Double circulation: explain it, don't just name it
Humans have a double circulatory system: blood passes through the heart twice for each complete circuit of the body. One circuit (pulmonary) carries blood between the heart and the lungs; the other (systemic) carries blood between the heart and the rest of the body.
The exam mark is explaining the advantage: blood pressure is boosted again after passing through the lungs, so blood travels to the body at higher pressure and faster, delivering oxygen and glucose more efficiently. Contrast this with a single circulation (e.g. fish) where pressure drops after the gills. A common error is describing the route without stating the advantage of higher pressure. Use 'oxygenated' and 'deoxygenated' carefully: the right side of the heart handles deoxygenated blood to the lungs; the left side handles oxygenated blood to the body.
Heart structure: the side that trips people up
Know the four chambers and which blood each holds:
- Right atrium → right ventricle → pulmonary artery → lungs (deoxygenated blood).
- Lungs → pulmonary vein → left atrium → left ventricle → aorta → body (oxygenated blood).
Two high-value marks: (1) the left ventricle wall is thicker/more muscular than the right because it must pump blood at higher pressure all the way around the whole body, whereas the right ventricle only pumps to the nearby lungs. (2) Valves (atrioventricular and semilunar) prevent the backflow of blood, ensuring one-way flow. The single most common slip is labelling the left and right sides as you look at the diagram. The heart's left is on the diagram's right. Remember the pulmonary artery is the one artery carrying deoxygenated blood, and the pulmonary vein the one vein carrying oxygenated blood.
Blood vessels: the comparison that scores
Compare arteries, veins and capillaries as matched features, each linked to function:
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Wall | Thick, muscular, elastic | Thinner | One cell thick |
| Lumen | Narrow | Wide | Very narrow |
| Valves | No | Yes (prevent backflow) | No |
| Blood pressure | High | Low | Falling |
Link each to function: arteries have thick elastic walls to withstand high pressure from the heart; veins have valves because blood is at low pressure and could flow backwards; capillaries have one-cell-thick walls and narrow lumen giving a short diffusion distance for exchange of substances with tissues. State that arteries carry blood away from the heart and veins carry it towards the heart. Defined by direction, not oxygen content (remember the pulmonary exceptions).
Blood components: functions, not just names
Blood has four parts; examiners want function, not just identification:
- Red blood cells: contain haemoglobin which binds oxygen to form oxyhaemoglobin in the lungs and releases it in respiring tissues; biconcave and no nucleus for more haemoglobin and surface area.
- White blood cells: defence: phagocytes engulf pathogens; lymphocytes produce antibodies.
- Platelets: involved in blood clotting, sealing wounds and preventing pathogen entry.
- Plasma: liquid that transports blood cells, nutrients, carbon dioxide, urea, hormones and heat.
The phrase 'haemoglobin combines with oxygen to form oxyhaemoglobin' is a reliable mark, as is naming what plasma transports. Iron's role in making haemoglobin links to human nutrition. A common loss is saying white blood cells 'kill germs' without specifying engulfing (phagocytosis) or antibody production.
Coronary heart disease and exercise (application marks)
(Extended only and Core application) Coronary heart disease (CHD) is caused by the build-up of fatty material (containing cholesterol) in the coronary arteries, narrowing them and reducing blood/oxygen supply to the heart muscle. The exam asks for risk factors (diet high in saturated fat, smoking, lack of exercise, stress, obesity, genetic factors) and prevention (healthy diet, regular exercise, not smoking).
For exercise questions, explain that heart rate increases to deliver more oxygen and glucose to muscles for increased respiration, and to remove carbon dioxide faster. The mark-scheme link is 'more oxygen/glucose for respiration to release energy for muscle contraction'. Avoid saying exercise 'gives the muscles more energy' directly. Energy is released by respiration. To practise these applied answers with a specialist, take a free trial class.