19. The Cardiovascular System II

Blood Vessels, Blood Pressure, and Circulation

Learning Objectives

19.1 The Structure of Blood Vessels

One design, endlessly re-proportioned

There are roughly 100,000 kilometres of blood vessels in an adult human, and almost all of them are built from the same three concentric layers, called tunics. What changes from vessel to vessel is not the ingredients but the proportions — and the proportions are always a statement about the job.

Tunica intima (innermost). A single layer of simple squamous endothelium on a basement membrane, plus a thin subendothelial layer of areolar connective tissue in larger vessels. The endothelium is continuous with the endocardium of the heart, and it is the only layer present in every vessel without exception, from the aorta to the smallest capillary. Its surface is smooth and actively antithrombotic — which is what keeps blood liquid (Chapter 17). In arteries, the outer boundary of the intima is marked by a fenestrated sheet of elastin, the internal elastic lamina.

Tunica media (middle). Circularly arranged smooth muscle and sheets of elastin. This is the layer that varies most dramatically and the layer that does things: sympathetic vasomotor fibres innervate its smooth muscle, and contraction (vasoconstriction) or relaxation (vasodilation) of this layer changes vessel radius and therefore resistance, flow, and blood pressure. In muscular arteries an external elastic lamina marks its outer edge.

Tunica externa (outermost, also adventitia). Loose collagenous connective tissue that anchors the vessel to surrounding structures. In vessels larger than about 1 mm it contains its own tiny blood supply, the vasa vasorum ("vessels of the vessel"), because the wall has become too thick to be nourished by diffusion from the lumen — and, in a preview of §19.9, the vasa vasorum are the route by which inflammatory cells reach a growing plaque. It also carries the nervi vasorum, the sympathetic nerves supplying the media.

   THE THREE TUNICS ACROSS THE VESSEL TYPES
   (wall drawn in cross-section; not to a single scale)

  ELASTIC ARTERY          MUSCULAR ARTERY         ARTERIOLE
  aorta, carotid,         femoral, radial,        10–300 µm
  subclavian, pulm.       splenic, brachial       ═══════════════
  trunk · 1–2.5 cm        0.3 mm–1 cm             ▒▒ ext (thin)
  ═════════════════       ═════════════════       ██ media: 1–3
  ▒▒▒▒ externa            ▒▒▒▒ externa               SMC layers
  ████████████████        ██████████ media         ──── intima
  ████ media: 40–70       ██████████ up to 40      ═══════════════
  ████ ELASTIC            ██████████ SMC layers    ► THE RESISTANCE
  ████ LAMINAE with       ────── intima              VESSEL: the tap
  ████ SMC between        ═════════════════
  ─────── intima          ► DISTRIBUTES to
  ═════════════════         named organs;        CAPILLARY
  ► CONDUCTS + RECOILS      strongest vaso-      4–10 µm
    (the Windkessel)        constrictor          ═══════════
                            response             ─ endothelium
                                                 ─ basement mbr
  VEIN                    VENULE                 ═══════════
  0.5 mm–3 cm             8–100 µm               ► EXCHANGE ONLY
  ═════════════════       ═══════════            (0.5 µm thick,
  ▒▒▒▒▒▒▒▒ externa        ▒▒ externa              1 mm long)
  ▒▒▒▒▒▒▒▒ THICKEST       ─ endothel.
  ██ media (thin)         + pericytes
  ─ intima → VALVES       ═══════════
  ═════════════════       ► LEAKIEST vessel:
  ► CAPACITANCE:            site of leukocyte
    holds 60–65% of         exit + inflammatory
    total blood volume      fluid loss

  ═══════ WHERE THE BLOOD ACTUALLY IS, AT REST ═══════════════════════
     Systemic VEINS and venules  ████████████████████████  60–65%
     Systemic arteries           █████                     13%
     Pulmonary circuit           ████                      9–12%
     Heart chambers              ███                       7–8%
     Systemic CAPILLARIES        ██                        5%
  ═══════════════════════════════════════════════════════════════════
   Only 5% of your blood is where exchange happens. Everything else
   is transport, storage, or reserve.

Elastic arteries: the second pump

The aorta and its largest branches — the brachiocephalic trunk, common carotids, subclavians, common iliacs, and the pulmonary trunk — have a media dominated by concentric elastic laminae, up to seventy of them, with smooth muscle interleaved. They contain proportionally so much elastin that they are pale yellow in a fresh dissection.

Here is what that elastin does, and it is one of the most beautiful pieces of mechanical design in the body. The left ventricle ejects 70 mL in about 0.25 s — a violent, intermittent event. Yet flow in your capillaries is smooth and continuous. The conversion happens in the aorta.

During systole, the aorta and great arteries distend, absorbing roughly 50–60% of the stroke volume into their expanded walls rather than passing it downstream. Elastic energy is stored in stretched elastin. During diastole, when the aortic valve has shut and the ventricle is contributing nothing, that stored energy is released as the walls recoil, squeezing the stored blood onward. The arteries function as a secondary, passive pump.

This is called the Windkessel effect, after the air chamber in old fire engines that turned a pulsatile hand pump into a steady jet. Its consequences are enormous:

• Diastolic pressure is maintained at around 80 mm Hg instead of falling toward zero between beats. Since the left coronary artery fills during diastole (§18.4), the aorta's elastic recoil is what perfuses the heart itself.
• Flow becomes continuous by the level of the arterioles, so capillary exchange is steady.
• The ventricle's workload falls, because it ejects into a compliant reservoir rather than a rigid pipe.

Hold on to all three. When elastin degenerates and the aorta stiffens, all three benefits are lost — and that is the mechanism behind Case File Question 2.

Muscular arteries, arterioles, and the location of resistance

Muscular (distributing) arteries carry blood to specific organs. Their media is the most muscular of any vessel relative to lumen size, with up to forty layers of smooth muscle, and they are active vasoconstrictors — which is why the brachial artery under a blood pressure cuff can be occluded and why the radial artery is the usual access route for coronary angiography.

Arterioles are the smallest arteries, from about 300 µm down to 10 µm. The largest have all three tunics; the smallest are little more than a single smooth muscle cell spiralling around an endothelial tube. They are called resistance vessels, and they deserve the title: arterioles account for roughly 50–60% of total peripheral resistance and are responsible for the largest pressure drop anywhere in the circulation, from about 80 mm Hg at their entrance to about 35 mm Hg where they feed capillaries.

Two features make them the control point. First, resistance depends on radius to the fourth power (§19.4), so a vessel small enough to change its radius by a factor of two changes its resistance by a factor of sixteen. Second, they are densely innervated by sympathetic vasomotor fibres and richly supplied with receptors for circulating hormones and local metabolites. Arterioles are where the nervous system, the endocrine system, and the tissues themselves all get a vote on blood flow.

Capillaries, venules, and veins

Capillaries are 8–10 µm across — just wide enough that red blood cells traverse them in single file, often deforming to do so — and their wall is a single endothelial cell rolled into a tube on a basement membrane, about 0.5 µm thick. Everything about that is optimized for diffusion, and §19.2 takes them in detail.

Venules collect from capillaries. The smallest, postcapillary venules, are barely distinguishable from capillaries but are the leakiest vessels in the body — their interendothelial junctions separate readily under the influence of histamine and other inflammatory mediators. This is where white cells exit the bloodstream (diapedesis) and where the fluid of inflammatory swelling comes from (Chapter 21).

Veins have a thin media, a thick tunica externa, and a large lumen relative to their wall thickness. The pressure they carry is low — around 15 mm Hg in venules falling to near 0 mm Hg at the right atrium — so they do not need thick walls, and their large distensible lumens make them capacitance vessels: at any moment 60–65% of your entire blood volume is sitting in the systemic veins. They are the reservoir. Sympathetic venoconstriction can mobilize several hundred millilitres of that reserve into the active circulation within seconds, which is one of the fastest and most effective responses to haemorrhage.

Veins in the limbs contain valves — infoldings of the intima forming paired cusps — that permit flow toward the heart and block flow away from it. They are most numerous in the lower limbs, where gravity is the problem, and absent in the great veins of the thorax and abdomen.

Anastomoses

Where two vessels join without an intervening capillary bed, the junction is an anastomosis.

• Arterial anastomoses create collateral supply: the cerebral arterial circle (§19.8), the coronary anastomoses, the mesenteric arcades, and the abundant arterial networks around joints where a vessel is repeatedly kinked. Where they are absent or trivial — the retinal artery, the splenic and renal arteries, and functionally the coronary arteries — the vessel is a true end artery and its occlusion causes infarction.
• Venous anastomoses are so abundant that occlusion of a single vein rarely blocks drainage.
• Arteriovenous anastomoses (shunts) connect an arteriole directly to a venule, bypassing the capillary bed. The most important are in the skin — in the fingers, palms, soles, ears, and nose — where they open to dump warm blood at the surface for heat loss and close to conserve it (§19.7 and Chapter 5).

19.2 Capillaries and the Capillary Bed

Every cell is within 100 micrometres of one

There are somewhere near 10 billion capillaries in an adult, with a combined surface area of around 6,000 square metres — larger than a football pitch. They are the reason the cardiovascular system exists. Everything else is plumbing.

The organizing constraint is diffusion. The distance a molecule diffuses in a given time scales with the square root of that time, so doubling the distance quadruples the delay. Beyond about 100 µm, diffusion is too slow to supply a working cell with oxygen. Every cell in your body is therefore within about 100 µm of a capillary, and tissue capillary density tracks metabolic rate precisely: cardiac muscle has capillaries almost one-to-one with myocytes; cartilage, tendon, the cornea and the lens have none at all and are supplied entirely by diffusion from elsewhere — which is exactly why they heal so badly (Chapter 4 and Chapter 7).

Three capillary designs

The wall is a single endothelial cell, but there are three versions of it, and the location of each is entirely predictable from what the tissue needs to move.

Notice how completely this table is generated by function. The blood–brain barrier is tighter than a normal capillary because the brain's requirement is stability. The glomerulus is leakier because the kidney's requirement is bulk filtration. The bone marrow is leakiest of all because it has to export cells. The structure is never arbitrary.

The capillary bed

   THE CAPILLARY BED — the precapillary sphincters as taps

   ═══ SPHINCTERS OPEN (tissue is active: metabolites accumulated) ═══

      ARTERIOLE                                          VENULE
         ║                                                  ║
         ▼      METARTERIOLE                                ▼
      ═══╬══════◄►══════◄►══════◄►═════ thoroughfare ═══════╣
         ║      ▲       ▲       ▲       channel             ║
         ║      │       │       │   (the proximal end has   ║
         ║   ○ open  ○ open  ○ open    scattered smooth     ║
         ║   sphincter                 muscle; the distal   ║
         ║      │       │       │      end has none)        ║
         ║   ┌──┴──┬────┴──┬────┴──┐                        ║
         ║   │TRUE CAPILLARIES     │  ► ALL PERFUSED        ║
         ║   │ ~ 8–10 µm lumen     │  ► exchange maximal    ║
         ║   └─────────────────────┘                        ║
         ═══════════════════════════════════════════════════

   ═══ SPHINCTERS CLOSED (tissue at rest: metabolites washed out) ═══

      ARTERIOLE                                          VENULE
         ║                                                  ║
         ▼      METARTERIOLE                                ║
      ═══╬═══════════════════════════ thoroughfare ═════════╣
         ║      ●       ●       ●       channel             ║
         ║   CLOSED  CLOSED  CLOSED                         ║
         ║      ╳       ╳       ╳                           ║
         ║   ┌──────────────────────┐                       ║
         ║   │  TRUE CAPILLARIES    │  ► BYPASSED           ║
         ║   │  no flow             │  ► no exchange        ║
         ║   └──────────────────────┘                       ║
         ═══════════════════════════════════════════════════

   VASOMOTION: sphincters open and close 5–10 times per minute,
   independently, in response to LOCAL metabolites — not to nerves.
   At rest only ~25% of capillary beds are perfused at any instant.

