Endocrine system
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What is the endocrine system?
The endocrine system is made of glands and tissues that create and secrete hormones. Glands such as the thyroid, ovaries, and testes are discrete organs with defined borders and endocrine functions. Other glands, such as the islets of Langerhans, are embedded within structures.
The hypothalamus produces neurohormones that stimulate hormone secretion from the anterior pituitary gland. The pituitary gland has an anterior, posterior, and intermediate lobe, each of which produces its own hormones. The adrenal glands have a medulla and cortex, which secrete hormones including epinephrine, cortisol, aldosterone, and androgens.
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What are hormones and what do they do?
Hormones are organic substances secreted by plants and animals that regulate physiological activities and maintain homeostasis. Hormones elicit responses from specific organs or tissues adapted to react to minute quantities of them.
The traditional view is that hormones are transmitted to their targets in the bloodstream after being discharged from the glands that secrete them, known as endocrine secretion. However, the meaning of the term has broadened to include similar regulatory substances distributed by diffusion across cell membranes instead of by a blood system.
The endocrine system consists of an endocrine gland, the hormone it secretes, a responding tissue containing a specific receptor to which the hormone binds, and an action that results after the hormone binds to its receptor, termed the postreceptor response.
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Which major glands make up the endocrine system?
The endocrine system consists of glands and tissues that create and secrete hormones. These hormones regulate various bodily functions by traveling through the bloodstream to target organs.
Major glands of the endocrine system:
* Adrenal glands: The cortex of these glands produces hormones like cortisol and aldosterone, while the medulla produces epinephrine and norepinephrine.
* Ovaries: Produce estrogen, progesterone, inhibin, and relaxin.
* Pancreas: The islets of Langerhans in the pancreas secrete insulin, glucagon, somatostatin and pancreatic polypeptide.
* Pineal gland: Secretes melatonin.
* Pituitary gland: The anterior lobe produces hormones such as growth hormone, and the posterior lobe releases oxytocin and vasopressin.
* Testes: Produces testosterone.
* Thyroid: Secretes thyroxine and triiodothyronine.Other tissues that function as endocrine organs include the hypothalamus, kidneys, skin, liver, heart, lungs, gastrointestinal tract, and skeletal muscles.
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Where is the pituitary gland located and what is its function?
The pituitary gland is located at the base of the skull, behind the nose, directly beneath the hypothalamus, within a bony structure called the sella turcica.
The anterior pituitary gland produces hormones like thyrotropin, luteinizing hormone, follicle-stimulating hormone, adrenocorticotropic hormone, growth hormone, and prolactin. The hypothalamus regulates the production and secretion of these anterior pituitary hormones.
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How do endocrine glands send messages throughout the body?
The endocrine system sends messages throughout the body using hormones. Endocrine glands, such as the anterior pituitary gland, synthesize and secrete specific hormones into the bloodstream. These hormones then travel to target tissues or organs, where they bind to specific receptors.
These receptors, located either on the surface or within the cells of the target tissue, are activated by the hormone-receptor complex, triggering a chain of chemical responses within the cells. This process ultimately leads to a specific action or effect within the target tissue.
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How does the endocrine system interact with other body systems to keep you healthy?
The endocrine system works with the nervous and immune systems to direct the functions of the human body, defend against threats, and maintain health. The endocrine system uses hormones to regulate processes, working with the nervous system to maintain internal stability, or homeostasis.
Endocrine glands, such as the anterior pituitary gland, adrenal glands, and pancreas, create and secrete specific hormones. The hormones released affect target organs or activities in many organs. This system monitors and responds to changes in the body, like energy storage, growth, and development.
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Endocrine system, any of the systems found in animals for the production of hormones, substances that regulate the functioning of the organism. Such a system may range, at its simplest, from the neurosecretory, involving one or more centres in the nervous system, to the complex array of glands found in the human endocrine system.
