Transport in Mammals — A-Level Biology Study Guide

For: A-Level Biology candidates sitting A-Level Biology.

Covers: Blood vessel structure and function, heart anatomy and the cardiac cycle, blood components, tissue fluid formation, oxygen dissociation curves and the Bohr effect, plus exam-focused practice and revision resources.

You should already know: IGCSE Biology, basic chemistry.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the A-Level Biology style for educational use. They are not reproductions of past Cambridge International examination papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official Cambridge mark schemes for grading conventions.

1. What Is Transport in Mammals?

Transport in mammals refers to the closed, double circulatory system that moves blood, respiratory gases, nutrients, waste products, hormones and immune cells throughout the body to support cellular function, homeostasis and response to external stimuli. It is a core A-Level Biology topic in the A-Level Biology syllabus, assessed in both AS and A2 papers, with frequent 3-6 mark structured questions and 8 mark extended response questions. Common synonyms include the mammalian cardiovascular system and circulatory system. The double circulatory structure means blood passes through the heart twice per full circuit: once through the pulmonary circuit to the lungs, and once through the systemic circuit to the rest of the body.

2. Blood vessels — arteries, veins, capillaries

All blood vessels are lined with a single layer of smooth endothelial cells to reduce friction as blood flows, but their other structural features are directly adapted to their function and the pressure of blood they carry:

• Arteries: Carry blood away from the heart at high pressure (80-120 mmHg). They have thick layers of elastic fibres and smooth muscle, plus a narrow lumen. The elastic fibres recoil between heartbeats to maintain consistent pressure, while smooth muscle can constrict or dilate to regulate blood flow to specific tissues. The only artery carrying deoxygenated blood is the pulmonary artery, which carries blood to the lungs.
• Veins: Carry blood back to the heart at low pressure (<10 mmHg). They have thin walls, a wide lumen, and pocket valves to prevent backflow of blood as it moves against gravity, for example in the legs. The only vein carrying oxygenated blood is the pulmonary vein, which carries blood from the lungs to the heart.
• Capillaries: The smallest blood vessels, forming networks called capillary beds between arteries and veins. They are only one endothelial cell thick, with porous walls and a lumen roughly the same diameter as a red blood cell. This structure minimizes diffusion distance for exchange of oxygen, carbon dioxide, glucose, urea and other substances between blood and surrounding tissue.

Worked example (2 mark exam-style): Explain one structural difference between arteries and veins linked to blood pressure. Solution: Arteries have far thicker elastic tissue layers than veins (1 mark). This is because arteries carry blood at much higher pressure from the heart, so the elastic tissue recoils to maintain blood flow between contractions and prevent vessel damage (1 mark).

3. Heart structure and the cardiac cycle

The mammalian heart is a hollow, muscular organ made of specialized myogenic muscle that contracts without external nerve stimulation. Key structural features include:

• Four chambers: Two thin-walled atria that receive blood from veins, and two thick-walled ventricles that pump blood out of the heart. The left ventricle has a 3x thicker wall than the right ventricle, as it pumps blood around the entire systemic circuit at high pressure, while the right ventricle only pumps blood to the low-resistance pulmonary circuit.
• Valves: Atrioventricular (AV) valves separate atria and ventricles (bicuspid on the left, tricuspid on the right), and semilunar valves separate ventricles from the aorta and pulmonary artery. All valves open and close passively in response to pressure differences to prevent backflow.
• Coronary arteries: Branch off the aorta to supply oxygen and glucose to the heart muscle itself.

The cardiac cycle is the sequence of events that occurs in one full heartbeat, split into three phases:

1. Atrial systole (0.1s): Atria contract, pushing blood into relaxed ventricles, AV valves open, semilunar valves closed.
2. Ventricular systole (0.3s): Ventricles contract, pressure in ventricles rises above atrial pressure, closing AV valves (producing the first 'lub' heart sound). When ventricular pressure exceeds pressure in the aorta/pulmonary artery, semilunar valves open, and blood is ejected out of the heart.
3. Diastole (0.4s): All chambers relax, pressure in ventricles drops below aortic/pulmonary pressure, closing semilunar valves (producing the second 'dub' heart sound). AV valves open, and blood flows passively from veins into the atria and ventricles.

