Spleen
Overview
 
The spleen is an organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...

 found in virtually all vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 animals with important roles in regard to red blood cells (also referred to as erythrocytes) and the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

. 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
Human iron metabolism
Human iron metabolism is the set of chemical reactions maintaining human homeostasis of iron. Iron is an essential element for most life on Earth, including human beings. The control of this necessary but potentially toxic substance is an important part of many aspects of human health and disease...

. As a part of the mononuclear phagocyte system, it metabolizes hemoglobin removed from senescent erythrocytes.
Encyclopedia
The spleen is an organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...

 found in virtually all vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 animals with important roles in regard to red blood cells (also referred to as erythrocytes) and the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

. 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
Human iron metabolism
Human iron metabolism is the set of chemical reactions maintaining human homeostasis of iron. Iron is an essential element for most life on Earth, including human beings. The control of this necessary but potentially toxic substance is an important part of many aspects of human health and disease...

. 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
Bilirubin
Bilirubin is the yellow breakdown product of normal heme catabolism. Heme is found in hemoglobin, a principal component of red blood cells. Bilirubin is excreted in bile and urine, and elevated levels may indicate certain diseases...

, which is subsequently shuttled to the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...

 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 monocyte
Monocyte
Monocytes are a type of white blood cell and are part of the innate immune system of vertebrates including all mammals , birds, reptiles, and fish. Monocytes play multiple roles in immune function...

s within the red pulp. These monocytes, upon moving to injured tissue (such as the heart), turn into dendritic cell
Dendritic cell
Dendritic cells are immune cells forming part of the mammalian immune system. Their main function is to process antigen material and present it on the surface to other cells of the immune system. That is, dendritic cells function as antigen-presenting cells...

s and macrophage
Macrophage
Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...

s while promoting tissue healing. It is one of the centers of activity of the reticuloendothelial system
Reticuloendothelial system
"Reticuloendothelial system" is an older term for the mononuclear phagocyte system. The mononuclear phagocyte system consists primarily of monocytes and macrophages. The spleen is the largest unit of the mononuclear phagocyte system. The monocyte is formed in the bone marrow and transported by the...

 and can be considered analogous to a large lymph node, as its absence leads to a predisposition toward certain infection
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...

s.

Anatomy

The spleen, in healthy adult humans, is approximately 11 centimetres (4.3 in) in length. It usually weighs between 150 grams (5.3 oz) and 200 grams (7.1 oz) and lies in front of the 9th to the 12th thoracic ribs. 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 side.

Like the thymus
Thymus
The thymus is a specialized organ of the immune system. The thymus produces and "educates" T-lymphocytes , which are critical cells of the adaptive immune system....

, 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 germinal centers are supplied by arterioles called penicilliary radicles.

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
Mesenchyme
Mesenchyme, or mesenchymal connective tissue, is a type of undifferentiated loose connective tissue that is derived mostly from mesoderm, although some are derived from other germ layers; e.g. some mesenchyme is derived from neural crest cells and thus originates from the ectoderm...

. Specifically, the spleen forms within, and from, the dorsal mesentery
Dorsal mesentery
The portion of mesentery attached to the greater curvature of the stomach is named the dorsal mesentery , and the part which suspends the colon is termed the mesocolon....

. However, it still shares the same blood supply — the celiac trunk — as the foregut
Foregut
The foregut is the anterior part of the alimentary canal, from the mouth to the duodenum at the entrance of the bile duct. At this point it is continuous with the midgut...

 organs.

Function

Area Function Composition
>-
| red pulp 
Mechanical filtration of red blood cells. In mice: Reserve of monocyte
Monocyte
Monocytes are a type of white blood cell and are part of the innate immune system of vertebrates including all mammals , birds, reptiles, and fish. Monocytes play multiple roles in immune function...

s
  • "sinuses
    Sinus (anatomy)
    Sinus is Latin for "bay", "pocket", "curve", or "bosom". In anatomy, the term is used in various contexts.A sinus is a sack or cavity in any organ or tissue, or an abnormal cavity or passage caused by the destruction of tissue...

    " (or "sinusoid
    Sinusoid (blood vessel)
    A sinusoid is a small blood vessel similar to a capillary but with a fenestrated endothelium. Fenestrations are pores in the endothelial cells that greatly increase their permeability. In addition, permeability is increased by large inter-cellular clefts and fewer tight junctions...

    s"), which are filled with blood
  • "splenic cords" of reticular fiber
    Reticular fiber
    Reticular fibers or reticulin is a histological term used to describe a type of fiber in connective tissue composed of type III collagen. Reticular fibers crosslink to form a fine meshwork...

    s
  • "marginal zone
    Marginal zone
    The marginal zone is the region at the interface between the non-lymphoid red pulp and the lymphoid white-pulp of the spleen. A marginal zone also exists in lymph nodes.-Composition and markers:It is composed of cells derived...

