The spleen is an organ found in virtually all vertebrate animals with important roles in regard to red blood cells and the immune system In humans, it is located in the left upper quadrant of the abdomen. It removes old red blood cells, holds a reserve in case of hemorrhagic shock, especially in animals like horses (not in humans) and recycles iron. It synthesizes antibodies in its white pulp, removes from the circulation antibody-coated bacteria and antibody-coated blood cells. Recently it has been found to contain in reserve half the body's monocytes in its red pulp that upon moving to injured tissue such as the heart turn into dendritic cells and macrophages and aid wound healing. It is one of the centers of activity of the reticuloendothelial system, and can be considered analogous to a large lymph node. Its absence leads to a predisposition to certain infections.
Spleen Anatomy
The spleen is found in the upper left quadrant of the human abdomen. Spleens in healthy adult humans are approximately 11 centimeters in length. It usually weighs 150 grams and lies beneath the 9th to the 12th rib.
Like the thymus, the spleen possesses only efferent lymphatic vessels.
The spleen is part of the lymphatic system.
The germinal centers are supplied by arterioles called ''penicilliary radicles''.
The spleen is unique with 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.
Spleen Function
| Area | Function |
| red pulp | Mechanical filtration of red blood cells. Reserve of monocytes In humans, it does not act as a reservoir of blood cells. It can also store platelets in case of an emergency. - Storage of half the body's monocytes so that upon injury they can migrate to the injured tissue and transform into dendritic cells and macrophages and so assist wound healing.
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Spleen Disorders
Disorders include splenomegaly, where the spleen is enlarged for various reasons, and asplenia, where the spleen is not present or functions abnormally.
Spleen Etymology and Cultural Views
The word spleen comes from the Greek σπλήν, and is the idiomatic equivalent of the heart in English, i.e. to be good-spleened (εὔσπλαγχνος) means to be good-hearted or compassionate.
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 in the Romantic literature (18th century). The word for the organ is "la 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, possibly suggesting a link with the humoral view of the organ. In eighteenth- and nineteenth-century England, women in bad humour 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.
Spleen is also the formal name for an elite social group with headquarters in South Georgia.
Spleen Variation Among Vertebrates
In cartilagenous and ray-finned fish the spleen is normally a somewhat elongated organ, consisting primarily of red pulp, with only a small amount of white pulp. In lungfish, the spleen is not a distinct 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 the adult. Many mammals possess tiny spleen-like structures known as haemal nodes throughout the body, which presumably have the same function as the spleen proper.
The only vertebrates to lack 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 presumably homologous with the spleen of higher vertebrates.
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