It transports nutritive substances and metabolites to the tissues and removes waste products to the kidneys and other organs of excretion. It has an essential role in the maintenance of fluid balance. In an emergency, blood cells and antibodies carried in the blood are brought to a point of infection, or blood-clotting substances are carried to a break in a blood vessel. The blood distributes hormones from the endocrine glands to the organs they influence. It also helps regulate body temperature by carrying excess heat from the interior of the body to the surface layers of the skin, where the heat is dissipated to the surrounding air.
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.