Organ (anatomy)

Organs are groups of tissues with similar functions. Plant and animal life relies on many organs that coexist in organ systems.[1]

Organs are composed of main tissue, parenchyma, and "sporadic" tissues, stroma. The main tissue is that which is unique for the specific organ, such as the myocardium, the main tissue of the heart, while sporadic tissues include the nerves, blood vessels, and connective tissues. The main tissues that make up an organ tend to have common embryologic origins, such as arising from the same germ layer. Functionally-related organs often cooperate to form whole organ systems. Organs exist in all organisms. In single-celled organisms such as bacteria, the functional analogue of an organ is known as an organelle. In plants there are three main organs.[2] A hollow organ is an internal organ that forms a hollow tube, or pouch such as the stomach, intestine, or bladder.

In the study of anatomy, the term viscus is used to refer to an internal organ, and viscera is the plural form.[3][4] 79 organs have been identified in the human body.[5]

Internal organs
Many of the internal organs of the human body
SystemOrgan systems
Anatomical terminology



In biology, tissue is a cellular organizational level between cells and complete organs. A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues.

The study of human and animal tissues is known as histology or, in connection with disease, histopathology. For plants, the discipline is called plant morphology. Classical tools for studying tissues include the paraffin block (agarose gel is often used with plants[6][7]) in which tissue is embedded and then sectioned, the histological stain, and the optical microscope. In the last couple of decades, developments in electron microscopy, immunofluorescence, and the use of frozen tissue sections have enhanced the detail that can be observed in tissues. With these tools, the classical appearances of tissues can be examined in health and disease, enabling considerable refinement of medical diagnosis and prognosis.

Organ systems

Two or more organs working together in the execution of a specific body function form an organ system, also called a biological system or body system. The functions of organ systems often share significant overlap. For instance, the nervous and endocrine system both operate via a shared organ, the hypothalamus. For this reason, the two systems are combined and studied as the neuroendocrine system. The same is true for the musculoskeletal system because of the relationship between the muscular and skeletal systems.

Common organ system designations in plants includes the differentiation of shoot and root. All parts of the plant above ground (in non-epiphytes), including the functionally distinct leaf and flower organs, may be classified together as the shoot organ system.[8]



Leber Schaf
The liver and gallbladder of a sheep

Animals such as humans have a variety of organ systems. These specific systems are also widely studied in human anatomy.


Red Hibiscus in Chennai during Spring
The flower is the angiosperm's reproductive organ. This Hibiscus flower is hermaphroditic, and it contains stamen and pistils.
Equisetum telmateia strob
Strobilus of Equisetum telmateia

The study of plant organs is referred to as plant morphology, rather than anatomy – as in animal systems. Organs of plants can be divided into vegetative and reproductive. Vegetative plant organs include roots, stems, and leaves. The reproductive organs are variable. In flowering plants, they are represented by the flower, seed and fruit. In conifers, the organ that bears the reproductive structures is called a cone. In other divisions (phyla) of plants, the reproductive organs are called strobili, in Lycopodiophyta, or simply gametophores in mosses.

The vegetative organs are essential for maintaining the life of a plant. While there can be 11 organ systems in animals, there are far fewer in plants, where some perform the vital functions, such as photosynthesis, while the reproductive organs are essential in reproduction. However, if there is asexual vegetative reproduction, the vegetative organs are those that create the new generation of plants (see clonal colony).

Society and culture

Many societies have a system for organ donation, in which a living or deceased donor's organ is transplanted into a person with a failing organ. The transplantation of larger solid organs often requires immunosuppression to prevent organ rejection or graft-versus-host disease.

There is considerable interest throughout the world in creating laboratory-grown or artificial organs.


