| Endocrine system | |
|---|---|
Main
glands of the endocrine system. Note that the thymus is no longer
considered part of the endocrine system, as it does not produce
hormones.
| |
| Details | |
| Identifiers | |
| Latin | Systema endocrinum |
| MeSH | D004703 |
| FMA | t6uk5iii8iu67i6i6ii78ii8 9584, t6uk5iii8iu67i6i6ii78ii8 |
The endocrine system is a chemical messenger system comprising feedback loops of hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In humans, the major endocrine glands are the thyroid gland and the adrenal glands. In vertebrates, the hypothalamus is the neural control center for all endocrine systems. The study of the endocrine system and its disorders is known as endocrinology. Endocrinology is a branch of internal medicine.
A number of glands that signal each other in sequence are usually referred to as an axis, for example, the hypothalamic-pituitary-adrenal axis. In addition to the specialized endocrine organs mentioned above, many other organs that are part of other body systems, inluding bone, kidney, liver, heart and gonads, have secondary endocrine functions. For example, the kidney secretes endocrine hormones such as erythropoietin and renin. Hormones can consist of either amino acid complexes, steroids, eicosanoids, leukotrienes, or prostaglandins.
The endocrine system can be contrasted to both exocrine glands, which secrete hormones to the outside of the body using ducts and paracrine signalling between cells over a relatively short distance. Endocrince glands have no ducts, are vascular and commonly have intracellular vacuoles or granules that store their hormones. In contrast, exocrine glands, such as salivary glands, sweat glands, and glands within the gastrointestinal tract, tend to be much less vascular and have ducts or a hollow lumen.
The word endocrine derives via New Latin from the Greek words ἔνδον, endon, "inside, within," and "crine" from the κρίνω, krīnō, "to separate, distinguish".
Structure
Major endocrine systems
The human endocrine system consists of several systems that operate via feedback loops. Several important feedback systems are mediated via the hypothalamus and pituitary.
- TRH – TSH – T3/T4
- GnRH – LH/FSH – sex hormones
- CRH – ACTH – cortisol
- Renin – angiotensin – aldosterone
- leptin vs. insulin
Glands
Endocrine glands are glands of the endocrine system that secrete their products, hormones,
directly into interstitial spaces and then absorbed into blood rather
than through a duct. The major glands of the endocrine system include
the pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus and adrenal glands. The hypothalamus and pituitary gland are neuroendocrine organs.
Cells
There are
many types of cells that comprise the endocrine system and these cells
typically make up larger tissues and organs that function within and
outside of the endocrine system.
- Thyroid Gland
- Follicular cells of the thyroid gland produce and secrete T3 and T4 in response to elevated levels of TRH, produced by the hypothalamus, and subsequent elevated levels of TSH, produced by the anterior pituitary gland, which further regulates the metabolic activity and rate of all cells, including cell growth and tissue differentiation.
- Parathyroid Gland
- Epithelial cells of the parathyroid glands are richly supplied with blood from the inferior and superior thyroid arteries and secrete parathyroid hormone (PTH). PTH acts on bone, the kidneys, and the GI tract to increase calcium reabsorption and phosphate excretion. In addition, PTH stimulates the conversion of Vitamin D to its most active variant, 1,25-dihydroxyvitamin D3, which further stimulates calcium absorption in the GI tract.
Function
Hormones
A hormone is any of a class of signaling molecules produced by glands in multicellular organisms that are transported by the circulatory system to target distant organs to regulate physiology and behaviour. Hormones have diverse chemical structures, mainly of 3 classes: eicosanoids, steroids, and amino acid/protein derivatives (amines, peptides, and proteins).
The glands that secrete hormones comprise the endocrine system. The
term hormone is sometimes extended to include chemicals produced by
cells that affect the same cell (autocrine or intracrine signalling) or nearby cells (paracrine signalling).
Hormones are used to communicate between organs and tissues for physiological regulation and behavioral activities, such as digestion, metabolism, respiration, tissue function, sensory perception, sleep, excretion, lactation, stress, growth and development, movement, reproduction, and mood.
Hormones affect distant cells by binding to specific receptor
proteins in the target cell resulting in a change in cell function.
This may lead to cell type-specific responses that include rapid changes
to the activity of existing proteins, or slower changes in the expression of target genes. Amino acid–based hormones (amines and peptide or protein hormones) are water-soluble and act on the surface of target cells via signal transduction pathways; steroid hormones, being lipid-soluble, move through the plasma membranes of target cells to act within their nuclei.