   ══ THE THREE ROUTES ACROSS THE WALL ═══════════════════════════════
    1 DIFFUSION      lipid-soluble (O2, CO2, steroids) → straight
                     THROUGH the membrane, anywhere on the cell
                     water-soluble (glucose, amino acids, ions) →
                     through CLEFTS or FENESTRAE only
    2 TRANSCYTOSIS   large molecules (insulin, some antibodies)
                     packaged into vesicles, ferried across
    3 BULK FLOW      water + dissolved solutes pushed out (filtration)
                     or drawn in (reabsorption) by the pressure
                     balance of §19.3.  Distributes EXTRACELLULAR
                     FLUID — it is NOT how nutrients are delivered.

A capillary bed is not a random tangle. Blood enters from a terminal arteriole and passes into a metarteriole, a vessel intermediate between an arteriole and a capillary whose proximal portion carries scattered smooth muscle cells. The metarteriole continues as a thoroughfare channel straight through to a postcapillary venule. Branching from the metarteriole are 10–100 true capillaries, each guarded at its origin by a cuff of smooth muscle, the precapillary sphincter.

Those sphincters are taps, and they respond almost entirely to local chemical conditions rather than to nerves. When a tissue is working, its accumulating metabolites — low oxygen, high carbon dioxide, low pH, adenosine, potassium — relax the sphincters and the bed opens. When flow has washed those metabolites away, the sphincters constrict again. This continual cycling, vasomotion, occurs 5–10 times per minute in each bed independently, and it means that at rest only about a quarter of your capillaries are carrying blood at any given instant. Skeletal muscle at rest may have only one capillary in fifty open; during exercise, essentially all of them perfuse.

This is a critical economy. If all capillaries were open simultaneously, their combined capacity would far exceed total blood volume and the circulation would collapse for want of blood to fill it — which is, incidentally, close to what happens in septic shock (§19.9).

The three routes across the wall

Diffusion is the workhorse and accounts for essentially all nutrient and gas exchange. Lipid-soluble substances — oxygen, carbon dioxide, steroid hormones, fatty acids, anaesthetic gases — diffuse straight through the endothelial cell membranes across the entire cell surface, which makes exchange extremely efficient. Water-soluble substances — glucose, amino acids, ions — cannot cross the lipid bilayer and must pass through intercellular clefts or fenestrae, which occupy a tiny fraction of the surface. This asymmetry has a large practical consequence: oxygen delivery is limited by blood flow, while glucose delivery is limited by surface area and permeability.

Transcytosis carries molecules too large for the clefts. The endothelial cell engulfs them at the luminal surface in a vesicle, ferries the vesicle across, and releases the contents on the other side. Insulin reaches muscle and fat cells this way, and the rate of transcytosis is itself a regulated step in insulin action — one of the less-known contributors to insulin resistance (Chapter 16).

Bulk flow is the movement of water with everything dissolved in it, driven by pressure differences. It is not how nutrients are delivered — it moves far too little material for that. Its job is to distribute extracellular fluid between the plasma compartment and the interstitial compartment, and it is the subject of the next section, which is arguably the single highest-yield page in this book.

19.3 Capillary Exchange and the Starling Forces

Two pressures, opposite directions

At every point along a capillary, fluid is being pushed out by hydrostatic pressure and pulled in by osmotic pressure. Whether fluid actually leaves or enters is decided by which of those wins, and by how much. That balance has a name — the Starling forces, after the physiologist Ernest Starling, who is also the Starling of the Frank–Starling mechanism — and almost every swollen tissue you will ever see is explained by it.

There are four forces, two pushing out and two pulling in.

Pushing fluid OUT of the capillary:

• Capillary hydrostatic pressure (HPc) — the blood pressure inside the capillary, pressing outward on the wall. This is the dominant filtration force. Critically, it falls along the length of the capillary because of resistance to flow: about 35 mm Hg at the arterial end, about 17 mm Hg at the venous end.
• Interstitial fluid colloid osmotic pressure (OPif) — the small osmotic pull exerted by the few proteins that have escaped into the interstitium. About 1 mm Hg, and constant.

Pulling fluid INTO the capillary:

• Capillary colloid osmotic pressure (OPc), also called oncotic pressure — the osmotic pull of the plasma proteins, roughly 65% of it from albumin. About 26 mm Hg. Because proteins cannot cross the wall, this force does not change along the capillary's length — and that single fact drives everything that follows.
• Interstitial fluid hydrostatic pressure (HPif) — the pressure of the fluid already in the tissue, pressing back. Approximately 0 mm Hg in most tissues (slightly negative in some).

   THE FOUR STARLING FORCES ACROSS ONE SYSTEMIC CAPILLARY

    ARTERIOLE                                              VENULE
       ║                                                      ║
   ════╬══════════════════ CAPILLARY ═════════════════════════╣
       ║   ↑↑↑ filtration            reabsorption ↓↓↓         ║
       ▼                                                      ▼
              ARTERIAL END                    VENOUS END
              ────────────                    ──────────
   FORCES PUSHING OUT
     HPc   capillary hydrostatic     35   ══════►►►    17   mm Hg
           (falls along the capillary — resistance to flow)
     OPif  interstitial oncotic       1                 1   mm Hg
                                    ────              ────
                       TOTAL OUT     36                18   mm Hg

   FORCES PULLING IN
     OPc   capillary oncotic         26   ══════════   26   mm Hg
           (ALBUMIN — cannot cross, so this does NOT change)
     HPif  interstitial hydrostatic   0                 0   mm Hg
                                    ────              ────
                        TOTAL IN     26                26   mm Hg

   NET FILTRATION PRESSURE = (forces out) − (forces in)
   ┌────────────────────────────────────────────────────────────────┐
   │  ARTERIAL END:  36 − 26 = +10 mm Hg  →  NET FILTRATION (out)   │
   │  VENOUS END:    18 − 26 =  −8 mm Hg  →  NET REABSORPTION (in)  │
   └────────────────────────────────────────────────────────────────┘

   THE DAILY BALANCE
     filtered out across all capillaries ........  ~20 L / day
     reabsorbed at venous ends ..................  ~17 L / day
     ───────────────────────────────────────────────────────────
     NOT reabsorbed → collected by LYMPHATICS ...   ~3 L / day
     (≈ 15% of what was filtered; returned to the subclavian
      veins, carrying with it the escaped plasma protein)

   ══ THE FOUR MECHANISMS OF EDEMA — one per force ══════════════════

    1  ↑ HPc      more push out       heart failure, venous
                                      obstruction, DVT, pregnancy,
                                      salt and water retention
    2  ↓ OPc      less pull in        liver failure (no albumin made),
                                      nephrotic syndrome (albumin
                                      lost in urine), malnutrition
    3  ↑ permeab. protein escapes,    burns, sepsis, anaphylaxis,
       (↑OPif,↓OPc) so the gradient   inflammation (histamine opens
                    reverses locally  postcapillary venule junctions)
    4  lymphatic  the 3 L/day has     filariasis, tumour, surgical
       obstruction  nowhere to go     node removal, radiation fibrosis

The arithmetic, and why it comes out the way it does

At the arterial end:

Forces out = HPc + OPif = 35 + 1 = 36 mm Hg Forces in = OPc + HPif = 26 + 0 = 26 mm Hg Net filtration pressure = 36 − 26 = +10 mm Hg → fluid leaves the capillary.

At the venous end:

Forces out = 17 + 1 = 18 mm Hg Forces in = 26 + 0 = 26 mm Hg Net filtration pressure = 18 − 26 = −8 mm Hg → fluid re-enters the capillary.

The whole mechanism turns on a single asymmetry: hydrostatic pressure falls along the capillary; oncotic pressure does not. Blood pressure drops because the capillary offers resistance. Oncotic pressure holds steady because albumin is too large to leave. So the same capillary filters at one end and reabsorbs at the other, purely because one of the two competing forces declines and the other does not.

Note that filtration exceeds reabsorption: +10 out against −8 back, and the arterial end of the capillary is shorter than the venous end. About 20 litres per day are filtered out across all the capillaries in the body, and only about 17 litres are reabsorbed. The remaining 3 litres per day — roughly 15% — is left behind in the interstitium, along with the small amount of plasma protein that has escaped.

Why the lymphatic system is not optional

Three litres a day, unreturned, would exsanguinate the circulation within about two days and drown every tissue in the body. The lymphatic system collects it.

Blind-ended lymphatic capillaries permeate almost every tissue. Their endothelial cells overlap like roof shingles, forming one-way mini-valves that open when interstitial pressure rises and close when it falls, so fluid and protein enter easily and cannot leave. Lymph is propelled by smooth muscle in the lymphatic vessel walls, by valves, and by the same muscular and respiratory pumps that move venous blood; it passes through lymph nodes and is finally returned to the bloodstream at the junctions of the internal jugular and subclavian veins — back into the cardiovascular system, at the point of lowest pressure in it.

Two consequences deserve emphasis now, before Chapter 20 develops them. First, the lymphatic system is the only route by which escaped plasma protein returns to the blood; if it were not, interstitial oncotic pressure would rise steadily until reabsorption stopped altogether. Second, because lymph passes through nodes, the lymphatic system is also the surveillance route by which the immune system samples the tissues — and, unfortunately, the route by which cancers metastasize.

Edema: four mechanisms, one equation

Edema is excess fluid in the interstitium. Every case of it is a failure of one of the four Starling terms, or of lymphatic drainage. This is one of the highest-yield frameworks in physiology because it converts a huge clinical category into four questions.

1 · Increased capillary hydrostatic pressure. More push out. Because capillary pressure is determined largely by venous pressure — the downstream end of the capillary — anything that obstructs venous drainage or raises venous pressure will do it: heart failure (a failing left ventricle raises left atrial and hence pulmonary capillary pressure; a failing right ventricle raises systemic venous pressure and causes ankle edema), deep vein thrombosis (unilateral leg swelling), pregnancy, prolonged standing, and any state of salt and water retention.

This is Amara's mechanism. Her stiff left ventricle requires a high filling pressure (§18.9). That pressure is transmitted backwards to the left atrium, the pulmonary veins, and the pulmonary capillaries. Pulmonary capillary hydrostatic pressure is normally only about 8–10 mm Hg — much lower than systemic, because the pulmonary circuit is low-pressure — and that low value is precisely why the lungs stay dry. Raise it above roughly 18–20 mm Hg and filtration overwhelms pulmonary lymphatic drainage. Fluid enters the interstitium and then the alveoli: pulmonary edema, and breathlessness. Chapter 22 develops the gas-exchange consequences.

2 · Decreased plasma colloid osmotic pressure. Less pull in. Albumin is made by the liver and lost by the kidney, so the two big causes are liver failure (albumin not synthesized) and nephrotic syndrome (albumin lost through a damaged glomerular filter). Severe protein malnutrition does the same, which is why children with kwashiorkor have swollen abdomens despite starving — the distended belly is ascites, not food.

