Endocrine glands are glands of the
endocrine systemThe endocrine system is a system of glands that involve the release of extracellular signaling molecules known as hormones.The endocrine system is instrumental in regulating metabolism, growth, development, puberty, tissue function, internal environment and also plays a part in determining...
that secrete their products,
hormoneA hormone is a chemical released by one or more cells that affects cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. It is essentially a chemical messenger that transports a signal from one cell to another. All multicellular organisms...
s, directly into the
bloodBlood is a specialized bodily fluid that delivers necessary substances to the body's cells — such as nutrients and oxygen — and transports waste products away from those same cells....
rather than through a duct. The main endocrine glands include the
pituitary glandThe pituitary gland, or hypophysis, is an endocrine gland about the size of a pea and weighing 0.5 g . It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity covered by a dural fold...
,
pancreasThe pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as an exocrine gland, secreting pancreatic juice containing digestive enzymes that pass to...
,
ovariesThe ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in females are homologous to testes in males, in that they are both gonads and endocrine glands.-Human anatomy:...
,
testesThe testicle is the male generative gland in animals.The etymology of the word is based on Roman law...
,
thyroid glandThe thyroid is one of the largest endocrine glands in the body. This gland is found in the neck inferior to the thyroid cartilage and at approximately the same level as the cricoid cartilage...
, and
adrenal glandIn mammals, the adrenal glands are the star-shaped endocrine glands that sit on top of the kidneys; their name indicates that position...
s. The
hypothalamusThe hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland ....
is a
neuroendocrineNeuroendocrine [IPA nʊəroʊˈɛndəkrɪn] cells are cells that release a hormone into the circulating blood in response to a neural stimulus. These hormones may be amines, neuropeptides, or specialized amino acids. They package the hormones in vesicles and send these packages via long processes to...
organ. Other
organIn biology and anatomy, an organ is a collection of tissues joined in structural unit to serve a common function ....
s which are not so well known for their endocrine activity include the
stomachIn most mammals, the stomach is a hollow, muscular organ of the gastrointestinal tract , between the esophagus and the small intestine. It is involved in the second phase of digestion, following mastication . The word stomach is derived from the Latin stomachus, which derives from the Greek word...
, which produces such hormones as
ghrelinGhrelin is a hormone produced mainly by P/D1 cells lining the fundus of the human stomach and epsilon cells of the pancreas that stimulates hunger. Ghrelin levels increase before meals and decrease after meals. It is considered the counterpart of the hormone leptin, produced by adipose tissue,...
.
Local chemical messengers, not generally considered part of the endocrine system, include
autocrineAutocrine signaling is a form of signalling in which a cell secretes a hormone or chemical messenger that binds to autocrine receptors on the same type of cell, leading to changes in the cells...
s, which act on the cells that secrete them, and
paracrineParacrine signaling is a form of cell signaling in which the target cell is near the signal-releasing cell.A distinction is sometimes made between paracrine and autocrine signaling...
s, which act on a different cell type nearby.
Most hormones are steroid or amino acid based
Hormones alter cell activity by stimulating or inhibiting characteristic cellular processes of their target cells=].
Cell responses to hormone stimulation may involve changes in membrane permeability; enzyme synthesis, activation, or inhibition; secretory activity; gene activation; and mitosis.
Second-messenger mechanismsIn cell physiology, a secondary messenger system is a method of cellular signaling whereby a diffusable signaling molecule is rapidly produced/secreted, which can then go on to activate effector proteins within the cell to exert a cellular response...
employing intracellular messengers and transduced by
G proteinG proteins, short for guanine nucleotide-binding proteins, are a family of proteins involved in second messenger cascades.G proteins are so called because they function as "molecular switches," alternating between an inactive guanosine diphosphate and active guanosine triphosphate bound state,...
s are a common means by which
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
–based hormones interact with their target cells.
Endocrine glands are glands of the
endocrine systemThe endocrine system is a system of glands that involve the release of extracellular signaling molecules known as hormones.The endocrine system is instrumental in regulating metabolism, growth, development, puberty, tissue function, internal environment and also plays a part in determining...
that secrete their products,
hormoneA hormone is a chemical released by one or more cells that affects cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. It is essentially a chemical messenger that transports a signal from one cell to another. All multicellular organisms...
s, directly into the
bloodBlood is a specialized bodily fluid that delivers necessary substances to the body's cells — such as nutrients and oxygen — and transports waste products away from those same cells....
rather than through a duct. The main endocrine glands include the
pituitary glandThe pituitary gland, or hypophysis, is an endocrine gland about the size of a pea and weighing 0.5 g . It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity covered by a dural fold...
