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A liquid which, in man and all the higher animals, circulates through the system, being propelled into the arteries by the heart and received back by that organ through the veins. Histologically considered, b. is a tissue, with solid or semisolid cellular elements and a liquid intercellular substance. In mammalian b., the cellular elements are red blood corpuscles, white blood corpuscles, and the bloodplates. Chemically considered, b. consists principally of proteins, sugar, lecithin, cholesterin, fat, fatty acids, carbonic acid, hydrochloric acid, and phosphoric acid, as well as the inorganic bases, sodium, potassium, and magnesium in combination with one of the above mentioned acids. Ammonium carbonate, urea, uric acid, amino-acids, and other compounds have been found in normal blood. When drawn from the b. vessels, b. tends to form a semisolid mass (clot, coagulum). It consists of a clear liquid, called plasma, in which the corpuscular elements are suspended,