AS 8 Transport in mammals | A Level Biology 9700

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🎯What you need to be able to do

Describe the mammalian closed double circulation and the functions of the pulmonary artery and vein, aorta and vena cava.
Recognise arteries, veins and capillaries in sections and relate the structure of each to its function.
Recognise and draw red blood cells, monocytes, neutrophils and lymphocytes.
State the functions of tissue fluid and describe its formation in a capillary network.
Describe oxygen and carbon dioxide transport, including haemoglobinic acid, carbaminohaemoglobin, carbonic anhydrase and the chloride shift.
Describe and explain the oxygen dissociation curve and its importance in the lungs and in respiring tissues.
Describe the Bohr shift and explain its importance.
Describe the structure of the heart and the cardiac cycle, and explain the roles of the SAN, AVN and Purkyne tissue.

📚The biology

A closed double circulation

Closed: blood stays inside vessels. Double: blood passes through the heart twice for each complete circuit — once through the pulmonary circulation to the lungs, once through the systemic circulation to the body. The advantage is that blood can be returned to the heart and re-pressurised after the lungs, so it reaches the body at high pressure and therefore flows quickly, which suits a high metabolic rate.

Pulmonary artery — right ventricle to lungs; the only artery carrying deoxygenated blood.
Pulmonary vein — lungs to left atrium; the only vein carrying oxygenated blood.
Aorta — left ventricle to the body.
Vena cava — body to right atrium.

Vessels

Elastic arteries (aorta and its large branches) — a thick wall with much elastic tissue that stretches during systole and recoils during diastole. This recoil smooths the flow, converting the pulse into a more continuous stream, and helps maintain pressure between beats.
Muscular arteries — more smooth muscle, less elastic tissue; they constrict and dilate to distribute blood to the tissues that need it.
Capillaries — a wall one endothelial cell thick, giving a very short diffusion distance; a very large total surface area; and a narrow lumen that slows the flow, allowing time for exchange.
Veins — a thin wall with little muscle or elastic tissue because pressure is low; a wide lumen reducing resistance; and semilunar valves preventing backflow, with blood moved by the contraction of surrounding skeletal muscle.

In section, an artery has a thick wall and a small, often crumpled circular lumen; a vein has a thin wall and a wide irregular lumen. That is the identification examiners want.

Blood cells you must recognise

Red blood cells — small, biconcave discs, no nucleus, packed with haemoglobin. Biconcave gives a large surface area to volume ratio for gas exchange; losing the nucleus makes room for more haemoglobin and lets the cell deform through capillaries.
Neutrophils — a multi-lobed nucleus and granular cytoplasm; phagocytes.
Monocytes — the largest, with a kidney-shaped nucleus; become macrophages.
Lymphocytes — a large round nucleus filling almost the whole cell, leaving only a thin rim of cytoplasm.

Tissue fluid

At the arterial end of a capillary the hydrostatic pressure is high, and it exceeds the opposing pressure created by the plasma proteins. Fluid is therefore forced out through the capillary wall — but the pores are small, so plasma proteins and blood cells stay in. The fluid that leaves, containing water, oxygen, glucose, amino acids and ions, is tissue fluid. It bathes the cells, and exchange between it and the cells occurs by diffusion.

Along the capillary, hydrostatic pressure falls. Meanwhile the remaining plasma proteins have made the blood’s water potential more negative. At the venous end the water potential gradient outweighs the reduced hydrostatic pressure, so most of the fluid returns by osmosis. The remainder drains into the lymphatic system and is eventually returned to the blood.

Water is the main component of both blood and tissue fluid; its solvent action allows substances to be transported dissolved, and its high specific heat capacity lets blood distribute heat without large temperature swings.

