The plasma is a warm liquid. It picks up much of its heat as it passes through the liver. A great many processes take place in the liver and each one produces heat. The plasma carries the heat to all parts of the body to keep them warm. The body must keep to a steady 37°C if it is to stay healthy.

The red blood cells are doughnut-shaped. Each one is packed with haemoglobin. This is the substance that makes the cell look red. Haemoglobin picks up oxygen when the blood passes through the lungs. It holds onto the oxygen until it reaches cells which have used up their oxygen supply. The haemoglobin simply passes the oxygen to the cell and when the cell returns to the lungs it picks up some more oxygen.

White blood cells feed on any particles in the blood which do not belong to the body. These particles are germs. When a white blood cell attacks, it encloses the victim inside itself then digests it.

Plasma is a watery solution that contains many dissolved substances. It contains digested food on its way to the cells. It also contains carbon dioxide on its way back to the lungs.

The blood also contains platelets. They help the blood to clot. Platelets have gather at wounds and help to make a net of fibres to hold in the blood cells. White cells also gather at a wound to digest the hundreds of germs that are teeming into the wound from the skin. As the white cells attack and eat the germs they die and their bodies pile up at the wound to form pus. In time the wound will heal and the dried pus will fall away.