The heart pumps blood, a fluid consisting of red cells, white cells and plasma, around the body. The red cells carry oxygen to the tissues and carbon dioxide from the tissues; the white cells are important in fighting disease and the plasma transports nutrients to the tissues and waste products from the tissues, which are metabolised in the liver or excreted by the kidneys. Blood remains fluid because of a fine balance between factors which cause the blood to clot and factors which cause clots to dissolve. When a small hole appears in the circulation, the body has the ability to repair this: first, by stopping the leak and, secondly, by repairing the hole. Leaks from the circulation are stopped by platelet plugs and the blood’s ability to clot. When clotting does occur, the body has the ability to dissolve clots using an active fibrinolytic system. The body is better able to deal with clots from the venous side of the circulation than the arterial side.