organ (anatomy)
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biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of Cell (biology), cells that proce ...
, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an
organ system An organ system is a biological system consisting of a group of organ (biology), organs that work together to perform one or more functions. Each organ has a specialized role in a plant or animal body, and is made up of distinct Tissue (biology) ...
. Tissues are formed from same type cells to act together in a function. Tissues of different types combine to form an organ which has a specific function. The intestinal wall for example is formed by epithelial tissue and
smooth muscle tissue Smooth muscle is an involuntary non-Striated muscle tissue, striated muscle, so-called because it has no sarcomeres and therefore no striated muscle tissue, striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit smoo ...
. 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. An organ's tissues can be broadly categorized as parenchyma, the functional tissue, and stroma, the structural tissue with supportive, connective, or ancillary functions. For example, the
gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream (Endocrine system, endocrine gland) or into cavities inside the body or its outer surface (exocrine gland ...
's tissue that makes the
hormone A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and beh ...
s is the parenchyma, whereas the stroma includes the
nerves A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the E ...
that innervate the parenchyma, the
blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide aw ...
s that oxygenate and nourish it and carry away its metabolic wastes, and the
connective tissue Connective tissue is one of the four primary types of animal tissue (biology), tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Con ...
s that provide a suitable place for it to be situated and anchored. The main tissues that make up an organ tend to have common embryologic origins, such as arising from the same
germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans (animals that are sister taxa to the sponges) produce t ...
. Organs exist in most multicellular
organism In biology, an organism () is any life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multicellular o ...
s. In single-celled organisms such as
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
, the functional analogue of an organ is known as an
organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to the ...
. In plants, there are three main organs. In the study of
anatomy Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having its ...
, viscera (singular viscus) refers to the internal organs of the
abdominal The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the torso. ...
, thoracic, and pelvic cavities. The abdominal organs may be classified as solid organs, or hollow organs. The solid organs are the
liver The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...
,
pancreas The pancreas is an Organ (anatomy), organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdominal cavity, abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine ...
,
spleen The spleen is an organ (biology), organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes .
,
kidney The kidneys are two reddish-brown bean-shaped organ (anatomy), organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal ...
s, and
adrenal gland The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys. Each gland has an outer cortex whic ...
s. The hollow organs of the abdomen are the
stomach The stomach is a muscular, Organ (anatomy), hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the Digestion, dige ...
,
intestines The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syste ...
,
gallbladder In vertebrates, the gallbladder, also known as the cholecyst, is a small hollow Organ (anatomy), organ where bile is stored and concentrated before it is released into the small intestine. In humans, the pear-shaped gallbladder lies beneath th ...
,
bladder The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine enters ...
, and
rectum The rectum is the final straight portion of the large intestine in humans and some other mammals, and the Gastrointestinal tract, gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the s ...
. In the
thoracic cavity The thoracic cavity (or chest cavity) is the body cavity, chamber of the body of vertebrates that is protected by the thoracic wall (rib cage and associated skin, muscle, and fascia). The central compartment of the thoracic cavity is the mediast ...
the
heart The heart is a muscular organ Organ may refer to: Biology * Organ (biology), a part of an organism Musical instruments * Organ (music), a family of keyboard musical instruments characterized by sustained tone ** Electronic organ, an el ...
is a hollow, muscular organ. The number of organs in any organism depends on the definition used. By one widely adopted definition, 79 organs have been identified in the human body.


