spleen [Gk, splen] , a soft, highly vascular, roughly ovoid organ situated between the stomach and the diaphragm in the left hypochondriac region of the body. It is considered part of the lymphatic system because it contains localized lymphatic nodules. It is dark purple and varies in shape in different individuals and within the same individual at different times. The precise function of the spleen has baffled physiologists for more than 100 years, but research indicates it performs various tasks, such as defense, hemopoiesis, blood storage, and destruction and recycling of red blood cells and platelets. The spleen also produces leukocytes, monocytes, lymphocytes, and plasma cells in response to an infectious agent. It produces red cells before birth and is believed to produce red cells after birth only in extreme and hemolytic anemia. If the body suffers severe hemorrhage, the spleen can contract and increase the blood volume from 350 mL to 550 mL in less than 60 seconds. In the adult the spleen is usually about 12 cm long, 7 cm wide, and 3 cm thick. Its weight increases from 17 g or less in the first year to about 170 g at 20 years of age, then slowly decreases to about 122 g at 75 to 80 years of age. The variation in the weight of adult spleens is 100 to 250 g and, in extreme cases, 50 to 400 g. The size of the spleen increases during and after digestion and often increases during illness. Compare thymus. −splenic, adj.
