The spleen is a small organ in the abdominal cavity that is considered to be part of the lymphatic system, which also includes the lymph nodes. The lymphatic system consists of a series of vessels that parallel the circulatory system to allow drainage of excess tissue fluid for eventual return to the blood. The system also allows immune cells in the lymph node tissue to survey the components of the fluid for infection and other foreign particles as they filter the tissue fluid, known as lymph. Lymphocytes, immature white blood cells that mature to T cells and B cells, reside in the nodes and are activated when foreign material is recognized. The spleen acts in a similar manner, but by filtering the blood.
The spleen is made of lymphatic tissue and contains lymphocytes, but it is not organized as well as lymph nodes. The spleen also contains macrophages, large white blood cells that engulf foreign particles and infectious agents when they are recognized. Two types of splenic tissue filter the blood from the arterial feed and return it to the venous feed, white and red pulp, respectively. Unlike other organs in the body, the spleen does not have lymph vessels that enter it. Instead, lymphatics arise in the white pulp and exit via the hilus of the spleen.
The spleen is highly vascularized, and as blood passes through it, old and damaged red blood cells (erythrocytes) are recycled. Red blood cells have a normal lifespan of approximately 120 days. They are produced in the bone marrow, cycling through the circulatory system until being filtered out by the spleen. As red blood cells age they undergo antigenic changes that allow them to be recognized by the immune cells in the spleen. They also become more permeable to water, swelling and becoming too large to pass through the splenic vessels. The breakdown process is called hemolysis, which results in the release of hemoglobin into the tissue fluid. The hemoglobin is broken down into hemosidirin, which is sometimes found in the splenic macrophages. The iron from the red blood cells is then stored as ferritin or bilirubin before being returned to the bone marrow to make more hemoglobin.
Blood cell balance
The spleen also removes red blood cells if there are too many of them in the circulation, and recycles platelets and granulocytes. During embryonic stages, the spleen produces red blood cells, platelets, and granulocytes, though it produces lymphocytes throughout life. Another function of the spleen was also recently discovered – it acts as a storage container for the circulatory system, sometimes harboring platelets in addition to white blood cells, particularly monocytes.
Function of the spleen
Although it is not as well known an organ as the heart or kidneys, the spleen is an important part of how the body functions. As part of the lymphatic system the spleen functions in immunity. But it also processes blood cells to maintain blood cell balance and remove red blood cells via hemolysis. By filtering the blood the spleen assists the lymph nodes in maintaining the body's health.