Carrying oxygen

Haemoglobin (see Topic 2) binds oxygen at the Fe²⁺ in each of four haem groups:

\[ \mathrm{Hb} + 4\,\mathrm{O_2} \rightleftharpoons \mathrm{Hb(O_2)_4} \]

The oxygen dissociation curve plots percentage saturation against partial pressure of oxygen, and it is sigmoid (S-shaped). That shape is the consequence of cooperative binding: the first oxygen binds with difficulty, but binding changes the quaternary structure so the remaining sites are more accessible, and the second and third bind readily. The last one is again harder, which flattens the top.

The biology of the shape:

In the lungs, partial pressure of oxygen is high and the curve is on its upper plateau, so haemoglobin is 95–100% saturated and loads fully. Because the curve is flat here, a fall in the partial pressure of oxygen — at altitude, or with mild lung disease — makes surprisingly little difference to loading.
In respiring tissues, partial pressure is low and the curve is on its steep part, so a small fall in partial pressure causes a large fall in saturation: a lot of oxygen is unloaded for a small change in conditions. The steepness is the point.

Carrying carbon dioxide, and the chloride shift

Carbon dioxide is transported three ways: about 5% dissolved in the plasma, about 10% as carbaminohaemoglobin (combined with the amino groups of haemoglobin), and about 85% as hydrogencarbonate ions. The last route is the one to learn in sequence:

CO₂ diffuses into the red blood cell.
Carbonic anhydrase catalyses CO₂ + H₂O → H₂CO₃ (carbonic acid).
Carbonic acid dissociates into H⁺ and HCO₃⁻.
HCO₃⁻ diffuses out into the plasma.
To maintain electrical neutrality, chloride ions move in — the chloride shift.
The H⁺ would make the cell dangerously acidic, so haemoglobin binds it, forming haemoglobinic acid (HHb). Haemoglobin acts as a buffer.

The Bohr shift

Binding H⁺ changes the shape of haemoglobin and reduces its affinity for oxygen, so oxygen is released. On the graph, a higher partial pressure of carbon dioxide shifts the whole dissociation curve down and to the right: at any given partial pressure of oxygen, haemoglobin is less saturated.

Its importance is elegant. The tissues that are respiring hardest produce the most carbon dioxide, so they generate the largest Bohr shift, so they cause haemoglobin to unload the most oxygen — exactly where oxygen is most needed, automatically, with no control system required.

The heart

Four chambers; the septum separates oxygenated from deoxygenated blood. Atrioventricular valves (tricuspid on the right, bicuspid on the left) prevent backflow into the atria; semilunar valves in the aorta and pulmonary artery prevent backflow into the ventricles. Coronary arteries supply the cardiac muscle itself.

The left ventricle wall is much thicker than the right because it must generate enough pressure to force blood round the whole body, whereas the right pumps only to the lungs, which are close and whose delicate capillaries would be damaged by high pressure. Atria have thin walls because they only pump blood into the ventricles a few centimetres away.

The cardiac cycle

Atrial systole — atria contract, atrial pressure exceeds ventricular, AV valves are open, ventricles fill completely.
Ventricular systole — ventricles contract, ventricular pressure rises above atrial so AV valves close (the first heart sound); when it exceeds arterial pressure the semilunar valves open and blood is ejected.
Diastole — ventricles relax, pressure falls below arterial so semilunar valves close (the second sound); when it falls below atrial pressure the AV valves open and filling begins again.

Valves open and close because of pressure differences alone — they are never actively opened. Every question on a cardiac cycle graph is answered by comparing the two pressure curves and asking which is higher.

Control of the heartbeat

Cardiac muscle is myogenic: it contracts without external stimulation.

The sinoatrial node (SAN) in the right atrium wall initiates a wave of electrical excitation; it is the pacemaker.
The wave spreads across both atria, causing atrial systole. It cannot cross to the ventricles directly because a layer of non-conducting tissue separates them.
The atrioventricular node (AVN) picks it up and imposes a delay, so the atria finish emptying before the ventricles contract.
The AVN passes it to the Purkyne tissue in the septum, which carries it to the apex of the ventricles, so contraction begins at the bottom and sweeps upwards, pushing blood up and out into the arteries.

Nervous and hormonal control of heart rate is not required at this level.

