A marathon runner's heart adapts over years of training, becoming larger and able to pump more blood per beat. Explain how the structure of the cardiovascular system allows oxygenated blood to be delivered to the muscles during a race.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During a marathon, muscles require a greatly increased supply of oxygen. The cardiovascular system must work efficiently to meet this demand. Blood travels through two circuits in a full body cycle before returning to deliver more oxygen.
Model answer (5 marks)
The heart is the pump that drives blood through the body. In a double‑circulatory system the blood passes through the heart twice in one full circuit. First, deoxygenated blood from the body enters the right heart and is pumped through the pulmonary artery to the lungs, where it picks up oxygen. Oxygenated blood then returns via the pulmonary vein to the left heart, which pumps it into the systemic circulation through the aorta. The aorta and its branches are arteries that carry blood away from the heart under high pressure, allowing oxygen‑rich blood to reach the muscles quickly during a marathon.
Examiner tips
- Mention the heart as the pump first; include the double‑circulatory system; describe pulmonary and systemic routes; state arteries carry blood under high pressure; keep answer concise and use correct terminology.
Common mistakes
- Mixing up pulmonary and systemic routes; forgetting that arteries carry blood away from the heart; not mentioning the high pressure of arteries; using vague terms like "blood flow" instead of "circulation".
Mark scheme (5 marks)
- The heart pumps blood around the body / heart is the organ that pumps blood
- The double circulatory system means blood passes through the heart twice in a full body circuit
- The pulmonary circuit carries deoxygenated blood to the lungs via the pulmonary artery, where it picks up oxygen
- Oxygenated blood returns to the heart via the pulmonary vein and is then pumped to the body through the aorta
- Arteries carry blood away from the heart under high pressure, ensuring rapid delivery of oxygenated blood to the muscles
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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