Biology editorial review is supported by the
named review panel.
Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
In double circulation, blood passes through the heart twice per complete circuit: once through the pulmonary circuit (heart to lungs and back) and once through the systemic circuit (heart to body and back). This maintains high blood pressure to the body, delivering oxygen more efficiently than single circulation.
Double circulation is a short but high-value topic. The key exam point is the advantage: oxygenated blood returns to the heart before being pumped to the body, so it reaches tissues at a higher pressure than in fish-style single circulation.
The two circuits
The pulmonary circuit carries deoxygenated blood from the right ventricle to the lungs via the pulmonary artery. In the lungs, carbon dioxide is released and oxygen is picked up. Oxygenated blood returns to the left atrium via the pulmonary vein.
The systemic circuit carries oxygenated blood from the left ventricle to the body via the aorta. Oxygen and nutrients are delivered to tissues, and carbon dioxide and waste are collected. Deoxygenated blood returns to the right atrium via the vena cava.
Why double circulation is an advantage
In a single circulation (as in fish), blood passes through the heart only once per circuit. After picking up oxygen in the gills, blood flows directly to the body at reduced pressure (because resistance in the gill capillaries slows it). This limits how quickly oxygen can be delivered to active tissues.
In double circulation, oxygenated blood returns to the heart and is pumped a second time to the body. The left ventricle generates high pressure, so blood reaches body tissues quickly and at sufficient pressure to maintain rapid delivery of oxygen and glucose to active cells. This supports the higher metabolic rates of mammals.
Separation of oxygenated and deoxygenated blood
The septum (wall) between the left and right sides of the heart ensures that oxygenated and deoxygenated blood do not mix. The left side handles oxygenated blood; the right side handles deoxygenated blood. This separation maximises the oxygen concentration of blood reaching body tissues.
If the septum has a hole (a septal defect), some mixing occurs, reducing the efficiency of oxygen delivery. This is a congenital heart condition that may require surgical repair.
Try a focused question (3 marks)
Explain the advantage of a double circulation compared to a single circulation.
Show the answer and marking guidance
In double circulation, oxygenated blood returns to the heart from the lungs before going to the body.
The heart pumps it a second time, generating higher pressure for the systemic circuit.
Higher blood pressure delivers oxygen to body tissues more efficiently, supporting a higher metabolic rate.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying double circulation means blood goes around twice: it passes through the heart twice per circuit.
Confusing pulmonary and systemic circuits.
Forgetting to explain why higher pressure is beneficial (faster delivery of oxygen to tissues).
Not comparing to single circulation when the question asks for an advantage.