Cambridge International AS & A Level Biology 9700: Transport in mammals – Study Guide

This study guide teaches Topic 8, Transport in mammals, from the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It covers sections 8.1 The circulatory system, 8.2 Transport of oxygen and carbon dioxide and 8.3 The heart. This is AS Level content: it is examined on Paper 1 (Multiple Choice) and Paper 2 (AS Level Structured Questions), and Paper 4 can draw on it because A Level questions require knowledge of the AS content.

Course hub: Cambridge A Level Biology · Printable checklist: 9700 checklist · Short on time? Use the revision notes · Test yourself with the practice questions.

What this topic covers

Section	What you must be able to do
8.1 The circulatory system	Double circulation; four main vessels; vessel structure and function; plan diagrams; blood cells; water; tissue fluid
8.2 Transport of oxygen and carbon dioxide	Haemoglobin, carbonic anhydrase, haemoglobinic acid, carbaminohaemoglobin; chloride shift; plasma; dissociation curve; Bohr shift
8.3 The heart	Structure; wall thickness; cardiac cycle and valves; SAN, AVN and Purkyne tissue

Haemoglobin’s structure is taught in Topic 2: see the biological molecules study guide.

8.1 The circulatory system

A closed double circulation

The mammalian circulatory system is a closed double circulation. It consists of a heart, blood and blood vessels: arteries, arterioles, capillaries, venules and veins.

• Closed: blood stays inside vessels.
• Double: blood passes through the heart twice per circuit: the pulmonary circulation (heart to lungs and back) and the systemic circulation (heart to body and back).

The four main vessels you must know:

Vessel	Carries blood	From → to
Pulmonary artery	Deoxygenated	Right ventricle → lungs
Pulmonary vein	Oxygenated	Lungs → left atrium
Aorta	Oxygenated	Left ventricle → body
Vena cava	Deoxygenated	Body → right atrium

Arteries carry blood away from the heart and veins carry it towards the heart. Define them by direction, not oxygen content.

Structure and function of vessels

Artery and vein walls have three layers: an inner endothelium (tunica intima), a middle layer of smooth muscle and elastic fibres (tunica media) and an outer layer mainly of collagen (tunica externa). Capillary walls are endothelium only.

• Elastic arteries (such as the aorta), near the heart, have many elastic fibres. The wall stretches as blood surges in, then recoils between beats, smoothing the pressure and keeping blood moving forward.
• Muscular arteries, further out, have more smooth muscle. Contraction narrows the lumen and relaxation widens it, controlling blood flow to each organ.
• Veins carry blood at low pressure. They have a thin wall with little muscle or elastic tissue, a wide lumen that offers little resistance to flow, and valves that stop blood flowing backwards.
• Capillaries have a wall one endothelial cell thick (short diffusion distance) and form dense networks close to every cell.

Recognising vessels and drawing plan diagrams

In transverse section (TS), an artery looks round with a thick wall and small lumen; a vein looks flattened, with a thin wall and large lumen; a capillary fits red blood cells in single file. A longitudinal section (LS) of a vein may show a valve.

A plan diagram shows tissue-layer outlines only: no cells, no shading, clear continuous lines and the correct proportions of layers.

Worked example 1. In a photomicrograph at ×12, the wall of an elastic artery measures 18 mm thick and the lumen is 30 mm across. Calculate the actual wall thickness and the ratio of wall thickness to lumen diameter.

actual size = image size ÷ magnification
wall  = 18 mm ÷ 12 = 1.5 mm
lumen = 30 mm ÷ 12 = 2.5 mm
ratio = 1.5 : 2.5 = 0.6 : 1   (or 3 : 5)

The ratio can also come straight from the image: 18 ÷ 30 = 0.6.

Blood cells

You must recognise and draw four types:

• Red blood cells: small, biconcave discs with no nucleus; they appear paler in the centre.
• Neutrophils: a lobed nucleus (several lobes joined by thin strands) and granular cytoplasm.
• Monocytes: the largest white cells, with a large kidney-shaped (bean-shaped) nucleus.
• Lymphocytes: small, with a large round nucleus that fills most of the cell and a thin rim of cytoplasm.

