriting rather than engineering.

Note the epithelial change at the soft palate. The nasopharynx handles air and uses ciliated columnar epithelium built for mucus transport; the oropharynx handles swallowed boluses and switches to stratified squamous, which is built for abrasion. The epithelium changes exactly where the mechanical demand changes. You will see this rule again three more times in this chapter.

The larynx

The larynx has three jobs, in this order of importance: it keeps food out of the airway, it provides an open airway, and it produces sound. Nine cartilages hold it open.

• The thyroid cartilage — the laryngeal prominence, or Adam's apple — is a shield of hyaline cartilage, larger in adults with more testosterone exposure because the whole larynx grew.
• The cricoid cartilage is the only complete ring of cartilage anywhere in the airway. That completeness is why it is the landmark for cricoid pressure and for emergency cricothyrotomy: it cannot collapse.
• The epiglottis is elastic cartilage, not hyaline, because it must bend and spring back thousands of times a day.
• The paired arytenoid, cuneiform, and corniculate cartilages anchor and move the vocal folds.

The true vocal folds (vocal cords) are the medial pair, containing the vocal ligaments; the space between them is the glottis. Air forced between them sets them vibrating. Pitch rises when the folds are stretched tighter and shorter; loudness rises with the force of the air. The false vocal folds above them play no part in sound — they close during swallowing and during a Valsalva maneuver.

Swallowing is a coordinated shutdown. The larynx is pulled superiorly and anteriorly by suprahyoid muscles, which tips the epiglottis down over the glottis; simultaneously the true folds adduct. Put a finger on your thyroid cartilage and swallow: the upward jump you feel is that mechanism firing.

The trachea

The trachea runs about 10–12 cm from the larynx to the carina, where it bifurcates at the level of the sternal angle (T4/T5 — you found that landmark in Chapter 1). Its wall contains 16–20 C-shaped rings of hyaline cartilage, open posteriorly, with the gap bridged by the trachealis smooth muscle.

Students routinely memorize "C-shaped, open posteriorly" without asking why, and the why is excellent. The cartilage exists to prevent collapse: the trachea sits in a thorax whose pressure swings negative on every inspiration, and a floppy tube would suck shut. But the esophagus lies directly posterior, and the esophagus must distend — sometimes to 2 cm — every time you swallow. A complete cartilage ring would make swallowing a fight against rigid cartilage. So the tube is rigid where rigidity is needed and soft exactly where a neighbour needs room. Meanwhile the trachealis can contract during a cough, narrowing the tracheal lumen and raising the velocity of expelled air to speeds approaching 100 mph (160 km/h), which is what makes a cough effective at clearing mucus.

The bronchial tree: 23 generations

From the carina the airway branches, and branches, and branches — roughly 23 generations of dichotomous division from trachea (generation 0) to alveolar sacs (generation 23).

Two trends run down that table in opposite directions, and both are structural statements about function.

Cartilage disappears; smooth muscle takes over. In the large airways the mechanical threat is collapse, so cartilage holds them open. In the small airways the mechanical threat is the opposite — the need to regulate how much air goes where — so cartilage is replaced by a circular sleeve of smooth muscle that can constrict or dilate. This single anatomical fact is why asthma is a small-airway disease: bronchioles are the only airways with enough muscle and little enough cartilage to close themselves.

The wall gets thinner. By the alveolus the wall is a single squamous cell. Everything about the distal tree is a march toward diffusion.

 ══ CONDUCTING ZONE ══ (no gas exchange · ~150 mL anatomic dead space) ══════

  NOSE ──► NASAL CAVITY ──► NASOPHARYNX ──► OROPHARYNX ──► LARYNGOPHARYNX
  warm      conchae:         [air only]      [air+food]     [air+food]
  filter    turbulence                        epithelium switches here:
  humidify  + contact                         pseudostrat. → strat. squamous
                                                    │
                                                    ▼
                        LARYNX  ── epiglottis closes glottis on swallowing
                     (gen 0) │     thyroid · CRICOID = only complete ring
                             ▼
                        TRACHEA  ── 16–20 C-RINGS, open POSTERIORLY
                             │       so the ESOPHAGUS behind can distend
                             ▼
                       CARINA (T4/T5 · sternal angle)
                    ┌────────┴────────┐
              R MAIN BRONCHUS    L MAIN BRONCHUS   (gen 1)
              wider/shorter/     narrower/longer/
              25° vertical       45° horizontal
              ◄── aspiration     (heart is in the way)
                  lands here
                    │
              LOBAR (3 R / 2 L) ──► SEGMENTAL ──► smaller BRONCHI  (gen 2–11)
                                    cartilage ↓↓   smooth muscle ↑↑
                    │
              BRONCHIOLES (<1 mm) ──► TERMINAL BRONCHIOLES  (gen 12–16)
              NO cartilage · smooth muscle sleeve = the asthma site
  ═════════════════════════════════════════════════════════════════════════
  ══ RESPIRATORY ZONE ══ (gas exchange · ~2,500 mL) ═══════════════════════
                    │
              RESPIRATORY BRONCHIOLES (gen 17–19) ── first alveoli bud here
                    │
              ALVEOLAR DUCTS (gen 20–22)
                    │
              ALVEOLAR SACS ──► ALVEOLI (gen 23)
              ~480 million · ~70 m² · wall = ONE squamous cell

22.2 The Alveoli and the Respiratory Membrane

Everything upstream was preparation. Here is where the system actually does its job.

An alveolus is a thin-walled sac roughly 200–300 µm across — about the width of two or three human hairs laid side by side. There are approximately 480 million of them in a pair of adult lungs, and because they are packed as clusters of interconnected sacs rather than as separate balloons, their combined internal surface is roughly 70 square metres, close to the area of a singles tennis court, folded into a space you can carry in your chest.

That number is not a curiosity. It is half of a two-part engineering argument.

The structure-to-function argument, stated explicitly

Diffusion of a gas across a sheet obeys a simple relationship, usually called Fick's law. In words rather than symbols: the rate at which a gas crosses a membrane is proportional to the surface area available, proportional to the pressure difference driving it, and inversely proportional to the thickness it must cross.

You cannot change the pressure gradient much — that is set by the atmosphere and by metabolism. So if you need to move a lot of gas, you have exactly two levers: make the area enormous, and make the barrier vanishingly thin. The lung pulls both levers as hard as physics allows.

• Area: ~70 m². Achieved by subdividing rather than enlarging. A single sphere holding the lung's 2.5 L of alveolar gas would have a surface area of about 0.9 m². Splitting the same volume into 480 million small spheres multiplies the surface roughly eightyfold. This is the same trick as the intestinal villus and the renal tubule, and it is the most reliable structural motif in the entire body: when you need surface, you subdivide.
• Thickness: 0.2–0.6 µm. Thinner than a red blood cell is wide. Thinner than the wavelength of some visible light. A red cell squeezing through a pulmonary capillary is deformed against a barrier a fraction of its own diameter.

Now put numbers on the consequence. A red blood cell spends about 0.75 seconds in a pulmonary capillary at rest. Equilibration of oxygen between alveolar gas and capillary blood is complete in about 0.25 seconds. The lung finishes its job in the first third of the available time and then has two-thirds of a second in hand. That reserve is why you can triple your cardiac output during exercise — cutting transit time to roughly 0.25 s — and still leave the lung fully saturated. It is also why gas exchange fails so late in lung disease: you must destroy a great deal of reserve before arterial oxygen falls.

The three cell types of the alveolus

The division of labour is exact. Type I cells are so specialized for thinness that they have given up the ability to reproduce; when they die, type II cells divide and differentiate to replace them. Notice the trade-off — this is the same bargain cardiac muscle made in Chapter 9, and the same one Amara's infarcted myocardium lost.

Alveolar macrophages deserve a moment. Roughly 2 million of them die and are swept up the mucociliary escalator every hour, and their contents are a record of what you have breathed. A smoker's macrophages are visibly black with tar.

The respiratory membrane, layer by layer

The respiratory membrane (or blood–air barrier) is what oxygen actually crosses. It is built of three structural layers, and gas crosses six things in total.

1. A film of alveolar fluid lining the airspace, containing surfactant.
2. The type I alveolar cell membrane and cytoplasm.
3. The fused basement membranes of the alveolar epithelium and the capillary endothelium — over most of the surface these two basal laminae are not merely adjacent but fused into a single sheet, which removes an entire interstitial layer from the diffusion path.
4. The capillary endothelial cell.
5. The plasma of the capillary.
6. The red blood cell membrane, and then hemoglobin.

Total: 0.2 to 0.6 µm over the thin portion of the barrier. The thicker portion, where the basement membranes separate and a little interstitial connective tissue intervenes, is where fluid and lymphatics live — and, when things go wrong, where fluid accumulates first. Hold that thought; it is the anatomical basis of Amara's second question.

   ALVEOLAR AIR SPACE          PO2 104 mm Hg    PCO2 40 mm Hg
   ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪
 ┌───────────────────────────────────────────────────────────────────┐
 │ 1  SURFACTANT + ALVEOLAR FLUID FILM   ~0.01 µm                    │  ▲
 │    (from type II cells — lowers surface tension)                  │  │  O2
 ├───────────────────────────────────────────────────────────────────┤  │  IN
 │ 2  TYPE I ALVEOLAR CELL   0.05–0.2 µm — flattest cell in the body │  │
 ├───────────────────────────────────────────────────────────────────┤  │
 │ 3  FUSED BASEMENT MEMBRANES  ~0.05 µm                             │  │
 │    epithelial BM + endothelial BM fused into ONE sheet            │  │
 ├───────────────────────────────────────────────────────────────────┤  │
 │ 4  CAPILLARY ENDOTHELIAL CELL  0.04–0.2 µm                        │  │
 └───────────────────────────────────────────────────────────────────┘  │
 │ 5  PLASMA                                                         │  │
 │        ╭─────────────╮                                            │  ▼ CO2
 │ 6      │ RBC  Hb ●●● │  ◄── 0.75 s transit; equilibrium at 0.25 s │    OUT
 │        ╰─────────────╯                                            │
   TOTAL BARRIER THICKNESS ─────────────► 0.2 – 0.6 µm
   TOTAL AREA ─────────────────────────► ~70 m² (≈480 million alveoli)

   ── THE THICK SIDE (other face of the capillary) ─────────────────────
   Basement membranes SEPARATE here; interstitium + fibroblasts +
   collagen sit between them.  Gas exchange is poor here — but this is
   where INTERSTITIAL FLUID collects and where lymphatics drain.
   ► In left heart failure, fluid fills THIS side first (interstitial
     edema, Kerley B lines) before spilling into the airspace.

Surfactant and the tyranny of surface tension

The alveolar surface is wet, and a wet surface has surface tension — water molecules at an air–water interface pull toward each other, which makes the surface behave like a stretched elastic sheet trying to shrink. In a sphere, that inward pull generates a pressure pushing the sphere closed. Roughly two-thirds of the lung's total elastic recoil comes not from elastin fibres but from this water film.

The physics of that pressure is Laplace's law, and although it is usually written as an equation, the idea is entirely graspable in words:

The pressure trying to collapse a bubble is proportional to its surface tension and inversely proportional to its radius. Small bubbles, at the same surface tension, generate higher collapsing pressure than large ones.

Now consider the consequence for a lung that has 480 million interconnected bubbles of different sizes, all sharing common airways. A small alveolus generates a higher internal pressure than a large one, so gas flows from the small alveolus into the large one — down its pressure gradient. Every small alveolus would empty itself into its larger neighbour, and the lung would collapse into a few enormous cavities with a tiny fraction of the surface area it needs. This is not hypothetical; it is exactly what happens in surfactant deficiency.

Surfactant is the solution, and it is a lovely one. It is a detergent-like mixture, about 90% phospholipid (chiefly dipalmitoylphosphatidylcholine) plus four surfactant proteins, secreted by type II cells. Its molecules insert between water molecules at the surface and break up their mutual attraction, reducing surface tension by up to twelvefold.