Heart rate (beats per minute, bpm) is calculated with the formula: $$\text{Heart rate}=\frac{60}{\text{Duration of one cardiac cycle (s)}}$$

Worked example (1 mark exam-style): Calculate the heart rate of a person with a cardiac cycle duration of 0.8s. Solution: $\frac{60}{0.8}=75$ bpm

4. Blood — red and white cells, plasma, platelets

Blood is a specialized tissue made of 55% plasma and 45% cellular components:

• Plasma: 90% water, plus dissolved solutes including glucose, amino acids, mineral ions, urea, carbon dioxide, hormones, antibodies, and large plasma proteins (e.g. albumin, fibrinogen). It acts as the main transport medium for dissolved substances.
• Red blood cells (erythrocytes): Biconcave disc-shaped cells with no nucleus, packed with the iron-containing protein haemoglobin. Their adaptations for oxygen transport include a large surface area to volume ratio for fast diffusion, and no nucleus to maximize space for haemoglobin.
• White blood cells (leukocytes): Larger than red blood cells, with a nucleus, and part of the immune system. Phagocytes (neutrophils, macrophages) engulf and digest pathogens via phagocytosis, while lymphocytes produce antibodies and coordinate specific immune responses.
• Platelets: Small cell fragments with no nucleus, involved in blood clotting. They release chemicals that trigger the conversion of fibrinogen to insoluble fibrin, which forms a mesh to trap red blood cells and form a clot at sites of vessel damage.

Worked example (2 mark exam-style): Explain why people with a low red blood cell count often feel tired during exercise. Solution: Fewer red blood cells means less haemoglobin to carry oxygen (1 mark). Less oxygen is delivered to muscle cells for aerobic respiration, so less energy is released, leading to fatigue (1 mark).

5. Tissue fluid formation

Tissue fluid is the watery fluid that surrounds all cells in mammalian tissues, acting as the medium for exchange of substances between blood and cells. It forms via the following process:

1. At the arterial end of the capillary bed, the hydrostatic pressure of blood (≈4.6 kPa) is higher than the hydrostatic pressure of the interstitial fluid around cells (≈1.3 kPa). This pressure gradient forces small molecules (water, glucose, oxygen, mineral ions, urea) out of the porous capillary walls into the interstitial space. Large plasma proteins, red blood cells and platelets are too large to pass through the pores, so they remain in the blood.
2. At the venous end of the capillary bed, blood hydrostatic pressure has dropped to ≈2.3 kPa due to friction against capillary walls. The retention of plasma proteins in the blood gives the blood a lower (more negative) water potential than the tissue fluid. As a result, water moves back into the capillary by osmosis, carrying dissolved waste products like urea and carbon dioxide with it.
3. Around 10% of the tissue fluid does not re-enter the venous capillary. It drains into blind-ended lymphatic vessels, where it is called lymph, and is eventually returned to the circulatory system near the heart, via a vein in the neck.

Worked example (2 mark exam-style): Explain why a diet very low in protein can cause oedema (swelling of tissues due to excess tissue fluid). Solution: Low protein intake reduces the concentration of plasma proteins in the blood, raising its water potential (1 mark). Less water moves back into the capillary by osmosis at the venous end, so excess tissue fluid accumulates in tissues (1 mark).

6. Oxygen dissociation curve and the Bohr effect

Haemoglobin (Hb) is the protein in red blood cells that binds reversibly to oxygen. Each Hb molecule can bind up to 4 oxygen molecules, forming oxyhaemoglobin ($Hb{O}_8$) when oxygen partial pressure ($p{O}_2$) is high: $$Hb+4O_2\rightleftharpoons Hb{O}_8$$ The oxygen dissociation curve plots the percentage saturation of Hb with oxygen against $p{O}_2$, and has a characteristic sigmoid (S-shaped) curve. This shape is due to positive cooperativity: when the first oxygen molecule binds to Hb, it changes the shape of the protein, making it easier for the next 3 oxygen molecules to bind. This means Hb becomes almost fully saturated at the high $p{O}_2$ found in the lungs (≈13 kPa, 97% saturation), and releases most of its oxygen at the low $p{O}_2$ found in respiring tissues (≈4 kPa, 50% saturation at rest).

The Bohr effect describes the shift of the oxygen dissociation curve to the right when partial pressure of carbon dioxide ($pC{O}_2$) is high, or pH is low. High $pC{O}_2$ (e.g. in actively respiring muscle tissue) forms weak carbonic acid in blood, lowering pH, which reduces Hb's affinity for oxygen. This means Hb releases more oxygen at the same $p{O}_2$ to meet the higher energy demand of respiring tissues.

Worked example (1 mark exam-style): At a $p{O}_2$ of 3 kPa, Hb is 60% saturated at rest, but only 35% saturated during exercise. What percentage extra oxygen is released to muscle tissue during exercise? Solution: $60-35=25\%$ extra oxygen released.