Cell signalling
The typical mode of cell signalling
in the endocrine system is endocrine signaling, that is, using the
circulatory system to reach distant target organs. However, there are
also other modes, i.e., paracrine, autocrine, and neuroendocrine signaling. Purely neurocrine signaling between neurons, on the other hand, belongs completely to the nervous system.
Autocrine
Autocrine signaling is a form of signaling in which a cell secretes a
hormone or chemical messenger (called the autocrine agent) that binds
to autocrine receptors on the same cell, leading to changes in the
cells.
Paracrine
Some endocrinologists and clinicians include the paracrine system as
part of the endocrine system, but there is not consensus. Paracrines are
slower acting, targeting cells in the same tissue or organ. An example
of this is somatostatin which is released by some pancreatic cells and targets other pancreatic cells.
Juxtacrine
Juxtacrine signaling is a type of intercellular communication that is
transmitted via oligosaccharide, lipid, or protein components of a cell
membrane, and may affect either the emitting cell or the immediately
adjacent cells.
It occurs between adjacent cells that possess broad patches of
closely opposed plasma membrane linked by transmembrane channels known
as connexons. The gap between the cells can usually be between only 2
and 4 nm.
Clinical significance
Disease
Disability-adjusted life year for endocrine disorders per 100,000 inhabitants in 2002.
no data
less than 80
80–160
160–240
240–320
320–400
400–480
480–560
560–640
640–720
720–800
800–1000
more than 1000
Diseases of the endocrine system are common, including conditions such as diabetes mellitus, thyroid disease, and obesity.
Endocrine disease is characterized by misregulated hormone release (a productive pituitary adenoma), inappropriate response to signaling (hypothyroidism), lack of a gland (diabetes mellitus type 1, diminished erythropoiesis in chronic renal failure), or structural enlargement in a critical site such as the thyroid (toxic multinodular goitre). Hypofunction of endocrine glands can occur as a result of loss of reserve, hyposecretion, agenesis, atrophy, or active destruction. Hyperfunction can occur as a result of hypersecretion, loss of suppression, hyperplastic or neoplastic change, or hyperstimulation.
Endocrinopathies are classified as primary, secondary, or
tertiary. Primary endocrine disease inhibits the action of downstream
glands. Secondary endocrine disease is indicative of a problem with the
pituitary gland. Tertiary endocrine disease is associated with
dysfunction of the hypothalamus and its releasing hormones.
As the thyroid, and hormones have been implicated in signaling distant tissues to proliferate, for example, the estrogen receptor has been shown to be involved in certain breast cancers. Endocrine, paracrine, and autocrine signaling have all been implicated in proliferation, one of the required steps of oncogenesis.
Other common diseases that result from endocrine dysfunction include Addison's disease, Cushing's disease and Graves' disease.
Cushing's disease and Addison's disease are pathologies involving the
dysfunction of the adrenal gland. Dysfunction in the adrenal gland could
be due to primary or secondary factors and can result in
hypercortisolism or hypocortisolism . Cushing's disease is characterized
by the hypersecretion of the adrenocorticotropic hormone (ACTH) due to a
pituitary adenoma that ultimately causes endogenous hypercortisolism by
stimulating the adrenal glands. Some clinical signs of Cushing's disease include obesity, moon face, and hirsutism.
Addison's disease is an endocrine disease that results from
hypocortisolism caused by adrenal gland insufficiency. Adrenal
insufficiency is significant because it is correlated with decreased
ability to maintain blood pressure and blood sugar, a defect that can
prove to be fatal.
Graves' disease involves the hyperactivity of the thyroid gland which produces the T3 and T4 hormones. Graves' disease effects range from excess sweating, fatigue, heat intolerance and high blood pressure to swelling of the eyes that causes redness, puffiness and in rare cases reduced or double vision.
Other animals
A neuroendocrine system has been observed in all animals with a nervous system and all vertebrates have an hypothalamus-pituitary axis. All vertebrates have a thyroid, which in amphibians is also crucial for transformation of larvae into adult form. All vertebrates have adrenal gland tissue, with mammals unique in having it organized into layers. All vertebrates have some form of a renin–angiotensin axis, and all tetrapods have aldosterone as a primary mineralocorticoid.
Additional images
Female endocrine system.
Male endocrine system