The critical detail is that surfactant does not simply reduce surface tension by a fixed amount — it reduces it more when the alveolus is small. As an alveolus shrinks, the surfactant molecules on its surface are crowded closer together, and the more concentrated they are, the more effectively they interrupt the water film. So surface tension falls exactly as radius falls, and the two effects cancel. The pressure in the small alveolus stops exceeding the pressure in the large one, the gradient disappears, and every alveolus stays open at its own size.

Surfactant does two other things worth naming. It reduces the muscular work of inspiration, because you are no longer fighting the full surface tension of water on each breath — without it, the work of breathing rises several-fold. And it keeps the alveolus dry: the same inward surface-tension pull that tries to collapse the alveolus also sucks fluid out of the capillaries into the airspace, so reducing it protects against alveolar flooding.

22.3 The Lungs, the Pleurae, and the Mechanics of Breathing

Gross anatomy in brief

The lungs occupy the two pleural cavities flanking the mediastinum (Chapter 1). The right lung has three lobes divided by horizontal and oblique fissures; the left has two, plus a cardiac notch and its associated lingula, because the heart is displaced leftward and the lung must accommodate it. Each lung's hilum admits the main bronchus, pulmonary artery, pulmonary veins, bronchial vessels, lymphatics, and nerves.

There are two separate blood supplies, and confusing them is a classic error.

• The pulmonary circulation — pulmonary arteries carrying deoxygenated blood from the right ventricle, pulmonary veins returning oxygenated blood to the left atrium — is the functional circulation. It carries the entire cardiac output, roughly 5 L/min, at a mean pressure of only about 15 mm Hg (Chapter 19). It is a low-pressure, low-resistance, high-compliance circuit, and everything about Amara's pulmonary edema depends on that.
• The bronchial circulation — bronchial arteries off the thoracic aorta — is the nutritive supply, feeding the lung tissue itself at systemic pressure. It amounts to 1–2% of cardiac output.

The pleurae and the pressure that holds you open

Each lung is enclosed by a serous membrane with two layers: visceral pleura glued to the lung surface, parietal pleura lining the thoracic wall, diaphragm, and mediastinum, continuous with each other at the hilum. Between them is the pleural cavity, a potential space holding only 5–15 mL of serous fluid.

That film of fluid does two things, and both are mechanical.

• Lubrication. The two layers slide over each other with almost no friction through perhaps half a billion breaths.
• Adhesion. Two wet surfaces separated by a thin fluid film resist being pulled apart, exactly as two wet glass slides do. The lung is therefore coupled to the chest wall without being attached to it.

Now the central problem of respiratory mechanics. The lung's natural state is collapsed. Its elastic fibres and its alveolar surface tension both pull inward, continuously. Remove a lung from a chest and it shrinks to roughly the size of a fist. The chest wall's natural state is expanded — its ribs and costal cartilages spring outward, and an isolated chest wall would sit at about 60% of total lung capacity rather than at the resting position it actually holds.

So you have two elastic structures pulling in opposite directions, coupled by a fluid film. The outward pull of the chest wall and the inward pull of the lung stretch the fluid between them, and the result is a subatmospheric intrapleural pressure.

Transpulmonary pressure is the single most important number in respiratory mechanics. It is the difference between the pressure inside the alveoli and the pressure outside the lung, and it is what physically holds the lung inflated against its own recoil. When transpulmonary pressure equals the lung's recoil pressure, the lung sits still at that volume. If transpulmonary pressure ever reaches zero — as it does when air enters the pleural space — nothing opposes recoil, and the lung collapses.

One quiet breath, in numbers

Air flows down pressure gradients, always. To breathe in, you must make alveolar pressure lower than atmospheric; the only way to do that, per Boyle's law, is to increase the volume of the thoracic cavity, since pressure and volume are inversely related in a closed container.

Inspiration (quiet, ~2 seconds):

1. The diaphragm contracts and descends 1–2 cm (up to 10 cm in forced inspiration), flattening its dome. It is responsible for roughly 75% of the volume change in quiet breathing.
2. The external intercostals contract, elevating the ribs. The upper ribs swing like a pump handle, increasing the anteroposterior diameter; the lower ribs swing like a bucket handle, increasing the transverse diameter.
3. Thoracic volume rises. Because the pleural fluid couples lung to chest wall, the lung is pulled outward with it, and intrapleural pressure falls from −4 to about −6 cm H₂O.
4. Transpulmonary pressure therefore rises from +4 to +6, expanding alveoli. Alveolar volume rises, so by Boyle's law alveolar pressure falls to about −1 cm H₂O — a single centimetre of water below atmospheric.
5. Air flows in down that tiny gradient. About 500 mL enters. As volume reaches its peak, alveolar pressure returns to 0 and flow stops.

Expiration (quiet, ~3 seconds) is passive. The inspiratory muscles simply relax. The stretched elastic tissue of the lung and the surface tension of the alveoli recoil inward, thoracic volume falls, alveolar pressure rises to about +1 cm H₂O, and air flows out. No muscle contracts. Quiet expiration costs you nothing but the release of stored elastic energy — which is one reason quiet breathing consumes only about 3% of your resting metabolic rate.

Forced breathing recruits more muscle. Forced inspiration adds the scalenes, sternocleidomastoid, pectoralis minor, and erector spinae — the "accessory muscles" whose visible use at the neck is a clinical sign of respiratory distress. Forced expiration becomes active, driven by the abdominal muscles (which push viscera up against the diaphragm) and the internal intercostals (which pull ribs down); intrapleural pressure can then become transiently positive, and alveolar pressure can exceed +30 cm H₂O during a cough.

        ONE QUIET BREATH · 5 SECONDS · SHARED TIME AXIS
        INSPIRATION (2 s) ────────►│◄──── EXPIRATION (3 s) ────►

 VOLUME     +500 ┤                 ╭──╮
 change     (mL) ┤              ╭──╯  ╰──╮
 from FRC        ┤           ╭──╯        ╰──╮
                 ┤        ╭──╯              ╰───╮
              0  ┼─────╭──╯                     ╰────────────────
                 └─────┴──────┴──────┴──────┴──────┴──────┴─────
                 0     1      2      3      4      5   seconds

 ALVEOLAR   +1   ┤                    ╭────╮
 PRESSURE        ┤                   ╭╯    ╰─╮
 (cm H2O)     0  ┼──╮──────────────╭─╯       ╰──╮────────────────
                 ┤   ╰─╮        ╭──╯             ╰──╮         ╭──
              -1 ┤     ╰────────╯                   ╰─────────╯
                  AIR FLOWS IN ◄──┘                └──► AIR FLOWS OUT
                  (alveolar P below atm)      (alveolar P above atm)

 INTRA-      -4  ┼──╮                              ╭───────────────
 PLEURAL         ┤   ╰──╮                      ╭───╯
 PRESSURE        ┤       ╰──╮              ╭───╯
 (cm H2O)    -6  ┤           ╰─────────────╯
                  NEVER reaches 0 in a healthy person.
                  If it does → transpulmonary P = 0 → LUNG COLLAPSES

 TRANSPULMONARY PRESSURE = P(alveolar) − P(intrapleural)
   at rest:  0 − (−4) = +4 cm H2O   ── holds the lung open
   at peak:  0 − (−6) = +6 cm H2O   ── lung is more distended

 DRIVER:  diaphragm ↓ 1–2 cm (75% of the work) + external intercostals
          ► thoracic VOLUME up ► intrapleural P down ► transpulmonary P up
          ► alveoli expand ► alveolar P down ► AIR IN   (Boyle's law)

Compliance: how easily the lung stretches

Compliance is the change in lung volume produced per unit change in transpulmonary pressure — a measure of stretchiness. A normal lung is remarkably compliant: about 200 mL per cm H₂O. High compliance means a small pressure change produces a large volume change, which means cheap breathing.

Two things determine it: the elasticity of the lung tissue (elastin and collagen in the alveolar septa) and the surface tension of the alveolar film, which surfactant tames.

Compliance can also be pathologically increased, which sounds good and is not. In emphysema, elastin is destroyed by proteases, so the lung stretches too easily and, more importantly, recoils too weakly. Since quiet expiration is powered entirely by recoil, a lung with no recoil cannot empty. That is the mechanism of air trapping.

Airway resistance, and where it lives

Resistance to airflow is governed by the same relationship you met for blood flow in Chapter 19: resistance is inversely proportional to the fourth power of the radius. Halve a tube's radius and resistance rises sixteenfold.

That relationship makes a prediction that turns out to be wrong, and the reason it is wrong is instructive. The bronchioles are the narrowest airways, so they should dominate resistance. In fact, the medium-sized bronchi — roughly generations 4 to 8 — account for most airway resistance, and the terminal bronchioles account for very little.

The resolution is total cross-sectional area. Each generation of branching roughly doubles the number of tubes while reducing each tube's radius by only about 20%. The result is that total cross-sectional area increases explosively down the tree — from about 2.5 cm² at the trachea to more than 5,000 cm² at the terminal bronchioles. The airways are arranged in parallel, and resistances in parallel add as reciprocals: a thousand narrow tubes side by side offer less resistance than one wide tube. By the respiratory zone, air velocity has fallen so low that bulk flow essentially stops and diffusion takes over.

This has a hard clinical consequence. Because the small airways contribute so little to total resistance, you can lose a great deal of small-airway function before anyone can measure it. The small airways are called the lung's "silent zone," and early COPD hides there for years.

22.4 Respiratory Volumes and Capacities

You cannot measure the lung's function by looking at it. You measure it by asking it to move air and recording how much and how fast. Spirometry is that measurement, and its vocabulary is worth learning precisely because each term isolates one physiological question.

A volume is a single, non-overlapping quantity of air. A capacity is the sum of two or more volumes. There are four of each.

Two of these deserve emphasis because they carry most of the physiology.

Residual volume cannot be measured by spirometry, because by definition you cannot blow it out — it requires helium dilution, nitrogen washout, or body plethysmography. Its existence is essential: it keeps alveoli from collapsing between breaths and keeps gas exchange continuous rather than intermittent. Without it, your arterial oxygen would oscillate with every breath.

Functional residual capacity is the volume your lungs sit at between breaths — the equilibrium point where the lung's inward recoil exactly balances the chest wall's outward recoil. It is the lung's oxygen reservoir, and it is why you do not desaturate during the few seconds between breaths. Anything that reduces FRC — supine posture, obesity, pregnancy, abdominal surgery, pulmonary edema — shrinks that reservoir and shortens the time to desaturation. This is why pre-oxygenation before intubation works, and why an obese supine patient desaturates in seconds when a lean upright one takes minutes.

  SPIROGRAM · volume (mL) against time ─────────────────────────────────►

  6000 ┤ ▁▁▁▁▁▁▁▁▁▁▁ TOTAL LUNG CAPACITY (TLC) 6000 ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁
       │        ╭───╮                    ╭─╮
       │       ╱     ╲                  ╱   ╲          ▲
  5000 ┤      ╱       ╲                ╱     ╲         │
       │     ╱  IRV    ╲              ╱       ╲        │  I
       │    ╱  3100 mL  ╲            ╱         ╲       │  C
  4000 ┤   ╱             ╲          ╱           ╲      │ 3600
       │  ╱               ╲        ╱             ╲     │  │      V
       │ ╱                 ╲      ╱               ╲    │  │      C
  3000 ┼╯ ─╮ ╭─╮ ╭─╮ ╭─╮ ╭──╲────╯                 ╲   ▼  │    4800
       │TV │ │ │ │ │ │ │ │   ╲                      ╲     │      │
       │500│ │ │ │ │ │ │ │    ╲   quiet breathing    ╲    │      │
  2500 ┤ ──╯ ╰─╯ ╰─╯ ╰─╯ ╰─────╲───────────────────── ╲───▲      │
       │  ◄─ resting expiratory ╲ level = FRC 2400 mL  ╲  │      │
  2000 ┤                         ╲                      ╲ │ E    │
       │                          ╲       ERV 1200 mL    ╲│ R    │
       │                           ╲                      │ V    ▼
  1200 ┼────────────────────────────╲─────────────────────▼──────
       │                             ╲   maximal expiration
       │        RESIDUAL VOLUME (RV) 1200 mL — CANNOT be exhaled,
       │        cannot be measured by spirometry (needs He dilution)
     0 ┴──────────────────────────────────────────────────────────

  CAPACITIES = sums of volumes
    IC  = TV + IRV                 = 3600 mL
    FRC = ERV + RV                 = 2400 mL  ◄ the O2 reservoir
    VC  = TV + IRV + ERV           = 4800 mL
    TLC = TV + IRV + ERV + RV      = 6000 mL
  (Values are typical adult male; female values run ~20–25% lower.)