View of Viscera Page 82
Human viscera

The English word "organ" dates back to the twelfth century, in reference to any musical instrument. By the late 14th century, the musical term's meaning had narrowed to refer specifically to the keyboard-based instrument. At the same time, a second meaning arose, in reference to a "body part adapted to a certain function".[9]

Plant organs are made from tissue composed of different types of tissue. When three or more organs are present, it is called an organ system.[10]

The adjective visceral, also splanchnic, is used for anything pertaining to the internal organs. Historically, viscera of animals were examined by Roman pagan priests like the haruspices or the augurs in order to divine the future by their shape, dimensions or other factors. This practice remains an important ritual in some remote, tribal societies.

The term "visceral" is contrasted with the term "parietal", meaning "of or relating to the wall of a body part, organ or cavity"[11] The two terms are often used in describing a membrane or piece of connective tissue, referring to the opposing sides.


Aristotle used the word frequently in his philosophy, both to describe the organs of plants or animals (e.g. the roots of a tree, the heart or liver of an animal), and to describe more abstract "parts" of an interconnected whole (e.g. his logical works, taken as a whole, are referred to as the "organon").

Some alchemists (e.g. Paracelsus) adopted the Hermetic Qabalah assignment between the seven vital organs and the seven classical planets as follows:[12]

Planet Organ
Sun Heart
Moon Brain
Mercury Lungs
Venus Kidneys
Mars Gall bladder
Jupiter Liver
Saturn Spleen

Modern times

The variations in natural language definitions of what constitutes an organ, their degree of precision, and the variations in how they map to ontologies and taxonomies in information science (for example, to count how many organs exist in a typical human body) are topics explored by writer Carl Engelking of Discover magazine in 2017 as he analyzed the science journalism coverage of the evolving scientific understanding of the mesentery.[13] He explored a challenge now faced by anatomists: as human understanding of ontology generally (that is, how things are defined, and how the relationship of one thing to another is defined) meets applied ontology and ontology engineering, unification of varying views is in higher demand.[13] However, such unification always faces epistemologic frontiers, as humans can only declare computer ontologies with certainty and finality to the extent that their own cognitive taxonomy (that is, science's understanding of the universe) is certain and final. For example, the fact that the tissues of the mesentery are continuous was something that was simply not known for sure until it was demonstrated with microscopy.[14] Because humans cannot predict all future scientific discoveries, they cannot build a unified ontology that is totally certain and will never again change. However, one of the points made by an anatomist interviewed by Engelking is that, finality aside, much more could be done even now to represent existing human knowledge more clearly for computing purposes.

Organ Procedures

Beginning in the 20th century[15] transplants began to occur as scientists knew more about the anatomy of organs. These came later in time as Procedures were often dangerous and difficult.[16]

Origin and evolution

Origin of major organs on the animal phylogeny
Relationship of major animal lineages with indication of how long ago these animals shared a common ancestor. On the left, important organs are shown, which allows us to determine how long ago these may have evolved.

The organ level of organisation in animals can be first detected in flatworms and the more derived phyla. The less-advanced taxa (like Placozoa, Sponges and Radiata) do not show consolidation of their tissues into organs.

Complex animals are composed of organs and many of these organs evolved a very long time ago. For example, the liver evolved in the stem vertebrates more than 500 million years ago, while the gut and brain are even more ancient, arising in the ancestor of vertebrates, insects, and worms more than 600 million years ago.

Given the ancient origin of most vertebrate organs, researchers have looked for model systems, where organs have evolved more recently, and ideally have evolved multiple times independently. An outstanding model for this kind of research is the placenta, which has evolved more than 100 times independently in vertebrates, has evolved relatively recently in some lineages, and exists in intermediate forms in extant taxa.[17] Studies on the evolution of the placenta have identified a variety of genetic and physiological processes that contribute to the origin and evolution of organs, these include the re-purposing of existing animal tissues, the acquisition of new functional properties by these tissues, and novel interactions of distinct tissue types.[17]

