The spleen lies behind the stomach, high up on the left side of the abdomen, and corresponds to the position of the ninth, tenth and 11th ribs, from which it is separated by the diaphragm. It is a soft, highly vascular, plum-coloured organ, and has a smooth surface. It is usually about 12·5–15 cm (5–6 inches) long, and weighs about 170 grams.

The spleen is enveloped by peritoneal membrane around a strong elastic tunic, which allows it to expand and contract according to the amount of blood within it. The spleen contains numerous blood vessels, with venous blood leaving by the splenic vein, which enters the PORTAL VEIN from the liver.

It produces lymphocytes (see LYMPHOCYTE) and acts as a reservoir of red blood cells for use in emergencies. Useless or worn-out red and white blood cells and blood PLATELETS are broken up by this organ. This results in the production of BILIRUBIN, which is conveyed to the liver, and of iron, which is used in the bone marrow for the production of new red blood cells. In the fetus it also manufactures red blood cells. The spleen also has an immunological function, helping to clear bacteria from the blood.