The circulatory system

Animals show different patterns of circulation depending on body size, level of activity and metabolic demand.

Open circulation (e.g. insects). Blood ('haemolymph') is pumped by a simple dorsal heart into a body cavity (the haemocoel) where it bathes the organs directly before slowly returning to the heart through pores (ostia). Pressure is low and flow is sluggish. This is adequate because insects have a separate tracheal system for delivering O₂ directly to tissues — their blood does not transport O₂.

Closed circulation (e.g. mammals, fish, all vertebrates, annelids, cephalopods). Blood is always contained within vessels. Pressure is much higher and flow is faster, so O₂, nutrients and waste are distributed quickly. The closed system allows for precise control of flow to particular tissues (by vasoconstriction/vasodilation of arterioles).

Single circulation (e.g. fish). Blood passes through the heart once per complete circuit: heart → gills → body → heart. After exchange at the gills, the blood is at low pressure when it reaches the body — fine for an ectothermic fish but not for an active endothermic mammal.

Double circulation (e.g. mammals, birds). Blood passes through the heart twice per circuit:

• Pulmonary circuit: right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium (low pressure to protect the alveolar capillaries).
• Systemic circuit: left ventricle → aorta → body → vena cavae → right atrium (high pressure for fast delivery to all tissues).

Because the two circuits are separated by the septum of the heart, oxygenated and deoxygenated blood do not mix, and the systemic circulation can operate at much higher pressure than the pulmonary circulation — both essential for the high metabolic rate of an endotherm.

Cambridge expects you to be able to name and place the major blood vessels:

• Aorta — leaves the left ventricle; supplies the whole systemic circulation.
• Vena cavae (superior + inferior) — return deoxygenated blood from the body to the right atrium.
• Pulmonary artery — leaves the right ventricle; carries deoxygenated blood to the lungs.
• Pulmonary vein — returns oxygenated blood from the lungs to the left atrium. (The pulmonary vessels are the exception to the 'arteries carry oxygenated, veins carry deoxygenated' rule.)
• Renal artery / renal vein — to / from the kidneys.
• Hepatic artery — oxygenated blood from the aorta to the liver.
• Hepatic vein — drains the liver into the inferior vena cava.
• Hepatic portal vein — special vessel carrying blood from the gut directly to the liver. Carries absorbed nutrients (glucose, amino acids) for processing.
• Mesenteric artery — supplies the intestines.
• Coronary arteries — branch off the aorta just above the aortic valve; supply the heart muscle (myocardium) itself with oxygenated blood. Blockage causes a heart attack (myocardial infarction).

Portal vessels are vessels that begin and end in capillary beds (rather than ending in the heart). The hepatic portal vein is the classic example — it starts in the capillaries of the gut wall and ends in the sinusoids of the liver.

Every blood vessel (except capillaries) has three layers in its wall:

• Tunica intima — inner lining of squamous endothelium plus elastic tissue; smooth to minimise friction.
• Tunica media — middle layer of smooth muscle and elastic fibres. Its thickness varies hugely between vessel types.
• Tunica externa (adventitia) — outer connective tissue layer; collagen for tensile strength.

Arteries carry blood away from the heart at high, pulsatile pressure. Their wall has a thick tunica media rich in elastin that stretches as the ventricle ejects blood and recoils between beats — this 'Windkessel effect' smooths the pulsatile flow and maintains pressure during diastole. The lumen is narrow relative to wall thickness, helping maintain pressure. Smooth muscle allows some vasoconstriction.

Arterioles are smaller arteries leading into capillary beds. Their walls contain a high proportion of smooth muscle (and proportionally less elastin), allowing precise vasoconstriction or vasodilation. Arterioles are the main resistance vessels and are responsible for most of the pressure drop between aorta (~120 mmHg) and capillary (~35 mmHg).

Capillaries are microscopic vessels (lumen ~7-10 μm) where exchange occurs. Their walls are a single endothelial cell layer (~0.1 μm thick) with fenestrations between cells. They have NO smooth muscle or elastin. RBCs are forced through in single file, slowing flow and maximising exchange.

Venules are small veins draining capillary beds. They have very thin walls.

Veins carry blood back to the heart at low, steady pressure. Their wall is thin (much less muscle and elastin than arteries) and the lumen is wide to reduce resistance. Veins contain semilunar valves at intervals — these prevent backflow of low-pressure blood. Return is aided by:

• Skeletal muscle pump — contracting limb muscles compress the veins, pushing blood towards the heart;
• Respiratory pump — pressure changes in the thorax during breathing draw blood back to the right atrium;
• Gravity assists — for blood returning from the head when standing.

