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Heart and circulation

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The blood and blood clotting

If you cut yourself you bleed. Your blood looks red but it is really a yellow liquid with lots of red cells floating in it! Your blood is the transport medium for the circulatory system.


The structure of the blood

The components of the blood


Plasma is a yellow liquid which transports all of the blood cells around your body. It carries waste carbon dioxide from the cells of the body to the lungs to be removed. It takes urea from the liver where it is made to the kidneys to be excreted in the urine. It carries the small dissolved food molecules from the gut to the cells where they are needed.

Red blood cells are small biconcave discs. They contain the red pigment haemoglobin which picks up oxygen in your lungs and carries it to the body cells where it is needed. Mature red blood cells don't have a nucleus so they can pack in more haemoglobin and carry more oxygen.


Oxygen is taken up in the lungs where it is at a relatively high concentration,
and released into cells where it is at a relatively low concentration.

White blood cells are bigger than red blood cells but there are fewer of them. They are an important part of the way your body defends itself against disease. There are lots of different types of white blood cells. Some of them make antibodies against disease-causing organisms. Some of them engulf bacteria and digest them. They all have a nucleus. The diagram opposite shows different types of white blood cells.

Platelets are small fragments of cells that have no nucleus. They are very important in the successful clotting of your blood. If you cut yourself it is very important that the blood forms a solid clot as fast as possible to prevent you from bleeding to death. The clotting of your blood involves a series of enzyme-controlled reactions which produce a net of fibres made of a protein called fibrin. These fibres then trap lots of red blood cells and platelets and form a jelly like clot which dries to form a scab. The skin heals under the scab.


A blood clot forming



Healing of a wound