7. Common Pitfalls (and how to avoid them)

• Wrong move: Mixing up artery and vein structural features, e.g. claiming veins have thick elastic walls. Why students do it: They memorize features without linking to function. Correct move: Always connect structure to pressure: high pressure = thick elastic/muscle walls (arteries), low pressure = valves, thin walls (veins). Examiners regularly ask for structure-function links, so you will lose marks if you do not explicitly connect the two.
• Wrong move: Stating left and right ventricles have the same wall thickness. Why students do it: They forget the different circuits each ventricle supplies. Correct move: Left ventricle walls are 3x thicker than right, as it pumps blood around the entire body at high pressure, while the right only pumps to the low-pressure lungs.
• Wrong move: Claiming all blood components leave capillaries during tissue fluid formation. Why students do it: They forget the size filter of capillary pores. Correct move: Only small molecules (water, glucose, oxygen, urea) leave; large plasma proteins, red blood cells and platelets remain in the capillary, as they are too large to pass through pores.
• Wrong move: Describing the oxygen dissociation curve as linear. Why students do it: They ignore positive cooperativity of haemoglobin binding. Correct move: The curve is S-shaped (sigmoid) because binding of the first oxygen molecule increases haemoglobin's affinity for subsequent oxygen molecules.
• Wrong move: Stating the Bohr shift moves the dissociation curve left. Why students do it: They mix up high and low haemoglobin oxygen affinity. Correct move: High $pC{O}_2$/low pH reduces affinity, so the curve shifts right, releasing more oxygen to respiring tissues.

8. Practice Questions (A-Level Biology Style)

Question 1 (3 marks)

Explain three ways the structure of a capillary is adapted for its function of exchanging substances between blood and tissue fluid.

Worked Solution

Award 1 mark per valid point, max 3:

1. Capillaries are only one endothelial cell thick, which reduces the diffusion distance for oxygen, glucose and other substances, speeding up exchange.
2. Capillary walls have small pores that allow small dissolved molecules and water to pass through, but retain large plasma proteins and cells in the blood.
3. Capillary beds have a very large total surface area, providing more space for exchange to occur across.

Question 2 (4 marks)

a) A patient has a resting heart rate of 68 bpm. Calculate the duration of one full cardiac cycle for this patient, giving your answer to 2 decimal places. Show your working. (2 marks) b) Explain why semilunar valves close at the start of diastole. (2 marks)

Worked Solution

a) Rearrange the heart rate formula to solve for cycle duration: $$\text{Cardiac cycle duration}=\frac{60}{\text{Heart rate}}=\frac{60}{68}=0.88\text{ s}$$ 1 mark for correct formula/rearrangement, 1 mark for correct answer to 2 decimal places. b) When diastole begins, the ventricles relax, so pressure inside the ventricles drops below the pressure in the aorta and pulmonary artery (1 mark). The pressure difference forces the semilunar valves closed to prevent backflow of blood from the arteries back into the ventricles (1 mark).

Question 3 (5 marks)

Describe how tissue fluid is formed and returned to the circulatory system.

Worked Solution

Award 1 mark per valid point, max 5:

1. At the arterial end of the capillary bed, blood hydrostatic pressure is higher than the hydrostatic pressure of interstitial fluid.
2. This pressure gradient forces small molecules including water, glucose, oxygen and mineral ions out of the capillary into the interstitial space, forming tissue fluid.
3. Large plasma proteins remain in the capillary, so blood has a lower water potential than tissue fluid.
4. At the venous end of the capillary bed, blood hydrostatic pressure is lower, so water moves back into the capillary by osmosis, carrying waste products with it.
5. Excess tissue fluid drains into lymphatic vessels, forming lymph, which is returned to the circulatory system via veins in the neck.

9. Quick Reference Cheatsheet

10. What's Next

This topic forms the foundation for multiple higher-level A-Level Biology syllabus topics, including gas exchange in the lungs, immune response, exercise physiology, and homeostasis. A strong grasp of transport in mammals is also required to answer extended response questions on respiratory and circulatory system adaptations to extreme environments, and common disorders like coronary heart disease, sickle cell anaemia, and emphysema that are regularly assessed in A2 papers. Understanding the oxygen dissociation curve will also help you when you study photosynthesis and respiration, as you will encounter similar partial pressure and affinity concepts in those topics.

If you have any questions about specific concepts, calculation steps, or exam marking conventions for this topic, you can ask Ollie, our AI tutor, at any time for personalized explanations and extra practice questions. You can also find more topic guides, past paper walkthroughs, and revision resources on the homepage to help you prepare for your A-Level Biology exams.

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