The red fluid that circulates in the body’s veins, arteries, and capillaries. Blood is pumped by the heart via the arteries to the lungs and all other tissues and is then returned to the heart in veins (see circulatory system). Blood is the body’s transport system and plays an important role in the defence against infection. An average adult has about 5 litres of blood. Almost half of the volume of blood consists of blood cells; these include red blood cells (erythrocytes), which carry oxygen to tissues; white blood cells (leukocytes), which fight infection; and platelets (thrombocytes), which are involved in blood clotting. The remainder of the blood volume is a watery, strawcoloured fluid called plasma, which contains dissolved proteins, sugars, fats, salts, and minerals. Nutrients are transported in the blood to the tissues after absorption from the intestinal tract or after release from storage depots such as the liver. Waste products, including urea and bilirubin are carried in the plasma to the kidneys and liver respectively. Plasma proteins include fibrinogen; which is involved in blood clotting; immunoglobulins (also called antibodies) and complement, which are part of the immune system; and albumin. Hormones are also transported in the blood to their target organs.
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