Dead space, and why it is the whole game

Anatomic dead space is the volume of the conducting zone — the air that fills nose, trachea, and bronchi at the end of inspiration and never touches an alveolus. A serviceable rule of thumb: 1 mL per pound of ideal body weight, or about 150 mL in a typical adult.

Alveolar dead space is alveolar volume that is ventilated but not perfused — alveoli receiving air that no blood is passing. In healthy lungs it is negligible. In pulmonary embolism it can be enormous.

Physiologic dead space is the sum of the two, and in health it equals anatomic dead space.

Now the arithmetic that matters more than any other calculation in this chapter.

• Minute ventilation (V̇_E) = tidal volume × respiratory rate. It is what you can measure at the mouth, and it is misleading.
• Alveolar ventilation (V̇_A) = (tidal volume − dead space) × respiratory rate. It is what actually reaches gas-exchanging surface, and it is the number the body regulates.

Work the example promised in §22.1. Three patterns, all with an identical minute ventilation of 6,000 mL/min, dead space 150 mL:

The same total air movement produces effective ventilation ranging from 5,100 down to 2,400 mL/min — a difference of more than twofold. The reason is that dead space is a fixed toll paid on every breath. Take fewer, bigger breaths and you pay the toll fewer times. Take many small breaths and you spend most of your effort moving the same 150 mL of air back and forth in your trachea.

Push it to the limit: at a tidal volume of 150 mL, alveolar ventilation is zero no matter how fast you breathe. A patient panting at 40 breaths a minute with a tidal volume of 180 mL looks like they are working furiously at ventilation and is in fact barely ventilating at all. This is why rapid shallow breathing is an ominous sign, why the rapid shallow breathing index predicts failure to wean from a ventilator, and why "breathe slowly and deeply" is genuinely good physiological advice rather than a calming platitude.

Spirometry, FEV₁, and the two great patterns

Ask a patient to inhale maximally and then blow out as hard and fast as possible, and you record a forced vital capacity (FVC) manoeuvre. Two numbers come out of it.

• FVC — the total volume forcefully exhaled. A measure of how much lung there is.
• FEV₁ — the forced expiratory volume in one second, the fraction of that total blown out in the first second. A measure of how fast it can come out.

Their ratio, FEV₁/FVC, is normally about 0.75–0.80 in a healthy adult (higher in children, declining with age). It separates the two fundamental patterns of lung disease, and the separation is clean enough to be nearly diagnostic on its own.

The counterintuitive line is the ratio in restriction. A fibrotic lung is small, so both FEV₁ and FVC fall — but they fall together, and a stiff lung with high recoil actually empties briskly, so the ratio is preserved or even elevated. Obstruction is defined by the ratio; restriction is defined by the volume.

In COPD, a post-bronchodilator FEV₁/FVC below 0.70 is the diagnostic criterion, and the FEV₁ as a percentage of predicted then grades severity. In asthma, the same obstruction is present but reverses — an improvement in FEV₁ of at least 12% and 200 mL after a bronchodilator is the classic demonstration that the obstruction was muscular and reversible rather than structural and fixed.

22.5 Gas Exchange: Partial Pressures and the Gradients That Drive Them

Air is a mixture, and each gas in a mixture behaves as though the others were not there. That is Dalton's law of partial pressures: in a mixture, the total pressure is the sum of the pressures each gas would exert alone, and each gas's partial pressure is the total pressure times that gas's fractional concentration.

At sea level, atmospheric pressure is 760 mm Hg and dry air is 20.9% oxygen, so:

• P_O₂ of dry atmospheric air = 760 × 0.209 = 159 mm Hg
• P_CO₂ of atmospheric air = 760 × 0.0004 = 0.3 mm Hg, effectively zero
• P_N₂ = 760 × 0.781 = 597 mm Hg

But alveolar gas is not atmospheric gas, and the difference explains a great deal. Two things happen on the way in.

Humidification. Air is saturated with water vapour in the airway, and water vapour at 37 °C exerts a partial pressure of 47 mm Hg regardless of anything else. That 47 mm Hg comes off the top of the available pressure before any other gas gets a share: the pressure of inspired gas at the trachea is (760 − 47) = 713 mm Hg, so inspired P_O₂ falls to 713 × 0.209 = 150 mm Hg.

Mixing and exchange. Alveolar gas is not replaced wholesale on each breath — 500 mL of tidal volume, of which 350 mL reaches alveoli, mixes into a functional residual capacity of 2,400 mL. Alveolar gas composition is therefore heavily damped and stable, which is exactly what you want for a partner in diffusion. Meanwhile oxygen is continuously being removed by blood and carbon dioxide continuously added. The equilibrium result:

Alveolar P_O₂ = 104 mm Hg. Alveolar P_CO₂ = 40 mm Hg.

Those two numbers are the anchors of the entire chapter.

Henry's law, and the twenty-fold fact

Henry's law: the amount of a gas that dissolves in a liquid is proportional to that gas's partial pressure and to its solubility in that liquid. Partial pressure sets the driving force; solubility sets the exchange rate for a given force.

And here is the fact that reorganizes respiratory physiology once you take it seriously: carbon dioxide is about 20 times more soluble in plasma than oxygen is (24 times, by the usual figures). Three enormous consequences follow.

1. CO₂ diffuses out far more easily than O₂ diffuses in, despite having a much smaller partial-pressure gradient to work with. At the alveolus, oxygen's gradient is 104 − 40 = 64 mm Hg, while carbon dioxide's is only 45 − 40 = 5 mm Hg. A gradient thirteen times smaller, and yet CO₂ crosses the membrane just as completely, because solubility more than compensates.
2. In lung disease, oxygen fails first. Thicken the respiratory membrane with fluid or fibrosis and diffusion slows for both gases — but CO₂ has such a large solubility margin that it continues to equilibrate while oxygen does not. This is why early respiratory disease produces hypoxemia with a normal or low PaCO₂, and why a rising PaCO₂ in a struggling patient is such an alarming sign: it means the reserve is finally gone. Look again at Amara's blood gas — PaO₂ is low at 74, PaCO₂ is low at 31. Textbook.
3. CO₂ is carried in blood in enormous quantities and, because it forms carbonic acid, its transport is inseparable from pH regulation. That is §22.6 and §22.9.

The gradients, station by station

One line in that table deserves explanation. Blood leaves the pulmonary capillary at P_O₂ 104, yet systemic arterial blood measures 95–100. Where did 5–10 mm Hg go? Two small physiologic shunts dilute it: bronchial venous blood, which has perfused lung tissue and given up oxygen, drains into the pulmonary veins, and some cardiac venous blood drains directly into the left ventricle through the thebesian veins. A few percent of deoxygenated blood mixed into fully oxygenated blood drops the P_O₂ measurably. The lung is not quite perfect, and the imperfection is anatomical.

What controls the rate of external respiration

Three factors, all of them in Fick's law, all of them clinically manipulable.

• Partial pressure gradient. The steeper, the faster. Supplemental oxygen works by raising alveolar P_O₂: 100% oxygen raises it to roughly 673 mm Hg, turning a 64 mm Hg gradient into a 630 mm Hg one.
• Surface area. ~70 m² in health. Emphysema destroys it; pneumonectomy halves it; atelectasis and pneumonia take regions offline.
• Membrane thickness. 0.2–0.6 µm in health. Pulmonary edema and fibrosis increase it, and because the relationship is inverse, doubling thickness halves the diffusion rate. This is the mechanism of Amara's exertional hypoxemia.

Ventilation–perfusion coupling

Here is a problem that the numbers above quietly conceal. Gas exchange requires that air and blood arrive at the same alveolus. Delivering air to an alveolus with no blood flow accomplishes nothing; delivering blood to an alveolus with no air accomplishes nothing worse — it actively harms, because that blood returns to the left heart still deoxygenated and dilutes everything else.

The ideal ratio of alveolar ventilation to perfusion — V̇/Q̇ — is about 0.8 overall (4.2 L/min of alveolar ventilation against 5.0 L/min of cardiac output). But it is not uniform. In an upright lung, gravity makes both ventilation and perfusion greater at the base than at the apex, and perfusion varies more steeply than ventilation does. So the apex is relatively over-ventilated (V̇/Q̇ ≈ 3.0) and the base relatively over-perfused (V̇/Q̇ ≈ 0.6). The lung tolerates this, and tuberculosis famously exploits it — the organism is an obligate aerobe and favours the high-P_O₂ apex.

What makes the system work locally is a pair of autoregulatory reflexes, and they are elegant because they run in opposite directions from their systemic counterparts.

Hypoxic pulmonary vasoconstriction. When alveolar P_O₂ falls in a region, the arterioles supplying that region constrict, diverting blood to better-ventilated regions. Note that this is exactly backwards from every systemic vascular bed you met in Chapter 19, where local hypoxia causes vasodilation to bring more blood to a struggling tissue. In the lung, that response would be counterproductive: the tissue is not short of oxygen, the air supply is, and sending more blood to an unventilated alveolus only produces more deoxygenated blood. The pulmonary circulation is the only vascular bed in the body where hypoxia constricts, and the reason is that its job is not to feed the lung but to be matched to ventilation.

Bronchiolar responses to CO₂. The mirror-image reflex acts on the airway. When alveolar P_CO₂ rises in a region — indicating that region is receiving more blood than air — the local bronchioles dilate, increasing ventilation to match the perfusion. When P_CO₂ falls, they constrict.

Together these two local loops continuously steer air and blood toward each other without any neural or hormonal input at all. They are pure local control, and they are the reason a healthy lung achieves such precise matching across 480 million units.

  PARTIAL PRESSURES AROUND THE CIRCUIT  (all values mm Hg)

   ATMOSPHERE          INSPIRED (humidified)        ALVEOLAR GAS
   PO2  159            PO2  150                     PO2  104
   PCO2   0.3          PCO2   0.3                   PCO2  40
   (760 × 0.209)       (760−47 water vapour)        (damped by FRC mixing)
                                    │
                                    │  EXTERNAL RESPIRATION
                                    ▼  O2 gradient = 104−40 = 64 mm Hg
   ┌──────────────────────────────────────────────────────────────────┐
   │  PULMONARY CAPILLARY                                             │
   │  arriving:  PO2  40   PCO2 45   ──0.25 s──►  PO2 104  PCO2 40    │
   │             ▲                                        │           │
   └─────────────┼────────────────────────────────────────┼───────────┘
                 │                                        ▼
        PULMONARY ARTERY                          PULMONARY VEIN
        (from RIGHT ventricle)                    ──► LEFT ATRIUM
        DEOXYGENATED                              + shunt dilution
                 ▲                                        ▼
                 │                              SYSTEMIC ARTERIAL BLOOD
                 │                              PO2 95–100   PCO2 40
                 │                              SaO2 ~98%
                 │                                        │
   ┌─────────────┼────────────────────────────────────────┼───────────┐
   │  SYSTEMIC CAPILLARY          INTERNAL RESPIRATION    ▼           │
   │  TISSUE (rest):     PO2 40   PCO2 45                             │
   │  TISSUE (exercise): PO2 <20  PCO2 >50  ◄ steeper gradient        │
   │  O2 gradient at rest = 100−40 = 60 mm Hg                         │
   └──────────────────────────────────────────────────────────────────┘
                 │
                 ▼
        SYSTEMIC VENOUS BLOOD:  PO2 40   PCO2 45   SvO2 ~75%
        ► 75% saturated means only 25% of carried O2 was used at rest.
          THE VENOUS RESERVE is the safety margin exercise draws on.

