In biology, an organ (Latin: organum, "instrument, tool", from Greek ὄργανον, organon, "organ, instrument, tool"[1]) is a collection of tissues joined in structural unit to serve a common function. [2]
Usually there is a "main" tissue, parenchyma, and "sporadic" tissues, stroma. 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 tissues include the nerves, blood and connective tissues. Functionally related organs often cooperate to form whole organ systems. Organs exist in all higher biological organisms, in particular they are not restricted to animals, but can also be identified in plants. An example of this is the bladder. In single-cell organisms like bacteria, the functional analogues of organs are called organelles.
A hollow organ is a visceral organ that is a hollow tube or pouch, such as the stomach or intestine, or that includes a cavity, like the heart or urinary bladder).[3]
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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.
Organs of plants can be divided into vegetative and reproductive. Vegetative plant organs are roots, stems, and leaves. The reproductive organs are variable. In flowering plants, they are represented with the flower, seed and fruit. In conifers, the organ that bears the reproductive structures is called a cone. In other divisions (phylums) 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).
The three main organ systems in vascular plants are the root, shoot and reproductive systems.
The organ level of organisation in animals can be first detected in flatworms and the more advanced phyla. The less-advanced taxons (like Placozoa, Porifera and Radiata) do not show consolidation of their tissues into organs.
There are eleven major organ systems found in mammals.
Mammals such as humans have a variety of organ systems. These specific systems are also widely studied in human anatomy.
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