3 · Increased capillary permeability. Protein leaks out, so OPc falls and OPif rises: the osmotic gradient that normally pulls fluid back is not merely reduced, it is partly reversed. This is the mechanism of inflammatory swelling — histamine and other mediators separate the junctions of postcapillary venules (Chapter 20) — and of the massive fluid loss in burns, sepsis, and anaphylaxis. Note that inflammatory edema is protein-rich (an exudate), while edema from raised hydrostatic pressure is protein-poor (a transudate), and analyzing the protein content of a fluid sample is therefore a direct test of which mechanism is operating.

4 · Lymphatic obstruction. The three litres a day has nowhere to go. Causes: lymphatic filariasis (parasitic worms blocking lymphatics, the classic cause of elephantiasis), tumour invasion, surgical removal of lymph nodes (lymphedema of the arm after axillary dissection for breast cancer), and radiation fibrosis. Lymphedema is protein-rich, tends to be non-pitting once chronic fibrosis develops, and is notoriously difficult to treat, because there is no drug that rebuilds lymphatic channels.

19.4 Blood Flow, Blood Pressure, and Resistance

The equation, in plain language first

Everything in this section is one relationship:

Flow depends on how hard you push and how hard it is to push.

Formally: F = ΔP ÷ R — flow equals the pressure difference between two points divided by the resistance between them.

Three points about that sentence are worth dwelling on, because each is a common error.

Flow depends on the pressure gradient, not on the absolute pressure. Blood does not move because the pressure in your aorta is 100 mm Hg; it moves because the pressure in your aorta is 100 and the pressure in your right atrium is 0. If both rose to 200, flow would be unchanged. This is why a patient can be profoundly hypertensive and still hypoperfuse an organ downstream of an obstruction.

Resistance is where control lives. The heart sets the pressure; the vessels set the resistance. And because the body needs to redirect flow between organs constantly — to muscle during exercise, to gut after a meal, to skin when hot — almost all distribution is managed by changing resistance locally rather than by changing the pressure globally.

The same equation applies to the whole circulation. Applied to the systemic circuit as a whole, F becomes cardiac output, ΔP becomes mean arterial pressure minus right atrial pressure (and right atrial pressure is nearly zero), and R becomes total peripheral resistance. Rearranged, this gives the master equation of circulatory physiology:

MAP = CO × TPR

Every antihypertensive drug ever invented lowers one of those two terms. Every form of shock is a failure of one of them.

The three determinants of resistance

Resistance to flow in a tube is described by Poiseuille's relationship. In words: resistance is proportional to fluid viscosity multiplied by tube length, divided by the fourth power of the tube's radius.

Viscosity — the "thickness" of the fluid. Blood is about 3–5 times as viscous as water, almost entirely because of the red cells and, to a lesser extent, plasma proteins. It changes little day to day, but pathologically it matters: polycythemia (a raised haematocrit, as in chronic hypoxia or at altitude) increases viscosity and therefore resistance and cardiac work, while severe anaemia lowers it, which is one reason anaemic patients have flow murmurs and bounding pulses. Amara's mild anaemia (Chapter 17) very slightly reduces her viscosity, which is a small mercy in a system otherwise loaded against her.

Vessel length — longer tube, more resistance, in direct proportion. Length is essentially fixed in an adult, with one important exception: new tissue requires new vessels. Every kilogram of adipose tissue gained is perfused by kilometres of additional capillaries and arterioles, adding to total vascular length and therefore to total peripheral resistance. This is one of several mechanisms linking obesity to hypertension.

Radius — and this is the one that matters. Resistance varies with the inverse fourth power of the radius. Make that concrete:

Read the last row against the Case File. Amara's proximal circumflex artery had a 90% stenosis. That is not a 90% reduction in flow capacity; it is a reduction by roughly four orders of magnitude in the vessel's maximal conducting capacity. The reason she was still working twelve-hour shifts is that resting coronary flow is only about a fifth of maximal flow, so even a devastated artery can meet resting demand — right up until the moment demand rises or a thrombus forms on the plaque.

The same fourth power explains the opposite phenomenon. A modest arteriolar dilation — say a 20% increase in radius, which smooth muscle achieves easily — roughly doubles flow through that bed. The body's flow-control system does not need large anatomical changes because the physics amplifies small ones.

Systolic, diastolic, pulse pressure, and MAP

Systolic pressure is the peak arterial pressure reached during ventricular ejection. Diastolic pressure is the minimum reached just before the next ejection. Blood pressure is reported as systolic over diastolic, and the two numbers report different things.

Pulse pressure (PP) is the difference between them:

PP = systolic − diastolic. Normally about 40 mm Hg (120 − 80). Amara at presentation: 168 − 98 = 70 mm Hg.

Pulse pressure is, to a good approximation, stroke volume divided by arterial compliance — how much volume was thrown in, divided by how willing the arteries were to accept it. Hold that; it is the whole of Question 2.

Mean arterial pressure (MAP) is the average pressure over the whole cardiac cycle, and it is the pressure that actually determines tissue perfusion. It is not the arithmetic mean of systolic and diastolic, because at resting heart rates the heart spends roughly twice as long in diastole as in systole, so the diastolic value must be weighted twice as heavily:

MAP ≈ diastolic + (pulse pressure ÷ 3)

Normal: 80 + (40 ÷ 3) = 80 + 13 = 93 mm Hg Amara at presentation: 98 + (70 ÷ 3) = 98 + 23.3 = 121 mm Hg Amara on treatment: 84 + (54 ÷ 3) = 84 + 18 = 102 mm Hg

MAP must stay above roughly 60 mm Hg for organs to be perfused at all, and the normal range is 70–100 mm Hg. Note that the formula is only valid at ordinary heart rates: during tachycardia diastole shortens disproportionately, so MAP approaches the arithmetic mean.

   DOWN THE SYSTEMIC CIRCUIT: PRESSURE, CROSS-SECTIONAL AREA, VELOCITY

            AORTA    LARGE    ARTERI-   CAPILL-   VENULES    VENAE
                    ARTERIES   OLES      ARIES    + VEINS    CAVAE
   ════════════════════════════════════════════════════════════════════
   PRESSURE (mm Hg)
   120 ┤ ▲▲▲▲▲▲   ▲▲▲▲▲
       │ ┊pulse┊  ┊pulse┊╲
   100 ┤ ▼▼▼▼▼▼   ▼▼▼▼▼   ╲    ◄══ THE BIG DROP ══►
    80 ┤ (120/80) (118/78) ╲   80 → 35 mm Hg across the arterioles.
    60 ┤                    ╲  ~50–60% of TOTAL peripheral resistance
    40 ┤                     ╲ lives here, in vessels you cannot see.
    35 ┤                      ╲╌╌╌╌╌╌╌   ← pulsation is DAMPED OUT
    20 ┤                              ╲╌╌ 17
    10 ┤                                  ╲╌╌╌╌ 15 ╌╌╌╌╌
     0 ┤                                            ╌╌╌╌╌ 2 → 0
   ════════════════════════════════════════════════════════════════════
   TOTAL CROSS-SECTIONAL AREA (cm2)  — all vessels of that type, summed
  5000 ┤                              ██████
  4000 ┤                              ██████
  3000 ┤                              ██████
  2000 ┤                    ████      ██████    ████
  1000 ┤                    ████      ██████    ████
     0 ┤ ▪ 2.5   ▪ 20      ████       ██████    ████     ▪ 8
       │                   (400)      (4500)    (2000)
   ════════════════════════════════════════════════════════════════════
   VELOCITY (cm/s)  — INVERSELY proportional to the area above
    40 ┤ ████
    30 ┤ ████ 40
    20 ┤ ████   ████ 20                                   ████ 15
    10 ┤ ████   ████        ██ 2                    ████  ████
     0 ┤ ████   ████        ██        ▪ 0.03   ████ ████  ████
   ════════════════════════════════════════════════════════════════════
   WHY VELOCITY FALLS: flow (mL/min) is the SAME through every level —
   it must be, they are in series.  Velocity = flow ÷ cross-sectional
   area.  The capillaries have 1,800× the area of the aorta, so blood
   crawls through them at 0.3 mm/s and each red cell spends 1–3 s in
   a capillary — exactly the time exchange requires.  Same river,
   same water per second, but wide and slow instead of narrow and fast.

Reading the pressure profile

The figure repays study, because four separate ideas are visible in it at once.

Pressure falls continuously, and it must: flow requires a gradient, so every millimetre of vessel consumes some pressure.

The steepest fall is across the arterioles — from about 80 mm Hg to about 35. That is where the resistance is, and it is why arterioles are called resistance vessels. Note the design logic: by placing most of the resistance upstream of the capillaries, the body protects those fragile 0.5 µm-thick tubes from arterial pressure. If arteriolar tone were lost, capillary hydrostatic pressure would rise toward 80 mm Hg and every capillary in the body would filter massively — which is precisely what happens locally in inflammation and globally in some forms of shock.

Pulsation is damped out across the arterioles too. Blood leaves the heart in violent spurts and arrives at the capillaries as a steady stream. Elastic recoil starts the job (§19.1) and arteriolar resistance finishes it.

Velocity is inversely related to total cross-sectional area. This is the single most counterintuitive fact in vascular physiology. Blood moves fastest in the aorta (about 40 cm/s) and slowest in the capillaries (about 0.03 cm/s, roughly 0.3 mm/s) — a thousandfold difference — even though exactly the same number of millilitres passes through each level per minute. The reason is that the levels are in series, so volume flow must be identical, and velocity is flow divided by area. A single aorta of 2.5 cm² becomes billions of capillaries totalling about 4,500 cm². The same river, spread 1,800 times wider, must run 1,800 times slower.

And that slowness is the point. A red cell spends 1–3 seconds crossing a capillary. That is the time available for oxygen to leave and carbon dioxide to enter, and it is neither more nor less than the amount that diffusion requires. The geometry of the vascular tree is calibrated to the physics of diffusion.

19.5 Short-Term Regulation · The Neural Reflexes

The cardiovascular centre

Blood pressure must be defended on two timescales that require completely different machinery. Second to second, the nervous system adjusts cardiac output and peripheral resistance. Day to day and year to year, the kidney adjusts blood volume. This section covers the first; §19.6 covers the second, and the distinction between them is one of the most important in physiology.

The cardiovascular centre is a group of interconnected nuclei in the medulla oblongata (Chapter 12) with three functional parts:

• The cardioacceleratory centre, which drives sympathetic outflow to the SA node, AV node, and ventricular myocardium.
• The cardioinhibitory centre, which drives parasympathetic (vagal) outflow to the SA and AV nodes.
• The vasomotor centre, which drives sympathetic outflow to vascular smooth muscle.

The vasomotor centre deserves particular attention because it is tonically active. It discharges continuously at about one impulse per second, keeping the arterioles of the body in a state of partial constriction called vasomotor tone. This is a crucial design choice: it means the centre can produce vasodilation simply by firing less, without needing a separate set of dilator nerves. One outflow, bidirectional control. (There is a small exception — sympathetic cholinergic vasodilator fibres to skeletal muscle in some species and sweat glands in humans — but for practical purposes vascular control is achieved by modulating a single constrictor tone.)