✏️Worked example

Two oxygen dissociation curves were plotted for human haemoglobin, one at a partial pressure of carbon dioxide of 5 kPa and one at 9 kPa. At an oxygen partial pressure of 4 kPa, the saturations were 62% and 40% respectively. In the lungs, at 13 kPa oxygen, both curves read 98%. (a) Calculate the percentage of oxygen unloaded in a tissue at 4 kPa under each condition. (b) Name the effect and explain its cause. (c) Explain why the curve is sigmoid, and why that shape suits the tissues.

(a) Haemoglobin arrives from the lungs 98% saturated in both cases. At 5 kPa CO₂ it falls to 62%, so the oxygen unloaded is \( 98 - 62 = 36\% \) of its capacity. At 9 kPa CO₂ it falls to 40%, so it unloads \( 98 - 40 = 58\% \).

The higher carbon dioxide concentration causes an extra 22 percentage points of oxygen to be released to that tissue — over half as much again.

(b) This is the Bohr shift. Carbon dioxide entering the red blood cell is converted by carbonic anhydrase into carbonic acid, which dissociates to give H⁺ ions. Haemoglobin binds these H⁺ as haemoglobinic acid, and doing so changes its shape so that its affinity for oxygen falls. The dissociation curve therefore shifts down and to the right: at any given partial pressure of oxygen, saturation is lower.

Its importance: the tissues respiring most rapidly produce the most carbon dioxide, so they generate the largest shift, so they receive the most oxygen — the supply matches demand automatically.

(c) The curve is sigmoid because of cooperative binding. The first oxygen molecule binds with difficulty, because the four haem groups are relatively inaccessible — hence the shallow lower part of the curve. Binding it changes the quaternary structure, making the remaining haem groups more accessible, so the second and third bind much more readily — the steep middle section. The fourth site is harder again, and saturation approaches a ceiling, giving the flat top.

The steep middle is exactly the range of oxygen partial pressures found in respiring tissues, so a small fall in partial pressure produces a large fall in saturation and a large release of oxygen. The flat top covers the range found in the lungs, so haemoglobin loads almost fully even if lung oxygen partial pressure varies.

Check it. Both readings must be taken at the same oxygen partial pressure, or the comparison is meaningless — that is why the question fixes it at 4 kPa. Check the direction of the shift makes biological sense: more carbon dioxide must mean less saturation, so the 9 kPa curve must lie below the 5 kPa one at every point, and 40% is indeed below 62%. If your reading put the high-CO₂ curve above the other, you have read the key backwards.

Confusing ‘shifts to the right’ with ‘carries less oxygen’. The Bohr shift does not reduce how much oxygen haemoglobin can hold — loading in the lungs is still 98% on both curves, because the plateau is barely affected. What changes is the affinity, so more oxygen is released at tissue partial pressures. Describing the Bohr shift as impairing oxygen transport inverts its purpose: it improves delivery precisely where it is needed.

📝Practise

Work through these, then reveal the answer. Each question targets a different objective from the list above.

1. Explain why the wall of the left ventricle is thicker than that of the right ventricle, and why atrial walls are thin.

The left ventricle pumps blood into the aorta and around the entire systemic circulation, so it must generate a high pressure to overcome the resistance of a long vascular network and to ensure blood reaches the extremities. More cardiac muscle means a more forceful contraction, so the wall is thick. The right ventricle pumps only to the lungs, which are close by, and the delicate alveolar capillaries would be damaged by high pressure; a lower pressure also allows blood to move slowly enough for gas exchange. The atria have thin walls because they only push blood a few centimetres into the ventricles below them, which requires very little pressure and is assisted by gravity and by the ventricles relaxing.

