In a multicellularorganism, anorgan is a collection oftissues joined in a structural unit to serve a common function.[1] In thehierarchy of life, an organ lies betweentissue and anorgan system. Tissues are formed from same typecells to act together in a function. Tissues of different types combine to form an organ which has a specific function. Theintestinal wall for example is formed byepithelial tissue andsmooth muscle tissue.[2] Two or more organs working together in the execution of a specific body function form an organ system, also called abiological system or body system.
An organ's tissues can be broadly categorized asparenchyma, the functional tissue, andstroma, the structural tissue with supportive, connective, orancillary functions. For example, thegland's tissue that makes thehormones is theparenchyma, whereas the stroma includes thenerves that innervate the parenchyma, theblood vessels that oxygenate and nourish it and carry away itsmetabolic wastes, and theconnective tissues that provide a suitable place for it to be situated and anchored. The main tissues that make up an organ tend to have commonembryologic origins, such as arising from the samegerm layer. Organs exist in most multicellularorganisms. Insingle-celled organisms such as members of theeukaryotes, thefunctional analogue of an organ is known as anorganelle. In plants, there are three main organs.[3]
The number of organs in any organism depends on the definition used. There are approximately 79 organs in the human body; the precise count is debated.[4]
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.
More complex animals are composed of different organs, which have evolved over time. For example, the liver and heart evolved in thechordates about 550-500 million years ago, while the gut and brain are even more ancient, arising in the ancestor of vertebrates, insects,molluscs, and worms about 700–650 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 theplacenta, 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.[11] 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.[11]
The study of plant organs is covered inplant morphology. Organs ofplants can be divided into vegetative and reproductive. Vegetative plant organs includeroots,stems, andleaves. The reproductive organs are variable. Inflowering plants, they are represented by theflower,seed andfruit.[citation needed] Inconifers, the organ that bears the reproductive structures is called acone. In other divisions (phyla) of plants, the reproductive organs are calledstrobili, inLycopodiophyta, or simply gametophores inmosses. Common organ system designations in plants include 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.[12]
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 asphotosynthesis, while the reproductive organs are essential inreproduction. However, if there isasexualvegetative reproduction, the vegetative organs are those that create the new generation of plants (seeclonal colony).
Beginning in the 20th century,[13]organ transplants began to take place as scientists knew more about the anatomy of organs. These came later in time as procedures were often dangerous and difficult.[14] Both the source and method of obtaining the organ to transplant are major ethical issues to consider, and because organs as resources for transplant are always more limited than demand for them, various notions of justice, includingdistributive justice, are developed in the ethical analysis. This situation continues as long as transplantation relies upon organ donors rather than technological innovation, testing, and industrial manufacturing.[citation needed]
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The English word "organ" dates back to the twelfth century and refers to any musical instrument. By the late 14th century, the musical term's meaning had narrowed to refer specifically to thekeyboard-based instrument. At the same time, a second meaning arose, in reference to a "body part adapted to a certain function".[15]
Plant organs are made from tissue composed of different types of tissue. The three tissue types are ground, vascular, and dermal.[16] When three or more organs are present, it is called an organ system.[17]
The adjectivevisceral, alsosplanchnic, is used for anything pertaining to the internal organs. Historically, viscera of animals were examined byRoman paganpriests like theharuspices or theaugurs in order to divine the future by their shape, dimensions or other factors.[18] 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"[9] The two terms are often used in describing a membrane or piece of connective tissue, referring to the opposing sides.[19]
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) because, in ancient Greek, the word 'organon' means 'tool', and Aristotle believed that the organs of the body were tools for us by means of which we can do things. For similar reasons, his logical works, taken as a whole, are referred to as theOrganon because logic is atool for philosophical thinking.[20] Earlier thinkers, such as those who wrote texts in theHippocratic corpus, generally did not believe that there were organs of the body but only differentparts of the body.[21]
The Chinese associated the five elements with the five planets (Jupiter, Mars, Venus, Saturn, and Mercury) similar to the way the classical planets were associated with different metals. Theyin andyang distinction approximates the modern notion of solid and hollow organs.
^Widmaier, E P; Raff, H; Strang, KT (2014).Vander's Human Physiology (12th ed.). McGraw-Hill Higher Education.ISBN978-0-07-128366-3.[page needed]
^Kent, Michael (2000).Advanced biology. Oxford: Oxford University Press. p. 81.ISBN0199141959.
^"Botany/Plant structure".en.wikibooks.org.Archived from the original on 2018-02-07. Retrieved2018-02-06 – via Wikibooks, open books for an open world.
^Lennox, James (31 Jan 2017)."Aristotle's Biology".Plato. Stanford University.Archived from the original on 7 May 2019. Retrieved23 March 2019.Section 2: Aristotle's Philosophy of Science
^Gundert 1992: 465. Gundert, Beate. 1992. “Parts and Their Roles in Hippocratic Medicine,”Isis 83: 453–65.
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