Animals

Except for
placozoa The Placozoa are a basal form of marine free-living (non-parasitic) multicellular organism. They are the simplest in structure of all animals. Three genera have been found: the classical '' Trichoplax adhaerens'', '' Hoilungia hongkongensis'', ...
ns,
multicellular A multicellular organism is an organism that consists of more than one cell (biology), cell, in contrast to unicellular organism. All species of animals, Embryophyte, land plants and most fungi are multicellular, as are many algae, whereas a few ...
animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage i ...
s including humans have a variety of
organ system An organ system is a biological system consisting of a group of organ (biology), organs that work together to perform one or more functions. Each organ has a specialized role in a plant or animal body, and is made up of distinct Tissue (biology) ...
s. These specific systems are widely studied in
human anatomy The human body is the structure of a Human, human being. It is composed of many different types of Cell (biology), cells that together create Tissue (biology), tissues and subsequently organ systems. They ensure homeostasis and the life, viabi ...
. The functions of these organ systems often share significant overlap. For instance, the nervous and
endocrine system The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neur ...
both operate via a shared organ, the
hypothalamus The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalam ...
. For this reason, the two systems are combined and studied as the neuroendocrine system. The same is true for the
musculoskeletal system The human musculoskeletal system (also known as the human locomotor system, and previously the activity system) is an Organ (anatomy), organ system that gives humans the ability to move using their Muscular system, muscular and skeletal systems. ...
because of the relationship between the muscular and
skeletal system A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...
s. *
Cardiovascular system The blood circulatory system is a system of organs that includes the heart The heart is a muscular organ Organ may refer to: Biology * Organ (biology), a part of an organism Musical instruments * Organ (music), a family of keybo ...
: pumping and channeling
blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the Cell (biology), cells, and transports Metabolic waste, metabolic waste products away from th ...
to and from the body and
lung The lungs are the primary Organ (anatomy), organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the vertebral co ...
s with
heart The heart is a muscular organ Organ may refer to: Biology * Organ (biology), a part of an organism Musical instruments * Organ (music), a family of keyboard musical instruments characterized by sustained tone ** Electronic organ, an el ...
,
blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the Cell (biology), cells, and transports Metabolic waste, metabolic waste products away from th ...
and
blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide aw ...
s. *
Digestive system The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food into smaller and smaller compone ...
:
digestion Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intest ...
and processing food with
salivary gland The salivary glands in mammals are exocrine glands that produce saliva through a system of Duct (anatomy), ducts. Humans have three paired major salivary glands (Parotid gland, parotid, Submandibular gland, submandibular, and sublingual gland, sub ...
s,
esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an Organ (anatomy), organ in vertebrates through which food passes, aided by Peristalsis, peristaltic contracti ...
,
stomach The stomach is a muscular, Organ (anatomy), hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the Digestion, dige ...
,
liver The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...
,
gallbladder In vertebrates, the gallbladder, also known as the cholecyst, is a small hollow Organ (anatomy), organ where bile is stored and concentrated before it is released into the small intestine. In humans, the pear-shaped gallbladder lies beneath th ...
,
pancreas The pancreas is an Organ (anatomy), organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdominal cavity, abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine ...
,
intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syste ...
s, colon,
rectum The rectum is the final straight portion of the large intestine in humans and some other mammals, and the Gastrointestinal tract, gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the s ...
and
anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, de ...
. *
Endocrine system The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neur ...
: communication within the body using
hormone A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and beh ...
s made by
endocrine gland Endocrine glands are ductless gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream (Endocrine system, endocrine gland) or into cavities inside the b ...
s such as the
hypothalamus The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalam ...
,
pituitary In vertebrate Vertebrates () comprise all animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, ...
gland, pineal body or pineal gland,
thyroid The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobe (anatomy), lobes. The lower two thirds of the lobes are connected by a thin band of Connective tissue, tissue cal ...
, parathyroids and adrenals, i.e., adrenal glands. * Excretory system:
kidney The kidneys are two reddish-brown bean-shaped organ (anatomy), organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal ...
s,
ureter The ureters are tubes made of Smooth muscle tissue, smooth muscle that propel urine from the kidneys to the urinary bladder. In a human adult, the ureters are usually long and around in diameter. The ureter is lined by urothelium, urothelial c ...
s,
bladder The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine enters ...
and
urethra The urethra (from Greek language, Greek οὐρήθρα – ''ourḗthrā'') is a tube that connects the urinary bladder to the urinary meatus for the removal of urine from the body of both females and males. In human females and other primates, ...
involved in fluid balance,
electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electron The electron ( or ) is a subatomic particle with a negative one elementary charge, elementary electr ...
balance and excretion of
urine Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excretion, excreted from the body through the urethra. Cel ...
. *
Lymphatic system The lymphatic system, or lymphoid system, is an organ system in vertebrates that is part of the immune system, and complementary to the circulatory system. It consists of a large network of lymphatic vessels, lymph nodes, lymphatic or lymphoid o ...
: structures involved in the transfer of
lymph Lymph (from Latin, , meaning "water") is the fluid that flows through the lymphatic system, a system composed of lymph vessels (channels) and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues to ...
