Blood

One function of the circulation system is to transport substances around the body in the blood.

Blood consists of a fluid called plasma containing white blood cells, red blood cells and platelets.

Many substances are transported in their dissolved form in the plasma.  These include:

• Carbon dioxide from the organs to the lungs
• Soluble products of digestion from the small intestine to other organs
• Urea from the liver to the kidneys

White blood cells form part of the body’s defence system against micro-organisms.  They have a nucleus.

Platelets are fragments of cells.  Platelets help blood to clot at the site of a wound.  They do not have a nucleus.

Red blood cells transport oxygen from the lungs to the organs.  They do not have a nucleus.  They are packed with a red pigment called haemoglobin.  In the lungs, oxygen combines with haemoglobin to form oxyhaemoglobin.  In other organs the oxygen is released as the oxyhaemoglobin splits up into oxygen and haemoglobin.

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The diagram below shows the structure of the heart.

Blood returns to the heart from the body in veins.  It enters the right atrium, which contracts, forcing the blood into the right ventricle. 

When this contracts, it forces blood out of the heart through the pulmonary artery to the lungs, where carbon dioxide is removed and oxygen is absorbed.

Blood from the lungs enters the left atrium of the heart and is forced into the left ventricle.  This chamber has a very thick muscular wall to push blood out of the heart into the main artery, the aorta, and around the body. 

There are valves in the heart to make sure that blood flows in the correct direction.

The heart is said to be a double pump, because the right side of the heart pumps blood to the lungs, and the left side pumps it around the body.  This is called double circulation.

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Arteries have thick walls containing elastic fibres, so they can stretch as blood is forced through, and muscle to pull them back into shape.

Veins have thinner walls because the pressure inside them is much lower than in the arteries.  They often have valves to prevent the back flow of blood.

Capillaries join arteries to veins.  They are very narrow, thin-walled blood vessels.  Substances, like oxygen and food needed by the cells in body tissues, pass out of the blood through the capillary walls.  Substances produced by the cells, for example, carbon dioxide, diffuse in the opposite direction.

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