The baroreceptor reflex

   THE BARORECEPTOR REFLEX — a complete negative feedback loop

   STIMULUS ▸ MEAN ARTERIAL PRESSURE RISES  (e.g. 93 → 120 mm Hg)
        │
        ▼
   ① RECEPTOR ── CAROTID SINUS (at the bifurcation of the common
                 carotid) and AORTIC ARCH.  Stretch-sensitive nerve
                 endings in the tunica externa.  Firing rate rises
                 as the wall is stretched.  MOST SENSITIVE 80–160
                 mm Hg — steepest response near the normal value.
        │
        ▼  AFFERENT PATHWAY
   ② CN IX  GLOSSOPHARYNGEAL  ◄─ from the carotid sinus
     CN X   VAGUS             ◄─ from the aortic arch
        │
        ▼
   ③ CONTROL CENTRE ── CARDIOVASCULAR CENTRE, MEDULLA OBLONGATA
        nucleus of the solitary tract (NTS) receives the input, then
          ├──► INHIBITS the vasomotor centre
          └──► EXCITES  the cardioinhibitory centre
        │
        ▼  EFFERENT PATHWAY
   ④  SYMPATHETIC OUTFLOW  ↓↓          PARASYMPATHETIC (vagal)  ↑↑
        │                                        │
        ▼                                        ▼
   ⑤ EFFECTORS
        SA node ......... rate ↓          SA node ..... rate ↓↓
        myocardium ...... contractility ↓  AV node ..... conduction ↓
        arterioles ...... DILATE  → TPR ↓
        veins ........... DILATE  → venous return ↓ → preload ↓ → SV ↓
        adrenal medulla . epinephrine release ↓
        │
        ▼
   ⑥ RESPONSE ▸  CO ↓  and  TPR ↓   →   MAP = CO × TPR   →   FALLS
        │
        └──────────► variable returns toward set point ──► LOOP CLOSES

   ═══ THE MIRROR IMAGE (this is the half that keeps you alive) ═══
   MAP FALLS → LESS stretch → LESS afferent firing → the NTS stops
   inhibiting → vasomotor centre DISINHIBITED → sympathetic ↑,
   vagal ↓ → HR ↑, contractility ↑, arteriolar + venous constriction
   → CO ↑ and TPR ↑ → MAP RISES.   "Baroreceptor UNLOADING."

   ═══ TIMESCALE AND ITS LIMIT ════════════════════════════════════
   Onset: 1–2 SECONDS.  Complete: 10–30 seconds.
   BUT the receptors RESET to whatever pressure they experience for
   1–3 days.  They defend against ACUTE change only.  They are
   USELESS against chronic hypertension — which is why long-term
   control must be done somewhere else entirely (§19.6).

The baroreceptor reflex is the fastest and most important short-term regulator of arterial pressure, and it is worth noting how neatly it maps onto the four-box architecture from §1.5: receptor, afferent pathway, control centre, efferent pathway, effector, response.

The resetting property in the last line of the figure is not a footnote. It is the reason this section cannot explain Amara's hypertension. Baroreceptors adapt: sustained exposure to a pressure of 168/98 shifts their operating range upward within a few days, after which they defend that pressure as vigorously as they once defended 120/80. They are exquisite regulators of change and completely indifferent to level. Any explanation of chronic hypertension must therefore live somewhere the baroreceptors are not, and the next section identifies where.

Chemoreceptor reflexes

Peripheral chemoreceptors sit in the carotid bodies (at the carotid bifurcation, adjacent to but distinct from the carotid sinus) and the aortic bodies. They sense arterial oxygen tension, carbon dioxide tension, and pH, and their primary job is respiratory (Chapter 22). But when arterial PO₂ falls below about 60 mm Hg, or PCO₂ rises sharply, or pH falls, they also drive a powerful sympathetic vasoconstrictor response, raising blood pressure to improve perfusion of the tissues that are starving.

Two applications worth carrying forward. First, this reflex is much weaker than the baroreflex under ordinary conditions and only becomes dominant in severe hypoxia or acidosis — which is why it matters in respiratory failure and in obstructive sleep apnea, where dozens of nocturnal hypoxic episodes each drive a sympathetic surge. Amara's non-dipping nocturnal blood pressure is partly explained by exactly this, and Chapter 22 makes the diagnosis explicit.

Second, there is a last-ditch CNS ischemic response. When cerebral blood flow falls so far that medullary neurons themselves become hypoxic and acidotic, the vasomotor centre discharges maximally, producing the highest arterial pressures the body can generate. When it is caused by rising intracranial pressure, the result is the Cushing reflex: hypertension, reflex bradycardia (the baroreceptors responding to that hypertension), and irregular breathing. It is an ominous sign of impending brain herniation, and it is a good illustration of a homeostatic mechanism sacrificing everything else to defend the one organ that cannot wait.

Higher centres

The cortex and limbic system reach the medullary centres directly. Blood pressure rises in anticipation of exercise before a muscle has contracted, falls during sleep, and can do remarkable things under emotional stress. Vasovagal syncope — the common faint — is a striking example: an emotional or painful stimulus triggers a paradoxical burst of vagal outflow with simultaneous sympathetic withdrawal, producing abrupt bradycardia and vasodilation, a collapse of blood pressure, and loss of consciousness. It is the opposite of a defence reaction, and it resolves rapidly once the person is horizontal, because being horizontal removes the hydrostatic column the circulation was fighting.

The hypothalamus also intervenes for thermoregulation, directing cutaneous vasodilation when hot and vasoconstriction when cold — which is why the same effector, skin arteriolar tone, is under competing control from two different systems, exactly as Chapter 1 described for Amara's pale skin.

Worked example · What happens when you stand up

Standing is a homeostatic emergency that you survive dozens of times a day without noticing. Work through it as a complete reflex.

t = 0. You are supine. Blood is distributed relatively evenly along the body's long axis.

t = 0–2 s. You stand. Gravity immediately pulls 500–800 mL of blood into the highly compliant veins of the legs, pelvis, and splanchnic circulation. That blood is no longer returning to the heart.

t = 2–5 s. Venous return falls. Right and then left ventricular end-diastolic volume falls. By the Frank–Starling mechanism (§18.8) stroke volume falls by up to 40%. Cardiac output falls. Mean arterial pressure begins to drop, and — critically — arterial pressure at the level of the brain falls further still, because the brain is now 30–40 cm above the heart and the hydrostatic column costs another 20–30 mm Hg.

t = 5–10 s. Carotid sinus baroreceptors, sitting at neck level, detect reduced stretch: baroreceptor unloading. Afferent firing in CN IX falls. The nucleus of the solitary tract stops inhibiting the vasomotor centre.

t = 10–30 s. The efferent response: - Vagal tone withdrawn and sympathetic outflow increased → heart rate rises by 10–20 beats/min. - Arteriolar constriction, especially in the splanchnic bed, skin, and kidney → total peripheral resistance rises. - Venoconstriction → the venous reservoir is squeezed and preload partially restored. - Contractility rises, partially defending stroke volume.

Simultaneously, the muscle pump engages as you shift your weight, and the venoarteriolar reflex — a local axon reflex triggered by venous distension — constricts arterioles in the dependent limb, protecting its capillaries from the raised hydrostatic pressure.

Result: systolic pressure falls transiently by about 5–10 mm Hg, diastolic rises by about 5 mm Hg (because TPR has risen), and MAP is essentially unchanged within half a minute. You notice nothing.

When it fails: blood pressure does not recover, cerebral perfusion falls, and you feel lightheaded — or faint.

19.6 Hormonal Control, and Why Long-Term Regulation Is a Kidney Problem

The short-term hormonal players

Neural reflexes act in seconds; hormones act in minutes to hours and persist far longer.

Epinephrine and norepinephrine from the adrenal medulla. Released with sympathetic activation, they reinforce and prolong everything the nerves are doing. Their effects are receptor-dependent, which is why the same molecule can constrict and dilate: α₁ receptors on most vascular smooth muscle cause constriction; β₁ receptors on the heart raise rate and contractility; β₂ receptors on skeletal muscle, coronary, and hepatic arterioles cause dilation. Epinephrine has greater β₂ affinity than norepinephrine, so at low concentrations it can lower diastolic pressure while raising cardiac output — redirecting blood to muscle. This is the fight-or-flight distribution, written in receptor subtypes.

Antidiuretic hormone (ADH, vasopressin) from the hypothalamus via the posterior pituitary (Chapter 16). At ordinary concentrations it acts on V2 receptors in the renal collecting duct to insert aquaporin-2 channels and reabsorb water, raising blood volume. At the much higher concentrations reached in severe haemorrhage it also acts on V1 receptors on vascular smooth muscle to cause powerful vasoconstriction — which is why its alternative name is vasopressin, and why it is used as a vasopressor drug in refractory shock.

Atrial natriuretic peptide (ANP) and its ventricular relative BNP. Released by cardiac myocytes when the atria or ventricles are stretched — that is, when blood volume is high. ANP is the body's only significant antihypertensive hormone, and it opposes the entire renin–angiotensin–aldosterone system: it promotes sodium and water excretion (natriuresis and diuresis), dilates arterioles and veins, and directly inhibits renin, aldosterone, and ADH release. It is a clean negative feedback loop for volume — high volume stretches the heart, the heart tells the kidney to get rid of some.

It is also why NT-proBNP is measured in patients like Amara. Her value of 940 pg/mL is a biochemical readout of myocardial stretch: a heart chronically working at elevated filling pressures secretes proportionally more, so the number is essentially a blood test for a mechanical problem.

Angiotensin II is the most powerful of them all, and it needs its own treatment.

The renin–angiotensin–aldosterone system

   THE RENIN–ANGIOTENSIN–ALDOSTERONE SYSTEM (RAAS)
   organ by organ, enzyme by enzyme, with the drug targets marked

   ┌── LIVER ────────────────────────────────────────────────┐
   │  ANGIOTENSINOGEN — an α2-globulin, continuously secreted│
   │  into plasma.  Inactive.  Never the rate-limiting step. │
   └──────────────────────────┬─────────────────────────────-┘
                              │
   ┌── KIDNEY · JUXTAGLOMERULAR CELLS of the afferent arteriole ──┐
   │  Secrete RENIN in response to THREE signals:                 │
   │   1  RENAL PERFUSION PRESSURE FALLS (intrarenal baroreceptor)│
   │   2  NaCl DELIVERY TO THE MACULA DENSA FALLS                 │
   │   3  SYMPATHETIC β1 STIMULATION                              │
   │            ◄══ β-BLOCKERS reduce renin release here          │
   └──────────────────────────┬───────────────────────────────────┘
                              │ RENIN is an ENZYME; it cleaves
                              ▼ angiotensinogen
                    ANGIOTENSIN I  (10 amino acids, INACTIVE)
                              │
   ┌── LUNG · ACE on the PULMONARY CAPILLARY ENDOTHELIUM ────────┐
   │  (also on renal and systemic endothelium, but the lung has  │
   │   the largest endothelial surface in the body and the WHOLE │
   │   cardiac output passes through it every minute)            │
   │                                                             │
   │   ANGIOTENSIN-CONVERTING ENZYME does TWO things:            │
   │    • removes 2 amino acids  →  ANGIOTENSIN II               │
   │    • DEGRADES BRADYKININ, a vasodilator                     │
   │                                                             │
   │        ◄══════ ACE INHIBITORS ("-pril": lisinopril,         │
   │                enalapril, ramipril) BLOCK HERE              │
   │                → less angiotensin II AND more bradykinin    │
   │                → the cough and angioedema side effects      │
   └──────────────────────────┬──────────────────────────────────┘
                              ▼
            ANGIOTENSIN II  (8 amino acids)  acting at AT1 receptors
                              │
                     ◄════ ARBs ("-sartan": losartan, valsartan)
                              │      BLOCK THE RECEPTOR HERE
   ┌────────┬────────┬────────┼─────────┬──────────┬─────────────┐
   ▼        ▼        ▼        ▼         ▼          ▼             ▼
 ARTERI-  ADRENAL  KIDNEY  KIDNEY   HYPOTHAL-   SYMPATHETIC   HEART
 OLES     CORTEX   proximal efferent  AMUS       NERVE          and
 CONSTRICT zona    tubule:  arteriole thirst +   TERMINALS      VESSEL
 (the most glomer- Na+      CONSTRICTS ADH from  facilitate     GROWTH
 potent    ulosa   reabs.   → holds   posterior  NE release   hypertrophy
 physio-     │     ↑        GFR up    pituitary               + FIBROSIS
 logical     │              when flow
 vaso-       ▼              is low        ◄══ MINERALOCORTICOID
 constrictor)│                                RECEPTOR ANTAGONISTS
   │    ALDOSTERONE                           (spironolactone,
   │         │                                 eplerenone) BLOCK HERE
   │         ▼
   │  COLLECTING DUCT principal cells:
   │  ENaC channels + Na/K-ATPase ↑
   │  → Na+ (and water with it) RETAINED
   │  → K+ and H+ SECRETED  (hence hypokalaemia + alkalosis, Ch. 31)
   │         │
   │         ▼
   │   BLOOD VOLUME ↑ ──► PRELOAD ↑ ──► STROKE VOLUME ↑ ──► CO ↑ ─┐
   │                                                               │
   └── TPR ↑ ──────────────────────────────────────────────────────┤
                                                                   ▼
                                                       MAP = CO × TPR ↑