between tissues and the blood stream, the lymph and the nodes and vessels that transport it including the
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as Tumor immunology, cancer cells and objects such ...
: defending against
disease A disease is a particular abnormal condition that negatively affects the structure or function (biology), function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medica ...
-causing agents with
leukocyte White blood cells, also called leukocytes or leucocytes, are the cell (biology), cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and de ...
s, tonsils,
adenoid In anatomy, the adenoid, also known as the pharyngeal tonsil or nasopharyngeal tonsil, is the anatomical terms of location#Superior and inferior, superior-most of the tonsils. It is a mass of Lymphatic tissues, lymphatic tissue located behind t ...
s,
thymus The thymus is a specialized primary Lymphatic system#Structure, lymphoid organ of the immune system. Within the thymus, T cell, thymus cell lymphocytes or ''T cells'' mature. T cells are critical to the adaptive immune system, where the body ada ...
and
spleen The spleen is an organ (biology), organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes .
. *
Integumentary system The integumentary system is the set of organs forming the outermost layer of an animal's body. It comprises the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: ...
:
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...
,
hair Hair is a protein filament that grows from hair follicle, follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick ter ...
and nails of mammals. Also scales of
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
,
reptile Reptiles, as most commonly defined are the animals in the Class (biology), class Reptilia ( ), a paraphyletic grouping comprising all sauropsid, sauropsids except birds. Living reptiles comprise turtles, crocodilians, Squamata, squamates (lizar ...
s, and
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s, and
feather Feathers are epidermis (zoology), epidermal growths that form a distinctive outer covering, or plumage, on both Bird, avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in ...
s of birds. *
Muscular system The muscular system is an organ (anatomy), organ system consisting of skeletal muscle, skeletal, smooth muscle, smooth, and cardiac muscle, cardiac muscle. It permits movement of the body, maintains posture, and circulates blood throughout the bo ...
: movement with
muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other ...
s. *
Nervous system In Biology, biology, the nervous system is the Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, sensory information by transmitting action potential, signals to and from different parts of its ...
: collecting, transferring and processing information with
brain A brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as Visual perception, vision. I ...
,
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spinal ...
and
nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the E ...
s. *
Reproductive system The reproductive system of an organism, also known as the genital system, is the biological system made up of all the anatomical sex organs, organs involved in sexual reproduction. Many non-living substances such as fluids, hormones, and pherom ...
: the
sex organ A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, a ...
s, such as
ovaries The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. T ...
,
fallopian tube The fallopian tubes, also known as uterine tubes, oviducts or salpinges (singular salpinx), are paired tubes in the human female that stretch from the uterus to the Ovary, ovaries. The fallopian tubes are part of the female reproductive system ...
s,
uterus The uterus (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around p ...
,
vulva The vulva (plural: vulvas or vulvae; derived from Latin for wrapper or covering) consists of the external sex organ, female sex organs. The vulva includes the mons pubis (or mons veneris), labia majora, labia minora, clitoris, bulb of vestibu ...
,
vagina In mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region o ...
,
testes A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is Homology (biology), homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, pr ...
,
vas deferens The vas deferens or ductus deferens is part of the male reproductive system of many vertebrates. The ducts transport sperm from the epididymis to the ejaculatory ducts in anticipation of ejaculation. The vas deferens is a partially coiled tube w ...
,
seminal vesicle The seminal vesicles (also called vesicular glands, or seminal glands) are a pair of two convoluted tubular glands that lie behind the urinary bladder of some Sexual reproduction#Male, male mammals. They secrete fluid that partly composes the se ...
s,
prostate The prostate is both an Male accessory gland, accessory gland of the male reproductive system and a muscle-driven mechanical switch between urination and ejaculation. It is found only in some mammals. It differs between species anatomically, ...
and
penis A penis (plural ''penises'' or ''penes'' () is the primary sexual organ that male animals use to inseminate females (or hermaphrodites) during Copulation (zoology), copulation. Such organs occur in many animals, both #Vertebrates, vertebra ...
. *
Respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies grea ...
: the organs used for
breathing Breathing (or ventilation) is the process of moving air into and from the lungs to facilitate gas exchange with the Milieu intérieur, internal environment, mostly to flush out carbon dioxide and bring in oxygen. All Aerobic respiration, aer ...
, the
pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity The nasal cavity is a large, air-filled space above and behind the human nose, nose in the middle of the face. The nasal septum divides the cavity int ...
,
larynx The larynx (), commonly called the voice box, is an organ (anatomy), organ in the top of the neck involved in breathing, producing sound and protecting the trachea against food aspiration. The opening of larynx into pharynx known as the laryngeal ...
,
trachea The trachea, also known as the windpipe, is a Cartilage, cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. The trachea extends ...
, bronchi,
lung The lungs are the primary Organ (anatomy), organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the vertebral co ...
s and diaphragm. *
Skeletal system A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...
: structural support and protection with
bone A bone is a rigid organ Organ may refer to: Biology * Organ (biology), a part of an organism Musical instruments * Organ (music), a family of keyboard musical instruments characterized by sustained tone ** Electronic organ, an electronic ...
s,
cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the Epiphysis, ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, th ...
,
ligament A ligament is the Connective tissue#Types, fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body inc ...
s and
tendon A tendon or sinew is a tough, high-tensile-strength band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skele ...
s.


