Hormone
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What are hormones?
Hormones are organic substances secreted by plants and animals. In animals, hormones regulate physiological activities and maintain homeostasis. Hormones trigger responses from specific organs or tissues adapted to react to minute quantities of them.
Traditionally, hormones are transmitted through the bloodstream after being discharged from glands, known as endocrine secretion. However, the definition has broadened to include regulatory substances distributed by diffusion across cell membranes instead of the bloodstream.
Most hormones are either protein hormones (including peptides and modified amino acids) or steroid hormones. Protein hormones are generally water-soluble for easy transport through the blood. Steroid hormones, synthesized from cholesterol, circulate in the blood both freely and bound to specific proteins.
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How do hormones work in the human body?
Hormones are organic substances that regulate physiological activities and maintain homeostasis in the body. Endocrine glands secrete hormones into the bloodstream, which then travel to target organs or tissues. Hormones trigger responses in these specific areas, even in minute quantities, by binding to receptors. Protein and peptide hormones bind to receptors on the surface of target cells, while steroid and thyroid hormones bind to receptors inside the cells. This hormone-receptor complex then activates chemical responses within the target tissue, leading to changes such as enzyme activation or protein synthesis.
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Where are hormones produced in the body?
Hormones are produced in various secretory organs that make up the human endocrine system, including the anterior pituitary gland, the adrenal glands, the pancreas, the thyroid gland, ovaries, and testes.
Additionally, other tissues such as the lungs, heart, skeletal muscles, kidneys, gastrointestinal tract lining, and nerve cell bundles can also function as endocrine organs. The hypothalamus produces neurohormones affecting the anterior pituitary gland, as well as vasopressin and oxytocin, which act on distant target organs.
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What are the main types of hormones based on their chemical structure?
Hormones can be categorized based on their chemical structure into a few major types.
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Peptide and protein hormones These hormones consist of amino acids linked together. They can range in size from small peptides to large, complex proteins. Examples include insulin and glucagon.
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Steroid hormones These are derived from cholesterol and have a characteristic ring structure. Examples include estrogen, testosterone, and cortisol.
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Amino acid derivatives Some hormones are modified from single amino acids. For instance, thyroxine is derived from tyrosine.
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Eicosanoids These are a class of molecules derived from polyunsaturated fatty acids.
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How do hormones help maintain balance in the body (homeostasis)?
Hormones are essential for homeostasis, helping the body maintain a stable internal environment. The endocrine system uses hormones to integrate body activities and ensure that the composition of fluids bathing cells remains constant.
Hormones regulate electrolyte concentrations, plasma volume, and nutrients/fuels for body metabolism. For instance, insulin is secreted after eating to stimulate glucose uptake by tissues and inhibit glucose production by the liver, maintaining blood glucose levels. This ensures glucose is available when needed and stored when in abundance.
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What can happen if the body makes too much or too little of a certain hormone?
Hormones are secreted by specific cells in endocrine glands and regulate chemical reactions by binding to receptors on target cells. Too much or too little of a hormone can disrupt the body's balance, leading to various disorders.
Overproduction (Hyperfunction):
* Hyperfunction may result from hyperplasia (increase in cell number) or neoplasia (tumor growth) in an endocrine gland. Tumors can be benign or malignant and may cause excess hormone secretion, leading to conditions like hyperthyroidism or Cushing's syndrome. In Graves disease, antibodies stimulate thyroid hormone receptors, causing uncontrolled hormone production and thyroid hyperplasia.Underproduction (Hypofunction):
* Hypofunction often results from the destruction of hormone-secreting cells due to infection, tissue death, cancer, or autoimmune diseases. It can also stem from congenital defects, genetic mutations, or substances blocking fetal hormone production. Hypofunction can be compensatory, like decreased thyroid hormone production during starvation, or true endocrine hypofunction, such as hypothyroidism due to iodine deficiency.3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
News •
Hormone, organic substance secreted by plants and animals that functions in the regulation of physiological activities and in maintaining homeostasis. Hormones carry out their functions by evoking responses from specific organs or tissues that are adapted to react to minute quantities of them. The classical view of hormones is that they are transmitted to their targets in the bloodstream after discharge from the glands that secrete them. This mode of discharge (directly into the bloodstream) is called endocrine secretion. The meaning of the term hormone has been extended beyond the original definition of a blood-borne secretion, however, to include similar regulatory substances that are distributed by diffusion across cell membranes instead of by a blood system.
General features
Relationships between endocrine and neural regulation
Hormonal regulation is closely related to that exerted by the nervous system, and the two processes have generally been distinguished by the rate at which each causes effects, the duration of these effects, and their extent; i.e., the effects of endocrine regulation may be slow to develop but prolonged in influence and widely distributed through the body, whereas nervous regulation is typically concerned with quick responses that are of brief duration and localized in their effects. Advances in knowledge, however, have modified these distinctions.
Nerve cells are secretory, for responses to the nerve impulses that they propagate depend upon the production of chemical transmitter substances, or neurotransmitters, such as acetylcholine and norepinephrine (noradrenaline), which are liberated at nerve endings in minute amounts and have only a momentary action. It has been established, however, that certain specialized nerve cells, called neurosecretory cells, can translate neural signals into chemical stimuli by producing secretions called neurohormones. These secretions, which are often polypeptides (compounds similar to proteins but composed of fewer amino acids), pass along nerve-cell extensions, or axons, and are typically released into the bloodstream at special regions called neurohemal organs, where the axon endings are in close contact with blood capillaries. Once released in this way, neurohormones function in principle similar to hormones that are transmitted in the bloodstream and are synthesized in the endocrine glands.
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The distinctions between neural and endocrine regulation, no longer as clear-cut as they once seemed to be, are further weakened by the fact that neurosecretory nerve endings are sometimes so close to their target cells that vascular transmission is not necessary. There is good evidence that hormonal regulation occurs by diffusion in plants and (although here the evidence is largely indirect) in lower animals (e.g., coelenterates), which lack a vascular system.
The evolution of hormones
Hormones have a long evolutionary history, knowledge of which is important if their properties and functions are to be understood. Many important features of the vertebrate endocrine system, for example, are present in the lampreys and hagfishes, modern representatives of the primitively jawless vertebrates (Agnatha), and these features were presumably present in fossil ancestors that lived more than 500 million years ago. The evolution of the endocrine system in the more advanced vertebrates with jaws (Gnathostomata) has involved both the appearance of new hormones and the further evolution of some of those already present in agnathans; in addition, extensive specialization of target organs has occurred to permit new patterns of response.
The factors involved in the first appearance of the various hormones is largely a matter for conjecture, although hormones clearly are only one mechanism for chemical regulation, diverse forms of which are found in living things at all stages of development. Other mechanisms for chemical regulation include chemical substances (so-called organizer substances) that regulate early embryonic development and the pheromones that are released by social insects as sex attractants and regulators of the social organization. Perhaps, in some instances, chemical regulators including hormones appeared first as metabolic by-products. A few such substances are known in physiological regulation: carbon dioxide, for example, is involved in the regulation of the respiratory activity of which it is a product, in insects as well as in vertebrates. Substances such as carbon dioxide are called parahormones to distinguish them from true hormones, which are specialized secretions.