The endocrine mechanism comprises glands and tissues that develop hormones for regulating and also coordinating an important bodily functions. This article, the first in one eight-part series, is review of the system


The endocrine device is made up of glands and tissues that produce and also secrete hormones to regulate and also coordinate critical bodily functions. This article – the first in an eight-part series on the anatomy and also physiology the the endocrine mechanism – explores the nature that endocrine glands and also tissues, and the function of hormones as chemical signals the are carried in the blood. It likewise highlights the varying duties of hormone in regulating and coordinating physiological processes, as well as maintaining homoeostasis in the body.

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Citation: article J (2021) Endocrine mechanism I: summary of the endocrine system and also hormones. Nursing Times ; 117: 5, 38-42.

Author: john Knight is combine professor in biomedical science, university of Human and Health Sciences, Swansea University.

This post has to be double-blind peer reviewed


The endocrine mechanism is a series of glands and tissues that produce and secrete hormones, i m sorry are supplied by the body to regulate and coordinate crucial bodily functions, including growth and development, metabolism, sexual duty and reproduction, sleep and also mood. This article – the an initial in one eight-part collection on the anatomy and also physiology that the endocrine device – provides an overview of the system, focusing on endocrine glands and tissues, and the duty of hormones together chemical signals that are bloodborne. It also explains the diverse roles of hormone in regulating and also coordinating physiological processes, and also maintaining homoeostatic balance in the body.

The endocrine device (Fig 1) is very complex: it is composed of dedicated, specialty endocrine glands – such together the thyroid, parathyroids and also adrenal glands – in addition to tissues such as fat (adipose tissue) and bone that have actually a second endocrine function and additionally secrete a range of hormones. It has been argued that the microbial biology (the diverse plethora the micro-organisms colonising the person body) additionally functions together a “virtual endocrine organ”, secreting a cocktail that chemical signal that further influences human being physiology (O’Callaghan et al, 2016).


Endocrine and also exocrine glands

By definition, every glandular tissues produce secretions. Most glandular structures are epithelial in origin, and also many are folded and also organised into recognisable glands through a central duct. Glands own a duct are exocrine glands (Fig 2); the duct acts as a conduit right into which secretions are released prior to being lugged away to their sites of action. Exocrine glands include plenty of of the cradle glands in the gut, sweat glands in the skin and mucus-producing glands in the mucous membranes of the mouth and reproductive tracts.


In contrast, endocrine glands have no duct, yet release their secretions, referred to as hormones, straight into the blood (Fig 2). For this reason, many endocrine glands are extremely vascularised, and also many of their component cells are in direct contact with blood capillaries. This nearby association with blood vessels facilitates the straight release that hormones right into the blood and enables the blood to be repeatedly monitored because that physiological changes that have the right to initiate hormone release. As an instance of this, the insulin-producing cell of the pancreas will certainly release insulin when they detect boost in blood–glucose concentration after the usage of carbohydrate-rich food.

The extremely vascular nature that endocrine glands also permits for the distribution of signal (usually various other hormones) from various other glands to regulate release of their very own hormones. For example, the thyroid gland releases hormone that manage metabolism, such as thyroxine, in response to the thyroid-stimulating hormone, which is produced by the anterior pituitary gland.

The significant endocrine glands

Fig 1 shows the place of the major endocrine glands in the body; however, that is necessary to be mindful that numerous other organs and tissues have actually a second endocrine function, consisting of the heart, kidneys, bone and also adipose organization (Knight et al, 2020; Moser and also van der Eerden, 2019).

The hypothalamus

The hypothalamus is a crucial region the the brain, i m sorry plays crucial role in:

Thermoregulation;Behavioural and also emotional responses;Regulation that appetite;Coordination that the autonomic nervous system;Generating a selection of hormone that regulate the activity of endocrine glands.

Indeed, the hypothalamus have the right to be believed of as the key crossover point between the concerned system and the endocrine system.

The pituitary gland

The pituitary gland is a pea-sized structure, commonly weighing approximately 500mg; that is situated at the basic of the brain, just behind the sleep cavity, wherein it is defended by the sphenoid bone of the skull (Ganapathy and also Tadi, 2020). It has actually two major regions:

The posterior (back portion) –essentially, an expansion of the hypothalamus, the posterior of the pituitary gland stores and concentrates 2 neuropeptide hormones referred to as anti-diuretic hormone (ADH) and oxytocin, i m sorry are developed by the neurons (nerve cells) that the hypothalamus. ADH help regulate fluid balance and blood pressure, while oxytocin – among other things – initiates parturition (childbirth).The anterior (front portion) – this creates from the epithelial tissues in the roof that the embryonic oral cavity, i beg your pardon bulges up into the skull, fusing through the posterior pituitary. That produces several key hormones such together somatotropin (growth hormone) and also melanocyte-stimulating hormone, which helps to manage skin pigmentation. The anterior pituitary additionally produces number of stimulating hormone that regulate the relax of hormones from other endocrine glands. As an instance of this, adrenocorticotropic hormone regulates the release of the long-term stress hormone, cortisol, indigenous the adrenal cortex.

As the pituitary gland regulates hormone release from other endocrine glands, the is regularly referred to as the ‘master’ gland. This is other of a misnomer together the release of stimulating hormone from the pituitary gland is, itself, under the manage of hormones developed by the hypothalamus; this will certainly be discover in component 2.

