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Human body


The heart, with parts cut away so we can see insideThe heart, with parts cut away so we can see inside All of the body's cells, tissues and organs must be continuously supplied with food and oxygen in order to stay alive. This job is carried out by the circulatory system. The heart lies at the centre of the circulatory system and pumps blood around the body. About the size of your fist, it is an incredibly strong organ, made almost entirely of muscle. It beats more than two and a half billion times during the average life span of a person and pumps about 340 litres (75 gallons) of blood every hour—enough to fill a car’s petrol tank every seven minutes. The heart has four chambers (left and right atria, left and right ventricles). Flaps, called valves, slam shut to prevent blood leaking back once it has entered each chamber. This closing of valves produces the heartbeat that can be heard in a person’s chest. 

Diagram of blood circulationDiagram of blood circulation

What the heart does

Blood rich in oxygen travels from the lungs to the left side of the heart through the pulmonary veins (1). At the same time, blood low in oxygen from the body’s tissues returns to the right side of the heart through large veins called the inferior (lower) and superior (upper) vena cava (2). The left side of the heart then pumps the oxygen-rich blood to body tissues along a large artery, the aorta (3). At the same time, the right side of the heart pumps oxygen-poor blood to the lungs along the pulmonary arteries (4) to take in oxygen and to get rid of waste carbon dioxide

The heart pumps blood at high pressure along the arteries. You can feel this pumping action, or pulse, by pressing two fingers on the thumb side of your wrist where a main artery lies just under your skin.


The first heartbeat happens in the fourth week of pregnancy.

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