blood A fluid body tissue that acts as a transport medium within an animal. It is contained within a blood vascular system and in vertebrates is circulated by means of contractions of the heart. Oxygen and food are carried to tissues, and carbon dioxide and chemical (nitrogenous) waste are transported from tissues to excretory organs for disposal (excretion). In addition blood carries hormones and also acts as a defence system. Blood consists of a liquid (see blood plasma) containing blood cells (see erythrocyte; leucocyte) and platelets (see illustration).

blood Various blood cells suspended in plasma; carries nutrients and oxygen to tissues, and removes metabolic products. Oxygenated blood travels from the heart in arteries, while deoxygenated blood returns in veins; in the tissues the blood from the arteries enters smaller vessels, the arterioles, then capillaries, which then drain into venules, then the veins. See also blood plasma.