The general pattern of blood circulation in a mammal.

The idea

Mammals have a double circulatory system: blood passes through the heart twice for every complete circuit of the body. In the pulmonary circuit, deoxygenated blood leaves the right ventricle via the pulmonary artery to the lungs and returns, oxygenated, via the pulmonary vein to the left atrium. In the systemic circuit, oxygenated blood leaves the left ventricle via the aorta to the body and returns, deoxygenated, via the vena cava to the right atrium. Blood flows through these circuits in series. Because blood returns to the heart after being oxygenated, its pressure is restored before it is pumped to the body. This ensures blood reaches respiring tissues quickly, supporting a mammal's high metabolic rate. The septum keeps oxygenated and deoxygenated blood separate.

What earns the marks

• A mammal has four chambers: two atria and two ventricles.
• Blood passes through the heart twice for every complete circuit of the body, defining the double circulation.
• Blood returns to the heart after being oxygenated, restoring its pressure before it is pumped to the systemic circuit.
• The right side of the heart pumps deoxygenated blood to the lungs via the pulmonary artery, while the left side pumps oxygenated blood to the body via the aorta.
• Oxygenated and deoxygenated blood are kept separate by the septum.
• Name the major vessels in sequence: vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body.

Common mistakes and how to put them right

• Writing double circulation without saying that blood passes through the heart twice per circuit.

  Put it right: The definition needs the twice-per-circuit idea.

• Confusing the simultaneous contraction of the ventricles with the overall flow of blood.

  Put it right: Blood flows through the pulmonary and systemic circuits in series (one after the other), even though the two ventricles pump simultaneously.

• Assuming the pulmonary artery carries oxygenated blood because arteries usually do.

  Put it right: The pulmonary artery carries deoxygenated blood to the lungs.

• Omitting the names of major vessels when tracing the circulation.

  Put it right: Include vena cava, pulmonary artery, pulmonary vein and aorta in the correct sequence.

Exam tips

• In a comparison table, every row must state both sides of the same difference; unlinked statements score nothing.
• The pressure mark is about blood reaching the body capillaries, so explicitly state that pressure is restored at the heart before entering the systemic circuit.
• When asked to trace the circulation, name the chambers and major vessels in order: vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body.