Comparative endocrinologists investigate the evolution of endocrine systems and the role of these systems in animals’ adaptation to their environments and their production of offspring. Studies of nonmammalian animalshave provided information that has furthered research in mammalian endocrinology, including that of humans. For example, the actions of a pituitary hormone, prolactin, on the control of body water and salt content were first discovered in fishes and later led to the demonstration of similar mechanisms in mammals. The mediating role of local ovarian secretions (paracrine function) in the maturation of oocytes (eggs) was discovered in starfishes and only later extended to vertebrates. The important role of thyroid hormones during embryonic development was first studied thoroughly in tadpoles during the early 1900s. In addition, the isolation and purification of many mammalian hormones was made possible in large part by using other vertebrates as bioassay systems; that is, primitive animals have served as relatively simple, sensitive indicators of the amount of hormone activity in extracts prepared from mammalian endocrine glands. Finally, some vertebrate and invertebrate animals have provided “model systems” for research that have yielded valuable information on the nature of hormone receptors and the mechanisms of hormone action. For example, one of the most intensively studied systems for understanding hormone actions on target tissues has been the receptors for progesterone and estrogens (hormones secreted by the gonads) from the oviducts of chickens.
An understanding of how the endocrine system is regulated in nonmammals also provides essential information for regulating natural populations or captive animals. Artificial control of salmon reproduction has had important implications for the salmon industry as a whole. Some successful attempts at reducing pest insect species have been based on the knowledge of pheromones. Understanding the endocrinology of a rare species may permit it to be bred successfully in captivity and thus prevent it from becoming extinct. Future research may even lead to the reintroduction of some endangered species into natural habitats.
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Evolution of endocrine systems
The most primitive endocrine systems seem to be those of the neurosecretory type, in which the nervous system either secretes neurohormones (hormones that act on, or are secreted by, nervous tissue) directly into the circulation or stores them in neurohemal organs (neurons whose endings directly contact blood vessels, allowing neurohormones to be secreted into the circulation), from which they are released in large amounts as needed. True endocrine glands probably evolved later in the evolutionary history of the animal kingdom as separate, hormone-secreting structures. Some of the cells of these endocrine glands are derived from nerve cells that migrated during the process of evolution from the nervous system to various locations in the body. These independent endocrine glands have been described only in arthropods (where neurohormones are still the dominant type of endocrine messenger) and in vertebrates (where they are best developed).
It has become obvious that many of the hormones previously ascribed only to vertebrates are secreted by invertebrates as well (for example, the pancreatic hormone insulin). Likewise, many invertebrate hormones have been discovered in the tissues of vertebrates, including those of humans. Some of these molecules are even synthesized and employed as chemical regulators, similar to hormones in higher animals, by unicellular animals and plants. Thus, the history of endocrinologic regulators has ancient beginnings, and the major changes that took place during evolution would seem to centre around the uses to which these molecules were put.
Vertebrate endocrine systems
Vertebrates (phylum Vertebrata) are separable into at least seven discrete classes that represent evolutionary groupings of related animals with common features. The class Agnatha, or the jawless fishes, is the most primitive group. Class Chondrichthyes and class Osteichthyes are jawed fishes that had their origins, millions of years ago, with the Agnatha. The Chondrichthyes are the cartilaginous fishes, such as sharks and rays, while the Osteichthyes are the bony fishes. Familiar bony fishes such as goldfish, trout, and bass are members of the most advanced subgroup of bony fishes, the teleosts, which developed lungs and first invaded land. From the teleosts evolved the class Amphibia, which includes frogs and toads. The amphibians gave rise to the class Reptilia, which became more adapted to land and diverged along several evolutionary lines. Among the groups descending from the primitive reptiles were turtles, dinosaurs, crocodilians (alligators, crocodiles), snakes, and lizards. Birds (class Aves) and mammals (class Mammalia) later evolved from separate groups of reptiles. Amphibians, reptiles, birds, and mammals, collectively, are referred to as the tetrapod (four-footed) vertebrates.
The human endocrine system is the product of millions of years of evolution. and it should not be surprising that the endocrine glands and associated hormones of the human endocrine system have their counterparts in the endocrine systems of more primitive vertebrates. By examining these animals it is possible to document the emergence of the hypothalamic-pituitary-target organ axis, as well as many other endocrine glands, during the evolution of fishes that preceded the origin of terrestrial vertebrates.
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