Water in transport

Water is the main component of blood and tissue fluid. Two properties matter:

• Solvent action: polar and charged substances such as glucose, amino acids, ions and urea dissolve in water, so they can be carried in plasma and tissue fluid.
• High specific heat capacity: blood temperature stays stable, and blood carries heat from active organs to the rest of the body.

Tissue fluid

Tissue fluid bathes the cells. It supplies them with oxygen, glucose and amino acids, removes carbon dioxide and other waste, and gives cells a stable environment.

Formation in a capillary network:

1. At the arterial end, high blood (hydrostatic) pressure forces plasma out through gaps between the endothelial cells.
2. Water and small solutes leave. Red blood cells and most plasma proteins are too large and stay in the blood. Some neutrophils squeeze out.
3. At the venous end, blood pressure is lower. The plasma proteins that stayed behind give the blood a lower water potential than the tissue fluid, so much of the water moves back into the capillary by osmosis.
4. The excess fluid that does not return drains into lymph vessels.

8.2 Transport of oxygen and carbon dioxide

Oxygen and haemoglobin

Each haemoglobin molecule has four haem groups, each binding one oxygen molecule, forming oxyhaemoglobin. Red blood cells have no nucleus, leaving more room for haemoglobin.

Carbon dioxide transport

In a red blood cell in a respiring tissue:

1. Carbon dioxide diffuses in. The enzyme carbonic anhydrase catalyses its reaction with water to form carbonic acid: CO₂ + H₂O → H₂CO₃
2. Carbonic acid dissociates: H₂CO₃ → H⁺ + HCO₃⁻
3. Hydrogen ions combine with haemoglobin to form haemoglobinic acid (HHb). This makes oxyhaemoglobin release its oxygen, and removes H⁺ so blood pH does not fall much.
4. Hydrogencarbonate ions diffuse out of the red blood cell into the plasma.
5. Some carbon dioxide combines directly with haemoglobin (with its amine groups) to form carbaminohaemoglobin.

The chloride shift. As hydrogencarbonate ions leave the red blood cell, chloride ions (Cl⁻) move in from the plasma, keeping the charges balanced. Its importance: hydrogencarbonate can keep leaving, so the cell can go on converting carbon dioxide.

The role of plasma. Most carbon dioxide is carried in the plasma as hydrogencarbonate ions. A small amount dissolves directly in the plasma. In the lungs these reactions reverse.

The oxygen dissociation curve

The curve plots percentage saturation of haemoglobin against the partial pressure of oxygen (pO₂). For adult haemoglobin it is S-shaped (sigmoid).

• At low pO₂ the curve is shallow: it is hard for the first oxygen molecule to bind.
• Once one oxygen molecule binds, the haemoglobin changes shape, making it easier for the next ones to bind. So the curve rises steeply.
• At high pO₂ the curve levels off as the haemoglobin nears full saturation.

Why the shape matters. In the lungs, pO₂ is high, so haemoglobin becomes almost fully saturated. In respiring tissues, pO₂ lies on the steep part of the curve, so a small fall in pO₂ releases a lot of oxygen.

The Bohr shift

A high partial pressure of carbon dioxide shifts the curve to the right. This is the Bohr shift. The extra carbon dioxide produces more H⁺, which forms haemoglobinic acid and lowers haemoglobin’s affinity for oxygen. At any given pO₂, saturation is lower, so more oxygen is unloaded in actively respiring tissues. In the lungs, where carbon dioxide is being removed, haemoglobin’s affinity is higher and it loads oxygen readily.

