Blood has a number of cell types in order to provide the different functions of blood. As well as carrying oxygen to the tissues, blood is responsible for the body's defence mechanism, and for clotting in response to injury.
The red blood cells, derived from the bone marrow, are highly specialised. During the course of maturation they lose their nucleus, so most of the internal space can be packed with the oxygen carrying protein - hæmoglobin. In addition, the cells have a characteristic biconcave shape: this ensures a large surface area for diffusion. Also, the distance the oxygen has to diffuse from the plasma to anywhere inside the cell is small. Erythrocytes also contain substantial amounts of the enzyme carbonic anhydrase. This is important in carbon dioxide transport from the tissues to the lungs: it speeds up the conversion of carbon dioxide to carbonic acid in the cells (this then dissociates into hydrogen ions - taken up by the hæmoglobin - and bicarbonate).
The white blood cells are vital in preventing and controlling infection.
This is a diverse group of cell types, each responsible for a different
aspect of fighting and preventing infection and tissue damage. They are
usually classified according to their morphological characteristics.