  CO2 note: its gradient is only 45→40 = 5 mm Hg, yet it fully
  equilibrates — because CO2 is ~20× more SOLUBLE than O2 (Henry's law).

22.6 Gas Transport in Blood

Dissolved oxygen is nowhere near enough. Run the numbers: at a P_O₂ of 100 mm Hg, plasma dissolves about 0.3 mL of O₂ per 100 mL of blood. With a cardiac output of 5 L/min, that delivers 15 mL of oxygen per minute to a body consuming 250 mL/min at rest. Dissolved oxygen supplies about 1.5% of resting demand. You would need a cardiac output of over 80 L/min to survive on dissolved oxygen, which is impossible.

Hemoglobin solves this. Each red cell holds about 250 million hemoglobin molecules; each hemoglobin has four heme groups; each heme binds one O₂ (Chapter 17). Fully saturated blood carries about 20 mL of O₂ per 100 mL — a seventyfold increase over what plasma alone could dissolve. Of the oxygen in arterial blood, 98.5% is bound to hemoglobin and 1.5% is dissolved.

One clarification that prevents a persistent confusion: only dissolved oxygen exerts partial pressure. P_O₂ — the number on a blood gas — measures dissolved gas only. Bound oxygen is chemically sequestered and invisible to a pressure sensor. This is why P_O₂ and oxygen content can dissociate completely, which is the entire point of Amara's first question and of carbon monoxide poisoning.

The oxyhemoglobin dissociation curve

Plot percent saturation of hemoglobin against P_O₂ and you do not get a straight line. You get an S-shaped (sigmoid) curve, and every feature of that shape does a job.

Why sigmoid? Cooperative binding. Hemoglobin has four subunits. When the first O₂ binds to one heme, it produces a conformational change in the whole molecule — from the tense (T) state to the relaxed (R) state — that makes the remaining three sites bind oxygen more readily. The second O₂ binds more easily than the first, the third more easily than the second, and the fourth most easily of all, with an affinity roughly 300 times that of the first. Unloading runs the same process in reverse: releasing one O₂ makes the rest easier to release. Binding affinity is therefore not fixed — it depends on how much is already bound, and that dependence is what bends the line into an S.

The flat upper portion (P_O₂ 60–100 mm Hg) is a safety plateau. Between 60 and 100 mm Hg, saturation changes only from 90% to 98%. A large drop in arterial P_O₂ costs almost no oxygen content. This protects you against altitude, against modest lung disease, and against the normal decline of PaO₂ with age. It also means supplemental oxygen buys very little for a patient who is already 95% saturated — you cannot fill a nearly full container. And it is why pulse oximetry is insensitive: an SpO₂ of 94% could correspond to a PaO₂ anywhere from about 70 to 80 mm Hg. Amara's SaO₂ of 94% with a PaO₂ of 74 sits exactly where the curve predicts.

The steep lower portion (P_O₂ 10–50 mm Hg) is the unloading zone. Here, a small fall in P_O₂ causes a large fall in saturation — which means a large release of oxygen. Tissue P_O₂ at rest is 40 mm Hg, where saturation is about 75%; exercising muscle drops it below 20 mm Hg, where saturation falls to 35% or less. The same drop in the flat region would release almost nothing. The curve is flat where you load and steep where you unload, which is the most economical possible arrangement.

Three reference points are worth committing to memory:

P₅₀ is the shorthand for the whole curve. A right shift raises P₅₀ (you need a higher P_O₂ to reach 50%, i.e. affinity is lower, i.e. oxygen is released more readily). A left shift lowers P₅₀ (higher affinity, oxygen held more tightly).

What shifts the curve, and why each shift is useful

Every shift factor has the same underlying logic: the curve shifts right — releasing more oxygen — under exactly the conditions that indicate a tissue needs more oxygen. It is a feedback loop with no nerves, no hormones, and no control centre. The chemistry itself is the sensor.

The Bohr effect is worth stating as a mechanism, not just a rule. Carbon dioxide entering a red cell is hydrated to carbonic acid, which dissociates to bicarbonate and a proton. That proton binds to specific amino acid residues on the globin chains, stabilizing the tense (T) state and lowering oxygen affinity. So the arrival of metabolic waste physically pries oxygen off hemoglobin at the precise location where the waste was produced. Meanwhile the same effect runs in reverse at the lung: as CO₂ leaves, protons come off the globin, affinity rises, and oxygen loading is enhanced. The two gases help each other in both directions. The reciprocal — oxygenation of hemoglobin reducing its affinity for CO₂ — is called the Haldane effect, and it is the reason blood picks up CO₂ efficiently at the tissues and releases it efficiently at the lung.

2,3-bisphosphoglycerate deserves its own note because it is the slow adjuster. It is produced by red cells as a side branch of glycolysis, binds in the central cavity of deoxyhemoglobin, and stabilizes the T state. Levels rise over 12–48 hours in response to chronic hypoxia — altitude, anemia, chronic lung disease, heart failure — and the rise right-shifts the curve, improving tissue unloading without any change in the lung. It is one of the fastest-acting adaptations to altitude and one of the reasons stored banked blood, which loses 2,3-BPG over weeks, unloads oxygen poorly for the first day after transfusion.

   OXYHEMOGLOBIN DISSOCIATION CURVE
   % saturation of Hb  vs.  PO2 (mm Hg)

 100 ┤                              ╭──────●══════════════════  ARTERIAL
     │                          ╭───╯      98% at PO2 100
  90 ┤                      ╭───╯   ◄══════ FLAT / PLATEAU REGION ══════►
     │                  ╭───╯       PO2 60→100 changes SaO2 only 90→98%
  80 ┤              ╭───╯           = a LOADING SAFETY MARGIN.
     │           ╭──╯               Big falls in PO2 cost little content.
  75 ┤        ╭──●  MIXED VENOUS at rest: PO2 40, SaO2 75%
     │      ╭─╯    ► only 25% of carried O2 extracted at rest
  60 ┤    ╭─╯
     │   ╭╯        ◄════════ STEEP REGION ════════►
  50 ┤  ●  P50 = 27 mm Hg   small ΔPO2 → LARGE Δsaturation
     │ ╱             = the UNLOADING zone. Tissue PO2 lives here.
  35 ┤╱   ● EXERCISING MUSCLE: PO2 <20 → SaO2 35% or less
  20 ┼╯
     └──┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬───►
        0   10   20   27   30   40   50   60   70   80  100  PO2

   SHIFTS ────────────────────────────────────────────────────────────
   RIGHT SHIFT (P50 ↑, O2 RELEASED more easily) — "an exercising muscle"
      ↓ pH (acidosis) ·  ↑ PCO2  ·  ↑ TEMPERATURE  ·  ↑ 2,3-BPG
      = the BOHR EFFECT (pH/CO2 components)
      LOGIC: every one of these is a SIGN of active metabolism.
             The blood delivers more O2 exactly where O2 is being used.

   LEFT SHIFT (P50 ↓, O2 HELD more tightly) — "the fetus, and the lung"
      ↑ pH (alkalosis) · ↓ PCO2 · ↓ TEMPERATURE · ↓ 2,3-BPG
      · FETAL Hb (γ chains bind 2,3-BPG poorly → steals O2 from mother)
      · CARBON MONOXIDE (also reduces total capacity — doubly lethal)

Carbon dioxide transport: three routes

Your cells produce about 200 mL of CO₂ per minute at rest, and every millilitre must get to the lung. It travels three ways.

1 · Dissolved in plasma — 7 to 10%. Small, but not trivial, and it is this fraction that generates the measured P_CO₂. CO₂'s high solubility means twenty times more of it than oxygen travels this way.

2 · Bound to hemoglobin as carbaminohemoglobin — about 20%. Note the critical difference from oxygen: CO₂ binds to the globin protein's terminal amino groups, not to the heme iron. The two gases are therefore not competing for the same site, and hemoglobin can carry both at once. Binding is favoured when hemoglobin is deoxygenated — the Haldane effect — which is exactly the state it is in at the tissues.

3 · As bicarbonate ion in plasma — about 70%. The dominant route, and the one that connects respiration to acid–base balance.

The bicarbonate route and the chloride shift

Follow one CO₂ molecule from a working muscle.

1. CO₂ diffuses out of the tissue cell, through interstitial fluid, into plasma, and then into a red blood cell. Almost all of the chemistry happens inside the red cell, and for one reason: that is where the enzyme is.
2. Inside, carbonic anhydrase catalyses CO₂ + H₂O → H₂CO₃ (carbonic acid). This reaction happens spontaneously in plasma but roughly 5,000 times faster inside the red cell. Reaction speed is the entire justification for routing the chemistry through a cell.
3. Carbonic acid immediately dissociates: H₂CO₃ → H⁺ + HCO₃⁻.
4. The H⁺ is buffered by hemoglobin, which is an excellent proton buffer — particularly in its deoxygenated form, which has just released its oxygen. This is the molecular basis of the Bohr effect: the proton binding to globin is what lowers oxygen affinity. Buffering the proton also prevents venous blood from becoming dangerously acidic; venous pH is only about 0.03 units below arterial.
5. The HCO₃⁻ moves out of the red cell into plasma, down its concentration gradient, through an anion exchanger in the membrane (band 3 protein). To preserve electrical neutrality, a Cl⁻ moves in as the bicarbonate moves out. This is the chloride shift. Without it, the red cell would rapidly accumulate negative charge and bicarbonate export would stop within milliseconds.

At the lung, every one of these steps runs backwards. The low alveolar P_CO₂ pulls dissolved CO₂ out of plasma; that lowers red-cell CO₂; carbonic anhydrase runs in reverse; bicarbonate re-enters the red cell as chloride leaves (the reverse chloride shift); protons come off hemoglobin, raising its oxygen affinity and helping it load oxygen; and CO₂ is regenerated and exhaled. Nothing in the sequence requires energy or regulation. It is driven end to end by concentration gradients that the lung and the tissues maintain.

Note the consequence that Chapter 31 will build on: hemoglobin is simultaneously the oxygen carrier, the main protein buffer of blood, and the vehicle of most CO₂ transport, and the three roles are mechanically coupled. Anemia is therefore not only an oxygen problem.

  CO2 TRANSPORT · what happens inside ONE red blood cell at the TISSUE

      TISSUE CELL ──► CO2 (200 mL/min produced at rest)
                       │
        ┌──────────────┼─────────── PLASMA ───────────────────────┐
        │              │                                          │
        │   ROUTE 1: stays DISSOLVED ────────────────► 7–10%      │
        │              │   (this fraction sets the PCO2 reading)  │
        └──────────────┼──────────────────────────────────────────┘
                       ▼
   ╭═══════════════════════════ RED BLOOD CELL ═══════════════════════╮
   ║                    CO2                                           ║
   ║          ┌──────────┴──────────┐                                 ║
   ║   ROUTE 2│                     │ROUTE 3                          ║
   ║          ▼                     ▼                                 ║
   ║   Hb-NH2 + CO2          CO2 + H2O                                ║
   ║      │                      │                                    ║
   ║      ▼               CARBONIC ANHYDRASE (~5,000× faster here)    ║
   ║  Hb-NH-COOH                 ▼                                    ║
   ║  CARBAMINO-              H2CO3                                   ║
   ║  HEMOGLOBIN                 │                                    ║
   ║   ~20%                      ▼                                    ║
   ║                        H+  +  HCO3−                              ║
   ║                         │        │                               ║
   ║          buffered by ◄──┘        │  ~70%                         ║
   ║          DEOXY-Hb                │                               ║
   ║          (= the BOHR effect:     │                               ║
   ║           H+ on globin lowers    │                               ║
   ║           O2 affinity → more     ▼                               ║
   ║           O2 released HERE)   ╔══════════════════╗               ║
   ║                               ║ band 3 exchanger ║               ║
   ╰═══════════════════════════════╣ HCO3− OUT ──────►╠═══════════════╯
                                   ║ ◄────── Cl− IN   ║   TO PLASMA
                                   ╚══════════════════╝
                                   = THE CHLORIDE SHIFT
                        (keeps the cell electrically neutral;
                         without it, HCO3− export stops at once)

   AT THE LUNG: every arrow reverses. HCO3− re-enters, Cl− leaves
   (reverse chloride shift), H+ comes off Hb → O2 affinity RISES
   → loading is enhanced. This is the HALDANE effect's other half.

