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The circulatory system in mammals transports essential chemical substances to and from all of the cells in our bodies. The heart pumps blood along a network of blood vessels to reach our cells, while the lungs replace the oxygen used during respiration.

The circulatory system

Humans - and all other mammals - have a double circulatory system which comprises two separate circuits.

Blood pumps through the heart twice during a complete journey around the body. Some blood is sent to the lungs and some blood is sent to the rest of the body each time the heart beats.

The blood in each circuit is kept separate. This is called double circulation and is a more efficient way of delivering oxygen to the tissues than single circulation.

The double circulatory system in humans

Arteries, veins and capillaries

Arteries are blood vessels that carry blood away from the heart. (It’s easy to remember this – ‘arteries away’.) Because arteries have to withstand the high pressure of the blood during each heart beat, they have thick, elastic walls which substances cannot pass through

Veins carry blood towards the heart. Veins have thinner walls which are less elastic, because the blood they transport is under lower pressure. They still do not allow substances to pass through their walls. Veins contain one-way valves which prevent blood flowing backwards.

Capillaries are very thin blood vessels which have walls that are only one cell thick. This means that substances such as glucose, oxygen, water and carbon dioxide can diffuse through between the blood and the tissues.

An investigation into the effect of a high-sugar meal on blood sugar levels

How does the circulatory system work?

The left side of the heart pumps oxygenated blood from the lungs, through the aorta to the body tissues (excluding the lungs).

Deoxygenated blood returns from the body tissues to the right side of the heart, via the vena cava.

It is then pumped to the lungs, via the pulmonary artery where it is oxygenated.

Blood returns to the left side of the heart, via the pulmonary vein and is pumped to the body tissues again.

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