    " bordering on white pulp

  • >-
    | white pulp 
    Active immune response through humoral and cell-mediated pathways. Composed of nodules, called Malpighian corpuscles. These are composed of:
    • "lymphoid follicles" (or "follicles"), rich in B-lymphocytes
      B cell
      B cells are lymphocytes that play a large role in the humoral immune response . The principal functions of B cells are to make antibodies against antigens, perform the role of antigen-presenting cells and eventually develop into memory B cells after activation by antigen interaction...

    • "periarteriolar lymphoid sheaths
      Periarteriolar lymphoid sheaths
      Periarteriolar lymphoid sheaths are a portion of the white pulp of the spleen. They are populated largely by T cells, which are presented with blood borne antigens by myeloid dendritic cells....

      " (PALS), rich in T-lymphocytes
      T cell
      T cells or T lymphocytes belong to a group of white blood cells known as lymphocytes, and play a central role in cell-mediated immunity. They can be distinguished from other lymphocytes, such as B cells and natural killer cells , by the presence of a T cell receptor on the cell surface. They are...



    Other functions of the spleen are less prominent, especially in the healthy adult:
    • Production of opsonin
      Opsonin
      An opsonin is any molecule that targets an antigen for an immune response. However, the term is usually used in reference to molecules that act as a binding enhancer for the process of phagocytosis, especially antibodies, which coat the negatively-charged molecules on the membrane. Molecules that...

      s, properdin
      Properdin
      Properdin or factor P is a globulin protein found in the blood serum of more complex animals. In the complement system, an innate-immunity series of proenzymes dissolved in the circulation, it is also called "Factor P".-Function:...

      , and tuftsin
      Tuftsin
      Tuftsin is a tetrapeptide produced by enzymatic cleavage of the Fc-domain of the heavy chain of immunoglobulin G. It is produced primarily in the spleen.-Function:Its biological activity is related primarily to the immune system function....

      .
    • Creation of red blood cells. While the bone marrow
      Bone marrow
      Bone marrow is the flexible tissue found in the interior of bones. In humans, bone marrow in large bones produces new blood cells. On average, bone marrow constitutes 4% of the total body mass of humans; in adults weighing 65 kg , bone marrow accounts for approximately 2.6 kg...

       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
      Erythropoiesis
      Erythropoiesis is the process by which red blood cells are produced. It is stimulated by decreased O2 in circulation, which is detected by the kidneys, which then secrete the hormone erythropoietin...

       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 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 (236.5ml) of red blood cells can be held in the spleen and released in cases of hypovolemia. It can also store platelets in case of an emergency.
    • In mice, the spleen stores half the body's monocyte
      Monocyte
      Monocytes are a type of white blood cell and are part of the innate immune system of vertebrates including all mammals , birds, reptiles, and fish. Monocytes play multiple roles in immune function...

      s so that upon injury they can migrate to the injured tissue and transform into dendritic cell
      Dendritic cell
      Dendritic cells are immune cells forming part of the mammalian immune system. Their main function is to process antigen material and present it on the surface to other cells of the immune system. That is, dendritic cells function as antigen-presenting cells...

      s and macrophage
      Macrophage
      Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...

      s and so assist wound healing
      Wound healing
      Wound healing, or cicatrisation, is an intricate process in which the skin repairs itself after injury. In normal skin, the epidermis and dermis exists in a steady-state equilibrium, forming a protective barrier against the external environment...

      .

    Effect of removal

    Surgical removal causes:
    • modest increases in circulating white blood cell
      White blood cell
      White blood cells, or leukocytes , are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a...

      s and platelet
      Platelet
      Platelets, or thrombocytes , are small,irregularly shaped clear cell fragments , 2–3 µm in diameter, which are derived from fragmentation of precursor megakaryocytes.  The average lifespan of a platelet is normally just 5 to 9 days...

      s,
    • diminished responsiveness to some vaccine
      Vaccine
      A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe or its toxins...

      s,
    • increased susceptibility to infection by bacteria
      Bacteria
      Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

       and protozoa
      Protozoa
      Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...

      ; in particular, there is an increased risk of sepsis from polysaccharide encapsulated bacteria
      Polysaccharide encapsulated bacteria
      Polysaccharide encapsulated bacteria, frequently referred to simply as encapsulated bacteria and less precisely called encapsulated organisms, are a group of bacteria that have an outer covering, a bacterial capsule, made of polysaccharide....

      .


    A 28-year follow-up of 740 veterans of World War II
    World War II
    World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

     who had their spleen removed on the battlefield found that those who had been splenectomize
    Splenectomy
    A splenectomy is a surgical procedure that partially or completely removes the spleen.-Indications:The spleen, similar in structure to a large lymph node, acts as a blood filter. Current knowledge of its purpose includes the removal of old red blood cells and platelets, and the detection and fight...

    d showed a significant excess of mortality from pneumonia
    Pneumonia
    Pneumonia is an inflammatory condition of the lung—especially affecting the microscopic air sacs —associated with fever, chest symptoms, and a lack of air space on a chest X-ray. Pneumonia is typically caused by an infection but there are a number of other causes...

