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Blood is the fluid which circulates through the heart, arteries, veins and capillaries. It is composed of red blood cells, white blood cells and blood platelets, and an interstitial fluid called plasma. It derives its reddish color from the iron within the hemoglobin.

Blood functions to provide nutrition and respiration for tissues located far from food and air supplies. It also transports waste from the tissues to the excretory organs. Blood provides chemical and thermal regulation to the body and helps in preventing infection by transporting antibodies.

Van Amerongen, C. The Way Things Work; Book Of The Body. New York: Simon and Schuster, 1979.