,
pancreasThe pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as an exocrine gland, secreting pancreatic juice containing digestive enzymes that pass to...
,
ovariesThe ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in females are homologous to testes in males, in that they are both gonads and endocrine glands.-Human anatomy:...
,
testesThe testicle is the male generative gland in animals.The etymology of the word is based on Roman law...
,
thyroid glandThe thyroid is one of the largest endocrine glands in the body. This gland is found in the neck inferior to the thyroid cartilage and at approximately the same level as the cricoid cartilage...
, and
adrenal glandIn mammals, the adrenal glands are the star-shaped endocrine glands that sit on top of the kidneys; their name indicates that position...
s. The
hypothalamusThe hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland ....
is a
neuroendocrineNeuroendocrine [IPA nʊəroʊˈɛndəkrɪn] cells are cells that release a hormone into the circulating blood in response to a neural stimulus. These hormones may be amines, neuropeptides, or specialized amino acids. They package the hormones in vesicles and send these packages via long processes to...
organ. Other
organIn biology and anatomy, an organ is a collection of tissues joined in structural unit to serve a common function ....
s which are not so well known for their endocrine activity include the
stomachIn most mammals, the stomach is a hollow, muscular organ of the gastrointestinal tract , between the esophagus and the small intestine. It is involved in the second phase of digestion, following mastication . The word stomach is derived from the Latin stomachus, which derives from the Greek word...
, which produces such hormones as
ghrelinGhrelin is a hormone produced mainly by P/D1 cells lining the fundus of the human stomach and epsilon cells of the pancreas that stimulates hunger. Ghrelin levels increase before meals and decrease after meals. It is considered the counterpart of the hormone leptin, produced by adipose tissue,...
.
Local chemical messengers, not generally considered part of the endocrine system, include
autocrineAutocrine signaling is a form of signalling in which a cell secretes a hormone or chemical messenger that binds to autocrine receptors on the same type of cell, leading to changes in the cells...
s, which act on the cells that secrete them, and
paracrineParacrine signaling is a form of cell signaling in which the target cell is near the signal-releasing cell.A distinction is sometimes made between paracrine and autocrine signaling...
s, which act on a different cell type nearby.
Chemistry
Most hormones are steroid or amino acid based
Hormones alter cell activity by stimulating or inhibiting characteristic cellular processes of their target cells=].
Cell responses to hormone stimulation may involve changes in membrane permeability; enzyme synthesis, activation, or inhibition; secretory activity; gene activation; and mitosis.
Second-messenger mechanismsIn cell physiology, a secondary messenger system is a method of cellular signaling whereby a diffusable signaling molecule is rapidly produced/secreted, which can then go on to activate effector proteins within the cell to exert a cellular response...
employing intracellular messengers and transduced by
G proteinG proteins, short for guanine nucleotide-binding proteins, are a family of proteins involved in second messenger cascades.G proteins are so called because they function as "molecular switches," alternating between an inactive guanosine diphosphate and active guanosine triphosphate bound state,...
s are a common means by which
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
–based hormones interact with their target cells. In the cyclic AMP system, the hormone binds to a plasma membrane receptor that couples to a G protein. When the G protein is activated it, in turn, couples to adenylate cyclase, which catalyzes the synthesis of cyclic AMP from ATP. Cyclic AMP initiates reactions that activate protein kinases and other enzymes, leading to cellular response. The PIP-calcium signal mechanism, involving phosphatidyl inositol, is another important second-messenger system. Other second messengers are cyclic GMP and calcium.
Steroid hormoneSteroid hormones are steroids that act as hormones. Steroid hormones can be grouped into five groups by the receptors to which they bind: glucocorticoids, mineralocorticoids, androgens, estrogens, and progestagens...
s (and
thyroid hormoneThe thyroid hormones, thyroxine and triiodothyronine , are tyrosine-based hormones produced by the thyroid gland. An important component in the synthesis of thyroid hormones is iodine. The major form of thyroid hormone in the blood is thyroxine , which has a longer half life than T3...
) enter their target cells and effect responses by activating DNA, which initiates messenger RNA formation leading to protein synthesis.
