Explain why the human circulatory system is described as a 'double circulatory system' and explain the advantage this arrangement gives over a single circulatory system.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The human heart pumps blood around the body continuously. Blood passes through the heart twice on each complete circuit of the body.
Model answer (5 marks)
The circulatory system is called a "double" system because blood passes through the heart twice in one complete circuit of the body.
1. One circuit is the pulmonary circulation: blood is pumped from the heart to the lungs and back to the heart.
2. The other circuit is the systemic circulation: blood is pumped from the heart to the rest of the body and back to the heart.
The advantage of this arrangement is that the heart re‑pressurises the blood after it leaves the lungs. Oxygenated blood therefore reaches the body tissues at a higher pressure, allowing oxygen and nutrients to be delivered more efficiently and quickly to cells.
1. One circuit is the pulmonary circulation: blood is pumped from the heart to the lungs and back to the heart.
2. The other circuit is the systemic circulation: blood is pumped from the heart to the rest of the body and back to the heart.
The advantage of this arrangement is that the heart re‑pressurises the blood after it leaves the lungs. Oxygenated blood therefore reaches the body tissues at a higher pressure, allowing oxygen and nutrients to be delivered more efficiently and quickly to cells.
Examiner tips
- Use the exact phrase "double circulatory system" and state the two circuits; mention re‑pressurisation and higher pressure for systemic flow; keep answer concise to fit 5 marks.
Common mistakes
- Confusing the order of the circuits or omitting the pulmonary circuit; not mentioning the re‑pressurisation advantage; using vague terms like "better" instead of "higher pressure".
Mark scheme (5 marks)
- The circulatory system is called 'double' because blood passes through the heart twice per complete circuit of the body
- One circuit carries blood from the heart to the lungs and back (pulmonary circulation)
- The other circuit carries blood from the heart to the rest of the body and back (systemic circulation)
- The advantage is that blood is repressurised/pumped again by the heart after the lungs before being sent to the body
- This means oxygenated blood reaches the body tissues at higher pressure, so oxygen and nutrients are delivered more efficiently/quickly to cells
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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