The RAAS is the body's principal defence against falling blood volume and falling blood pressure, and its architecture rewards careful reading, because seven distinct actions all push the same variable in the same direction.

Angiotensin II is the most potent physiological vasoconstrictor in the body — more potent, molecule for molecule, than norepinephrine — but constriction is only one of the things it does. It also recruits volume (through aldosterone, direct proximal tubular sodium reabsorption, thirst, and ADH), amplifies the sympathetic nervous system (by facilitating norepinephrine release), and protects glomerular filtration when renal flow is low (by constricting the efferent arteriole preferentially). This last action is why an ACE inhibitor causes a small, expected rise in serum creatinine, and why that rise is generally accepted rather than feared.

And it is a growth factor. Angiotensin II, acting through AT1 receptors, drives myocyte hypertrophy and stimulates cardiac and vascular fibroblasts to deposit collagen. Aldosterone does the same. This is the link back to Chapter 18: the same system that raised Amara's blood pressure also directly stiffened her ventricle, independently of the pressure. That is why blocking it produces regression of hypertrophy and fibrosis over months, and why ACE inhibitors and mineralocorticoid antagonists improve outcomes in heart failure by more than their effect on blood pressure alone would predict.

Long-term regulation is a volume problem, and therefore a kidney problem

Here is the argument that ties the whole chapter together, and it is worth stating carefully because it is the conceptual foundation of Chapter 26.

Recall from §19.5 that baroreceptors reset within one to three days. They are change-detectors, not level-detectors. So no neural mechanism can set the long-term operating pressure of the circulation.

The kidney can, and it does so through a mechanism with a property no other regulator possesses: infinite gain. The mechanism is pressure natriuresis. As arterial pressure rises, the kidney excretes more sodium and water — not because it has decided to, but because raised renal perfusion pressure directly increases sodium excretion. Excreting salt and water reduces extracellular fluid volume, which reduces plasma volume, which reduces venous return, which reduces preload, which reduces stroke volume and cardiac output, which reduces mean arterial pressure. And as long as pressure remains even slightly above the kidney's operating point, the kidney keeps excreting. It does not settle for partial correction. Given time and an intact kidney, arterial pressure returns to the point at which sodium intake and sodium output balance exactly.

Two enormous conclusions follow.

One: every case of sustained hypertension involves the kidney. Not necessarily as the original cause — but if the kidney's pressure–natriuresis relationship were operating normally, a raised pressure would simply be excreted away. A person with sustained hypertension is a person whose kidney is now defending a higher pressure, whether because of intrinsic renal disease, excess RAAS activity, excess sympathetic drive to the kidney, salt sensitivity, or the nephrosclerosis that hypertension itself causes over years.

Two: nearly every effective antihypertensive drug either reduces volume or blocks the RAAS. Diuretics act on the kidney directly. ACE inhibitors, ARBs, and mineralocorticoid antagonists block the hormonal system through which the kidney defends volume. Even calcium-channel blockers and beta-blockers have renal components — beta-blockers reduce renin release. The apparent variety of antihypertensive pharmacology conceals a striking uniformity of target.

19.7 Tissue Perfusion and Its Local Control

Every tissue negotiates for itself

Systemic regulation defends pressure. Local regulation determines flow — how much of the cardiac output each tissue actually receives. These are different problems, and they are solved by different machinery. The heart and the medulla maintain the pressure head; each tissue then sets its own arteriolar resistance and draws what it needs.

The result is that total blood flow to any organ is described by the same relation as always, F = ΔP ÷ R, with ΔP set centrally and R set locally.

Autoregulation

Autoregulation is the ability of a tissue to maintain a nearly constant blood flow despite changes in perfusion pressure. Two mechanisms produce it.

The myogenic mechanism is a property of vascular smooth muscle itself and requires no nerves, hormones, or metabolites. When an arteriole is stretched by rising intraluminal pressure, stretch-activated cation channels open, the smooth muscle cell depolarizes, voltage-gated calcium channels open, and the cell contracts. Higher pressure produces constriction, which raises resistance, which holds flow constant. When pressure falls, the muscle relaxes for the same reason in reverse. It is a purely mechanical negative feedback loop built into the wall.

The metabolic mechanism responds to the chemical consequences of tissue activity. Falling oxygen and rising carbon dioxide, hydrogen ion, potassium, adenosine, lactate, and osmolality all relax arteriolar smooth muscle and open precapillary sphincters. Adenosine is particularly important in the heart and brain: it is produced when ATP consumption outstrips supply, so it is a direct chemical signal that a tissue is running an energy deficit.

Together these keep flow roughly constant across a wide range of mean arterial pressures — about 60 to 160 mm Hg in the brain and kidney. Below the lower limit, autoregulation is exhausted and flow falls with pressure; above the upper limit, the vessels are forced open and flow rises dangerously, which is what produces hypertensive encephalopathy.

Active (functional) hyperemia is metabolic autoregulation seen from the other direction: increase a tissue's metabolic rate and its flow rises, sometimes twentyfold, because its metabolites accumulate. Reactive hyperemia is the transient overshoot in flow after a period of occlusion — release a tourniquet and the limb flushes red, because metabolites accumulated during ischemia have maximally dilated the bed. Measuring that overshoot is the basis of flow-mediated dilation testing, a research and clinical index of endothelial health.

The special circulations

Five vascular beds behave in ways that cannot be predicted from the general rules, and in each case the deviation exists to solve a specific problem.

Three of these deserve expansion.

The brain is the least negotiable. Total cerebral blood flow is held nearly constant under almost all conditions, because both too little (ischemia) and too much (raised intracranial pressure, since the brain is enclosed in a rigid box) are dangerous. Regional activity is met by shunting flow within the brain rather than by increasing the total, which is exactly what functional MRI images. Cerebral vessels are unusually responsive to carbon dioxide: a rise in PaCO₂ dilates them powerfully. This is why hyperventilating a patient with raised intracranial pressure lowers that pressure temporarily — blowing off CO₂ constricts cerebral vessels and reduces cerebral blood volume — and it is also why a patient who hyperventilates from anxiety becomes light-headed.

The heart has already been covered in §18.4, but its combination of features is worth restating because it is Amara's disease. Flow occurs during diastole, so anything that shortens diastole cuts supply. Extraction is already near-maximal, so the heart cannot compensate for reduced flow by taking more from each millilitre. Coronary anastomoses are functionally inadequate acutely. The heart is, in short, the tissue least able to tolerate a narrowed artery, which is why coronary artery disease is the leading cause of death in most of the world.

The lung reverses the rule, and the reason is beautiful. In every systemic tissue, low oxygen means the tissue needs more blood, so hypoxia causes vasoDILATION. In the lung, low oxygen in an alveolus means this alveolus is not being ventilated, so any blood sent there will not be oxygenated and is wasted — it returns to the left heart still deoxygenated, diluting the arterial blood. The correct response is therefore to send the blood somewhere better ventilated, which requires vasoCONSTRICTION.

Hypoxic pulmonary vasoconstriction does exactly that, matching perfusion to ventilation region by region. It is why a pneumonia in one lobe does not cause more hypoxemia than it does.

But the mechanism assumes the hypoxia is local. When hypoxia is global — at high altitude, in chronic lung disease, or during the repeated apneas of obstructive sleep apnea — every pulmonary vessel constricts at once. Pulmonary vascular resistance rises across the whole lung, producing pulmonary hypertension, and the right ventricle, built for 25 mm Hg, is forced to generate more. Over years it hypertrophies and eventually fails: cor pulmonale. A reflex that is precisely right locally is destructive globally. Chapter 22 will diagnose Amara's sleep apnea, and this paragraph is why that diagnosis belongs in her cardiovascular story rather than only in her respiratory one.

19.8 The Circulatory Pathways

Vessel names are the part of this subject that feels like rote learning, and there is no pretending otherwise. But two organizing ideas make most of it derivable: vessels are usually named for the region or bone they accompany, and arteries and their companion veins usually share a name. Learn the exceptions and the pattern carries the rest.

   ══════════ THE SYSTEMIC ARTERIES: THE AORTA AND ITS BRANCHES ═════════

   LEFT VENTRICLE
        │
   ASCENDING AORTA ──► RIGHT and LEFT CORONARY ARTERIES        [§18.4]
        │
   AORTIC ARCH ──┬──► BRACHIOCEPHALIC TRUNK ──┬──► R COMMON CAROTID
                 │      (right side only)     └──► R SUBCLAVIAN
                 ├──► LEFT COMMON CAROTID
                 └──► LEFT SUBCLAVIAN
        │                    │
        │              ┌─────┴──────────────────────────────┐
        │        VERTEBRAL a.          AXILLARY ─► BRACHIAL │
        │          │                     (BP cuff sits here)│
        │          └► BASILAR ─► CEREBRAL ARTERIAL CIRCLE   │
        │                                     ┌─► RADIAL ───┤
        │              COMMON CAROTID ─┬─ INT │              │
        │              (to the head)   └─ EXT └─► ULNAR ─────┤
        │                                        → PALMAR ARCHES
   THORACIC AORTA ──► intercostal, bronchial, esophageal
        │
   ══ passes through the diaphragm at T12 ══
        │
   ABDOMINAL AORTA ─┬──► CELIAC TRUNK ──► stomach, liver, spleen
                    ├──► SUPERIOR MESENTERIC ──► small intestine,
                    │                            proximal colon
                    ├──► RENAL (paired) ──► kidneys (~20% of CO)
                    ├──► GONADAL (paired)
                    └──► INFERIOR MESENTERIC ──► distal colon, rectum
        │
   bifurcates at L4 into
        │
   COMMON ILIAC ─┬─ INTERNAL ILIAC ──► pelvic organs, gluteal region
                 └─ EXTERNAL ILIAC ──► FEMORAL ──► POPLITEAL
                                                      │
                                       ┌──────────────┴──────────┐
                              ANTERIOR TIBIAL          POSTERIOR TIBIAL
                                    │                        │
                              DORSALIS PEDIS          MEDIAL + LATERAL
                              (pulse: dorsum          PLANTAR
                               of foot)               (pulse: behind
                                                       medial malleolus)

