The blood transports life-supporting food and oxygen to every cell of the body and removes their waste products. It also helps to maintain body temperature, transports hormones, and fights infections. The brain cells in particular are very dependent on a constant supply of oxygen. If the circulation to the brain is stopped, death shortly follows.
Blood has two main constituents.
The cells, or corpuscles, comprise about 45 percent, and the liquid portion, or plasma, in which the cells are suspended comprises 55 percent. Each type of cell has its own individual functions in the body. The plasma is a complex colorless solution, about 90 percent water, that carries different ions and molecules including proteins, enzymes, hormones, nutrients, waste materials such as urea, and fibrinogen, the protein that aids in clotting. Because the normal life span of red cells in the circulation is only about 120 days, more than 200 billion cells are normally destroyed each day by the spleen and must be replaced. Red blood cells, as well as most white cells and platelets, are made by the bone marrow.