22.7 Control of Respiration

Breathing is the only major autonomic function you can override at will, and that dual control — automatic by default, voluntary on demand — is built into the anatomy.

The medullary rhythm generator

The basic respiratory rhythm arises in the medulla oblongata (Chapter 12), from two interconnected cell groups.

• The ventral respiratory group (VRG) is the rhythm generator. Within it, a small cluster called the pre-Bötzinger complex contains pacemaker neurons that depolarize spontaneously and rhythmically, much as the sinoatrial node does in the heart (Chapter 18). The VRG contains both inspiratory and expiratory neurons; in quiet breathing only the inspiratory ones fire, driving the phrenic nerves (C3–C5, to the diaphragm) and intercostal nerves for about two seconds, then falling silent for three while the lung recoils passively. The expiratory neurons are recruited only for forced expiration.
• The dorsal respiratory group (DRG) is chiefly an integrator. It receives afferent input from peripheral chemoreceptors, from pulmonary stretch receptors, and from irritant and J-receptors, all arriving via the vagus and glossopharyngeal nerves, and it modulates the VRG's output accordingly.

The phrenic nerve's origin at C3–C5 is worth remembering with the old mnemonic C3, 4, 5 keeps the diaphragm alive: a spinal cord injury above C3 abolishes spontaneous breathing entirely, while an injury at C6 leaves the diaphragm working and costs only the intercostals and abdominals.

The pontine centres

The pontine respiratory group (historically the pneumotaxic and apneustic centres) does not generate rhythm; it smooths and shapes it. It fine-tunes the transition from inspiration to expiration, and its influence is what allows the breathing pattern to be modified by speech, exercise, sleep, and emotion. Damage produces characteristically abnormal patterns — prolonged gasping inspirations, or irregular ataxic breathing — which is why breathing patterns are used clinically to localize brainstem lesions.

Central chemoreceptors: the dominant drive

Here is the most consequential fact in the control of breathing, and it surprises nearly everyone: the moment-to-moment drive to breathe is set by carbon dioxide, not oxygen.

Central chemoreceptors lie on the ventral surface of the medulla, bathed in cerebrospinal fluid. They do not sense CO₂ directly. They sense the pH of the CSF, and here the blood–brain barrier does something clever.

• The blood–brain barrier is impermeable to H⁺ and HCO₃⁻ — charged ions cannot cross.
• It is freely permeable to CO₂, a small uncharged lipid-soluble molecule.

So when arterial P_CO₂ rises, CO₂ diffuses straight into the CSF. There it meets carbonic anhydrase, forms carbonic acid, and dissociates to H⁺ and HCO₃⁻. Because CSF has very little protein, it has very little buffering capacity, so the same amount of acid produces a larger pH change in CSF than it does in blood. The central chemoreceptors detect that sharpened pH signal and drive ventilation up.

The result is a control system of remarkable sensitivity. A rise in arterial P_CO₂ of just 1 mm Hg increases minute ventilation by roughly 2–3 L/min — a 30–50% increase in breathing for a 2.5% change in the variable. Around 70% of the total ventilatory response to CO₂ comes from these central receptors.

And this explains something you have experienced. The urge to breathe during a breath-hold is not an oxygen signal; it is a CO₂ signal. You can hold your breath much longer after hyperventilating, not because you stored extra oxygen — the flat top of the dissociation curve means you stored essentially none — but because you blew off CO₂ and it now takes longer for it to accumulate to the threshold that forces you to breathe.

Peripheral chemoreceptors: the oxygen backup

Peripheral chemoreceptors sit in the carotid bodies (at the bifurcation of the common carotid arteries, afferent via the glossopharyngeal nerve) and the aortic bodies (on the aortic arch, afferent via the vagus). They respond to three things: falling P_O₂, rising P_CO₂, and falling arterial pH.

The oxygen response has a threshold that follows directly from Figure 22.6. Peripheral chemoreceptors fire only weakly until arterial P_O₂ falls below about 60 mm Hg, below which their firing rate climbs steeply. That threshold is not arbitrary — it is the knee of the oxyhemoglobin dissociation curve. Above 60 mm Hg, falling P_O₂ barely reduces saturation and therefore barely reduces oxygen content, so there is nothing worth responding to. Below 60, content falls off a cliff. The sensor's threshold is placed exactly where the variable it protects starts to matter. That is one of the tidiest examples of design logic in the body.

Note the consequence for Amara. Her PaO₂ of 74 mm Hg is below normal but above the peripheral chemoreceptor threshold. Her hypoxemia is contributing very little to her respiratory drive. Something else is driving her.

Reflexes and higher control

The Hering-Breuer inflation reflex. Stretch receptors in the smooth muscle of the airways fire when the lung is substantially inflated, sending vagal afferents to the DRG that inhibit inspiration. It is a protective over-inflation cutoff, not a normal breath terminator — in adults it activates only above tidal volumes of roughly 1 L (it is more active in infants). It is why you cannot voluntarily inhale indefinitely.

Irritant receptors in the airway epithelium respond to dust, smoke, noxious gases, and mucus, triggering cough, bronchoconstriction, and breath-holding.

J-receptors (juxtacapillary receptors) sit in the alveolar walls next to capillaries and fire when the pulmonary interstitium becomes engorged with fluid. They produce rapid shallow breathing and the sensation of dyspnea. Remember this receptor — it is the direct link between Amara's Kerley B lines and her feeling of air hunger.

Higher centres. The hypothalamus and limbic system modify breathing with emotion (which is why anxiety produces hyperventilation), and the cerebral cortex can bypass the medulla entirely via direct corticospinal projections to the respiratory motor neurons — voluntary control, used for speech, singing, and breath-holding. That voluntary control has a hard limit: the chemoreceptor drive eventually overrides it, which is why a child threatening to hold their breath until they get their way will, at worst, faint and then breathe.

The exercise response remains partly unexplained, and honestly so. Ventilation rises almost instantaneously at the onset of exercise — before any measurable change in arterial blood gases — and arterial P_CO₂ and P_O₂ stay remarkably constant through moderate exercise. Since the blood gases are not changing, they cannot be what is driving the response. Three contributors are recognized: anticipatory central command (motor cortex signals descending to the respiratory centres as a copy of the movement command), proprioceptive feedback from moving joints and muscles, and later, at higher intensities, genuine chemical drive from the CO₂ generated by bicarbonate buffering of lactate. The first two are feed-forward rather than feedback control — the system responds to the prediction of a disturbance rather than to the disturbance itself, which is a strategy you will see again in Chapters 23 and 24.

     CONTROL OF BREATHING — THE FULL LOOP

   ┌──────────────────── HIGHER CENTRES ─────────────────────┐
   │ CEREBRAL CORTEX ──── voluntary override (speech, breath │
   │                      holding) via corticospinal tract   │
   │ HYPOTHALAMUS/LIMBIC ─ emotion, pain, temperature        │
   └──────────────────────────┬──────────────────────────────┘
                              │
   ┌──────────────────────────▼──────────────────────────────┐
   │  PONTINE RESPIRATORY GROUP — smooths the transitions    │
   ├─────────────────────────────────────────────────────────┤
   │  MEDULLA                                                │
   │   ┌──────────────────┐        ┌───────────────────────┐ │
   │   │ DRG              │◄───────│ VRG                   │ │
   │   │ integrates       │───────►│ pre-Bötzinger complex │ │
   │   │ afferent input   │        │ = RHYTHM GENERATOR    │ │
   │   └──────────▲───────┘        └───────────┬───────────┘ │
   └──────────────┼────────────────────────────┼─────────────┘
                  │ AFFERENT                   │ EFFERENT
                  │                            ▼
   ┌──────────────┴──────────┐    phrenic n. (C3–C5) ──► DIAPHRAGM
   │ SENSORS                 │    intercostal nn.    ──► INTERCOSTALS
   │                         │                            │
   │ ● CENTRAL CHEMO-        │                            ▼
   │   RECEPTORS (ventral    │                    VENTILATION CHANGES
   │   medulla, in CSF)      │                            │
   │   sense CSF pH ◄── CO2  │                            ▼
   │   crosses BBB; H+/HCO3− │              ┌──────────────────────────┐
   │   cannot. Low CSF       │              │ ARTERIAL PCO2 ▼          │
   │   buffering = sharp     │              │ ARTERIAL pH   ▲          │
   │   signal. ~70% of drive │              │ ARTERIAL PO2  ▲          │
   │   +1 mm Hg PCO2 →       │◄─────────────┤                          │
   │   +2–3 L/min            │   NEGATIVE   └──────────────────────────┘
   │                         │   FEEDBACK
   │ ● PERIPHERAL CHEMO-     │
   │   RECEPTORS (carotid +  │   ► THE DOMINANT EVERYDAY DRIVE IS CO2.
   │   aortic bodies)        │     Oxygen is the BACKUP, and it does
   │   PO2 < 60 mm Hg only!  │     not speak until PO2 < 60 — exactly
   │   = the knee of the O2  │     the knee of the dissociation curve.
   │   curve. ~30% of drive  │
   │                         │
   │ ● STRETCH receptors ──► Hering-Breuer: inhibits inspiration >1 L
   │ ● IRRITANT receptors ─► cough, bronchoconstriction
   │ ● J-RECEPTORS ────────► fire when INTERSTITIUM is congested
   │                         → rapid shallow breathing + DYSPNEA
   │                         ◄══ THIS IS AMARA'S DRIVE
   └─────────────────────────┘

22.8 Respiratory Adjustments and Disorders

Every disorder below is a failure of one specific mechanism from §22.1 through §22.7. Naming the failed mechanism is more useful than memorizing the disease.

Exercise

Ventilation rises from 6 L/min at rest to 100–150 L/min at maximum, achieved by increasing tidal volume first (up to about 60% of vital capacity) and respiratory rate second — the efficient order, given what §22.4 established about dead space. Arterial P_O₂, P_CO₂, and pH change remarkably little through moderate exercise. Pulmonary capillary recruitment and distension roughly triple the diffusing capacity, and pulmonary vascular resistance actually falls as cardiac output rises, so pulmonary artery pressure increases only modestly.

Altitude

At 3,000 m, atmospheric pressure is about 526 mm Hg, so inspired P_O₂ falls to about 100 mm Hg and alveolar P_O₂ to roughly 60. Acclimatization proceeds on a schedule.

Acute mountain sickness — headache, nausea, insomnia, fatigue — appears above roughly 2,500 m when ascent outpaces acclimatization. Its severe forms, high-altitude pulmonary edema and cerebral edema, are both mechanistically instructive: HAPE arises because hypoxic pulmonary vasoconstriction becomes global rather than local. What is an elegant local matching reflex at sea level becomes, when every alveolus is hypoxic, a uniform rise in pulmonary arterial pressure that forces fluid across the capillary wall. A protective mechanism applied indiscriminately turns into the disease.

COPD: two phenotypes, one ratio

Chronic obstructive pulmonary disease is defined physiologically by a post-bronchodilator FEV₁/FVC below 0.70 that does not fully reverse. Two classical phenotypes sit at either end of a spectrum, and most patients have some of both.

Chronic hypoxemia in either phenotype drives sustained hypoxic pulmonary vasoconstriction throughout the lung, raising pulmonary vascular resistance and afterloading the right ventricle. The result is cor pulmonale — right heart failure secondary to lung disease (Chapter 18) — with peripheral edema and a raised jugular venous pressure.