Target cell specificity
The ability of a target cell to respond to a hormone depends on the presence of receptors, within the cell or on its plasma membrane, to which the hormone can bind.
Hormone receptors are dynamic structures. Changes in number and sensitivity of hormone receptors may occur in response to high or low levels of stimulating hormones.
Blood levels of hormones reflect a balance between
secretionSecretion is the process of elaborating, releasing, and oozing chemicals from a cell, a secreted chemical substance or amount of substance. In contrast to excretion, the substance may have a certain function, rather than being a waste product....
and degradation/
excretionExcretion is the process of eliminating waste products of metabolism and other non-useful materials. It is an essential process in all forms of life. It contrasts secretion, where the substance may have specific tasks after leaving the cell....
. The
liverThe liver is a vital organ present in vertebrates and some other animals; it has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
and
kidneyThe kidneys are paired organs, which have the production of urine as their primary function. Kidneys are seen in many types of animals, including vertebrates and some invertebrates. They are part of the urinary system, but have several secondary functions concerned with homeostatic functions. ...
s are the major organs that degrade hormones; breakdown products are excreted in urine and feces.
Hormone half-life and duration of activity are limited and vary from hormone to hormone.
Interaction of hormones at target cells
PermissivenessIn biology, permissiveness is a certain relationship between hormones and the target cell. It can be applied to describe situations in which the presence of one hormone, at a certain concentration, is required in order to allow a second hormone to fully affect the target cell...
is the situation in which a hormone cannot exert its full effects without the presence of another hormone.
SynergismSynergism, in general, may be defined as two or more agents working together to produce a result not obtainable by any of the agents independently...
occurs when two or more hormones produce the same effects in a target cell and their results are amplified.
AntagonismAntagonism is hostility that results in active resistance, opposition, or contentiousness.Additionally, it may refer to:*Antagonism , where the involvement of multiple agents reduce their overall effect...
occurs when a hormone opposes or reverses the effect of another hormone.
Control of hormone release
Endocrine organs are activated to release their hormones by humoral, neural, or hormonal stimuli. Negative feedback is important in regulating hormone levels in the blood.
The
nervous systemThe nervous system is a network of specialized cells that communicate information about an organism's surroundings and itself. It processes this information and causes reactions in other parts of the body. It is composed of neurons and other specialized cells called glial cells that aid in the...
, acting through hypothalamic controls, can in certain cases override or modulate hormonal effects.
Pituitary gland (hypophysis)
The pituitary gland hangs from the base of the
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
by a stalk and is enclosed by bone. It consists of a hormone-producing glandular portion (anterior pituitary) and a neural portion (posterior pituitary), which is an extension of the
hypothalamusThe hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland ....
. The hypothalamus regulates the hormonal output of the anterior pituitary and synthesizes two hormones that it exports to the posterior pituitary of storage and later release.
Four of the six adenohypophyseal hormones are
tropic hormoneTropic hormones are hormones produced and secreted by the anterior pituitary which target endocrine glands. Tropic hormones include:*Thyroid-stimulating hormone – stimulates the thyroid gland to make and release thyroid hormone....
s that regulate the function of other endocrine organs. Most anterior pituitary hormones exhibit a diurnal rhythm of release, which is subject to modification by stimuli influencing the hypothalamus.
Somatotropic hormone or
Growth hormoneGrowth hormone is a protein-based poly-peptide hormone. It stimulates growth and cell reproduction and regeneration in humans and other animals. It is a 191-amino acid, single-chain polypeptide hormone that is synthesized, stored, and secreted by the somatotroph cells within the lateral wings of...
(GH) is an
anabolic hormoneAnabolism is the set of metabolic pathways that construct molecules from smaller units. These reactions require energy. One way of categorizing metabolic processes, whether at the cellular, organ or organism level is as 'anabolic' or as 'catabolic', which is the opposite...
that stimulates growth of all body tissues but especially skeletal muscle and bone. It may act directly, or indirectly via
insulin-like growth factorThe insulin-like growth factors are polypeptides with high sequence similarity to insulin. IGFs are part of a complex system that cells use to communicate with their physiologic environment...
s (IGFs). GH mobilizes fats, stimulates protein synthesis, and inhibits glucose uptake and metabolism. Secretion is regulated by
growth hormone releasing hormoneGrowth-hormone-releasing hormone , also known as growth-hormone-releasing factor or somatocrinin, is a releasing hormone for growth hormone...
