Spleen
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The spleen is a ductless, vertebrate gland that is closely associated with the circulatory system, where it functions in the destruction of old red blood cells in holding a reservoir of blood. It is regarded as one of the centres of activity of the reticuloendothelial system. Until recently, the purpose of the spleen was not known. It is increasingly recognized that its absence leads to a predisposition to certain infections.
Anatomy
The human spleen is located in the upper left part of the abdomen, behind the stomach and just below the diaphragm. In normal individuals this organ measures about 125 × 75 × 50 mm (5 × 3 × 2 in) in size.
The spleen is the largest organ derived from mesenchyme and lying in the mesentery. The spleen consists of masses of lymphoid tissue of granular appearance located around fine terminal branches of veins and arteries. These vessels are connected by modified capillaries called splenic sinuses. Cross sections of the spleen reveal a red soft surface which is divided into red pulp and white pulp. The red pulp is composed largely of sinuses which are filled with blood. The white pulp is composed of nodules, called Malpighian corpuscles. These are composed of lymphoid follicles, rich in B-lymphocytes, and "periarteriolar lymphoid sheaths" (PALS), rich in T-lymphocytes.
The red and white pulp correspond to the two functional roles of the spleen. The white pulp helps fight infections and the red pulp removes unwanted materials.
In certain animals such as dogs and horses, the spleen sequesters a large number of erythrocytes (red blood cells), which can be dumped into the bloodstream during periods of physical exertion.[1] These animals also have large hearts in relation to their body size to accommodate the higher-viscosity blood that results. Some athletes have tried doping themselves with their own stored red blood cells to try to achieve the same effect, but the human heart is not equipped to handle the higher-viscosity blood.
Approximately 10% of people have one or more accessory spleens. They may form near the hilum of the main spleen, the junction at which the splenic vessels enter and leave the organ.
Functions
The most important functions of the spleen are mechanical filtration, which removes senescent red blood cells and control of infection. It is also produces the opsonins properdin and tuftsin.
Until 5th month of gestation, when the bone marrow starts functioning, the spleen has important hematopoietic functions. After birth, no significant hematopoietic function is left in the spleen except in some hematologic disorders: e.g. myelodysplastic syndrome, hemoglobinopathies.
Disorders
Enlargement of the spleen is known as splenomegaly. It may be caused by malaria, bacterial endocarditis, leukaemia, pernicious anaemia, leishmaniasis, Hodgkin's disease, Banti's disease, hereditary spherocytosis, cysts, glandular fever (mononucleosis), and tumours. Primary tumours of the spleen include hemangiomas and hemangiosarcomas. Marked splenomegaly may result in the spleen occupying a large portion of the left side of the abdomen.
The spleen is the largest collection of lymphoid tissue in the body. It is normally palpable in preterm infants, in 30% of normal, full-term neonates, and in 5% to 10% of infants and toddlers. A spleen easily palpable below the costal margin in any child over the age of 3-4 years should be considered abnormal until proven otherwise.
Splenomegaly can result from antigenic stimulation (eg, infection), obstruction of blood flow (eg, portal vein obstruction), underlying functional abnormality (eg, hemolytic anemia), or infiltration (eg, leukemia or storage disease, such as Gaucher's disease). The most common cause of acute splenomegaly in children is viral infection, which is transient and usually moderate. Basic work-up for acute splenomegaly includes a complete blood count with differential, platelet count, and reticulocyte and atypical lymphocyte counts to exclude hemolytic anemia and leukemia. Assessment of IgM antibodies to viral capsid antigen (a rising titer) is indicated to confirm Epstein-Barr virus or cytomegalovirus. Other infections should be excluded if these tests are negative.
Absence
The absence of a spleen predisposes to some septicaemia infections. Vaccination and antibiotic measures are discussed under asplenia.
- Some people congenitally completely lack a spleen, although this is rare.
- Sickle-cell disease can cause a functional asplenia (or autosplenectomy) by causing infarctions of the spleen during repeated sickle-cell crises.
- It may be removed surgically (known as a splenectomy), and indeed often is. For example, it may be removed following abdominal injuries with rupture and hemorrhage of the spleen, or in the treatment of certain blood diseases (Idiopathic thrombocytopenic purpura, hereditary spherocytosis, etc.), certain forms of lymphoma or for the removal of splenic tumours or cysts.
Etymology and cultural views
The word spleen comes from the Greek splēn.
In French, spleen refers to a state of pensive sadness or melancholy. It has been popularized by the poet Charles-Pierre Baudelaire (1821-1867) but was already used before, in particular in the Romantic literature (18th century). The connection between spleen (the organ) and melancholy (the temperament) comes from the humoral medicine of the ancient Greeks. One of the humours (body fluid) was the black bile, secreted by the spleen organ and associated with melancholy. In contrast, the Talmud (tractate Berachoth 61b) refers to the spleen as the organ of laughter, possibly suggesting a link with the humoral view of the organ.
In German, the word "spleen", pronounced as in English, refers to a persisting somewhat cranky (but not quite lunatic) idea or habit of a person; however the organ is called "Milz", (cognate with Old English milte). In 19th century England women in bad humour were said to be afflicted by spleen, or the vapours of spleen. In modern English "to vent one's spleen" means to vent one's anger, e.g. by shouting, and can be applied to both males and females.
In China, the spleen '脾 (pí)' counts as the seat of one's temperament and is thought to influence the individual's willpower. Analogous to "venting one's spleen", "发脾气" is used as an expression, although in the view of Traditional Chinese Medicine, the view of "脾" does not correspond to the anatomical "spleen".
Footnotes
- ^ Carey, Bjorn (May 5, 2006). "Horse science: What makes a Derby winner - Spleen acts as 'natural blood doper,' scientist says". MSNBC.com. Microsoft. Retrieved 2006-05-09.