     (6 rather than the expected 1.3) and a significant excess of mortality from ischemic heart disease (4.1 rather than the expected 3) but not from other conditions.

    Disorders

    Disorders include splenomegaly
    Splenomegaly
    Splenomegaly is an enlargement of the spleen. The spleen usually lies in the left upper quadrant of the human abdomen. It is one of the four cardinal signs of hypersplenism, some reduction in the number of circulating blood cells affecting granulocytes, erythrocytes or platelets in any...

    , where the spleen is enlarged for various reasons, such as cancer
    Cancer
    Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

    , specifically blood
    Blood
    Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

    -based leukemias, and asplenia
    Asplenia
    Asplenia refers to the absence of normal spleen function and is associated with some serious infection risks. Hyposplenism is used to describe reduced splenic functioning, but not as severely affected as with asplenism.-Congenital:...

    , where the spleen is not present or functions abnormally.

    Etymology and cultural views

    The word spleen comes from the Greek
    Greek language
    Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

     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 English the word spleen was customary during the period of the 18th century. Authors like Richard Blackmore
    Richard Blackmore
    Sir Richard Blackmore , English poet and physician, is remembered primarily as the object of satire and as an example of a dull poet. He was, however, a respected physician and religious writer....

     or George Cheyne employed it to characterize the hypocondriacal and hysterical affections.

    In French
    French language
    French is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...

    , "splénétique" refers to a state of pensive sadness or melancholy. It has been popularized by the poet Charles Baudelaire
    Charles Baudelaire
    Charles Baudelaire was a French poet who produced notable work as an essayist, art critic, and pioneering translator of Edgar Allan Poe. His most famous work, Les Fleurs du mal expresses the changing nature of beauty in modern, industrializing Paris during the nineteenth century...

     (1821–1867) but was already used before in particular to the Romantic
    Romanticism
    Romanticism was an artistic, literary and intellectual movement that originated in the second half of the 18th century in Europe, and gained strength in reaction to the Industrial Revolution...

     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
    Talmud
    The Talmud is a central text of mainstream Judaism. It takes the form of a record of rabbinic discussions pertaining to Jewish law, ethics, philosophy, customs and history....

     (tractate Berachoth 61b) refers to the spleen as the organ of laughter
    Laughter
    Laughing is a reaction to certain stimuli, fundamentally stress, which serves as an emotional balancing mechanism. Traditionally, it is considered a visual expression of happiness, or an inward feeling of joy. It may ensue from hearing a joke, being tickled, or other stimuli...

     while possibly suggesting a link with the humoral view of the organ. In the 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
    Anger
    Anger is an automatic response to ill treatment. It is the way a person indicates he or she will not tolerate certain types of behaviour. It is a feedback mechanism in which an unpleasant stimulus is met with an unpleasant response....

    , 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.

    Variation among vertebrates

    In cartilagenous 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
    Intestine
    In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...

    . In many amphibian
    Amphibian
    Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

    s, especially frog
    Frog
    Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...

    s, it takes on the more rounded form and there is often a greater quantity of white pulp.

    In reptile
    Reptile
    Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

    s, birds, and mammal
    Mammal
    Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

    s, 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
    Haemal nodes
    Haemal nodes are lymphoid organs found in various mammals and some birds. Haemal nodes were first described by Gibbes in 1884. Haemal nodes are presumed to have the same function as the human spleen....

    throughout the body, which, it is presumed, have the same function as the spleen proper. The spleens of aquatic mammals are in some ways dissimilar to those of fully land-dwelling mammals. In general, the spleens of aquatic mammals are bluish in colour. In cetaceans and manatees it tends to be quite small, but in deep diving pinnipeds it can be quite massive, owing to its function of storing red blood cells.

    The only vertebrates lacking a spleen are the lamprey
    Lamprey
    Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...

    s and hagfish
    Hagfish
    Hagfish, the clade Myxini , are eel-shaped slime-producing marine animals . They are the only living animals that have a skull but not a vertebral column. Along with lampreys, hagfish are jawless and are living fossils whose next nearest relatives include all vertebrates...

    es. 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
    Homology (biology)
    Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

     with the spleen of higher vertebrates.

    External links

    - "The visceral surface of the spleen."
    • "spleen" from Encyclopædia Britannica Online
    • Spleen and Lymphatic System, Kidshealth.org (American Academy of Family Physicians
      American Academy of Family Physicians
      The American Academy of Family Physicians was founded in 1947 to promote the science and art of family medicine. It is one of the largest medical organizations in the United States, with over 100,000 members...

      )
    • Spleen Diseases from MedlinePlus
      MedlinePlus
      MedlinePlus is a free Web site that provides consumer health information for patients, families, and Health care providers. The site brings together information from the United States National Library of Medicine, the National Institutes of Health , other U.S. government agencies, and...

    • "Finally, the Spleen Gets Some Respect" New York Times piece on the spleen
    The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
     
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