BLOOD CELLS
Blood contains many types of cells, which perform quite different functions, from oxygen transport to producing of antibodies. Life cycles of all blood cells are similar to some extent. One part of the life they spend in tissues, and during the other part of the life they freely circulate in the blood currant. Lifetime of all blood cells is limited, and they are produced constantly during the whole life of an animal.
Blood cells can be divided into red cells (erythrocytes) and white cells (leucocytes). Erythrocytes deliver haemoglobin all over the body. Leucocytes fight infection and also devour and digest remains of destroyed cells, etc. Besides, there are thrombocytes in blood. They present not usual cells, but small cell fragments, which were separated from large cells called megacaryocytes. Thrombocytes control blood coagulation and help to restore damaged parts in blood vessel walls.