(GHRH) and
growth hormone inhibiting hormoneSomatostatin is a peptide hormone that regulates the endocrine system and affects neurotransmission and cell proliferation via interaction with G-protein-coupled somatostatin receptors and inhibition of the release of numerous secondary hormones.Somatostatin...
(GHIH), or somatostatin. Hypersecretion causes
gigantismGigantism, also known as gigantism, is a condition characterized by excessive growth and height significantly above average.-Types:As a medical term, gigantism can refer to:...
in children and
acromegalyAcromegaly is a syndrome that results when the pituitary gland produces excess growth hormone after epiphyseal plate closure at puberty...
in adults; hyposecretion in children causes
pituitary dwarfismGrowth Hormone Deficiency is a medical condition in which the body does not produce enough growth hormone . Growth hormone, also called somatotropin, is a polypeptide hormone which stimulates growth and cell reproduction...
.
Thyroid-stimulating hormoneThyroid-stimulating hormone is a peptide hormone synthesized and secreted by thyrotrope cells in the anterior pituitary gland which regulates the endocrine function of the thyroid gland....
(TSH) promotes normal development and activity of the thyroid gland.
Thyrotropin-releasing hormoneThyrotropin-releasing hormone , also called thyrotropin-releasing factor , thyroliberin or protirelin, is a tropic tripeptide hormone that stimulates the release of thyroid-stimulating hormone and prolactin by the anterior pituitary.- Synthesis :TRH is produced by the hypothalamus in medial neurons...
(TRH) stimulates its release; negative feedback of thyroid hormone inhibits it.
Adrenocorticotropic hormoneAdrenocorticotropic hormone , also known as corticotropin, is a polypeptide tropic hormone produced and secreted by the anterior pituitary gland. It is an important component of the hypothalamic-pituitary-adrenal axis and is often produced in response to biological stress...
(ACTH) stimulates the adrenal cortex to release corticosteroids. ACTH release is triggered by corticotropin-releasing hormone (CRH) and inhibited by rising glucocorticoid levels.
The
gonadotropinGonadotropins are protein hormones secreted by gonadotrope cells of the pituitary gland of vertebrates.Gonadotropin is sometimes abbreviated Gn. The British spelling is gonadotrophin.-Types:...
s—
follicle-stimulating hormoneFollicle-stimulating hormone is a hormone found in humans and other animals. It is synthesized and secreted by gonadotropes of the anterior pituitary gland. FSH regulates the development, growth, pubertal maturation, and reproductive processes of the body. FSH and Luteinizing hormone act...
(FSH) and
luteinizing hormoneLuteinizing hormone is a hormone produced by the anterior pituitary gland.* In the female, an acute rise of LH – the LH surge – triggers ovulation and corpus luteum development....
(LH) regulate the functions of the gonads in both sexes. FSH stimulates sex cell production; LH stimulates gonadal hormone production. Gonadotropin levels rise in response to
gonadotropin-releasing hormoneGonadotropin-releasing hormone , also known as Luteinizing-hormone releasing hormone and luliberin, is a tropic peptide hormone responsible for the release of FSH and LH from the anterior pituitary...
(GnRH). Negative feedback of gonadal hormones inhibits gonadotropin release.
ProlactinProlactin or Luteotropic hormone is a peptide hormone discovered by Dr. Henry Friesen, primarily associated with lactation. In breastfeeding, the act of an infant suckling the nipple stimulates the production of prolactin, which fills the breast with milk via a process called lactogenesis, in...
(PRL) promotes milk production in humans. Its secretion is prompted by
prolactin-releasing hormoneProlactin releasing hormone, also known as PRLH, is a human hormone.-Further reading:...
(PRH) and inhibited by prolactin-inhibiting hormone (PIH).
The
neurohypophysisThe posterior pituitary comprises the posterior lobe of the pituitary gland and is part of the endocrine system. Despite its name, the posterior pituitary gland is not a gland, per se; rather, it is largely a collection of axonal projections from the hypothalamus that terminate behind the anterior...
stores and releases two hypothalamic hormones:
- Oxytocin
Oxytocin is a mammalian hormone that also acts as a neurotransmitter in the brain.It is best known for its roles in female reproduction: it is released in large amounts after distension of the cervix and vagina during labor, and after stimulation of the nipples, facilitating birth and...
stimulates powerful uterine contractions, which trigger labor and delivery of an infant, and milk ejection in nursing women. Its release is mediated reflexively by the hypothalamus and represents a positive feedback mechanism.
