Human Circulation

IGCSE Edexcel Biology
2.59–2.69 Blood, the immune system, the heart and the circulatory system
Key Concepts: Blood has four components: red blood cells (carry O₂), white blood cells (immune defence), platelets (clotting), and plasma (transport). The heart pumps blood around a double circulation. Arteries carry blood away from the heart at high pressure; veins return it; capillaries allow exchange with tissues.

Section A — Blood Components

1. State the function of each component of blood. [8]
ComponentFunction(s)
Red blood cells
White blood cells (phagocytes)
White blood cells (lymphocytes)
Platelets
Plasma
2. Explain three adaptations of red blood cells for their function. [6]

Section B — The Immune Response

3. Describe how phagocytes destroy pathogens. [3]
4. Explain the role of lymphocytes in the immune response. Include the terms antigen, antibody and memory cells. [4]

Section C — The Heart

5. State the function of each structure of the heart. [6]
StructureFunction
Left ventricle
Right ventricle
Atrioventricular valves
Coronary arteries
Aorta
Pulmonary artery
6. Explain why the left ventricle has a thicker muscular wall than the right ventricle. [2]
7. State two factors that increase heart rate and explain the physiological benefit of this. [3]
8. State two risk factors for coronary heart disease. [2]

Section D — Blood Vessels and Circulation

9. Compare the structure of arteries, veins and capillaries. [6]
Feature Artery Vein Capillary
Wall thickness
Has valves?
Blood pressure
10. Explain two features of capillaries that make them well adapted for the exchange of substances with body cells. [4]
11. Describe the double circulation of blood in mammals. State why having two separate circuits is an advantage. [4]

Total marks: 48

Mark Scheme

1. RBC: carry oxygen (using haemoglobin) to cells [2]; Phagocytes: engulf and digest pathogens by phagocytosis [2]; Lymphocytes: produce specific antibodies against pathogens [2]; Platelets: involved in blood clotting to seal wounds [1]; Plasma: transports dissolved substances (glucose, amino acids, CO₂, urea, hormones, heat) [1] [8]
2. Any three: biconcave disc shape — increases surface area for O₂ absorption [2]; no nucleus — more space for haemoglobin [2]; haemoglobin binds O₂ reversibly to form oxyhaemoglobin [2]; flexible cell membrane — can squeeze through narrow capillaries [2] (max 6) [6]
3. Phagocytes move towards pathogens (attracted by chemicals) [1]; engulf them by phagocytosis (surrounding them with their cell membrane) [1]; enzymes in lysosomes digest and destroy the pathogen [1] [3]
4. Lymphocytes detect antigens (foreign proteins) on the surface of pathogens [1]; they produce specific antibodies complementary to the antigens [1]; antibodies bind to pathogens, neutralising them or marking them for destruction [1]; some lymphocytes become memory cells that respond faster to a future infection [1] [4]
5. Left ventricle: pumps oxygenated blood to the entire body (systemic circulation) [1]; Right ventricle: pumps deoxygenated blood to the lungs (pulmonary circulation) [1]; AV valves: prevent backflow from ventricles to atria [1]; Coronary arteries: supply heart muscle with oxygenated blood and glucose [1]; Aorta: carries oxygenated blood from left ventricle to the body [1]; Pulmonary artery: carries deoxygenated blood from right ventricle to the lungs [1] [6]
6. The left ventricle pumps blood around the entire body (systemic circulation) [1]; this requires much greater force/pressure than pumping blood to the nearby lungs only [1] [2]
7. Any two: exercise, stress/adrenaline, high CO₂ levels [1+1]; benefit: delivers more oxygenated blood and glucose to respiring muscles faster, removes CO₂ more quickly [1] [3]
8. Any two: high blood cholesterol, smoking, high blood pressure, obesity, lack of exercise, high fat diet, stress [2]
9. Wall thickness: thick/elastic walls / thin walls / walls one cell thick; Has valves: No / Yes / No; Blood pressure: high / low / very low [6 — 2 per column]
10. Any two: walls only one cell thick — very short diffusion distance for exchange [2]; large total surface area — allows rapid exchange with many cells [2]; slow blood flow — allows more time for exchange [2] (max 4) [4]
11. The pulmonary circulation carries blood from the heart to the lungs for oxygenation and back [1]; the systemic circulation carries oxygenated blood from the heart to the rest of the body and deoxygenated blood back [1]; advantage: fully oxygenated blood reaches body tissues at high pressure [1]; the two circuits are kept separate — no mixing of oxygenated and deoxygenated blood [1] [4]