Blood

Blood, thick red fluid pumped by the heart and flowing throughout the body in the blood vessels of the circulatory system. The blood serves many functions in the body, but principally it carries nutrients to and waste away from individual cells and helps regulate the body's metabolism. It carries oxygen from the lungs to the tissues, and carbon dioxide from the tissues to the lungs. It transports hormones to tissues that need them, carries nutrients absorbed from the intestine, and bears away the waste products of metabolism to the organs of excretion—the lungs, the kidneys, the intestines, and the skin. Blood also defends the body against infection, and the clotting mechanism minimizes the loss of blood after an injury.

Seen with the naked eye, the blood appears opaque and homogeneous, but upon microscopic examination it is seen to consist of cells. The most numerous are red corpuscles (erythrocytes), which normally outnumber white cells (leucocytes) by 500 to 1. Also present in the blood are minute circular bodies known as platelets, or thrombocytes, necessary for clotting. The blood cells subsist in an intercellular liquid called plasma. The volume of cells and plasma is approximately equal. Blood plasma itself is a complex fluid; 90% of it is water, but the balance consists of proteins, electrolytes, other minerals, and nutrients needed by the body's cells.

Common blood disorders are leukemia (excess white blood cells or leucocytes) and anemia (a lack of red blood cells). For medical purposes, principally transfusion, blood is usually categorized into one of 4 groups, A, B, AB, and O.

See also: Blood transfusion; Blood type; Circulatory system.