Topic 9 of IGCSE Biology (Cambridge 0610) is transport in animals. You need to label the heart (chambers, valves, major vessels), state the function of arteries, veins, and capillaries, state the function of red blood cells, white blood cells, platelets, and plasma, and describe double circulation (pulmonary and systemic). This revision guide takes you through each of these with clear descriptions and exam-style tips for full marks.

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The heart: chambers and valves

The heart has four chambers: two atria (singular: atrium) at the top and two ventricles at the bottom. The right atrium receives deoxygenated blood from the body (via the vena cava); the right ventricle pumps this blood to the lungs (via the pulmonary artery). The left atrium receives oxygenated blood from the lungs (via the pulmonary vein); the left ventricle pumps this blood to the body (via the aorta). Valves between the atria and ventricles (and in the major vessels) prevent backflow of blood. The left ventricle has a thicker wall than the right ventricle because it has to pump blood to the whole body (higher pressure), whereas the right ventricle only pumps to the lungs. You may be asked to label the heart (atria, ventricles, valves, aorta, vena cava, pulmonary artery, pulmonary vein) and to explain why the left ventricle has a thicker wall.

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Blood vessels: arteries, veins, capillaries

Arteries carry blood away from the heart. They have thick, muscular walls to withstand high pressure (blood is forced out by the heart). Veins carry blood towards the heart. They have thinner walls and often have valves to prevent backflow (blood is at lower pressure). Capillaries are very narrow vessels with thin walls (one cell thick) that allow exchange of substances (e.g. oxygen, glucose, carbon dioxide, waste) between the blood and the body cells. Do not say “arteries carry oxygenated blood and veins carry deoxygenated blood”: the pulmonary artery carries deoxygenated blood (to the lungs), and the pulmonary vein carries oxygenated blood (from the lungs). It is the direction (away from or towards the heart) and structure (thick vs thin wall; valves) that define arteries and veins.

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Blood: red cells, white cells, platelets, plasma

Red blood cells contain haemoglobin, which carries oxygen. They have no nucleus (to make more space for haemoglobin) and a biconcave shape (increases surface area). White blood cells are part of the immune system: they engulf pathogens (phagocytosis) or produce antibodies that bind to antigens. Platelets are small cell fragments that help blood clotting (to stop bleeding). Plasma is the liquid part of the blood; it carries dissolved substances (e.g. glucose, amino acids, carbon dioxide, hormones, waste such as urea) and the blood cells. You may be asked to state the function of each component. Always state that haemoglobin (in red blood cells) carries oxygen.

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Double circulation

Humans have double circulation: blood passes through the heart twice for each complete circuit of the body. The pulmonary circulation is: heart → lungs (to pick up oxygen and lose carbon dioxide) → heart. The systemic circulation is: heart → body (to deliver oxygen and nutrients; pick up carbon dioxide and waste) → heart. This separation means that oxygenated and deoxygenated blood are kept separate in the heart, and the body receives blood that has been fully oxygenated in the lungs. You may be asked to describe the path of blood through the heart and lungs (mention which chambers, which vessels, and where the blood is oxygenated or deoxygenated).

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Exam tips and command words

• Label the heart: Atria, ventricles, valves, aorta, vena cava, pulmonary artery, pulmonary vein.
• State the function: Of each vessel type (arteries, veins, capillaries) and each blood component.
• Describe the path: Of blood through the heart (mention valves and oxygenation).
• Explain: Why the left ventricle has a thicker wall than the right (pumps to whole body; higher pressure).

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Common mistakes to avoid

• Confusing pulmonary artery (deoxygenated blood to lungs) with aorta (oxygenated blood to body).
• Saying “arteries carry oxygenated blood” (pulmonary artery carries deoxygenated blood).
• Not stating that haemoglobin (in red blood cells) carries oxygen.
• Saying veins always carry deoxygenated blood (the pulmonary vein carries oxygenated blood from the lungs).

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Revision checklist

• Label a diagram of the heart and name the four chambers and main vessels.
• Compare arteries, veins, and capillaries (structure and function).
• State the function of red blood cells, white blood cells, platelets, and plasma.
• Describe double circulation and the route of blood through the heart and lungs.

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Next steps

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