IGCSE Biology / Blood and circulation

Double circulation revision notes

Edexcel International GCSE Biology (4BI1), chapter 5: Blood and circulation, specification point 2.69. Free lesson.

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In double circulation, blood passes through the heart twice during one complete journey around the body. This lets the heart restore blood pressure between the lungs and the body, so oxygen reaches tissues quickly.

The pulmonary and systemic circuits

There are two connected loops, with the heart between them. The pulmonary circuit carries blood between the heart and lungs. The systemic circuit carries blood between the heart and the rest of the body. The right side of the heart pumps deoxygenated blood to the lungs through the pulmonary artery. Blood picks up oxygen in the lungs and returns to the left side of the heart through the pulmonary vein. The left side pumps oxygenated blood out through the aorta, the largest artery, to the body. Deoxygenated blood returns from the body to the right side of the heart…

Other vessels to know

Some named vessels carry blood to or from particular organs. Their names tell you where blood is travelling, so learn the route as well as the vessel name. The hepatic portal vein carries food-rich blood from the gut to the liver. It is unusual because it does not carry blood towards the heart. Blood leaves the liver through the hepatic vein, which joins the vena cava. The renal artery and renal vein carry blood to and from the kidneys.

How one full circuit passes through the heart

Follow one red blood cell around both loops. It passes through the heart once after travelling from the lungs, then again after travelling around the body. That makes two heart passes for one complete circuit.

Why double circulation is an advantage

Blood pressure falls as blood passes through the lung capillaries. The heart pumps the blood again before it travels to the body, so its pressure is restored. This means blood reaches body tissues quickly and delivers oxygen fast enough for a warm, active animal.

How to tell arteries and veins apart

The names artery and vein describe the direction blood travels, not how much oxygen it carries. The pulmonary vessels are often tested because they do not fit the usual oxygen pattern. Arteries carry blood away from the heart. Veins carry blood towards the heart. The pulmonary artery carries deoxygenated blood, while the pulmonary vein carries oxygenated blood.