See also


  1. ^ Widmaier EP; Raff H; Strang KT (2014). Vander's Human Physiology (12th ed.). ISBN 978-0-07-128366-3.
  2. ^ "Botany/Plant structure - Wikibooks, open books for an open world".
  3. ^ "Viscus - Definition". Merriam-Webster Online Dictionary. Retrieved 14 December 2009.
  4. ^ "Viscera". MeSH. Retrieved 14 December 2009.
  5. ^ "New organ named in digestive system". BBC News. 2017. Retrieved 2018-02-05.
  6. ^ Lišková, Desana; Kollárová, Karin; Martinka, Michal; Sterckeman, Thibault; Lux, Alexander; Zelko, Ivan (2012-07-01). "An easy method for cutting and fluorescent staining of thin roots". Annals of Botany. 110 (2): 475–478. doi:10.1093/aob/mcs046. ISSN 0305-7364.
  7. ^ "Rapid Preparation of Transverse Sections of Plant Roots | Schiefelbein Lab". Retrieved 2019-03-19.
  8. ^ "The Plant Body | Boundless Biology". Retrieved 2019-03-19.
  9. ^ "organ (n.)". Online Etymology Dictionary. Retrieved 22 March 2019.
  10. ^ "Organ System - Definition and Examples | Biology Dictionary". Biology Dictionary. 2016-10-31. Retrieved 2018-02-10.
  11. ^ "Parietal – Learning brain structure, function and variability from neuroimaging data". Retrieved 2018-02-10.
  12. ^ Philip Ball, The Devil's Doctor: Paracelsus and the World of Renaissance Magic and Science, ISBN 978-0-09-945787-9
  13. ^ a b Engelking, Carl (2017-01-06), "We got the mesentery news all wrong", The Crux (a group blog by Discover writers).
  14. ^ Coffey, J Calvin; O'Leary, D Peter (2016). "The mesentery: structure, function, and role in disease". The Lancet Gastroenterology & Hepatology. 1 (3): 238–247. doi:10.1016/S2468-1253(16)30026-7.
  15. ^ "Timeline of Historical Events and Significant Milestones". Organ Donor Government Web. Retrieved 19 March 2019.
  16. ^
  17. ^ a b Griffith, Oliver W.; Wagner, G?nter P. (23 March 2017). "The placenta as a model for understanding the origin and evolution of vertebrate organs". Nature Ecology & Evolution. 1 (4): 0072. doi:10.1038/s41559-017-0072.

External links

Circumventricular organs

Circumventricular organs (CVOs) are structures in the brain characterized by their extensive and highly permeable microvasculature, unlike those in the rest of the brain where there exists a blood brain barrier (BBB). The term "circumventricular organs" was originally proposed in 1958 by Austrian anatomist Helmut Hofer, in recognition of the distribution of the structures around the ventricular system of the brain.The circumventricular organs are midline structures around the third and fourth ventricles that are in contact with blood and cerebrospinal fluid, and allow for the linkage between the central nervous system and peripheral blood. Additionally, they are an integral part of neuroendocrine function. The highly permeable capillaries allow the CVOs to act as an alternative route for peptides and hormones in the neural tissue to sample from and secrete to circulating blood. CVOs also have roles in body fluid regulation, cardiovascular functions, immune responses, thirst, feeding behavior and reproductive behavior.CVOs can be classified as either sensory or secretory organs serving homeostatic functions and body water balance. The sensory organs include the area postrema (AP), the subfornical organ (SFO) and the vascular organ of lamina terminalis (VOLT), all having the ability to sense signals in blood, then pass that information neurally to other brain regions. Through their neural circuitry, they provide direct information to the autonomic nervous system from the systemic circulation. The secretory organs include the subcommissural organ (SCO), the pituitary gland, the median eminence, and the pineal gland. These organs are responsible for secreting hormones and glycoproteins into the peripheral blood using feedback from both the brain environment and external stimuli.All of the circumventricular organs, except the subcommissural organ, contain extensive vasculature and permeable capillaries which define a sensory and secretory system within the brain. Furthermore, all CVOs contain neural tissue, enabling a neuroendocrine role. The choroid plexus, having permeable capillaries, does not contain neural tissue, but rather its primary role is to produce cerebrospinal fluid (CSF), and so is typically excluded from classification as a CVO.


