Describe the double circulatory system in humans, explaining why having two separate circuits is important for efficient transport of substances to body tissues.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The human heart pumps blood around two separate circuits simultaneously. One circuit supplies the lungs and the other supplies the rest of the body.
Model answer (5 marks)
The heart has two separate circuits. The first, pulmonary circulation, carries de‑oxygenated blood from the right ventricle to the lungs where it is oxygenated and then returns to the left atrium. The second, systemic circulation, carries oxygen‑rich blood from the left ventricle to the rest of the body and returns de‑oxygenated blood to the right atrium.
Because the two circuits are separate, the blood that reaches the body has already been fully oxygenated in the lungs. It also returns to the heart between circuits, so it is re‑pressurised by the left ventricle before it is pumped to the tissues. This high pressure and full oxygenation mean that oxygen and other substances are delivered to body tissues efficiently.
Because the two circuits are separate, the blood that reaches the body has already been fully oxygenated in the lungs. It also returns to the heart between circuits, so it is re‑pressurised by the left ventricle before it is pumped to the tissues. This high pressure and full oxygenation mean that oxygen and other substances are delivered to body tissues efficiently.
Examiner tips
- Use the terms pulmonary and systemic circulation; mention oxygenation in the lungs and re‑pressurisation by the left ventricle.
- Show the flow: right ventricle → lungs → left atrium → left ventricle → body → right atrium.
- Explain the benefit: fully oxygenated, high‑pressure blood reaches tissues, improving transport efficiency.
Common mistakes
- Confusing the direction of blood flow or mixing up which ventricle pumps to which circuit.
- Failing to mention that the blood is re‑pressurised before systemic circulation.
- Using vague terms like ‘two loops’ without naming pulmonary and systemic.
Mark scheme (5 marks)
- One circuit carries blood from the heart to the lungs (pulmonary circulation) and back to the heart
- The second circuit carries blood from the heart to the rest of the body (systemic circulation) and back to the heart
- Blood is oxygenated in the lungs / oxygen is picked up at the lungs before being pumped to the body
- Because blood returns to the heart between circuits, it is repressurised / pumped at high pressure to body tissues
- This means fully oxygenated blood reaches body tissues, making transport of oxygen (and other substances) more efficient
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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