Explain why the blood leaving the left ventricle must be maintained at a higher pressure than the blood leaving the right ventricle.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The heart pumps blood into two separate circulatory loops simultaneously. The left and right ventricles contract at the same time, yet the pressure they generate differs significantly.
Model answer (4 marks)
The left ventricle pumps blood into the systemic circulation, which must travel a much longer distance to reach peripheral tissues. This longer circuit has higher vascular resistance, so a higher pressure is required to overcome it and maintain adequate flow.
The right ventricle pumps into the pulmonary circuit, which is short and close to the heart. The pulmonary vessels have low resistance, so a lower pressure suffices.
If the pulmonary circulation were exposed to high pressure, the fragile capillaries in the alveoli would be damaged, impairing gas exchange.
The right ventricle pumps into the pulmonary circuit, which is short and close to the heart. The pulmonary vessels have low resistance, so a lower pressure suffices.
If the pulmonary circulation were exposed to high pressure, the fragile capillaries in the alveoli would be damaged, impairing gas exchange.
Examiner tips
- Use the terms ‘systemic’ and ‘pulmonary’ circulation to show understanding of the two loops.
- Explain that resistance is higher in the longer systemic circuit, requiring higher pressure.
- Mention the risk of damaging alveolar capillaries if pulmonary pressure is too high.
- Keep the answer concise – 4 marks can be earned with two short points.
Common mistakes
- Confusing the direction of blood flow or the ventricle that pumps to each circuit.
- Failing to mention the risk of alveolar damage from high pulmonary pressure.
- Using vague language such as ‘more pressure’ without linking to resistance or distance.
Mark scheme (4 marks)
- The left ventricle pumps blood to the body (systemic circulation), which is a much greater distance than the pulmonary circulation
- Greater pressure is needed to overcome the higher resistance of the longer systemic circuit
- The right ventricle only needs to pump blood to the lungs (pulmonary circulation), which are close to the heart
- High pressure in the pulmonary circulation would damage the delicate capillaries of the alveoli, impairing gas exchange
Key terms in this question
left ventricle · right ventricle
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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