Asthma

Reversible obstruction of the small airways, driven by chronic inflammation. Three components narrow the lumen simultaneously: bronchospasm of the smooth muscle sleeve, mucosal edema, and mucus hypersecretion. Because resistance varies with the fourth power of the radius, a lumen narrowed by half raises resistance sixteenfold.

The physiological signature is expiratory: the airway narrows further during expiration, because the positive intrapleural pressure of forced expiration compresses airways that have lost their cartilage. Hence wheeze on expiration, prolonged expiratory phase, air trapping, and a rising FRC. A silent chest in a severe attack is ominous rather than reassuring — no wheeze means no airflow. The blood gas in a severe attack initially shows a low PaCO₂ (hyperventilation of the unobstructed regions); a normalizing or rising PaCO₂ signals exhaustion and impending respiratory failure. The immunology behind it — the type I hypersensitivity reaction, IgE, mast cells, and the late-phase response — is in Chapter 20.

Pneumonia, ARDS, and pulmonary edema

Pneumonia fills alveoli with inflammatory exudate. Those alveoli are perfused but not ventilated — a shunt, V̇/Q̇ = 0 — which, as §22.5 established, responds poorly to supplemental oxygen. Consolidated lung also becomes stiff, lowering compliance.

Acute respiratory distress syndrome (ARDS) is diffuse alveolar injury from sepsis, trauma, aspiration, or severe pneumonia. The alveolar–capillary membrane becomes leaky, protein-rich fluid floods the airspaces, type II cells are damaged so surfactant is lost, and the lung becomes stiff and heterogeneous — some regions collapsed, some flooded, some relatively normal. The result is severe refractory hypoxemia with markedly reduced compliance. Note that ARDS attacks §22.2 directly: it is a surfactant and membrane disease, which is why the ventilation strategy is low tidal volumes plus positive end-expiratory pressure to recruit and hold open collapsed alveoli.

Cardiogenic pulmonary edema is different in origin and identical in mechanics, and it is Amara's. Take it in order, because it is the answer to her second question.

The pulmonary capillary is subject to the same Starling forces you met in Chapter 19. Fluid movement across its wall is set by the balance of hydrostatic pressure pushing out and oncotic pressure holding in.

• Normal pulmonary capillary hydrostatic pressure is about 10 mm Hg — far lower than the ~30 mm Hg of a systemic capillary, because the pulmonary circuit is a low-pressure system.
• Plasma oncotic pressure is about 25 mm Hg, the same as everywhere.
• The net balance therefore favours reabsorption, and the small amount of fluid that does filter is removed by pulmonary lymphatics, which can increase their flow up to tenfold (Chapter 20).

The lung is normally kept dry by a large safety margin. Now break it. A left ventricle that cannot relax properly — Amara's heart failure with preserved ejection fraction — needs a higher filling pressure to accept the same volume. That elevated left ventricular end-diastolic pressure is transmitted backwards: to the left atrium, then through the pulmonary veins, then into the pulmonary capillaries.

• At a pulmonary capillary pressure of about 18–20 mm Hg, filtration exceeds lymphatic drainage and fluid accumulates in the interstitium. This is interstitial edema, and it is visible on the chest radiograph as Kerley B lines — thickened interlobular septa — exactly as §22.2's "thick side" predicted. It is also where J-receptors are, so the patient becomes dyspneic and breathes rapidly and shallowly before any alveolus has flooded.
• Above about 25 mm Hg, exceeding plasma oncotic pressure, fluid crosses into the alveoli themselves. Now gas exchange fails outright: flooded alveoli are perfused but unventilated — shunt — and the patient becomes hypoxemic, produces frothy pink sputum, and has audible crackles.

Note what has not happened: nothing is wrong with Amara's lungs. The lung is the passive downstream victim of an upstream pressure problem. Left heart failure is diagnosed by listening to the lungs, which is one of the best arguments in this book for studying systems together.

Orthopnea, and why lying flat is the test

Amara cannot lie flat. The mechanism has two parts.

First, venous return redistributes. When you stand or sit, roughly 500–700 mL of blood pools in the veins of the legs and splanchnic circulation, held there by gravity (Chapter 19). Lie down and gravity stops holding it there; that volume returns to the central circulation over a few minutes. A healthy left ventricle simply pumps the extra volume — the Frank-Starling mechanism means a larger end-diastolic volume produces a larger stroke volume (Chapter 18). A failing left ventricle cannot. The extra preload arrives, the ventricle cannot accommodate it, filling pressure rises further, and the pressure is transmitted back into the pulmonary capillaries. Pulmonary capillary hydrostatic pressure climbs, filtration increases, and interstitial edema worsens — within minutes.

Second, mechanics worsen. Supine, the abdominal contents press the diaphragm cephalad, reducing FRC by several hundred millilitres (a larger effect at Amara's BMI of 29.3), and the edematous lung's already-reduced compliance makes each breath more expensive.

Sitting up reverses both. This is why patients with heart failure sleep on three pillows and why counting the pillows is a genuine, quantitative clinical measurement. Paroxysmal nocturnal dyspnea — waking abruptly at 02:00 gasping and needing to sit or stand at an open window — is the same mechanism operating more slowly during sleep, as reabsorbed interstitial fluid from the legs gradually loads the circulation over hours.

Obstructive sleep apnea

Amara's AHI of 32 events per hour, with desaturation to 84%, places her in the severe category (mild 5–15, moderate 15–30, severe > 30). The mechanism is purely mechanical and then comprehensively systemic.

The mechanical event. During sleep, muscle tone falls throughout the body, including in the genioglossus and other pharyngeal dilator muscles that hold the oropharynx open. Recall from §22.1 that the pharynx has no cartilage — it is a collapsible tube held open by muscle alone, a design compromise inherited from the need to share the corridor with food. During inspiration, the airway is subjected to negative pressure, and if dilator tone is insufficient the tube collapses. Airflow stops while respiratory effort continues — the definition of an obstructive apnea, as opposed to a central apnea where effort also stops. Risk rises with obesity (fat deposition in the parapharyngeal tissues), a large neck circumference, supine posture, nasal obstruction, alcohol and sedatives, and male sex.

The consequence, repeated 32 times an hour. Each event follows the same arc: airway collapses → PaO₂ falls and PaCO₂ rises → chemoreceptors drive increasing inspiratory effort against a closed airway (generating strikingly negative intrathoracic pressures, sometimes −60 cm H₂O) → a cortical arousal finally restores dilator tone → the airway snaps open with the characteristic gasp → the patient returns to sleep, and the cycle repeats.

Over a seven-hour night, that is more than 200 episodes of hypoxia, hypercapnia, and arousal.

22.9 Advanced Topic · Acid-Base and the Respiratory System

The body defends arterial pH at 7.40, within a range of 7.35 to 7.45. That is a hydrogen ion concentration of about 40 nanomoles per litre — a millionfold more dilute than sodium, and regulated a thousand times more tightly. Below 6.8 or above 7.8, enzymes and membrane proteins fail and life ends.

Three lines of defence hold that value, on three different timescales.

This section is about the middle row. Chapter 31 assembles all three.

Why ventilation is a pH control

The connection is the carbonic acid equilibrium you met in §22.6, now read as a chemical equation running in both directions:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

Read it left to right and you have CO₂ transport. Read it as a lever and you have acid–base control. Because CO₂ sits on one end and H⁺ on the other, anything that changes CO₂ changes pH in the opposite direction:

• Hypoventilation → CO₂ accumulates → equilibrium shifts right → H⁺ rises → pH falls. CO₂ behaves as an acid. It is often called a volatile acid precisely because you can exhale it.
• Hyperventilation → CO₂ is blown off → equilibrium shifts left → H⁺ is consumed → pH rises.

You therefore have, in your brainstem, a pH controller that works by changing how fast you breathe, and it can move arterial pH within a minute or two. Nothing else in the body adjusts pH that fast. It is also the reason a single number — PaCO₂ — appears in both the respiratory and the acid–base sections of every blood gas interpretation: it is simultaneously a measure of alveolar ventilation and a determinant of pH.

The regulation is genuinely closed-loop. Rising H⁺ stimulates peripheral chemoreceptors (which do sense arterial pH directly) and, via CO₂, the central ones; ventilation rises; CO₂ falls; H⁺ falls. It is textbook negative feedback (Chapter 1), with the lung as the effector.

The four primary disturbances

Every acid–base disorder is classified by two questions: is the pH acidic or alkaline, and is the primary problem respiratory (PaCO₂) or metabolic (HCO₃⁻)?

Respiratory compensation for a metabolic disturbance is fast and powerful. In diabetic ketoacidosis, ketoacids consume bicarbonate and pH falls; chemoreceptors respond within minutes, and the patient develops Kussmaul respirations — deep, laboured, sighing breathing — driving PaCO₂ down to 15–20 mm Hg and dragging pH back toward normal. The breathing pattern is a compensation, not a lung problem, and treating it as though it were one would be a serious error.

Metabolic compensation for a respiratory disturbance is slow and complete. The kidney takes 3–5 days to adjust bicarbonate handling. This distinction — minutes in one direction, days in the other — is why you can tell an acute disturbance from a chronic one by looking at whether compensation has occurred. A PaCO₂ of 60 with a bicarbonate of 24 is acute; a PaCO₂ of 60 with a bicarbonate of 34 has been present for a week.

Two rules that keep interpretation honest: compensation never fully corrects the pH (if pH is dead normal with abnormal PaCO₂ and HCO₃⁻, suspect two coexisting disorders), and compensation never overshoots past the midline.

A working method for any blood gas

1. Look at the pH. Below 7.35, acidemia. Above 7.45, alkalemia.
2. Look at the PaCO₂. Has it moved in the direction that would cause the pH abnormality? If yes, the problem is respiratory.
3. Look at the HCO₃⁻. Has it moved in the direction that would cause the pH abnormality? If yes, the problem is metabolic.
4. Check the other value for compensation — is it moving in the direction that would push pH back toward normal?
5. Assess oxygenation separately. PaO₂ and SaO₂ have nothing to do with the acid–base classification. Two independent readings on one report.

Amara's blood gas, interpreted completely

Interpretation: acute uncompensated respiratory alkalosis with mild hypoxemia.

Every element of that sentence carries information.

• Respiratory, because PaCO₂ is the value that moved in the direction that explains the pH.
• Alkalosis, because pH is above 7.45.
• Acute and uncompensated, because bicarbonate is still 23 — squarely normal. The kidney needs 3–5 days to excrete bicarbonate in response; this began yesterday. Had this been chronic, you would expect a bicarbonate around 18–20.
• With mild hypoxemia, a separate finding, caused by the thickened respiratory membrane and the low-V̇/Q̇ units of her interstitial pulmonary edema (§22.8).

And now the causal chain, which is the whole point. Her hyperventilation is not a respiratory disease. It is the symptom of a cardiac one. Interstitial fluid, driven across the pulmonary capillary wall by an elevated hydrostatic pressure that originates in a stiff left ventricle, stimulates J-receptors. J-receptors drive rapid shallow breathing and produce the sensation of air hunger. That increased alveolar ventilation cannot correct the hypoxemia — hemoglobin in her healthy alveoli is already 97% saturated and cannot hold more — but it very effectively removes CO₂, whose transport is not saturable. So PaCO₂ falls, pH rises, and the blood gas records a respiratory alkalosis.

The alkalosis is the fingerprint of a heart problem, left on a respiratory test.

Note also what the alkalosis costs her. Alkalosis left-shifts the oxyhemoglobin curve (§22.6), increasing hemoglobin's affinity for oxygen and making it less willing to release oxygen at the tissues. In a woman whose myocardium is already ischemic, that is not a helpful direction. It is a small effect, but it is real, and it is the sort of second-order consequence that separates knowing the facts from understanding the system.

Chapter 31 will take this apparatus and add the kidney, at which point Amara's story acquires a diuretic, a falling potassium, and a metabolic alkalosis layered on top of everything here.