- Antidiuretic hormone
Arginine vasopressin , also known as vasopressin, argipressin or antidiuretic hormone , is a hormone found in most mammals, including humans. Vasopressin is a peptide hormone. It is derived from a preprohormone precursor that is synthesized in the hypothalamus and stored in vesicles at the...
(ADH) stimulates the kidney tubules to reabsorb and conserve water, resulting in small volumes of highly concentrated urine and decreased plasma osmolality. ADH is released in response to high solute concentrations in the blood and inhibited by low solute concentrations in the blood. Hyposecretion results in diabetes insipidus.
Thyroid gland
The thyroid gland is located in the anterior throat. Thyroid follicles store colloid containing
thyroglobulinThyroglobulin is a 660 kDa, dimeric protein produced by and used entirely within the thyroid gland. In earlier literature, Tg was referred to as "colloid"....
, a glycoprotein from which thyroid hormone is derived.
Thyroid hormoneThe thyroid hormones, thyroxine and triiodothyronine , are tyrosine-based hormones produced by the thyroid gland. An important component in the synthesis of thyroid hormones is iodine. The major form of thyroid hormone in the blood is thyroxine , which has a longer half life than T3...
(TH) includes
thyroxineThyroxine, or 3,5,3',5'-tetraiodothyronine , a form of thyroid hormones is the major hormone secreted by the follicular cells of the thyroid gland. Thyroxine is synthesized via the iodination and covalent bonding of the phenyl portions of tyrosine residues found in an initial peptide,...
(T4) and
triiodothyronineTriiodothyronine, C
15H
12I
3NO
4, also known as T
3, is a thyroid hormone.Thyroid-stimulating hormone activates the production of thyroxine and T
3. This process is under regulation. In the thyroid, T
4 is converted to...
(T3), which increase the rate of
cellular metabolismMetabolism is the set of chemical reactions that occur in living organisms to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolism breaks down organic matter,...
. Consequently, oxygen use and heat production rise.
Secretion of thyroid hormone, prompted by TSH, requires reuptake of the stored colloid by the follicle cells and splitting of the hormones from the colloid for release. Rising levels of thyroid hormone feed back to inhibit the pituitary and hypothalamus.
Most T4 is converted to T3 (the more active form) in the target tissues. These hormones act by turning on gene transcription and protein synthesis.
Graves' diseaseGraves' disease is an autoimmune disease. It most commonly affects the thyroid, frequently causing it to enlarge to twice its size or more , become overactive, with related hyperthyroid symptoms such as increased heartbeat, muscle weakness, disturbed sleep, and irritability. It can also affect the...
is the most common cause of
hyperthyroidismHyperthyroidism is the term for overactive tissue within the thyroid gland,resulting in overproduction and thus an excess of circulating free thyroid hormones: thyroxine , triiodothyronine , or both...
; hyposecretion causes
cretinismCretinism is a condition of severely stunted physical and mental growth due to untreated congenital deficiency of thyroid hormones or from prolonged nutritional deficiency of iodine.-Etymology and use of cretin:...
in infants and
myxedemaHypothyroid typemyxedema describes a specific form of cutaneous and dermal non-pitting edema secondary to increased deposition of connective tissue matrix components...
in adults.
CalcitoninCalcitonin is a 32-amino acid linear polypeptide hormone that is producedin humans primarily by the parafollicular cells of the thyroid, and in many other animals in the ultimobranchial body. It acts to reduce blood calcium , opposing the effects of parathyroid hormone . It has been found in fish,...
, produced by the parafollicular cells of the thyroid gland in response to rising blood calcium levels, depresses blood calcium levels by inhibiting bone matrix resorption and enhancing calcium deposit in bone.
Parathyroid glands
The parathyroid glands, located on the dorsal aspect of the thyroid gland, secrete
parathyroid hormoneParathyroid hormone , or parathormone, is secreted by the parathyroid glands as a polypeptide containing 84 amino acids. It acts to increase the concentration of calcium in the blood, whereas calcitonin acts to decrease calcium concentration.PTH acts to increase the concentration...
