The spleen is an organ found in virtually all vertebrate animals Figure 1. Similar in structure to a large lymph node, the spleen acts primarily as a blood filter. As such, it is a non-vital organ: a healthy life is possible after removal. The spleen plays important roles in regards to red blood cells (also referred to as erythrocytes) and the immune system. In humans, it is located in the left upper quadrant of the abdomen. It removes old red blood cells and holds a reserve of blood in case of hemorrhagic shock, while also recycling iron. As a part of the mononuclear phagocyte system, it metabolizes hemoglobin removed from senescent erythrocytes. The globin portion of hemoglobin is degraded to its constitutive amino acids, and the heme portion is metabolized to bilirubin, which is subsequently shuttled to the liver for removal. It synthesizes antibodies in its white pulp and removes antibody-coated bacteria along with antibody-coated blood cells by way of blood and lymph node circulation. The spleen is brownish. Recently, it has been found to contain in its reserve half of the body's monocytes within the red pulp. These monocytes, upon moving to injured tissue (such as the heart), turn into dendritic cells and macrophages while promoting tissue healing. It is one of the centers of activity of the mononuclear phagocyte system and can be considered analogous to a large lymph node, as its absence leads to a predisposition toward certain infections.
The spleen, in healthy adult humans, is approximately 11 cm (4.3 in) in length. It usually weighs between 150 gms (5.3 oz) and 200 gms (7.1 oz). An easy way to remember the anatomy of the spleen is the 1x3x5x7x9x11 rule. The spleen is 1" by 3" by 5", weighs approximately 7 oz, and lies between the 9th and 11th ribs on the left hand side.
Like the thymus, the spleen possesses only efferent lymphatic vessels. The spleen is part of the lymphatic system. Both the short gastric arteries and the splenic artery supply it with blood. The spleen is unique in respect to its development within the gut. While most of the gut viscera are endodermally derived (with the exception of the neural-crest derived suprarenal gland), the spleen is derived from mesenchymal tissue. Specifically, the spleen forms within, and from, the dorsal mesentery. However, it still shares the same blood supply (the celiac trunk) as the foregut organs.
While the bone marrow is the primary site of hematopoiesis in the adult, the spleen has important hematopoietic functions up until the fifth month of gestation. After birth, erythropoietic functions cease, except in some hematologic disorders. As a major lymphoid organ and a central player in the mononuclear phagocyte system, the spleen retains the ability to produce lymphocytes and, as such, remains a hematopoietic organ. In humans, up to a cup (236.5ml) of red blood cells can be held in the spleen and released in cases of hypovolemia. It can store platelets in case of an emergency. Up to a quarter of lymphocytes can be stored in the spleen at any one time.
Surgical removal causes: modest increases in circulating white blood cells and platelets, diminished responsiveness to some vaccines, and increased susceptibility to infection by bacteria and protozoa. In particular, there is an increased risk of sepsis from polysaccharide-encapsulated bacteria.