Chapter Summary

§22.1 The respiratory system solves the distance problem with bulk flow followed by diffusion, and is divided accordingly into a conducting zone (nose through terminal bronchioles — warming, humidifying, filtering, and ~150 mL of anatomic dead space) and a respiratory zone (respiratory bronchioles, alveolar ducts, alveoli). Structure predicts function throughout: turbinates create turbulence for air conditioning; the cricoid is the only complete ring; tracheal cartilage is C-shaped and open posteriorly so the esophagus can distend; the right main bronchus is wider, shorter, and more vertical, so aspirated material lands there; and down the 23 generations of the bronchial tree, cartilage disappears while smooth muscle takes over, which is why asthma is a bronchiolar disease.

§22.2 Roughly 480 million alveoli present ~70 m² across a barrier of 0.2–0.6 µm — area maximized by subdivision, thickness minimized to the limit of what a cell can be. Type I cells are the barrier, type II cells make surfactant and serve as stem cells, and alveolar macrophages patrol the airspace. Surfactant solves the Laplace problem by lowering surface tension more when the alveolus is smaller, which is what keeps small alveoli from emptying into large ones. A red cell equilibrates in 0.25 s of a 0.75 s transit — a threefold reserve that exercise consumes and disease erodes.

§22.3 The lung's natural state is collapsed and the chest wall's is expanded; coupled by pleural fluid, they generate a subatmospheric intrapleural pressure of −4 cm H₂O and a transpulmonary pressure of +4 that holds the lung open. Quiet inspiration is active (diaphragm, 75% of the work), quiet expiration passive. Compliance falls with fibrosis, edema, surfactant loss, and chest-wall restriction, and rises destructively in emphysema, where lost recoil means the lung cannot empty. Most airway resistance lies in medium bronchi, not bronchioles, because total cross-sectional area explodes downstream — which is why small-airway disease is silent for years.

§22.4 Four volumes and four capacities. FRC is the oxygen reservoir between breaths; residual volume keeps gas exchange continuous. Alveolar ventilation, not minute ventilation, is the physiologically meaningful number, because dead space is a fixed toll paid on every breath — 6 L/min delivered as slow deep breaths gives 5,100 mL/min of alveolar ventilation and as rapid shallow breaths only 2,400. FEV₁/FVC below 0.70 defines obstruction; restriction reduces volumes while preserving or raising the ratio.

§22.5 Dalton's law sets partial pressures; humidification and mixing bring alveolar P_O₂ to 104 and P_CO₂ to 40. Henry's law plus CO₂'s twentyfold greater solubility explains why CO₂ crosses fully on a 5 mm Hg gradient while O₂ needs 64 — and why oxygen always fails first in lung disease. Exchange depends on gradient, area, and thickness. Ventilation–perfusion matching is maintained locally by hypoxic pulmonary vasoconstriction (unique to the lung) and by bronchiolar responses to CO₂.

§22.6 Hemoglobin carries 98.5% of blood oxygen and raises capacity seventyfold; only dissolved gas exerts partial pressure, which is why saturation, content, and delivery are three different things. The sigmoid dissociation curve is flat where it loads (a safety margin) and steep where it unloads. It shifts right — releasing more oxygen — with acidity, CO₂ (the Bohr effect), heat, and 2,3-BPG, every one of which is a marker of active metabolism. CO₂ travels dissolved (7–10%), as carbaminohemoglobin on globin (20%), and as bicarbonate (70%), generated by carbonic anhydrase inside the red cell and exported in exchange for chloride — the chloride shift.

§22.7 The VRG generates the rhythm and the DRG integrates afferents; the pons smooths. Central chemoreceptors sensing CSF pH provide ~70% of the drive and make CO₂ the dominant everyday stimulus; peripheral chemoreceptors are the only oxygen sensors and stay quiet until PaO₂ falls below 60 — the knee of the dissociation curve. Hering-Breuer limits over-inflation, J-receptors signal interstitial congestion, and the exercise response is largely feed-forward.

§22.8 Each disorder is one broken mechanism: COPD (fixed obstruction; emphysema loses surface and recoil, bronchitis loses ventilation), asthma (reversible bronchiolar narrowing), pneumonia and ARDS (shunt and surfactant loss), pulmonary embolism (dead space), cardiogenic pulmonary edema (a Starling-forces problem imported from the left heart), and sleep apnea (loss of pharyngeal dilator tone in a tube with no cartilage).

§22.9 Ventilation controls pH within minutes by moving CO₂ along the carbonic acid equilibrium. Four primary disturbances; respiratory compensation is fast, renal compensation takes days, and neither fully corrects pH. Amara's gas is an acute uncompensated respiratory alkalosis with mild hypoxemia — a cardiac disease leaving its fingerprint on a respiratory test.

Case File 22 · Resolution

Question 1 — Her SpO₂ was 96% on room air at triage, and she was breathing 24 times a minute. What was she actually trying to fix?

Not her oxygen. Three separate lines of evidence say so, and together they make the point that this chapter exists to make.

First, she could not have been fixing her oxygen even if she had tried. At an SpO₂ of 96%, hemoglobin sits on the flat plateau of the oxyhemoglobin dissociation curve (§22.6). Breathing harder raises alveolar P_O₂ somewhat, but hemoglobin that is already 96% saturated has almost no unoccupied binding sites left. The additional oxygen carried would be dissolved oxygen, which is 1.5% of the total. Hyperventilation is a nearly useless intervention for oxygen content, and the body does not deploy useless interventions.

Second, her oxygen sensor was not talking. Peripheral chemoreceptors in the carotid and aortic bodies — the only oxygen sensors she has — respond meaningfully only below a PaO₂ of about 60 mm Hg (§22.7). Her PaO₂ the next day was 74. At triage, with a saturation of 96%, it was higher still. The hypoxic drive was essentially silent.

Third, what she was fixing shows up on the blood gas. Her PaCO₂ is 31 mm Hg, well below the normal 35–45. She was ventilating well above her metabolic requirement, and the measurable consequence was a fall in CO₂ and a rise in pH to 7.47.

So what drove it? Two things, and they are separable.

The reflex driver was J-receptor stimulation. Fluid was accumulating in her pulmonary interstitium — the same fluid that later produced Kerley B lines on her chest film. J-receptors sit in the alveolar walls next to the capillaries and fire when that interstitium becomes engorged. Their output is stereotyped: rapid shallow breathing and the conscious sensation of air hunger. This is a direct reflex arc. It does not consult the blood gases, it does not wait for hypoxemia, and it fires while the oximeter still reads 96%.

The conscious driver was dyspnea itself. "I can't get a deep enough breath" is a precise description of the sensation those receptors produce, layered on top of the sympathetic activation of an ischemic myocardium (Chapter 1) and the anxiety that accompanies it. Dyspnea is not a measurement of oxygen; it is a sensation with its own afferent pathway.

Add the third contributor: rapid shallow breathing at a small tidal volume is inefficient, because dead space is a fixed toll on every breath (§22.4). Some of her increased effort was being spent moving the same 150 mL of tracheal air back and forth. The pattern feeding the sensation was making the ventilation less effective, which is a small, vicious circle of its own.

The general lesson. Amara's oximeter was reporting one of three numbers: saturation (the percentage of hemoglobin sites filled), which was normal. It was not reporting content — how much oxygen a volume of blood actually holds, which depends on her hemoglobin concentration — and it was not reporting delivery, which is content times blood flow and is the only quantity a mitochondrion experiences. A coronary artery narrowed to a trickle delivers almost nothing to the muscle downstream while the blood in her fingertip remains beautifully saturated. That distinction was previewed in the Chapter 1 Case File and you have now derived it from first principles.

Question 2 — Why does a failing heart put fluid in the lungs, and why does lying flat make it worse?

Because the pulmonary capillary is a Starling membrane (Chapter 19) plugged directly into the left atrium, and there is no valve in between.

The normal balance. Fluid movement across a capillary wall is set by hydrostatic pressure pushing out against oncotic pressure holding in. In the lung, capillary hydrostatic pressure is about 10 mm Hg — far below the ~30 mm Hg of a systemic capillary, because the pulmonary circulation is a low-pressure circuit fed by a thin-walled right ventricle. Plasma oncotic pressure is about 25 mm Hg everywhere. So the lung sits with a large net margin favouring reabsorption, and whatever does filter is carried away by pulmonary lymphatics that can increase their flow up to tenfold (Chapter 20). The lung is kept dry with an enormous safety factor.

How the failing heart consumes that margin. Amara has heart failure with preserved ejection fraction: her left ventricle contracts adequately but is stiff and relaxes poorly, so it requires a higher filling pressure to accept a normal volume. That elevated left ventricular end-diastolic pressure has nowhere to go but backwards. There is no valve between the left atrium and the pulmonary veins, and none between the pulmonary veins and the pulmonary capillaries. Left atrial pressure is therefore transmitted, essentially undamped, into the pulmonary capillary bed.

Raise pulmonary capillary hydrostatic pressure from 10 mm Hg to 18–20, and filtration outstrips lymphatic drainage. Fluid accumulates in the interstitium first, exactly on the thick side of the capillary described in §22.2, where the basement membranes separate and the lymphatics run. This is interstitial edema, and it produces three things at once: Kerley B lines on the radiograph (fluid-thickened interlobular septa), reduced compliance (a wet lung is a stiff lung), and J-receptor firing (dyspnea and rapid shallow breathing). Note that all three occur while oxygenation is still nearly normal.

Raise it above about 25 mm Hg — past plasma oncotic pressure — and fluid crosses into the alveoli. Now gas exchange fails: flooded alveoli are perfused but unventilated, a shunt that responds poorly to supplemental oxygen, and the patient becomes frankly hypoxemic with crackles and frothy sputum.

Why lying flat makes it worse, within minutes. Upright, 500–700 mL of blood is held by gravity in the veins of the legs and abdomen. Lie down and that pooled volume returns centrally over a few minutes. A healthy ventricle handles the extra preload by the Frank-Starling mechanism (Chapter 18) — more filling, more stretch, more stroke volume. A stiff ventricle cannot. The extra volume arrives and simply raises filling pressure further, which is transmitted backwards to the pulmonary capillaries, which increases filtration, which worsens interstitial edema. Simultaneously the abdominal contents press the diaphragm cephalad, reducing FRC — an effect magnified by her BMI of 29.3 — so each breath is taken from a smaller, stiffer lung.

Sitting up reverses both mechanisms, which is why she is on three pillows and why orthopnea counted in pillows is a real hemodynamic measurement. The same mechanism operating slowly overnight, as leg interstitial fluid is reabsorbed into the circulation, is paroxysmal nocturnal dyspnea.

The one-sentence version: her lungs are healthy and are drowning in a pressure problem imported from the chamber upstream.

Question 3 — How can sleep apnea, a night-time breathing problem, damage a heart?

By running the causation the other way, 32 times an hour, every night, for fifteen years.

The event. During sleep, tone falls in the genioglossus and the other pharyngeal dilators. The oropharynx has no cartilage — it is a collapsible tube held open by muscle alone, because it evolved to be shared with swallowing (§22.1). Inspiratory negative pressure then sucks it shut. Airflow stops; respiratory effort continues against a closed airway. Each event lasts 10–30 seconds and ends in a cortical arousal that restores muscle tone, opens the airway, and produces the gasp her husband has been listening to for years.

Four separate injuries follow from that, and they converge on the heart.

1 · Intermittent hypoxia and sympathetic surges. PaO₂ falls — in her case to a saturation of 84% — and PaCO₂ rises. Both stimulate chemoreceptors, which drive a powerful sympathetic discharge: vasoconstriction, tachycardia, and a surge in blood pressure often exceeding 240/130 mm Hg at the moment of arousal. This happens 32 times an hour, more than 200 times a night. Unlike a single stress, the pattern of repeated hypoxia–reoxygenation is a potent generator of oxidative stress and endothelial dysfunction (Chapter 19), and the sympathetic overactivity does not switch off with the sunrise — it persists into the daytime, resetting baseline sympathetic tone upward. Sustained hypertension is the result, and OSA is one of the leading identifiable causes of resistant hypertension. Amara's triage blood pressure of 168/98 has been rising for years for a reason nobody looked for.