Viscera

In the study of
anatomy Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having its ...
, viscera (singular viscus) refers to the internal organs of the
abdominal The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the torso. ...
, thoracic, and pelvic cavities. The abdominal organs may be classed as solid organs, or hollow organs. The solid organs are the
liver The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...
,
pancreas The pancreas is an Organ (anatomy), organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdominal cavity, abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine ...
,
spleen The spleen is an organ (biology), organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes .
,
kidney The kidneys are two reddish-brown bean-shaped organ (anatomy), organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal ...
s, and
adrenal gland The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys. Each gland has an outer cortex whic ...
s. The hollow organs are the
stomach The stomach is a muscular, Organ (anatomy), hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the Digestion, dige ...
,
intestines The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syste ...
,
gallbladder In vertebrates, the gallbladder, also known as the cholecyst, is a small hollow Organ (anatomy), organ where bile is stored and concentrated before it is released into the small intestine. In humans, the pear-shaped gallbladder lies beneath th ...
,
bladder The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine enters ...
, and
rectum The rectum is the final straight portion of the large intestine in humans and some other mammals, and the Gastrointestinal tract, gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the s ...
. In the
thoracic cavity The thoracic cavity (or chest cavity) is the body cavity, chamber of the body of vertebrates that is protected by the thoracic wall (rib cage and associated skin, muscle, and fascia). The central compartment of the thoracic cavity is the mediast ...
the
heart The heart is a muscular organ Organ may refer to: Biology * Organ (biology), a part of an organism Musical instruments * Organ (music), a family of keyboard musical instruments characterized by sustained tone ** Electronic organ, an el ...
is a hollow, muscular organ. Splanchnology is the study of the viscera. The term "visceral" is contrasted with the term "", meaning "of or relating to the wall of a body part, organ or cavity" The two terms are often used in describing a membrane or piece of connective tissue, referring to the opposing sides.


Origin and evolution

The organ level of organisation in
animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage i ...
s can be first detected in
flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a Phylum (biology), phylum of relati ...
s and the more derived phyla, i.e. the
bilateria The Bilateria or bilaterians are animals with symmetry (biology)#Bilateral symmetry, bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior ...
ns. The less-advanced
taxa In biology, a taxon (back-formation from ''Taxonomy (biology), taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known ...
(i.e. ''
Placozoa The Placozoa are a basal form of marine free-living (non-parasitic) multicellular organism. They are the simplest in structure of all animals. Three genera have been found: the classical '' Trichoplax adhaerens'', '' Hoilungia hongkongensis'', ...
'', ''
Porifera Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...
'', ''
Ctenophora Ctenophora (; ctenophore ; ) comprise a phylum of marine life, marine invertebrates, commonly known as comb jellies, that marine habitats, inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly ...
'' and ''
Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in Fresh water, freshwater and Marine habitats, marine environments, predominantly the latter. Their distinguishing feature is cnidocyt ...
'') do not show consolidation of their tissues into organs. More complex animals are composed of different organs, which have evolved over time. For example, the liver and heart evolved in the
chordate A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five Apomorphy and synapomorphy, synapomorphies, or primary physical characteristics, that distinguish them from al ...
s 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 the
placenta The placenta is a temporary embryonic and later fetal organ (anatomy), organ that begins embryonic development, developing from the blastocyst shortly after implantation (embryology), implantation. It plays critical roles in facilitating nutrien ...
, 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. 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.