Worked example 2. Blood leaving the lungs is 97% saturated. Fully saturated blood carries 20.0 cm³ of oxygen per 100 cm³. In a muscle at a pO₂ of 3 kPa, saturation falls to 45% at normal carbon dioxide levels, but to 30% when the muscle is working hard and pCO₂ is high. Calculate the oxygen released per 100 cm³ of blood in each case, and the percentage increase caused by the Bohr shift.

normal:     20.0 × (0.97 − 0.45) = 20.0 × 0.52 = 10.4 cm³
Bohr shift: 20.0 × (0.97 − 0.30) = 20.0 × 0.67 = 13.4 cm³
increase:   13.4 − 10.4 = 3.0 cm³
% increase: 3.0 ÷ 10.4 × 100 = 28.8%

8.3 The heart

Structure

Externally: two atria above two ventricles, coronary arteries on the surface supplying the heart muscle, and the aorta, pulmonary artery, pulmonary veins and venae cavae.

Internally:

• The septum separates the right side (deoxygenated blood) from the left side (oxygenated blood).
• Atrioventricular valves lie between each atrium and ventricle: the tricuspid valve on the right and the bicuspid (mitral) valve on the left. Tendons (tendinous cords) attached to papillary muscles stop these valves turning inside out.
• Semilunar valves sit at the base of the aorta and the pulmonary artery.

Wall thickness

• Atria vs ventricles: atria pump blood only into the ventricles, a short distance, so their walls are thin. Ventricles pump blood out through a whole circulation, so their walls are thicker.
• Left vs right ventricle: the left ventricle pumps through the whole systemic circulation, with much greater resistance, so it needs more muscle to produce a higher pressure. The right ventricle pumps only to the nearby lungs, and a lower pressure there avoids damaging lung capillaries.

The cardiac cycle

• Atrial systole: atria contract, pushing blood into the ventricles through the open atrioventricular valves.
• Ventricular systole: ventricles contract. As soon as ventricular pressure rises above atrial pressure, the atrioventricular valves close. When ventricular pressure rises above the pressure in the aorta and pulmonary artery, the semilunar valves open and blood is ejected.
• Diastole: heart muscle relaxes. When ventricular pressure falls below arterial pressure, the semilunar valves close. When it falls below atrial pressure, the atrioventricular valves open and the chambers refill.

Valves open and close only because of pressure differences.

Worked example 3. The table shows pressures (kPa) on the left side of the heart during a cycle lasting 0.75 s. State whether each valve is open or closed, and calculate the heart rate.

Time / s	Left atrium	Left ventricle	Aorta	AV valve	Semilunar valve
0.05	1.5	1.2	11.0	open	closed
0.15	0.8	6.0	10.6	closed	closed
0.25	0.6	15.5	15.0	closed	open
0.45	0.9	5.0	12.0	closed	closed
0.60	1.1	0.5	11.2	open	closed

Method: the AV valve is open only when atrial pressure exceeds ventricular pressure; the semilunar valve only when ventricular pressure exceeds aortic pressure.

heart rate = 60 s ÷ 0.75 s per beat = 80 beats per minute

Control of the cycle

Cardiac muscle is myogenic: it starts its own contractions.

1. The sinoatrial node (SAN), the pacemaker in the right atrium wall, sends waves of excitation across both atria, so they contract.
2. Non-conducting tissue between atria and ventricles stops the wave passing straight to the ventricles.
3. The atrioventricular node (AVN) passes the wave on after a short delay, so the atria finish emptying before the ventricles contract.
4. Purkyne tissue carries the wave down the septum to the base of the ventricles, then up their walls. The ventricles contract from the bottom upwards, squeezing blood into the arteries.

Knowledge of nervous and hormonal control is not expected.

Common errors

• Saying arteries carry oxygenated blood: the pulmonary artery does not.
• Saying valves “open to let blood through” without saying which pressure is higher.
• Describing the Bohr shift as “more carbon dioxide binds to haemoglobin” instead of lower affinity for oxygen.
• Saying plasma proteins leave the capillary in tissue fluid.
• Drawing cells or shading in plan diagrams.

Next steps

• Condensed recall and a self-test: Transport in mammals revision notes
• Exam-style questions: Transport in mammals practice
• Find your gaps across AS topics: free 9700 AS diagnostic
• Next topic: Gas exchange study guide

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027, Cambridge University Press & Assessment – Topic 8, Transport in mammals (sections 8.1–8.3).