(PTH), which causes an increase in blood calcium levels by targeting bone, the intestine, and the kidneys. PTH is the antagonist of
calcitoninCalcitonin is a 32-amino acid linear polypeptide hormone that is producedin humans primarily by the parafollicular cells of the thyroid, and in many other animals in the ultimobranchial body. It acts to reduce blood calcium , opposing the effects of parathyroid hormone . It has been found in fish,...
. PTH release is triggered by falling blood calcium levels and is inhibited by rising blood calcium levels.
HyperparathyroidismHyperparathyroidism is overactivity of the parathyroid glands resulting in excess production of parathyroid hormone . The parathyroid hormone regulates calcium and phosphate levels and helps to maintain these levels...
results in
hypercalcaemiaHypercalcaemia is an elevated calcium level in the blood. . It can be an asymptomatic laboratory finding, but because an elevated calcium level is often indicative of other diseases, a diagnosis should be undertaken if it persists...
and all its effects and in extreme bone wasting.
HypoparathyroidismIn medicine , hypoparathyroidism is decreased function of the parathyroid glands, leading to decreased levels of parathyroid hormone...
leads to
hypocalcaemiaIn medicine, hypocalcemia is the presence of low serum calcium levels in the blood, usually taken as less than 2.1 mmol/L or 9 mg/dl or an ionized calcium level mm of less than 1.1 mmol/L . It is a type of electrolyte disturbance...
, evidenced by tetany and respiratory paralysis.
Pancreas
The pancreas, located in the abdomen close to the stomach, is both an
exocrineExocrine glands are glands that secrete their products into ducts . They are the counterparts to endocrine glands, which secrete their products directly into the bloodstream or release hormones that affect only target cells nearby the release site.-Examples:Typical exocrine glands include sweat...
and an endocrine gland. The endocrine portion (pancreatic islets) releases insulin and glucagon and smaller amounts of other hormones to the blood.
GlucagonGlucagon is an important hormone involved in carbohydrate metabolism. Produced by the pancreas, it is released when blood glucose levels start to fall too low, causing the liver to convert stored glycogen into glucose and release it into the bloodstream, raising blood glucose levels and ultimately...
, released by alpha (α) cells when blood levels of glucose are low, stimulates the liver to release
glucoseGlucose , a monosaccharide also known as - grape sugar, blood sugar, or corn sugar, is a very important carbohydrate in biology. The living cell uses it as a source of energy and metabolic intermediate...
to the blood.
InsulinInsulin is a hormone that has extensive effects on metabolism and other body functions, such as vascular compliance. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle, and stopping use of fat as an energy...
is released by beta (β) cells when blood levels of glucose (and amino acids) are rising. It increases the rate of glucose uptake and metabolism by most body cells. Hyposecretion of insulin results in diabetes mellitus; cardinal signs are polyuria, polydipsia, and polyphagia.
Gonads
The ovaries of the female, located in the pelvic cavity, release two main hormones. Secretion of
estrogenEstrogens are a group of steroid compounds, named for their importance in the estrous cycle, and functioning as the primary female sex hormone, their name comes from estrus/oistros + gen/gonos = to generate.Estrogens are used as part of some oral contraceptives, in estrogen replacement...
s by the ovarian follicles begins at puberty under the influence of FSH. Estrogens stimulate maturation of the female reproductive system and development of the secondary sex characteristics.
ProgesteroneProgesterone also known as P4 is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy and embryogenesis of humans and other species...
is released in response to high blood levels of LH. It works with estrogens in establishing the
menstrual cycleThe menstrual cycle is a cycle of physiological changes that occurs in fertile females. Overt menstruation occurs primarily in humans and close evolutionary relatives such as chimpanzees...
.
The testes of the male begin to produce
testosteroneTestosterone is a steroid hormone from the androgen group. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands. It is the principal male sex hormone and an anabolic steroid.In men,...
at puberty in response to LH. Testosterone promotes maturation of the male reproductive organs, development of secondary sex characteristics, and production of sperm by the testes.
Pineal gland
The pineal gland is located in the
diencephalonThe diencephalon is the region of the brain that includes the thalamus, hypothalamus, epithalamus, prethalamus or subthalamus and pretectum. The diencephalon is located at the midline of the brain, above the mesencephalon of the brain stem...
. Its primary hormone is
melatoninMelatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring hormone found in animals and in some other living organisms, including algae. Circulating levels vary in a daily cycle, and melatonin is important in the regulation of the circadian rhythms of several...
, which influences daily rhythms and may have an antigonadotropic effect in humans.