The clitoris ( (listen) or (listen)) is a female sex organ present in mammals, ostriches and a limited number of other animals. In humans, the visible portion - the glans - is at the front junction of the labia minora (inner lips), above the opening of the urethra. Unlike the penis, the male homologue (equivalent) to the clitoris, it usually does not contain the distal portion (or opening) of the urethra and is therefore not used for urination. The clitoris also usually lacks a reproductive function. While few animals urinate through the clitoris or use it reproductively, the spotted hyena, which has an especially large clitoris, urinates, mates, and gives birth via the organ. Some other mammals, such as lemurs and spider monkeys, also have a large clitoris.The clitoris is the human female's most sensitive erogenous zone and generally the primary anatomical source of human female sexual pleasure. In humans and other mammals, it develops from an outgrowth in the embryo called the genital tubercle. Initially undifferentiated, the tubercle develops into either a penis or a clitoris during the development of the reproductive system depending on exposure to androgens (which are primarily male hormones). The clitoris is a complex structure, and its size and sensitivity can vary. The glans (head) of the human clitoris is roughly the size and shape of a pea, and is estimated to have about 8,000 sensory nerve endings.Sexological, medical, and psychological debate have focused on the clitoris, and it has been subject to social constructionist analyses and studies. Such discussions range from anatomical accuracy, gender inequality, female genital mutilation, and orgasmic factors and their physiological explanation for the G-spot. Although, in humans, the only known purpose of the clitoris is to provide sexual pleasure, whether the clitoris is vestigial, an adaptation, or serves a reproductive function has been debated. Social perceptions of the clitoris include the significance of its role in female sexual pleasure, assumptions about its true size and depth, and varying beliefs regarding genital modification such as clitoris enlargement, clitoris piercing and clitoridectomy. Genital modification may be for aesthetic, medical or cultural reasons.Knowledge of the clitoris is significantly impacted by cultural perceptions of the organ. Studies suggest that knowledge of its existence and anatomy is scant in comparison with that of other sexual organs, and that more education about it could help alleviate social stigmas associated with the female body and female sexual pleasure; for example, that the clitoris and vulva in general are visually unappealing, that female masturbation is taboo, or that men should be expected to master and control women's orgasms.

Cords of Billroth

The Cords of Billroth (also known as splenic cords or red pulp cords) are found in the red pulp of the spleen between the sinusoids, consisting of fibrils and connective tissue cells with a large population of monocytes and macrophages. These cords contain half of the mouse body's monocytes as a reserve so that after tissue injury these monocytes can move in and aid locally sourced monocytes in wound healing.Erythrocytes pass through the cords of Billroth before entering the sinusoids. The passage into the sinusoids may be seen as a bottleneck, where erythrocytes need to be flexible in order to pass through. In disorders of erythrocyte shape and/or flexibility, such as hereditary spherocytosis, erythrocytes fail to pass through and get phagocytosed, causing extravascular hemolysis.

Electric organ (biology)

In biology, the electric organ is an organ common to all electric fish used for the purposes of creating an electric field. The electric organ is derived from modified nerve or muscle tissue. The electric discharge from this organ is used for navigation, communication, mating, defense and also sometimes for the incapacitation of prey.

Inguinal lymph nodes

Inguinal lymph nodes are the lymph nodes in the inguinal region (groin). They are located in the femoral triangle, and are grouped into superficial lymph nodes, and deep lymph nodes. The superficial lymph nodes have three divisions – the superomedial, superolateral, and inferior superficial lymph nodes.

Lymph capillary

Lymph capillaries or lymphatic capillaries are tiny, thin-walled microvessels located in the spaces between cells (except in the central nervous system and non-vascular tissues) which serve to drain and process extracellular fluid. Upon entering the lumen of a lymphatic capillary, the collected fluid and associated cells (notably white blood cells) is known as lymph. Each lymphatic capillary carries lymph into a lymphatic vessel, which in turn connects to a lymph node. Lymph is ultimately returned to the venous circulation.