   ══════════════ THE SYSTEMIC VEINS: RETURN TO THE HEART ═══════════════

   DURAL VENOUS SINUSES (brain) ──► INTERNAL JUGULAR ─┐
   superficial scalp/face ──► EXTERNAL JUGULAR ───────┤
   arm: CEPHALIC (lateral) ─┐                         ├─ BRACHIOCEPHALIC
        BASILIC  (medial) ──┼─► AXILLARY ─► SUBCLAVIAN┘        │
        MEDIAN CUBITAL ─────┘   (the antecubital vein            ▼
                                 used for blood draws)     SUPERIOR
   thoracic wall ──► AZYGOS SYSTEM ─────────────────────────► VENA CAVA
                                                                 │
                                                                 ▼
                                                            RIGHT ATRIUM
                                                                 ▲
   HEPATIC VEINS (from liver) ──────────────────────────────┐    │
   RENAL VEINS ─────────────────────────────────────────────┤    │
   GONADAL (R → IVC directly; L → left renal vein) ─────────┤ INFERIOR
   COMMON ILIAC ◄─ EXTERNAL ILIAC ◄─ FEMORAL ◄─ POPLITEAL ──┤ VENA CAVA
                        ▲                          ▲             │
        GREAT SAPHENOUS ┘         SMALL SAPHENOUS ─┘             ┘
        (longest vein in the body; medial leg and thigh;
         the vein harvested for coronary artery bypass grafts)

   ══════════════ THE HEPATIC PORTAL SYSTEM — TWO CAPILLARY BEDS ════════

     stomach, intestines, pancreas, spleen
        │  (FIRST capillary bed — absorption)
        ▼
     SPLENIC VEIN ──┐   INFERIOR MESENTERIC VEIN ──► joins splenic
                    ├──► HEPATIC PORTAL VEIN
     SUPERIOR ──────┘
     MESENTERIC VEIN
        │
        ▼
     LIVER SINUSOIDS  (SECOND capillary bed — processing)
        │             • nutrients stored or converted
        │             • toxins and drugs metabolized ("first pass")
        │             • ammonia → urea
        ▼
     HEPATIC VEINS ──► INFERIOR VENA CAVA ──► right atrium

Points worth extracting from the figure

The arch is asymmetrical. The brachiocephalic trunk exists only on the right, where it divides into the right common carotid and right subclavian. On the left, the common carotid and subclavian arise directly from the arch. This is an embryological legacy of the aortic arch arteries and it is the reason the right and left subclavian arteries have different origins — with the incidental consequence that the right recurrent laryngeal nerve hooks around the right subclavian while the left hooks around the aortic arch, so left-sided chest pathology can cause hoarseness and right-sided pathology usually cannot.

Pulse points are anatomy you can verify. Temporal, facial, common carotid, brachial, radial, femoral, popliteal, posterior tibial, and dorsalis pedis are palpable because at those points an artery lies superficially over a bone. The brachial artery is where a blood pressure cuff listens; the radial is where you count a pulse and where a cardiac catheter is usually inserted; the posterior tibial and dorsalis pedis are where perfusion of the foot is assessed.

The renal arteries take about 20% of cardiac output — over a litre a minute for two organs weighing 300 g together. That is not because kidneys are metabolically extravagant; it is because their function is processing blood. Chapter 26 develops the consequence.

Superficial veins are visible and expendable. The cephalic, basilic, median cubital, and great saphenous veins run in the superficial fascia, are not accompanied by arteries, and are extensively interconnected with the deep system by perforating veins. This is why the median cubital vein is used for venepuncture and why the great saphenous vein can be harvested for a coronary bypass without compromising the leg.

The cerebral arterial circle (circle of Willis) is an anastomotic ring at the base of the brain formed by the two internal carotid arteries, the two posterior cerebral arteries (from the basilar), one anterior communicating artery, and two posterior communicating arteries. Its purpose is redundancy: if one supplying vessel is occluded gradually, flow can cross the ring from the others. In practice the circle is complete and symmetrical in only about 20–40% of people, so the protection it offers is highly variable — one of the reasons the same carotid occlusion causes a devastating stroke in one person and nothing in another.

The hepatic portal system is the most important physiological exception to the usual pattern. A portal system is one in which blood passes through two capillary beds in series before returning to the heart. Blood draining the stomach, intestines, pancreas, and spleen does not go straight back to the heart; it is routed to the liver first, through the hepatic portal vein, and passes through the hepatic sinusoids before entering the hepatic veins and the inferior vena cava.

The purpose is quality control. Everything absorbed from the gut — glucose, amino acids, drugs, toxins, bacteria, and ammonia produced by gut flora — is presented to the liver before it reaches the general circulation. The liver buffers the enormous post-meal glucose surge, converts ammonia to urea, and metabolizes drugs (the first-pass effect, which is why some drugs cannot be given orally and why oral doses often exceed intravenous ones several-fold).

The cost is that when the liver becomes fibrotic, portal pressure rises, and blood is forced backward through the small portosystemic anastomoses — at the lower esophagus, the rectum, and around the umbilicus — which dilate into varices. Esophageal varices bleed catastrophically, and they are a direct anatomical consequence of a physiological arrangement that is otherwise entirely beneficial.

The pulmonary circulation, finally, is the whole-body exception. The pulmonary trunk carries deoxygenated blood from the right ventricle and divides into right and left pulmonary arteries; four pulmonary veins return oxygenated blood to the left atrium. Pressures are low (25/10, mean 15 mm Hg), resistance is about one-tenth of systemic, and the vessels are thin-walled and highly distensible — able to accommodate a fivefold increase in flow during exercise with only a small rise in pressure, by recruiting collapsed capillaries and distending open ones. The lungs also receive their own nutrient supply through the bronchial arteries from the thoracic aorta, so the lung is one of the few organs with a genuine dual blood supply, which is why a pulmonary embolus does not always cause infarction.

19.9 Advanced Topic · Atherosclerosis, Hypertension, Aneurysm, and Shock

Atherosclerosis: the disease that has been happening to Amara since her twenties

Atherosclerosis is a chronic inflammatory disease of the arterial intima, driven by lipid retention and endothelial injury. It is not a passive accumulation of fat in a pipe, and thinking of it that way makes most of its clinical behaviour incomprehensible. It is an immune response to a lipid that should not be there, occurring inside a vessel wall.

   ATHEROSCLEROSIS IN FIVE STAGES  (cross-section of an arterial wall)

  ─── ① ENDOTHELIAL INJURY AND DYSFUNCTION ─────── (begins in the teens)
      LUMEN  ~~~~ shear ~~~~►
      ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬  endothelium: NO output ↓, permeability ↑,
      ░░ intima ░░       adhesion molecules (VCAM-1, ICAM-1) ↑
      ████ media ████
      CAUSES: hypertension (mechanical), hyperglycaemia + insulin
      resistance, oxidized LDL, smoking, inflammation, low/oscillatory
      shear at BRANCH POINTS and CURVES — which is why plaques appear
      at bifurcations and at the proximal segments of vessels.

  ─── ② LIPID ENTRY AND OXIDATION ──────────────────────────────────
      ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
      ░░ LDL ● ● ● ░░    LDL crosses the leaky endothelium, binds
      ░░  ↓ oxidised ░   intimal proteoglycans, and is TRAPPED.
      ░░ oxLDL ◉ ◉ ░░    Trapped LDL is oxidized → oxLDL, which is
      ████ media ████    both an irritant and an immune target.

  ─── ③ MONOCYTES → MACROPHAGES → FOAM CELLS → FATTY STREAK ────────
      ▬▬▬▬◄ monocyte ▬▬▬  Monocytes bind VCAM-1, migrate in, become
      ░░ ⊛ ⊛ ⊛ ⊛ ⊛ ░░    macrophages, and ingest oxLDL through
      ░░ FOAM CELLS ░░    SCAVENGER receptors — which, unlike the LDL
      ████ media ████     receptor, are NOT switched off by cholesterol
      T cells arrive too. The macrophage eats until it becomes a lipid-
      stuffed FOAM CELL and dies, spilling its contents.
      RESULT: the FATTY STREAK — present in most people by age 20.
      Still reversible at this stage.

  ─── ④ SMOOTH MUSCLE MIGRATION AND THE FIBROUS CAP ────────────────
      ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
      ░▓▓▓ FIBROUS CAP ▓▓░  Macrophages release PDGF; smooth muscle
      ░▓ ◉◉ NECROTIC ◉◉ ▓░  cells migrate from MEDIA into INTIMA and
      ░▓  LIPID CORE   ▓░   secrete COLLAGEN, forming a fibrous cap
      ████ media ████       over a necrotic lipid core. Calcification
      follows. The artery initially remodels OUTWARD (Glagov), so the
      LUMEN IS PRESERVED until plaque burden reaches ~40% — which is
      why angiography under-reads early disease.

  ─── ⑤ COMPLICATION — TWO ROUTES, TWO DISEASES ────────────────────
   (a) STABLE, STENOTIC          (b) UNSTABLE → RUPTURE
      ▬▬▬▬▬▬▬▬▬▬▬▬▬▬              ▬▬▬▬▬▬╱ CAP TEARS ╲▬▬▬▬
      ░▓▓▓ THICK cap ▓▓░          ░▓ THIN cap ▓░  ╲   ╱
      ░▓ small core   ▓░          ░▓ LARGE lipid core▓░
      ████ media ████             ████ media ████
      lumen narrowed >70%         core contact with blood → platelets
      → EXERTIONAL ANGINA         adhere → THROMBUS (Ch. 17)
      → predictable, stable       → partial: UNSTABLE ANGINA / NSTEMI
                                  → total:   STEMI
      Slow growth allows           Sudden. Unpredictable.
      COLLATERALS to develop       NO collaterals.

   ═══ THE CRITICAL POINT ══════════════════════════════════════════
   The plaques that RUPTURE are often NOT the most stenotic ones.
   Rupture risk depends on CAP THICKNESS, CORE SIZE, and INFLAMMATION
   — not on percent narrowing. Amara's 50% LAD lesion may be more
   dangerous over ten years than the 90% circumflex lesion that was
   stented, and that is why she leaves hospital on a statin and two
   antiplatelet drugs rather than with a stent in every vessel.

Two features of the mature plaque explain most of what happens clinically.

The Glagov phenomenon. For the first phase of its growth, a plaque expands outward, pushing the media and externa away rather than into the lumen. Lumen diameter is preserved until the plaque occupies roughly 40% of the vessel's cross-section. This is why atherosclerosis is clinically silent for decades, why an angiogram can look almost normal in a diffusely diseased artery, and why the first symptom of coronary disease is a myocardial infarction in a large minority of people.

Cap thickness, not lumen size, predicts rupture. A vulnerable plaque has a thin fibrous cap (under about 65 µm), a large necrotic lipid core, and abundant macrophages, which secrete matrix metalloproteinases that digest the cap's collagen while inflammatory cytokines inhibit the smooth muscle cells that would replace it. Rupture exposes the thrombogenic core — collagen and tissue factor — to flowing blood, and the platelet and coagulation cascades of Chapter 17 do exactly what they are built to do.

This is why statins reduce events far more than they reduce stenosis. Lowering LDL and inflammation converts vulnerable plaques into stable, fibrous, calcified ones. The artery may look no wider on angiography, and the patient is nonetheless considerably safer.

