Explain why the human circulatory system is described as a 'double circulatory system' and give two reasons why this type of system is an advantage to mammals.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Mammals such as humans have a double circulatory system, whereas some other animals have a single circulatory system in which blood passes through the heart only once per circuit.
Model answer (5 marks)
A double circulatory system means that 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, where it is oxygenated, and returns to the heart.
2. The other circuit is the systemic circulation: oxygenated blood is pumped from the heart to the rest of the body and returns deoxygenated.
Advantages for mammals:
• The separation of the two circuits allows the heart to maintain a high blood pressure, so oxygen and nutrients are delivered to tissues more quickly and efficiently.
• Oxygenated and deoxygenated blood are kept separate by the septum, ensuring that fully oxygenated blood reaches the body's cells.
Mammals are large organisms with a high demand for oxygen; a double system meets this demand.
1. One circuit is the pulmonary circulation: blood is pumped from the heart to the lungs, where it is oxygenated, and returns to the heart.
2. The other circuit is the systemic circulation: oxygenated blood is pumped from the heart to the rest of the body and returns deoxygenated.
Advantages for mammals:
• The separation of the two circuits allows the heart to maintain a high blood pressure, so oxygen and nutrients are delivered to tissues more quickly and efficiently.
• Oxygenated and deoxygenated blood are kept separate by the septum, ensuring that fully oxygenated blood reaches the body's cells.
Mammals are large organisms with a high demand for oxygen; a double system meets this demand.
Examiner tips
- Use the phrase "double circulatory system" to show you know the definition. Show the two circuits explicitly. Mention high pressure and separation of blood as the two advantages. Link the advantage to the large size and oxygen demand of mammals.
Common mistakes
- Confusing a single circulatory system with a double one. Failing to mention both pulmonary and systemic circuits. Not linking the advantages to the needs of mammals.
Mark scheme (5 marks)
- Blood passes through the heart twice in one full circuit of the body
- One circuit goes from the heart to the lungs (pulmonary circulation) and a separate circuit goes from the heart to the rest of the body (systemic circulation)
- Advantage 1: The system can maintain a high blood pressure, so oxygen and nutrients are delivered to tissues more quickly / efficiently
- Advantage 2: Oxygenated and deoxygenated blood are kept separate (by the septum), so fully oxygenated blood reaches the body's cells
- Mammals are large organisms with a greater need for oxygen (for respiration), so a double circulatory system meets this higher oxygen demand
Key terms in this question
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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