Lymphatic capillaries are slightly larger in diameter than blood capillaries, and have closed ends (unlike the loop structure of blood capillaries). Their unique structure permits interstitial fluid to flow into them but not out. The ends of the endothelial cells that make up the wall of a lymphatic capillary overlap. When pressure is greater in the interstitial fluid than in lymph, the cells separate slightly, like the opening of a one-way swinging door, and interstitial fluid enters the lymphatic capillary. When pressure is greater inside the lymphatic capillary, the cells adhere more closely, and lymph cannot escape back into interstitial fluid. Attached to the lymphatic capillaries are anchoring filaments, which contain elastic fibers. They extend out from the lymphatic capillary, attaching lymphatic endothelial cells to surrounding tissues. When excess interstitial fluid accumulates and causes tissue swelling, the anchoring filaments are pulled, making the openings between cells even larger so that more fluid can flow into the lymphatic capillary.Lymph capillaries have a greater internal oncotic pressure than blood capillaries, due to the greater concentration of plasma proteins in the lymph.

Mantle zone

The mantle zone (or just mantle) of a lymphatic nodule (or lymphatic follicle) is an outer ring of small lymphocytes surrounding a germinal center.It is also known as the "corona".It contains transient lymphocytes.It is the location of the lymphoma in mantle cell lymphoma.

Medial supraclavicular lymph node

A medial supraclavicular lymph node is a lymph node located above the collar bone and between the center of the body and a line drawn through the nipple to the shoulder.


The mesentery is a contiguous set of tissues that attaches the intestines to the posterior abdominal wall in humans and is formed by the double fold of peritoneum. It helps in storing fat and allowing blood vessels, lymphatics, and nerves to supply the intestines, among other functions.The mesocolon was thought to be a fragmented structure, with all named parts—the ascending, transverse, descending, and sigmoid mesocolons, the mesoappendix, and the mesorectum—separately terminating their insertion into the posterior abdominal wall. However, in 2012, new microscopic and electron microscopic examinations at the University of Limerick showed the mesocolon to be a single structure derived from the duodenojejunal flexure and extending to the distal mesorectal layer. Thus, the mesentery is an organ.

Periarteriolar lymphoid sheaths

Periarteriolar lymphoid sheaths (or periarterial lymphatic sheaths, or PALS) are a portion of the white pulp of the spleen. They are populated largely by T cells and surround central arteries within the spleen; the PALS T-cells are presented with blood borne antigens via myeloid dendritic cells.

In contrast, the lymphoid portions of the white pulp are dominated by B cells.

Systematized Nomenclature of Medicine

The Systematized Nomenclature of Medicine (SNOMED) is a systematic, computer-processable collection of medical terms, in human and veterinary medicine, to provide codes, terms, synonyms and definitions which cover anatomy, diseases, findings, procedures, microorganisms, substances, etc. It allows a consistent way to index, store, retrieve, and aggregate medical data across specialties and sites of care. Although now international, SNOMED was started in the U.S. by the College of American Pathologists (CAP) in 1973 and revised into the 1990s. In 2002 CAP's SNOMED Reference Terminology (SNOMED RT) was merged with, and expanded by, the National Health Service's Clinical Terms Version 3 (previously known as the Read codes) to produce SNOMED CT.Versions of SNOMED released prior to 2001 were based on a multiaxial, hierarchical classification system. As in any such system, a disease may be located in a body organ (anatomy), which results in a code in a topography axis and may lead to morphological alterations represented by a morphology code.

In 2002 the first release of SNOMED CT adopted a completely different structure. A sub-type hierarchy, supported by defining relationships based on description logic, replaced the axes described in this article. Versions of SNOMED prior to SNOMED CT are planned to be formally deprecated from 2017. Therefore, readers interested in current information about SNOMED are directed to the article on SNOMED CT.

Human systems and organs
Circulatory system
Nervous system
Integumentary system
Immune system
Respiratory system
Digestive system
Urinary system
Reproductive system
Endocrine system

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