The spleen is an organ found in almost all vertebrate animals. It mainly acts as a filter for purifying the blood, removing microbes and worn out or damaged red blood cells. It is also an important organ in the immune system, producing the white blood cells that fight infection and synthesize antibodies.
Although the spleen is important for many functions in the body, the spleen is not vital to survival and it is possible to live without a spleen. Without the spleen, other organs in the body (particularly the liver) adapt and increase their ability to fight infection and remove redundant red blood cells.
Anatomy and function of the spleen
The spleen is located in the left upper quarter of the abdomen, beneath the ninth to the twelfth rib. In healthy adults, the organ measures around 10 to 12 centimetres in length and weighs around 150 to 200 grams.
The spleen is divided into two main compartments, the red pulp and the white pulp, and is surrounded by a dense fibrous covering called the splenic capsule.
The white pulp is the infection-fighting lymphoid tissue where white blood cells are produced and mature. This pulp is made up of periarteriolar lymphoid sheaths (PALS) and lymphatic nodules. The sheaths surround central arteries within the spleen and contain T lymphocytes that attack foreign bodies as the blood is filtered into the spleen. The lymphatic nodules are where B-lymphocytes predominate, producing antibodies to coat pathogens and flag them up for removal by scavenger cells.
The splenic artery delivers blood to the spleen where it is filtered by the red pulp. The red pulp is made up of cords of connective tissue and wide blood vessels called splenic sinusoids. Blood passes through the cords and into the sinusoids where it is drained into the large trabecular veins of the spleen.
Blood is filtered through gaps in the sinusoid lining, which prevents old, damaged or abnormal red blood cells from passing into the bloodstream. These unwanted cells are targeted by phagocytes, which also ingest any invading pathogens that are present such as bacteria, fungi and viruses. Other debris such as denatured hemoglobin is also prevented from entering the blood stream by the sinusoids.
The channels of the red pulp also act as a reservoir for storing various blood components, particularly phagocytes and platelets, which can be released and travel to sites of injury to regulate inflammation and facilitate healing.