Hypertension

Hypertension is sustained elevation of arterial pressure, currently defined in most guidelines as 130/80 mm Hg or higher on repeated measurement. About 90–95% of cases are primary (essential) hypertension, meaning no single identifiable cause.

"No single cause" does not mean no mechanism. Primary hypertension is the sum of several interacting processes, all of which act on one of the two terms of MAP = CO × TPR, and most of which lead back to the kidney's volume set point (§19.6):

• Renal sodium handling. Salt sensitivity — a rightward shift of the pressure–natriuresis curve — is present in roughly half of hypertensive patients and is more common in people of African ancestry, in older adults, and in those with obesity or CKD.
• RAAS activity inappropriate for the prevailing volume.
• Sympathetic overactivity, from stress, obesity, insulin resistance, sleep apnea, or circadian disruption.
• Endothelial dysfunction, reducing NO-mediated vasodilation and raising basal tone.
• Arterial stiffening, which raises systolic pressure and pulse pressure specifically.
• Obesity and insulin resistance, which raise sympathetic tone, activate the RAAS through adipose-derived angiotensinogen, promote renal sodium retention, and add vascular length.

Amara has at least five of the six. And her ambulatory monitoring adds one more finding worth dwelling on.

Non-dipping. In a healthy person, blood pressure falls 10–20% during sleep, driven by withdrawal of sympathetic tone and increased vagal tone as part of the circadian rhythm. A person whose nocturnal pressure falls by less than 10% is a non-dipper, and non-dipping predicts left ventricular hypertrophy, chronic kidney disease, stroke, and cardiovascular events independently of daytime pressure. The reason is simple exposure: the ventricle and the arteries are being loaded 24 hours a day instead of 16.

Amara's nocturnal fall is 2.8%. Three of her established features explain it: twenty years of rotating night shift, which chronically disrupts the circadian control of sympathetic outflow; probable obstructive sleep apnea, in which each apneic episode produces a hypoxic chemoreceptor surge (§19.5); and salt-sensitive kidney physiology, in which sodium excretion is shifted into the night and requires a higher pressure to accomplish. Her non-dipping is therefore not an incidental finding. It is the ambulatory record of a specific occupational exposure, and it is part of why a 45-year-old has the arteries of someone considerably older.

End-organ damage is the reason hypertension matters, and it follows a predictable list.

Note the kidney row. Hypertension damages the kidney, and a damaged kidney raises blood pressure further. Amara's urine albumin-to-creatinine ratio is 62 mg/g, above the normal threshold of 30 — the earliest laboratory sign of glomerular injury, and evidence that this loop has already started turning. Chapter 26 follows it.

Chapter Summary

§19.1 Every vessel is built from three tunics — intima (endothelium), media (smooth muscle and elastin), and externa (collagen, vasa vasorum, nervi vasorum) — re-proportioned for the job. Elastic arteries store 50–60% of the stroke volume in systole and return it in diastole (the Windkessel effect), maintaining diastolic pressure, smoothing flow, and reducing cardiac work. Muscular arteries distribute. Arterioles are the resistance vessels, carrying 50–60% of total peripheral resistance. Capillaries exchange. Veins are capacitance vessels holding 60–65% of blood volume, with valves in the limbs. Anastomoses provide collateral supply where they exist; where they do not, occlusion causes infarction.

§19.2 Every cell lies within about 100 µm of a capillary, because that is the diffusion limit. Continuous capillaries handle ordinary exchange and, with continuous tight junctions, form the blood–brain barrier; fenestrated capillaries permit high-volume water and solute movement in gut, endocrine glands, and glomerulus; sinusoidal capillaries let whole cells and proteins through in liver, marrow, and spleen. Precapillary sphincters open and close in response to local metabolites (vasomotion), so only about 25% of beds are perfused at rest. Exchange occurs by diffusion, transcytosis, and bulk flow.

§19.3 The Starling forces: hydrostatic pressure pushes fluid out and falls along the capillary (35 → 17 mm Hg); oncotic pressure from albumin pulls it in and does not change (26 mm Hg). Net filtration pressure is +10 mm Hg at the arterial end and −8 mm Hg at the venous end. About 20 L/day is filtered, 17 L reabsorbed, and the remaining 3 L/day returned by the lymphatics. Edema arises from raised hydrostatic pressure, reduced plasma protein, increased permeability, or lymphatic obstruction.

§19.4 F = ΔP ÷ R, and for the whole circuit MAP = CO × TPR. Resistance depends on viscosity, length, and — decisively — the fourth power of the radius: halving the radius raises resistance sixteenfold. Pulse pressure is systolic minus diastolic, approximately stroke volume divided by arterial compliance; MAP ≈ diastolic + (pulse pressure ÷ 3). Pressure falls most steeply across the arterioles; velocity varies inversely with total cross-sectional area, so blood crawls through capillaries at 0.3 mm/s, giving the 1–3 s transit that diffusion requires.

§19.5 The medullary cardiovascular centre maintains tonic vasomotor tone. The baroreceptor reflex — carotid sinus and aortic arch, via CN IX and CN X, to the nucleus of the solitary tract, out through sympathetic and vagal efferents to heart and vessels — corrects pressure within seconds, and is the mechanism that lets you stand up. Chemoreceptors add a vasoconstrictor response to hypoxia and acidosis. Baroreceptors reset within 1–3 days, so they cannot correct chronic hypertension.

§19.6 Hormonal control adds epinephrine (receptor-subtype-dependent), ADH (water retention, and vasoconstriction at high concentration), ANP/BNP (the only significant antihypertensive hormone, released on cardiac stretch), and the RAAS: renin from juxtaglomerular cells → angiotensin I → ACE on pulmonary capillary endothelium → angiotensin II → vasoconstriction, aldosterone, proximal sodium reabsorption, efferent arteriolar constriction, thirst, ADH, sympathetic facilitation, and fibrosis. Long-term pressure control is achieved by the kidney through pressure natriuresis, a mechanism with infinite gain, which is why sustained hypertension always involves the kidney and why most antihypertensive drugs target volume or the RAAS.

§19.7 Local control is myogenic (stretch causes constriction) and metabolic (metabolites cause dilation), producing autoregulation across MAP 60–160 mm Hg, plus active and reactive hyperemia. The endothelium is an endocrine organ secreting nitric oxide, prostacyclin, and endothelin. The brain holds flow nearly constant and is CO₂-sensitive; the heart perfuses in diastole with near-maximal extraction; skeletal muscle has the largest dynamic range; skin is thermoregulatory and sacrificed first; and the lung uniquely responds to hypoxia with vasoconstriction, matching perfusion to ventilation — beneficial locally, damaging when hypoxia is global.

§19.8 The aorta and its named branches supply the body in a predictable pattern; superficial veins are visible and expendable; the cerebral arterial circle offers variable collateral protection; and the hepatic portal system routes gut blood through the liver, providing first-pass processing at the cost of varices when portal pressure rises. The pulmonary circulation is a low-pressure, low-resistance, highly distensible circuit with its own separate bronchial nutrient supply.

§19.9 Atherosclerosis proceeds from endothelial dysfunction through lipid retention, macrophage foam-cell formation, and fibrous cap formation to either stable stenosis or rupture and thrombosis — and rupture risk depends on cap thickness and inflammation, not on percent narrowing. Hypertension is a multifactorial disorder that always involves the kidney's volume set point and damages heart, arteries, brain, kidney, and eye. Aneurysms accelerate by Laplace's law; dissection splits the media. Shock is classified by which term of MAP = CO × TPR has failed: preload (hypovolemic), pump (cardiogenic), mechanical obstruction (obstructive), or resistance (distributive).

Case File 19 · Resolution

Question 1 — What, physically, is the top number, and what is the bottom number?

They measure two different events, and the reason there are two of them is that the heart is an intermittent pump feeding a continuous circuit.

The top number — systolic pressure — is the peak pressure reached in the arteries during ejection. It is set by three things: how much blood the ventricle throws in (stroke volume), how fast it throws it (ejection velocity, which depends on contractility), and how willing the arteries are to accept it (arterial compliance). A compliant aorta expands and absorbs 50–60% of the stroke volume, so the peak is blunted; a stiff aorta cannot, so the same stroke volume produces a higher peak. Systolic pressure is therefore a statement about the ejection and the conduit's willingness to stretch.

The bottom number — diastolic pressure — is the minimum pressure remaining just before the next ejection. During diastole the ventricle contributes nothing: the aortic valve is shut, and the only thing keeping pressure up is the elastic recoil of the great arteries squeezing their stored volume onward. Pressure falls throughout diastole as blood drains out through the arterioles. How far it falls depends on three things: how much elastic energy was stored (again, compliance), how fast blood runs off through the arterioles (total peripheral resistance — high resistance means slow runoff means a higher trough), and how long diastole lasts (heart rate). Diastolic pressure is therefore a statement about elastic storage and downstream resistance.

Two practical corollaries. Diastolic pressure is the pressure that perfuses the left ventricle's own coronary arteries (§18.4), so it is not merely the smaller number. And because the heart spends roughly two-thirds of the cycle in diastole at resting rates, the mean arterial pressure — the one that actually determines tissue perfusion — is weighted toward the bottom number: MAP ≈ diastolic + (pulse pressure ÷ 3). For Amara at presentation that gives 98 + 23 = 121 mm Hg, against a normal 93.

Question 2 — Why does a wide pulse pressure mean stiff arteries rather than a strong heart?

Because pulse pressure is not a measure of force; it is a measure of volume divided by compliance.

Pulse pressure ≈ stroke volume ÷ arterial compliance

Read that as a fraction. The pulse pressure can widen in exactly two ways: the numerator can rise (a bigger stroke volume) or the denominator can fall (a stiffer, less compliant aorta). The intuition that a wide pulse pressure means a powerful heart is picking the numerator. In a 45-year-old with twenty years of hypertension, a hypertrophied ventricle, a reduced stroke volume of 52 mL, and an ejection fraction of 48%, the numerator is not the explanation. Her stroke volume is small. The only remaining possibility is that the denominator has collapsed.

The mechanism is §19.9. Elastin in the aortic media is laid down before birth and never replaced. Across 2.5 billion cardiac cycles it fatigues and fractures, and its place in the wall is taken by collagen — 100 to 1,000 times stiffer — along with calcium and non-enzymatic cross-links between collagen molecules, a process accelerated by hyperglycaemia. Hypertension accelerates the whole sequence, because a wall cycling between higher pressures fatigues faster. Amara's carotid–femoral pulse wave velocity of 11.4 m/s is the direct measurement: pressure waves travel faster in stiffer tubes, and hers is travelling at a speed typical of someone in their sixties.

Now trace what stiffness does to each number:

1. Systolic rises, because the aorta cannot expand to buffer the stroke volume, so more of it must be pushed downstream instantly.
2. Diastolic falls, or fails to rise as much as it otherwise would, because less elastic energy was stored and therefore less is returned during diastole. (In Amara, diastolic pressure is nonetheless high at 98, because her resistance is also raised — stiffness and high resistance are separate problems, and she has both. In her mother Adwoa, at 78, resistance is not the dominant issue and the pattern is the classic isolated systolic hypertension: 158/72, a pulse pressure of 86.)
3. The reflected pressure wave returns early. The pulse wave travelling out to the arterioles is partly reflected back. In a compliant young aorta it returns slowly, arriving during diastole, where it usefully augments diastolic pressure and therefore coronary filling. In a stiff aorta it returns fast, arriving during late systole, where it augments the systolic peak instead. Stiffening therefore raises the ventricle's workload and simultaneously removes the diastolic boost to the coronary circulation. Two harms from one mechanical change.

