Circulation and the heart

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The blood circulation system

the blood vessels (tubes that carry blood around your body)

the heart (pumps blood around your body)

the blood (liquid that carries blood around your body)

Human beings have a double circulation system. One carries blood from your heart to your lungs and back and the other carries blood to all the other organs of your body and back.

The circulatory system and the heart

oxygenated

oxygenated

deoxygenated

Blood enters the top chambers of your heart (the atria). The blood coming into the right atrium from the vena cava is deoxygenated blood from your body. The blood coming into the left atrium in the pulmonary vein is oxygenated blood from your lungs. The atria contract and force blood down into the ventricles. Valves close to stop the blood flowing backwards out of the heart.

Blood vessels

Your arteries

carry blood away from your heart to the organs of your body. This blood is

usually bright-red oxygenated blood. The arteries stretch as the blood is

forced through them and go back into shape afterwards. You can feel this as a

pulse where the arteries run close to the skin’s surface (for example, at your

wrist). Arteries have thick walls containing muscle and elastic fibres. As the

blood in the arteries is under pressure, it is very dangerous if an artery is

cut, because the blood will spurt our rapidly every time the heart beats.

The veins

carry blood from the organs towards your heart. This blood is usually low in

oxygen and so is a deep purple-red colour. Veins do not have a pulse. They have

much thinner walls than arteries and often have valves to prevent the backflow

of blood as it returns to the heart.

Throughout the body, capillaries form a huge network of tiny vessels linking the

arteries and the veins. Capillaries are narrow with very thin walls. This

enables substances, such as oxygen and glucose, to easily diffuse out of your

blood and into your cells. The substances produced by your cells, such as

carbon dioxide, pass easily into the blood through the walls of the

capillaries.

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