2 · Increased afterload. Higher arterial pressure means the left ventricle ejects against greater resistance on every beat. Afterload is one of the three principal determinants of myocardial oxygen demand (Chapter 18), so this both increases her heart's work and worsens the supply–demand mismatch that produced her infarct.

3 · Pressure-overload remodelling. A ventricle chronically ejecting against high afterload hypertrophies concentrically — thicker walls, a smaller cavity, and, critically, impaired relaxation. That is diastolic dysfunction, and diastolic dysfunction is exactly the lesion behind her heart failure with preserved ejection fraction. Her sleep apnea helped build the stiff ventricle that is now flooding her lungs. The two halves of this Case File are the same story told at two ends.

4 · Mechanical strain during the event itself. Straining to inhale against a closed airway generates intrathoracic pressures as negative as −60 cm H₂O. That markedly negative pressure increases venous return to the right heart while simultaneously increasing left ventricular transmural pressure — effectively an added afterload on every obstructed breath, and a mechanical stimulus to atrial stretch that contributes to the well-documented association between OSA and atrial fibrillation.

Add the loss of restorative sleep, which raises cortisol and worsens insulin resistance (Chapter 16) — compounding twenty years of night-shift circadian disruption — and the picture is complete.

And the loop closes. Heart failure worsens sleep apnea, because pulmonary congestion and rostral fluid shift when supine narrow the upper airway further. Apnea worsens heart failure through the four mechanisms above. This is a positive feedback loop in the sense of Chapter 1 — a self-amplifying cycle with no built-in brake — and it is why treating the apnea, usually with continuous positive airway pressure, is a cardiac intervention as much as a respiratory one. CPAP is a pneumatic splint: it holds the collapsible pharyngeal tube open with a few centimetres of positive pressure, abolishes the events, and thereby removes the nightly sympathetic surges.

A woman who came to the emergency department with chest pain has, it turns out, been having a cardiovascular event every two minutes, all night, for fifteen years. Nobody asked, because snoring is not a symptom anybody takes to a doctor.

Systems Integration Case File · Entry 22

Review

Level 1 · Recall

19.1 The conducting zone ends at the:

a) respiratory bronchioles    b) terminal bronchioles    c) alveolar ducts    d) segmental bronchi

19.2 Surfactant reduces the tendency of small alveoli to collapse because it:

a) increases surface tension uniformly    b) reduces surface tension more as the alveolus gets smaller    c) thickens the alveolar wall    d) increases alveolar radius

19.3 Intrapleural pressure at the end of a quiet expiration is approximately:

a) +4 cm H₂O    b) 0 cm H₂O    c) −4 cm H₂O    d) −40 cm H₂O

19.4 A patient breathes 250 mL twenty-four times per minute. With a dead space of 150 mL, alveolar ventilation is:

a) 6,000 mL/min    b) 3,600 mL/min    c) 2,400 mL/min    d) 1,800 mL/min

19.5 Which shifts the oxyhemoglobin dissociation curve to the RIGHT?

a) decreased temperature    b) increased pH    c) increased 2,3-BPG    d) fetal hemoglobin

19.6 Most carbon dioxide is transported in blood as:

a) dissolved CO₂    b) carbaminohemoglobin    c) bicarbonate ion    d) carbonic acid

19.7 The dominant stimulus for the everyday drive to breathe is:

a) arterial PO₂    b) arterial PCO₂, sensed as CSF pH    c) arterial bicarbonate    d) lung stretch

19.8 A patient with COPD has an FEV₁/FVC of 0.55. A patient with pulmonary fibrosis would be expected to have an FEV₁/FVC that is:

a) below 0.70    b) normal or increased    c) exactly 0.55    d) unmeasurable

Level 2 · Comprehension

19.9 Explain why a person can hold their breath longer after hyperventilating, and why this is dangerous rather than useful.

19.10 Blood in a pulmonary capillary equilibrates with alveolar gas in about 0.25 seconds, but spends 0.75 seconds there. Why is that reserve important, and what happens to it in exercise and in disease?

19.11 A patient's arterial blood gas shows pH 7.30, PaCO₂ 60 mm Hg, HCO₃⁻ 29 mEq/L. Classify it and state whether it is acute or chronic. Justify each part.

19.12 Explain why hypoxic pulmonary vasoconstriction is the opposite of the systemic vascular response to hypoxia, and why both responses are correct.

Level 3 · Clinical Application

19.13 A 22-year-old is brought in from a house fire. He is confused and headachy but not cyanosed. His pulse oximeter reads 99% and his arterial PaO₂ is 98 mm Hg. His lungs are clear. Explain why he is dying, why every number looks normal, and what test you would order.

19.14 A 68-year-old with severe emphysema is admitted breathless. On 2 L/min of nasal oxygen his saturation is 89% and he is comfortable. A well-meaning helper turns the oxygen up to 10 L/min. Two hours later he is drowsy and his PaCO₂ has risen from 55 to 78 mm Hg. Explain the three mechanisms, and state what should be done.

19.15 A 34-year-old, 10 days post-partum, presents with sudden pleuritic chest pain and breathlessness. RR 30, HR 118, SpO₂ 91%. Chest radiograph is normal. ABG: pH 7.50, PaCO₂ 28, PaO₂ 58. Explain why a normal chest film supports rather than refutes the likely diagnosis, and explain the blood gas.

Level 4 · Integration and Synthesis

19.16 Construct the complete causal loop linking Amara's obstructive sleep apnea to her pulmonary edema, passing through at least four organ systems. Label which links are feedback and which are feed-forward, identify the point at which the loop becomes self-amplifying, and name the single intervention that breaks it and explain where in the loop it acts.

19.17 Nia runs a marathon in 3 hours 05 minutes on a 28 °C day, finishing dehydrated. Trace what happens to her oxyhemoglobin dissociation curve, her acid–base status, her ventilation, and her oxygen delivery over the course of the race. Then explain why her mother, doing 20 minutes on a stationary bicycle in cardiac rehabilitation, faces an entirely different limiting factor.

19.18 Amara's day-2 blood gas shows a respiratory alkalosis. Her overnight gas during an apnea would show a respiratory acidosis. Explain how the same patient produces opposite acid–base disturbances eight hours apart, what her kidney will do about it over the coming week, and why that renal response could become a problem when she is started on a loop diuretic in Chapter 31.

Concept Map to Complete

Copy this onto blank paper and fill in every bracket from memory before checking the chapter. Then, in a different colour, add what you missed.

                      OXYGEN FROM AIR TO MITOCHONDRION
                                    │
   ┌────────────────┬───────────────┼───────────────┬────────────────┐
   │                │               │               │                │
VENTILATION     DIFFUSION      TRANSPORT      PERFUSION       UTILIZATION
   │                │               │               │                │
 driven by      governed by     98.5% on       depends on      needs [ ____ ]
 [ _______ ]    [ ______'s ]    [ _______ ]    [ _______ ]     in mitochondria
 sensed as      law: rate ∝     1.5% [ ____ ]  (Ch 16, 17)
 [ _______ ]    [ ____ ] × Δ[ __ ]     │
 in the CSF          ÷ [ _______ ]     │
   │                                   ▼
   │                        DISSOCIATION CURVE is [ _______ ]-shaped
   │                        because binding is [ ____________ ]
   │                         │                        │
   │                    FLAT region              STEEP region
   │                    PO2 [ __ ]–100           PO2 10–[ __ ]
   │                    = safety for             = [ _________ ]
   │                      [ _________ ]            at the tissue
   │                         │
   │                    SHIFTS RIGHT with:  ↓[ __ ] · ↑[ ____ ]
   │                      ↑[ ___________ ] · ↑[ _______ ]
   │                    = all markers of [ _________________ ]
   │
   ├─── CONDUCTING ZONE ends at [ ___________________ ]
   │      = [ ______ ] mL of [ ______________ ] dead space
   │      ► ALVEOLAR ventilation = ([ __ ] − [ __ ]) × [ __ ]
   │
   ├─── RESPIRATORY MEMBRANE: [ _____ ] µm thick, [ __ ] m² area
   │      cells: type I = [ _________ ] · type II = [ __________ ]
   │      surfactant defeats [ _________'s ] law by lowering
   │      surface tension MORE when radius is [ _________ ]
   │
   └─── MECHANICS: transpulmonary P = P[ _____ ] − P[ ________ ]
          = +[ _ ] cm H2O at rest.  If it reaches 0 → [ ___________ ]

   FAILURE MODES ─────────────────────────────────────────────────────
     V/Q = 0        → [ _________ ]  → responds [ ______ ] to O2
     V/Q = infinity → [ _________ ]  → example: [ _______________ ]
     ↑ capillary hydrostatic P → [ ______________ ] → Kerley [ _ ] lines
     ↓ pharyngeal tone in sleep → [ ______________ ] → BP [ ____ ]

Lab / Self-Exploration

Do these on yourself or a willing partner. Physiology you have measured on your own body is retained far better than physiology you have read about.

1. Count a resting respiratory rate — someone else's, without telling them. Keep two fingers on their radial pulse as though still counting it, and watch their chest for 30 seconds. Now tell them what you are doing and count again. The second number is almost always different, usually higher and more regular. You have just demonstrated cortical override of a brainstem rhythm (§22.7), and incidentally learned why respiratory rate is the most frequently falsified vital sign in clinical practice.
2. Measure your own tidal volume and estimate your alveolar ventilation. Breathe normally into a balloon for exactly ten breaths, tie it off, and estimate its volume by water displacement in a measuring jug. Divide by ten for tidal volume. Now compute minute ventilation (× your respiratory rate) and alveolar ventilation (subtract 150 mL per breath first). Then deliberately breathe half as deeply, twice as fast, and recompute. Watch the alveolar number collapse while the minute number stays the same.
3. Breath-hold, twice — with a strict safety rule. Sit down. Time a comfortable breath-hold after a normal breath. Rest five minutes. Take three or four slightly deeper-than-normal breaths (do not hyperventilate hard) and time it again. It will be longer, and the reason is CO₂ washout, not oxygen storage (§22.7). Do this seated, on dry land, never in or near water, and never after deliberate hyperventilation — the combination causes shallow water blackout and kills strong swimmers every year.
4. Feel the two rib movements. Place your hands on your lower ribs, thumbs behind, fingers forward, and breathe deeply: you will feel the ribs swing outward — the bucket handle, increasing transverse diameter. Now place one hand flat on your upper sternum: you will feel it move forward and up — the pump handle, increasing anteroposterior diameter. Then place a hand just below your xiphoid and feel the diaphragm's descent push your abdomen out. Quiet inspiration should be mostly the third of these; if you are mostly using your neck muscles, you are recruiting accessory muscles at rest.
5. Find your own larynx in action. Put a finger lightly on your thyroid cartilage and swallow: feel it jump superiorly and anteriorly, the movement that tips the epiglottis over the glottis. Now hum and feel the vibration; hum a higher note and feel the cartilage tilt as the vocal folds are stretched. Finally, take a breath, close your glottis, and push gently as though lifting something heavy — that is a Valsalva. Release after two seconds only.
6. Test the paper-bag myth, on paper rather than in practice. Breathing into a paper bag was long recommended for hyperventilation, on the theory that rebreathing CO₂ corrects the respiratory alkalosis. Using §22.4 and §22.5, write down (a) why the theory is mechanistically sound and (b) two reasons it is nonetheless a bad idea — one involving what happens to the oxygen in a rebreathed bag, and one involving what happens if the hyperventilation was actually caused by a pulmonary embolism, an asthma attack, or diabetic ketoacidosis rather than by anxiety. Then look up current guidance and see whether you predicted it.

Key Terms

Next: Chapter 23 · The Digestive System — where the tube that shares its entrance with the airway is followed the other way, and where Amara's medications meet her gut.