Blood varies in color from a bright red in the arteries to a duller red in the veins. The total quantity of blood within an individual depends upon his body weight. A person who weighs 150 lb. has about 5 quarts of blood in his body. The plasma accounts for about 55 per cent of the total volume of the blood. Plasma from which the fibrinogen has been removed is called serum.
Plasma contains many specialized proteins such as serum albumin, the globulins, and fibrinogen.
Serum albumin is important to the nutrition of body cells and helps prevent the escape of body water from the intravascular fluid compartment. It probably originates in the liver, as does fibrinogen, which is essential to the normal clotting of blood. The globulins are divided into three major groups. The suspended particles of the blood constitute the other 45 per cent of the total volume of blood. The great majority of the cells in the blood are red blood cells.