2. Describe the formation of tissue fluid at the arterial end of a capillary and its return at the venous end.

At the arterial end, the hydrostatic pressure of the blood is high because it is close to the heart. This pressure exceeds the opposing force created by the plasma proteins, so fluid is forced out through the capillary wall. The gaps between endothelial cells are small, so plasma proteins and blood cells remain in the capillary while water and small solutes — oxygen, glucose, amino acids, ions — pass out. This fluid is tissue fluid. Along the capillary, hydrostatic pressure falls as fluid is lost and resistance is met. Meanwhile the plasma proteins left behind are now more concentrated, so the blood has a lower (more negative) water potential than the tissue fluid. At the venous end the water potential gradient outweighs the reduced hydrostatic pressure, so most of the fluid returns to the capillary by osmosis. The excess drains into lymph vessels and is eventually returned to the blood.

3. Describe the sequence of events by which carbon dioxide produced in a respiring muscle is carried to the lungs as hydrogencarbonate.

CO₂ diffuses from the respiring cell into the plasma and then into the red blood cell. There the enzyme carbonic anhydrase catalyses its combination with water to form carbonic acid (H₂CO₃). Carbonic acid dissociates into hydrogen ions (H⁺) and hydrogencarbonate ions (HCO₃⁻). The HCO₃⁻ diffuses out of the red blood cell into the plasma, where it is carried to the lungs; to maintain electrical neutrality, chloride ions diffuse in to replace the negative charge lost — the chloride shift. The H⁺ ions are taken up by haemoglobin to form haemoglobinic acid, which prevents the pH of the cell falling and simultaneously causes oxygen to be released (the Bohr shift). At the lungs the whole sequence reverses and CO₂ is exhaled.

4. Using a cardiac cycle pressure graph, explain how you would identify the moment the semilunar valves open and the moment the atrioventricular valves close.

Both are found by comparing pressure curves, because valves are opened and closed by pressure differences alone. The semilunar valves open at the point where the ventricular pressure curve rises above the aortic (or pulmonary artery) pressure curve — the two lines cross, and blood is then ejected. The atrioventricular valves close at the point where the ventricular pressure rises above the atrial pressure — where those two lines cross, early in ventricular systole. Similarly, the semilunar valves close where ventricular pressure falls back below arterial pressure, and the AV valves open where ventricular pressure falls below atrial. On any such graph, identify which curve is which first, then look only at the crossings.

5. Explain the role of the atrioventricular node and the Purkyne tissue, and predict the consequence if the AVN delay were absent.

The AVN receives the wave of excitation after it has spread across the atria, and imposes a short delay before passing it on. This ensures that atrial systole is complete and the ventricles are fully filled before ventricular contraction begins. The Purkyne tissue conducts the excitation rapidly down the septum to the apex of the ventricles, so that contraction begins at the bottom and sweeps upwards, forcing blood up and out through the arteries at the top rather than trapping it in the base. Without the AVN delay, the atria and ventricles would contract at almost the same time. The ventricles would contract before they were fully filled, so stroke volume and cardiac output would fall, and contracting atria would be pushing against closing AV valves, forcing blood back into the veins.

6. Fetal haemoglobin has a dissociation curve lying to the left of adult haemoglobin. Explain what this means and why it is necessary.

A curve to the left means that at any given partial pressure of oxygen the haemoglobin has a higher percentage saturation — that is, a higher affinity for oxygen. This is necessary because the fetus obtains oxygen at the placenta, from the mother’s blood, and the partial pressure of oxygen there is relatively low — it has already been partly unloaded to maternal tissues. If fetal haemoglobin had the same affinity as the mother’s, there would be no net transfer. Because fetal haemoglobin has the higher affinity, it takes up oxygen from maternal haemoglobin at the placenta and becomes well saturated. The same logic explains the curves of animals adapted to low oxygen environments, such as the llama at altitude or a diving mammal: shifted left for loading where oxygen is scarce.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

British Heart Foundation — clear animations of the cardiac cycle with the valves and pressures shown together
Any set of stained blood smear micrographs — distinguishing a monocyte from a lymphocyte by nucleus shape is a Paper 3 skill and needs practice on real images
Interactive oxygen dissociation curve simulators — drag the carbon dioxide level and watch the curve shift, which fixes the direction of the Bohr shift permanently