How the heart pumps blood

Transport in animals · The heart · note 2 of 4

Spec B9.2.5

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How the heart pumps bloodSpec B9.2.5

In short

Atria contract, then ventricles contract to force blood out while valves stop backflow; the left ventricle is thicker as it pumps blood round the body.

The pumping action

Blood flows into the atria from the veins. The atria contract and push blood through the open atrioventricular valves into the ventricles.
The ventricles contract. Pressure rises, the atrioventricular valves shut (stopping blood flowing back into the atria) and the semilunar valves open.
Blood is forced out of the ventricles into the aorta and pulmonary artery.
The ventricles relax. The semilunar valves shut, which stops blood flowing back into the ventricles. The atria fill again.

Three stages of a heartbeat. 1: the atria contract, the atrioventricular valves are open and the semilunar valves are shut, so blood moves into the ventricles. 2: the ventricles contract, the atrioventricular valves are shut and the semilunar valves are open, so blood leaves through the pulmonary artery and aorta. 3: the heart relaxes, the semilunar valves are shut and blood flows into the atria from the vena cava and pulmonary vein. — The valves open and shut because of blood pressure, so blood flows only one way.

Exam tip:

The valves open and shut because of the pressure of the blood, not because the valves can contract. Say which valve and which way the blood is pushed.

Written and checked against the Cambridge IGCSE Combined Science (0653) specification · Updated October 2026