Other hormone-producing structures
Many body organs not normally considered endocrine organs contain isolated cell clusters that secrete hormones. Examples include the
heartThe heart is a muscular organ found in all vertebrates that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
(
atrial natriuretic peptideAtrial natriuretic peptide , atrial natriuretic factor , atrial natriuretic hormone , or atriopeptin, is a powerful vasodilator, and a protein hormone secreted by heart muscle cells. It is involved in the homeostatic control of body water, sodium, potassium and fat...
);
gastrointestinal tractThe human gastrointestinal tract , digestive tract, guts or gut is the system of organs within humans that takes in food, digests it to extract energy and nutrients, and expels the remaining matter...
organs (
gastrinIn humans, gastrin is a hormone that stimulates secretion of gastric acid by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the stomach, duodenum, and the pancreas. Its release is stimulated by gastric luminal peptides...
,
secretinSecretin is a hormone that controls the secretions into the duodenum, and also separately, water homeostasis throughout the body.Acting as an anti-Müllerian hormone it is produced in the S cells of the duodenum in the crypts of Lieberkühn. Its effect is to regulate the pH of the duodenal contents...
, and others); the
placentaThe placenta is an organ unique to mammals that connects the developing fetus to the uterine wall. The placenta supplies the fetus with oxygen and food, and allows fetal waste to be disposed of via the maternal kidneys...
(hormones of pregnancy—
estrogenEstrogens are a group of steroid compounds, named for their importance in the estrous cycle, and functioning as the primary female sex hormone, their name comes from estrus/oistros + gen/gonos = to generate.Estrogens are used as part of some oral contraceptives, in estrogen replacement...
,
progesteroneProgesterone also known as P4 is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy and embryogenesis of humans and other species...
, and others); the
kidneyThe kidneys are paired organs, which have the production of urine as their primary function. Kidneys are seen in many types of animals, including vertebrates and some invertebrates. They are part of the urinary system, but have several secondary functions concerned with homeostatic functions. ...
s (
erythropoietinErythropoietin, or its alternative erythropoetin or EPO, is a glycoprotein hormone that controls erythropoiesis, or red blood cell production...
and
reninRenin , also known as Angiotensinogenase, is a circumsized enzyme that participates in the body's renin-angiotensin system that mediates extracellular volume , and arterial vasoconstriction...
);
skinThe skin is the outer covering of the body. In humans, it is the largest organ of the integumentary system made up of multiple layers of mesodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs. Skin of a different nature exists in amphibians, reptiles, birds...
(
cholecalciferolCholecalciferol is a form of Vitamin D, also called vitamin D
3 or calciol.It is structurally similar to steroids such as testosterone, cholesterol, and cortisol ....
); and
adipose tissueIn histology, adipose tissue or body fat or just fat is loose connective tissue composed of adipocytes. Adipose tissue is derived from lipoblasts. Its main role is to store energy in the form of fat, although it also cushions and insulates the body...
(
leptinLeptin is a 16 kDa protein hormone that plays a key role in regulating energy intake and energy expenditure, including appetite and metabolism. It is one of the most important adipose derived hormones...
and
resistinResistin is a cysteine-rich protein secreted by adipose tissue of mice and rats. In other mammals, at least primates, pigs and dogs, resistin is secreted by immune and epithelial cells. Resistin is also known as CEBPE regulated myeloid-specific secreted cysteine-rich protein precursor 1 , found in...
).
Developmental aspects of the endocrine system
Endocrine glands derive from all three germ layers. Those derived from mesoderm produce steroidal hormones; the others produce the amino acid–based hormones.
The natural decrease in function of the female’s ovaries during late middle age results in
menopauseMenopause is the time in a woman’s life when menstruation ends. It is part of a biological process that begins, for most women, in their mid-fifties. During this time, the ovaries gradually produce lower levels of natural sex hormones—estrogen and progesterone...
. The efficiency of all endocrine glands seems to decrease gradually as aging occurs. This leads to a generalized increase in the incidence of
diabetes mellitusDiabetes mellitus —often referred to simply as diabetes—is a condition in which the body either does not produce enough, or does not properly respond to, insulin, a hormone produced in the pancreas. Insulin enables cells to absorb glucose in order to turn it into energy...
and a lower
metabolic rateBasal metabolic rate , and the closely related resting metabolic rate , is the amount of energy expended while at rest in a [neutrally temperate environment, in the post-absorptive state...
.