Three quantities change predictably as blood moves around the circulation:

Pressure is highest where the left ventricle ejects blood (~120 mmHg systolic / 80 mmHg diastolic in the aorta) and falls progressively as blood encounters resistance:

• Aorta and large arteries: ~120/80 mmHg
• Arterioles: pressure drops sharply (most of the resistance is here)
• Capillaries: ~35 mmHg (arteriole end) → ~15 mmHg (venule end)
• Veins: ~10 mmHg
• Vena cava entering right atrium: ~5 mmHg

The fall from arteriole to capillary protects the thin-walled capillaries from rupture.

Velocity of blood flow depends on the total cross-sectional area at that point (continuity: $Av = \text{constant}$, where $A$ is total CSA and $v$ is velocity). The aorta has one large vessel with the highest velocity (~30 cm s⁻¹). But arteries branch into millions of capillaries with a combined CSA much greater than the aorta's, so velocity is lowest in capillaries (~0.05 cm s⁻¹). Low velocity gives time for exchange. Velocity rises again in venules and veins as the cross-section narrows back towards the vena cavae.

Pulse is felt in the arteries because the elastic recoil of the arterial wall creates a pressure wave with each heartbeat. By the time blood reaches capillaries, the elastic wall and rhythm have been damped out — flow is smooth and continuous. There is no pulse in veins.

Tissue fluid is plasma minus most of the proteins. It forms at the arteriole end of capillaries and is partly reabsorbed at the venule end. The forces driving this exchange were first described by Ernest Starling in 1896 (the 'Starling forces').

At the arteriole end:

• Hydrostatic pressure of blood (HP): ~32 mmHg (generated by the heart, maintained by narrow arteriole).
• Oncotic pressure of plasma (OP): ~25 mmHg (due to plasma proteins, especially albumin, that cannot leave the capillary).
• Net outward force = HP − OP ≈ +7 mmHg outward.

Water and small dissolved solutes (glucose, amino acids, ions, urea, O₂) are forced out through fenestrations between adjacent endothelial cells. Plasma proteins and blood cells stay behind. The fluid that emerges is called tissue fluid (interstitial fluid).

Bathing the cells. Tissue fluid surrounds the cells, allowing exchange by diffusion:

• O₂ and glucose diffuse FROM tissue fluid INTO cells.
• CO₂ and other waste diffuse FROM cells INTO tissue fluid.

At the venule end:

• Hydrostatic pressure has fallen to ~15 mmHg (resistance has consumed it).
• Oncotic pressure remains ~25 mmHg (proteins are still in the blood).
• Net inward force = OP − HP ≈ +10 mmHg inward.

So most of the water and dissolved waste is reabsorbed back into the capillary by osmosis.

The lymphatic system handles the excess. About 10% of the tissue fluid is not reabsorbed at the venule end. This excess fluid (plus any plasma proteins that have leaked out) drains into blind-ended lymph capillaries, becoming lymph. Lymph is propelled by skeletal muscle contraction (lymph vessels also have valves) and returns to the venous circulation via the subclavian veins in the neck.

Oedema is the build-up of excess tissue fluid in the spaces between cells. Causes:

• Low plasma protein (e.g. severe malnutrition / kwashiorkor; liver disease): less albumin → lower oncotic pressure → less reabsorption.
• Lymphatic blockage (e.g. filarial worm in elephantiasis; lymph node removal during cancer surgery): excess fluid cannot drain.
• High venous pressure (e.g. heart failure): pressure backs up; HP at venule end stays high → less reabsorption.
• Increased capillary permeability (e.g. inflammation, allergy): more proteins leak out → tissue fluid becomes more concentrated → less water drawn back in.

Capillaries vary in permeability depending on the tissue. Cambridge focuses on the general structure, but you should know that not all capillaries are the same.

• Continuous capillaries (most common — skin, muscle, lung, CNS). Endothelial cells form a continuous lining with tight junctions; small intercellular clefts allow water and small solutes to pass. In the brain, tight junctions are especially tight, forming the blood-brain barrier.
• Fenestrated capillaries (kidney, endocrine glands, intestine). Endothelial cells have actual pores (fenestrations) through them, allowing higher rates of filtration.
• Sinusoidal capillaries (liver, spleen, bone marrow). Large gaps between endothelial cells allow even proteins and whole cells to pass.

For 9700, the key adaptations to memorise are:

• One-cell-thick wall (squamous endothelium) → short diffusion distance.
• Narrow lumen (7-10 μm) → RBCs single file, slow flow, maximises exchange.
• Fenestrations / clefts between cells → allow water and small solutes through.
• Extensive network → enormous total surface area; every body cell is close to a capillary.

Most exchange happens at the level of the capillary because of the combination of large total surface area, short diffusion distance, and slow velocity giving plenty of time for diffusion and ultrafiltration.