The spleen is a dark purplish gland lying on the left side of the abdomen in the left hypochochondriac region beneath the ninth, tenth and eleventh ribs. It lies against the fundus of the stomach and its outer surface is in contact with the diaphragm. It touches the left kidney, the splenic flexure of the colon, and the tail of the pancreas.
The spleen consists of a structural connective tissue framework in the interstices of which is the splenic pulp formed from lymphoid tissue and numerous blood cells. It is covered by a capsule of collagenous and elastic tissue and a few smooth muscle fibres. These latter have little if any functional role in the human spleen. From the capsule processes called trabeculae pass into the substance of the gland breaking it up into compartments.
The splenic blood vessels enter and leave the gland at the hilum, which is on the inner surface. The blood vessels empty their contents directly into the splenic pulp, so that the blood comes into contact with the spleen substances, and is not as in other organs separated from it by blood vessels. There is no ordinary capillary system --- the blood comes into direct contact with the cells of the organ. The blood which flows through the spleen is collected in a system of venous sinuses which empty their blood into the branches which unite to form the splenic vein by which the blood is carried from the spleen to enter the portal circulation and be conveyed to the liver.
To begin with, the spleen is like a manufacturing shop. Even before the third month of development of a fetus the spleen begins to work, producing white and red blood cells. However, after birth a baby's spleen limits itself to the production of white cells called lymphocytes. But what a producer it is! It is said that the blood is sixty times richer in white cells when it leaves the spleen than when it enters it. As a manufacturing shop the spleen also produces antibodies, tiny particles in the blood that serve to build up the body's immunity. And the spleen produces a substance that helps the body in combating the effects of irradiation. The spleen is also a filtration unit. It shares with the liver in filtering out waste products in the blood, such as harmful organisms, worn-out red cells and platelets. It has a large artery seemingly all out of proportion to its size. But that it is very much needed is apparent from the fact that the body's entire blood supply, some five to six quarts, passes through the spleen every ninety minutes. This filtering is done largely by cells lining its blood channels.
