Blood is the fluid in the body that carries oxygen to all the living cells within that body. It is like a transport system and, in addition to oxygen, it carries food substances, hormones and waste matter and helps the body to fight disease. Blood flows around the human body in a network of blood vessels (arteries, veins and capillaries) that total about 100,000km. The human body has about four litres of blood and a single drop contains millions of cells.
Most of these blood cells are red blood cells which contain a protein called haemoglobin, a pigment that gives the red blood cells their colour. The haemoglobin contains iron and is able to make temporary bonds with gas molecules. It combines with the oxygen when the red blood cells travel through the lungs and gives up the oxygen in exchange for carbon dioxide in other parts of the body. When it reaches the lungs again, it releases the carbon dioxide and collects more oxygen.
Blood also contains white blood cells which are larger than red blood cells although not as numerous. Their function is to surround foreign cells such as harmful bacteria and attack intruders, such as viruses, by releasing antibodies. White blood cells also contain platelets, which help the blood to clot. See Circulatory System for details of how blood circulates in humans and other animals.
