|Precursor||Mesenchyme of dorsal mesogastrium|
The spleen plays important roles in regard to red blood cells (also referred to as erythrocytes) and the immune system. It removes old red blood cells and holds a reserve of blood, which can be valuable in case of hemorrhagic shock, and also recycles 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 removed in the liver.
The spleen synthesizes antibodies in its white pulp and removes antibody-coated bacteria and antibody-coated blood cells by way of blood and lymph node circulation. A study published in 2009 using mice found that the spleen contains, 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. The spleen is a center of activity of the mononuclear phagocyte system and can be considered analogous to a large lymph node, as its absence causes a predisposition to certain infections.
The spleen, in healthy adult humans, is approximately 7 centimetres (2.8 in) to 14 centimetres (5.5 in) in length. It usually weighs between 150 grams (5.3 oz) and 200 grams (7.1 oz). An easy way to remember the anatomy of the spleen is the 1×3×5×7×9×11 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.
The diaphragmatic surface of the spleen (or phrenic surface) is convex, smooth, and is directed upward, backward, and to the left, except at its upper end, where it is directed slightly to the middle. It is in relation with the under surface of the diaphragm, which separates it from the ninth, tenth, and eleventh ribs of the left side, and the intervening lower border of the left lung and pleura.
The visceral surface of the spleen is divided by a ridge into two regions: an anterior or gastric and a posterior or renal. The gastric surface (facies gastrica) is directed forward, upward, and toward the middle, is broad and concave, and is in contact with the posterior wall of the stomach. Below this it is in contact with the tail of the pancreas. Near to its mid-border is a long fissure, termed the hilum. This is pierced by several irregular openings, for the entrance and exit of vessels and nerves. The renal surface (facies renalis) is directed medialward and downward. It is somewhat flattened, considerably narrower than the gastric surface, and is in relation with the upper part of the anterior surface of the left kidney and occasionally with the left suprarenal gland.
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.
|red pulp||Mechanical filtration of red blood cells. In mice: Reserve of monocytes|
|white pulp||Active immune response through humoral and cell-mediated pathways.||Composed of nodules, called Malpighian corpuscles. These are composed of:|
Other functions of the spleen are less prominent, especially in the healthy adult:
- Production of opsonins, properdin, and tuftsin.
- Creation of red blood cells. 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 reticuloendothelial system, the spleen retains the ability to produce lymphocytes and, as such, remains an hematopoietic organ.
- Storage of red blood cells, lymphocytes and other formed elements. In horses, roughly 30% of the red blood cells are stored there. The red blood cells can be released when needed. In humans, up to a cup (236.5 ml) 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 and also clears old platelets from the circulation. Up to a quarter of lymphocytes can be stored in the spleen at any one time.
Disorders include splenomegaly, where the spleen is enlarged for various reasons, such as cancer, specifically blood-based leukemias, and asplenia, where the spleen is not present or functions abnormally.
Asplenia refers to a non-functioning spleen, which may be congenital, damaged by trauma or caused by disease such as sickle cell anaemia. Hyposplenia refers to a partially functioning spleen. These conditions may cause: a modest increase in circulating white blood cells and platelets; a diminished response to some vaccines and an increased susceptibility to infection. In particular, there is an increased risk of sepsis from polysaccharide encapsulated bacteria. Encapsulated bacteria inhibit binding of complement or prevent complement assembled on the capsule from interacting with macrophage receptors. Natural antibodies are required for phagocytosis, which are immunoglobulins that facilitate phagocytosis either directly or by complement deposition on the capsule. They are produced by IgM memory B cells in the marginal zone of the spleen.
A splenectomy (removal of the spleen) also causes asplenia and results in a greatly diminished frequency of memory B cells. A 28-year follow-up of 740 World War II veterans who had their spleens removed, on the battlefield, showed a significant increase in the usual death rate from pneumonia (6 rather than the expected 1.3) and an increase in the death rate from ischemic heart disease (4.1 rather than the expected 3) but not from other conditions.
An accessory spleen is a small splenic nodule extra to the spleen usually formed in early embryogenesis. Accessory spleens are found in approximately 10 percent of the population and are typically around 1 centimeter in diameter. Splenosis is a condition where displaced pieces of splenic tissue (often following trauma or splenectomy) autotransplant in the abdominal cavity as accessory spleens.
Polysplenia is a congenital disease manifested by multiple small accessory spleens, rather than a single, full-sized, normal spleen. Polysplenia sometimes occurs alone, but it is often accompanied by other developmental abnormalities such as intestinal malrotation or biliary atresia, or cardiac abnormalities, such as dextrocardia. These accessory spleens are non-functional.
Society and culture
The word spleen comes from the Ancient Greek σπλήν (splḗn), and is the idiomatic equivalent of the heart in English, i.e. to be good-spleened (εὔσπλαγχνος, eúsplankhnos) means to be good-hearted or compassionate.
In English the word spleen was customary during the period of the 18th century. Authors like Richard Blackmore or George Cheyne employed it to characterise the hypochondriacal and hysterical affections. William Shakespeare, in Julius Caesar uses the spleen to describe Cassius' irritable nature.
Must I observe you? must I stand and crouch
Under your testy humour? By the gods
You shall digest the venom of your spleen,
Though it do split you; for, from this day forth,
I'll use you for my mirth, yea, for my laughter,
When you are waspish.
In French, "splénétique" refers to a state of pensive sadness or melancholy. It has been popularized by the poet Charles Baudelaire (1821–1867) but was already used before in particular to the Romantic literature (19th century). The word for the organ is "rate".
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 while possibly suggesting a link with the humoral view of the organ. In eighteenth- and nineteenth-century England, women in bad humor were said to be afflicted by the spleen, or the vapours of the 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. Similarly, the English term "splenetic" is used to describe a person in a foul mood.
In cartilaginous and ray-finned fish it consists primarily of red pulp and is normally a somewhat elongated organ as it actually lies inside the serosal lining of the intestine. In many amphibians, especially frogs, it takes on the more rounded form and there is often a greater quantity of white pulp.
In reptiles, birds, and mammals, white pulp is always relatively plentiful, and in the latter two groups, the spleen is typically rounded, although it adjusts its shape somewhat to the arrangement of the surrounding organs. In the great majority of vertebrates, the spleen continues to produce red blood cells throughout life; it is only in mammals that this function is lost in adults. Many mammals have tiny spleen-like structures known as haemal nodes throughout the body that are presumed to have the same function as the spleen. The spleens of aquatic mammals differ in some ways from those of fully land-dwelling mammals. In general, they are bluish in colour. In cetaceans and manatees they tend to be quite small, but in deep diving pinnipeds, they can be quite massive, due to their function of storing red blood cells.
The only vertebrates lacking a spleen are the lampreys and hagfishes. Even in these animals, there is a diffuse layer of haematopoeitic tissue within the gut wall, which has a similar structure to red pulp and is presumed to be homologous with the spleen of higher vertebrates.
- This article uses anatomical terminology; for an overview, see anatomical terminology.
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- Spleen, Internet Encyclopedia of Science
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|Look up spleen in Wiktionary, the free dictionary.|
|Wikimedia Commons has media related to Spleen.|
- Anatomy figure: 38:03-01 at Human Anatomy Online, SUNY Downstate Medical Center—"The visceral surface of the spleen."
- Anatomy image:7881 at the SUNY Downstate Medical Center
- "spleen" from Encyclopædia Britannica Online
- Spleen and Lymphatic System, Kidshealth.org (American Academy of Family Physicians)
- Spleen Diseases from MedlinePlus
- "Finally, the Spleen Gets Some Respect" New York Times piece on the spleen