So a wide pulse pressure is a compliance measurement disguised as a pressure measurement, and it matters enormously at 45. A pulse pressure of 70 in a 45-year-old is not a variant; it is evidence that her conduit arteries have aged prematurely, and wide pulse pressure independently predicts stroke, heart failure, and cardiovascular death even after adjusting for mean pressure. It is also mechanistically upstream of the disease Chapter 18 described: the raised systolic load is what drove her concentric left ventricular hypertrophy, and the lost diastolic augmentation is one more reason a hypertrophied ventricle becomes subendocardially ischemic.

Question 3 — How does blocking an enzyme in the lung lower the pressure in her arm?

Because the enzyme is local but its product is a circulating hormone.

Angiotensin-converting enzyme is a membrane-bound ectoenzyme on the outer surface of endothelial cells, with its active site facing the bloodstream. It is present on endothelium everywhere, but it is most concentrated in the pulmonary capillaries, and for a good reason: the lung has the largest endothelial surface area of any organ, and the entire cardiac output passes through it every single minute. A single pass through the pulmonary circulation converts most of the angiotensin I in the blood to angiotensin II. The lung is a chemical processing plant that happens also to exchange gas.

The product, angiotensin II, is not a lung molecule. It is an octapeptide hormone that leaves the pulmonary veins, enters the left heart, and is distributed to the whole body, where it acts on AT1 receptors in at least seven places (Figure 19.6):

1. Systemic arterioles — the most potent physiological vasoconstrictor there is. Raises TPR.
2. Adrenal cortex (zona glomerulosa) — releases aldosterone, which drives sodium and water retention in the collecting duct. Raises blood volume, therefore preload, therefore cardiac output.
3. Renal proximal tubule — directly increases sodium reabsorption. Same effect.
4. Glomerular efferent arteriole — constricts, maintaining filtration when renal flow is low.
5. Hypothalamus — stimulates thirst and ADH release. More volume.
6. Sympathetic nerve terminals — facilitates norepinephrine release. Amplifies neural vasoconstriction.
7. Heart and vessel walls — acts as a growth factor, driving myocyte hypertrophy and fibroblast collagen deposition.

Block the enzyme and every one of those effects is reduced simultaneously. MAP = CO × TPR, and lisinopril lowers both terms: TPR falls because arterioles are less constricted and sympathetic transmission is less facilitated; CO falls modestly because aldosterone-driven volume retention is reduced. A third mechanism adds to it — ACE also degrades bradykinin, a vasodilator, so inhibiting ACE raises bradykinin levels and lowers resistance further. (That same accumulation is what causes the dry cough in roughly 10% of patients, which is why an ARB is substituted when it occurs.)

And there are effects beyond the pressure itself, which is why an ACE inhibitor is prescribed for Amara rather than simply any drug that lowers a number:

• Regression of hypertrophy and fibrosis. Removing angiotensin II removes a direct growth and profibrotic signal to her myocytes and cardiac fibroblasts. Over months this can genuinely soften a stiff ventricle — the one intervention that addresses her diastolic dysfunction at its mechanism rather than its symptoms.
• Renal protection. Dilating the efferent arteriole lowers glomerular capillary pressure, which reduces the glomerular hypertension that is currently pushing albumin into her urine. Expect her creatinine to rise slightly at first — that is the same efferent dilation, seen acutely — and her albumin-to-creatinine ratio of 62 mg/g to fall over months.
• Reduced left atrial pressure, and therefore less pulmonary capillary filtration, and therefore less breathlessness.

So the chain is: an enzyme on lung endothelium → less of a hormone in the blood → less constriction of arterioles everywhere, less sodium retained by the kidney, less growth signal to the heart → lower pressure in the brachial artery under a cuff. The site of the enzyme and the site of the effect are different because the messenger travels. That is the definition of a hormone, and it is why an organ system diagram with lines between the boxes explains more than any list of organs ever will.

Systems Integration Case File · Entry 19

Review

Level 1 · Recall

17.1 The tunica media consists chiefly of:

a) endothelium    b) smooth muscle and elastin    c) collagen and vasa vasorum    d) simple squamous epithelium

17.2 Which vessels contribute most to total peripheral resistance?

a) elastic arteries    b) capillaries    c) arterioles    d) veins

17.3 Halving the radius of a vessel increases its resistance approximately:

a) 2-fold    b) 4-fold    c) 8-fold    d) 16-fold

17.4 For a blood pressure of 150/90, the mean arterial pressure is approximately:

a) 100 mm Hg    b) 110 mm Hg    c) 120 mm Hg    d) 130 mm Hg

17.5 Net filtration pressure at the arterial end of a capillary is about +10 mm Hg mainly because:

a) oncotic pressure falls along the capillary    b) hydrostatic pressure exceeds oncotic pressure there    c) interstitial hydrostatic pressure is high    d) the lymphatics are closed

17.6 Baroreceptors are located in the:

a) carotid sinus and aortic arch    b) carotid body and aortic body    c) medulla oblongata    d) renal afferent arteriole

17.7 Angiotensin-converting enzyme is most concentrated on the endothelium of the:

a) kidney    b) liver    c) lungs    d) adrenal gland

17.8 Hypoxia in a region of the lung causes local:

a) vasodilation    b) vasoconstriction    c) no vascular response    d) capillary rupture

Level 2 · Comprehension

17.9 Explain, using the Starling forces, why a patient with left heart failure develops pulmonary edema while a patient with right heart failure develops ankle edema.

17.10 Why can the baroreceptor reflex not correct chronic hypertension, and what mechanism does the long-term job instead?

17.11 Blood velocity in the capillaries is about a thousand times slower than in the aorta, yet the same volume flows through both per minute. Explain, and explain why the slowness matters.

17.12 Distinguish arteriosclerosis from atherosclerosis, and explain why the first produces a wide pulse pressure while the second produces angina.

Level 3 · Clinical Application

17.13 A 22-year-old donates blood and stands up too quickly. She feels lightheaded, and her blood pressure drops from 118/76 to 92/60 with a heart rate rising from 68 to 96. Trace the entire sequence, naming each component of the reflex, and explain why her heart rate rose.

17.14 A 66-year-old with long-standing diabetes has a non-healing ulcer on his great toe, pain in his calves after walking 100 metres, and absent dorsalis pedis pulses. His ankle–brachial index is reported as 1.5. Explain the discrepancy and state what you would do.

17.15 A patient in the emergency department is hypotensive at 78/40, tachycardic at 130, with distended neck veins, muffled heart sounds, and a systolic pressure that falls 18 mm Hg on inspiration. Classify the shock and explain each finding.

Level 4 · Integration and Synthesis

17.16 Construct the complete cardiorenal loop for Amara: start from her hypertension, run it through the kidney and back to the heart, and identify every point at which one of her five medications intervenes. Then state which single organ, if it fails, makes the loop irreversible.

17.17 A pharmaceutical company proposes a drug that selectively increases arterial compliance without changing total peripheral resistance, heart rate, or blood volume. Predict its effects on systolic pressure, diastolic pressure, pulse pressure, MAP, left ventricular workload, and coronary perfusion. Would you want it for Amara?

17.18 Argue for or against this claim: "Blood pressure is a cardiovascular measurement." Use at least four systems.

Concept Map to Complete

Copy this onto blank paper and fill in every bracket from memory before checking the chapter.

                 MEAN ARTERIAL PRESSURE
                  MAP = [ _____ ] × [ _____ ]
                            │
        ┌───────────────────┴──────────────────────┐
   CARDIAC OUTPUT                        PERIPHERAL RESISTANCE
        │                                          │
   HR × SV  (Ch. 18)                    R ∝ [ ______ ] × [ ______ ]
        │                                          ÷ [ ______ ]^[ _ ]
   preload ← BLOOD VOLUME                          │
        │                                  main site = [ __________ ]
        │                                          │
   ═══════════════════════════════════════════════════════════════
   REGULATION ON TWO TIMESCALES
        │                                          │
   SECONDS: [ _____________ ] REFLEX      DAYS: the [ ________ ]
   receptors: [ ______ ] sinus            mechanism: [ ________ ]
              [ ______ ] arch                        [ ________ ]
   afferents: CN [ __ ] and CN [ __ ]     property: it does NOT
   centre:    [ ________________ ]                  [ _______ ]
   LIMITATION: it [ _______ ] in 1–3 d    → so it sets the LEVEL
   ═══════════════════════════════════════════════════════════════
   CAPILLARY EXCHANGE — the four Starling forces
        OUT: HPc [ __ ] → [ __ ] mm Hg  ·  OPif [ __ ]
        IN:  OPc [ __ ] (does it change? [ ___ ])  ·  HPif [ __ ]
        NFP arterial end = [ ____ ]   venous end = [ ____ ]
        the ~[ __ ] L/day not reabsorbed → [ ____________ ]
        FOUR CAUSES OF EDEMA: [ ___ ] [ ___ ] [ ___ ] [ ___ ]
   ═══════════════════════════════════════════════════════════════
   PULSE PRESSURE = [ _______ ] − [ _______ ]
                  ≈ [ ___________ ] ÷ [ ___________ ]
        so a WIDE pulse pressure with a SMALL stroke volume means
        [ ______________________ ] has fallen.

Lab / Self-Exploration

1. Find your own pulse points. Locate, in order: superficial temporal (in front of the ear), common carotid (beside the larynx — one side only, gently, and never both at once, because of the carotid sinus reflex in §19.5), brachial (medial antecubital fossa), radial (lateral wrist), femoral, popliteal, posterior tibial (behind the medial malleolus), and dorsalis pedis (dorsum of the foot). Note that each sits where an artery crosses superficially over bone.
2. Demonstrate venous valves. Let one arm hang down for thirty seconds until the superficial veins on the back of the hand distend. Press a fingertip firmly on a vein and, with a second fingertip, milk the blood away from the heart along the vein. The segment stays empty when you lift the second finger, because a valve prevents backfill. Now milk toward the heart and it refills instantly. You have just mapped a valve.
3. Watch the muscle pump work. Stand still for two minutes and note the visible distension of the hand and foot veins. Then walk briskly for one minute and look again. The veins collapse, because the calf muscle pump has emptied them upward through their valves.
4. Measure your own orthostatic response. Lie flat for five minutes and record heart rate and, if you have a cuff, blood pressure. Stand and record again immediately, at one minute, and at three minutes. A systolic fall over 20 mm Hg or a diastolic fall over 10 mm Hg meets the definition of orthostatic hypotension. Note the heart rate response and use §19.5 to say which part of the reflex it demonstrates.
5. Demonstrate reactive hyperemia. Squeeze one hand into a tight fist for 60 seconds while the arm is raised, then lower it and open the hand. Compare its colour with the other hand for the next 30 seconds. The flush is metabolite-driven vasodilation, and its duration is a crude index of how long it takes to wash the metabolites out.
6. Calculate your own numbers. Take your blood pressure. Compute pulse pressure and MAP. Compare your pulse pressure with Amara's 70, Adwoa's 86, and Nia's 38, and state which of the three you most resemble and what that implies about your arterial compliance.

Key Terms

Next: Chapter 20 · The Lymphatic System — where the three litres a day that this chapter left in the interstitium are collected, inspected, and returned, and where the inflammation that built Amara's plaque becomes a subject in its own right.