A patient is diagnosed with a faulty valve in their heart. Explain how the double circulatory system normally works and why a faulty valve would affect the delivery of oxygen to the body's cells.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The human heart pumps blood continuously around the body. It contains valves at each end of both ventricles, and blood travels through two separate circuits before completing a full body circuit.
Model answer (5 marks)
The double circulatory system means that blood passes through the heart twice during one full body circuit. The pulmonary circuit carries de‑oxygenated blood from the right ventricle to the lungs via the pulmonary artery and returns oxygenated blood to the left ventricle via the pulmonary vein. The systemic circuit carries oxygenated blood from the left ventricle to the body through the aorta and returns de‑oxygenated blood to the right ventricle via the venae cavae. Valves at the atrioventricular and semilunar positions prevent back‑flow, ensuring that blood moves in the correct direction through the heart. A faulty valve allows blood to flow backwards or reduces the efficiency of pumping, so less oxygenated blood reaches the body’s cells.
Examiner tips
- Use the exact terms ‘pulmonary circuit’, ‘systemic circuit’, ‘pulmonary artery/vein’, ‘aorta’, and ‘valves’.
- Explain the direction of flow for each circuit before describing the valve function.
- Show how valve failure leads to reduced oxygen delivery.
- Keep the answer concise – 5 marks, so one or two sentences per point.
Common mistakes
- Mixing up the directions of the pulmonary and systemic circuits.
- Omitting the role of valves in preventing back‑flow.
- Using vague language such as ‘blood flow’ instead of ‘oxygenated/de‑oxygenated’.
Mark scheme (5 marks)
- The double circulatory system means blood passes through the heart twice in a full body circuit
- The pulmonary circuit carries deoxygenated blood from the heart to the lungs (via the pulmonary artery) and returns oxygenated blood to the heart (via the pulmonary vein)
- The systemic circuit carries oxygenated blood from the heart to the body via the aorta
- Valves prevent the backflow of blood, ensuring blood flows in the correct direction through the heart
- A faulty valve allows blood to flow backwards / reduces the efficiency of pumping, so less oxygenated blood reaches the body's cells
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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