Plants

The study of plant organs is covered in
plant morphology Phytomorphology is the study of the morphology (biology), physical form and external structure of plants.Raven, P. H., R. F. Evert, & S. E. Eichhorn. ''Biology of Plants'', 7th ed., page 9. (New York: W. H. Freeman, 2005). . This is usually cons ...
. Organs of
plant Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...
s can be divided into vegetative and reproductive. Vegetative plant organs include
root In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often b ...
s, stems, and
leaves A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...
. The reproductive organs are variable. In
flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek language, Greek words ('container, vessel') and ('seed'), and refers to ...
s, they are represented by the
flower A flower, sometimes known as a bloom or blossom, is the reproduction, reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providin ...
,
seed A seed is an Plant embryogenesis, embryonic plant enclosed in a testa (botany), protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, includ ...
and
fruit In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in particul ...
. In conifers, the organ that bears the reproductive structures is called a
cone A cone is a three-dimensional Three-dimensional space (also: 3D space, 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called ''parameters'') are required to determine the position (geometry), ...
. In other divisions ( phyla) of plants, the reproductive organs are called
strobili A strobilus (plural: strobili) is a structure present on many land plant Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living thi ...
, in '' Lycopodiophyta'', or simply gametophores in
moss Mosses are small, non-vascular flower A flower, sometimes known as a bloom or blossom, is the reproduction, reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to ...
es. Common organ system designations in plants include the differentiation of shoot and root. All parts of the plant above ground (in non-
epiphyte An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphytes grow are called Phorophy ...
s), including the functionally distinct leaf and flower organs, may be classified together as the shoot organ system. 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 Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemica ...
, while the reproductive organs are essential in
reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual org ...
. However, if there is asexual
vegetative reproduction Vegetative reproduction (also known as vegetative propagation, vegetative multiplication or cloning) is any form of asexual reproduction Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or chan ...
, 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 are transplanted into a person with a failing organ. The transplantation of larger solid organs often requires
immunosuppression Immunosuppression is a reduction of the activation or efficacy of the immune system. Some portions of the immune system itself have immunosuppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse reacti ...
to prevent
organ rejection Transplant rejection occurs when Organ transplant, transplanted tissue is rejected by the recipient's immune system, which destroys the transplanted tissue. Transplant rejection can be lessened by determining the molecular similitude between don ...
or
graft-versus-host disease Graft-versus-host disease (GvHD) is a syndrome, characterized by inflammation in different organs. GvHD is commonly associated with bone marrow transplants and hematopoietic stem cell transplantation, stem cell transplants. White blood cells of t ...
. There is considerable interest throughout the world in creating Tissue engineering, laboratory-grown or artificial organs.


Organ transplants

Beginning in the 20th century Organ transplantation, 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. 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, including distributive 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.


History

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 the Organ (music), keyboard-based instrument. At the same time, a second meaning arose, in reference to a "body part adapted to a certain function". Plant organs are made from tissue composed of different types of tissue. The three tissue types are ground, vascular, and dermal. When three or more organs are present, it is called an organ system. The adjective ''wikt:visceral, visceral'', also ''splanchnic'', is used for anything pertaining to the internal organs. Historically, viscera of animals were examined by Rome, 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 "", meaning "of or relating to the wall of a body part, organ or cavity" The two terms are often used in describing a membrane or piece of connective tissue, referring to the opposing sides.


Antiquity

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:


See also

* Organoid * Organ-on-a-chip


References


External links

* {{Authority control Levels of organization (Biology) Organ systems, Organs (anatomy),