Thyroid gland and also associated parathyroids

The thyroid is a bilobed (two-lobed) organ that each other a bow tie in shape; it generally weighs 25-30g and is situated just listed below the larynx (Dorion, 2017). The thyroid itself has two significant populations the endocrine cells:

Follicular cell – these develop the iodine-containing hormones triiodothyronine (T3) and tetraiodothyronine (T4, likewise known as thyroxine), which regulate the body’s metabolism;Parafollicular cell – these develop the hormone calcitonin, which help to regulate blood–calcium concentration.

The parathyroid glands are found embedded in the posterior portion of the thyroid gland. Most civilization have four parathyroid glands (explored in component 3); these develop parathyroid hormone, which functions antagonistically come calcitonin throughout calcium homoeostasis.


The kidnize is a an important organ in both the digestive and also the endocrine systems; residing in the U-shaped loop the the duodenum, the is frequently 14-23cm in length and weighs roughly 100g (Longnecker, 2021).

The endocrine portions of the kidnize are recognized as the islets that Langerhans, which are small islands of glandular tissue discovered throughout the framework of the pancreas. The pancreatic islets save on computer several species of endocrine cells, including:

Alpha cells, which create glucagon;Beta cells, which create insulin.

These two hormones – glucagon and also insulin – beat a key role in regulation blood–glucose concentration, which will be debated in the section on homoeostasis later in this article.

Adrenal glands

There are two adrenal glands – one over each kidney. Lock are roughly triangular in shape, around 3cm in width and each weighs 4-6g (Lack and Paal, 2020). Adrenal glands have actually two significant regions:

Adrenal cortex (outer region) – this to produce steroid hormones, consisting of the permanent stress hormone cortisol, aldosterone (which regulates the level of sodium and potassium in the blood) and a team of testosterone-like hormones referred to as androgens;Adrenal medulla (inner region) – this produces adrenaline (epinephrine) and noradrenaline (norepinephrine). These ‘fight-or-flight’ hormones – that room usually developed when a person is under threat, fear or excited – role primarily come activate the forgiveness branch the the autonomic concerned system and also prepare the body for immediate action.

Ovaries and also testes

The ovaries room the main reproductive organs in females, responsible for creating ova. Tires ovaries are relatively irregular, lumpy and almond shaped, generally 3-5cm long and weigh 5-8g, back they have tendency to to decrease in dimension in later life (Wallace and also Kelsey, 2004). Ova construct in fluid-filled sacs called follicles; as follicles enlarge, they relax oestrogen, the woman sex hormone the promotes the thickening of the uterine lining (endometrium).

Once a follicle ruptures and also releases its maturation ovum into the fallopian tube throughout ovulation, the remnants of the follicle collapse right into a structure referred to as the corpus luteum (yellow body). This to produce the second major female sex hormone, progesterone, i m sorry prepares the endometrium because that implantation that a fertilised ovum (zygote) and, subsequently, maintains the verity of the endometrial lining, have to implantation occur.

The testes (testicles) room the paired main reproductive organs in males, responsible for creating spermatozoa. They space oval shame and, in adult males, are commonly 4.5-5.1cm long and weigh 15-19g (Silber, 2018). Each testis includes a specialised group of endocrine cells referred to as interstitial cells, which develop the male sex hormone testosterone. This is one anabolic steroid produced in greater amounts during puberty, as soon as it promotes muscle development, growth of facial and body hair, and expansion of the larynx, bring about a deepening of the voice.

“It has actually been said that the microbial biology (the diverse micro-organisms colonising the body) likewise functions together a digital endocrine organ”

Hormones together chemical signals

Hormones are traditionally characterized as chemistry signals, transported to their target organization in the blood; today, however, that definition is often increased to incorporate all chemistry messengers that tie to target cells through high affinity. Therefore far, an ext than 100 hormones have been determined in the human body, and also this rises to much more than 200 if hormone-like building materials are included (Silver and Kriegsfeld, 2001).

Hormones exert their physiological effects by binding to certain receptors connected with their target cells (Fig 3). Many drugs have been designed to target this receptor sites, either to mimic the actions of hormones (for example, in the situation of a hormone deficiency such together hypothyroidism, i beg your pardon is treated v levothyroxine) or to act together competitive antagonists to physically block the receptor, staying clear of the natural hormone native binding and also exerting that is effect. Hormones can be extensively divided into three major classes:

Peptide hormones;Steroid hormones;Amino acid-derived hormones.
Peptide hormones

These are the biggest hormones, with fairly high molecule weights. They space proteinaceous chemical signals, making up chains of amino mountain of differing lengths. Instances include:

Somatotropin (growth hormone);Insulin;Ghrelin (the hunger hormone).

Some peptide hormones space initially developed as inactive forms called prohormones; a an excellent example is insulin, i beg your pardon is an initial synthesised as a much larger molecule, called proinsulin, and then cleaved into its active, shorter form before gift released into the blood.

Peptide hormones have tendency to exert their effects by binding to receptors on the surface ar of the plasma membranes of target cells, as shown in Fig 3. This triggers a variety of transmembrane events, leading to the production of second messengers (such as cyclic adenosine monophosphate), which, subsequently, start the wanted effect that the hormone in the target cell (Foster et al, 2019).

Steroid hormones

Steroid hormones room lipids (fats), mostly acquired directly native cholesterol, i m sorry acts together a precursor molecule because that steroid biosynthesis. Instances include:


As steroid hormones room lipids, they quickly diffuse throughout the phospholipid double layer of their target cabinet membranes (Fig 3) and exert their effects by binding to receptors in the cytoplasm or nucleus (Ozawa, 2006). Steroid hormones often tend to precipitate their desired effects by modulating the task of details genes in cells.

Amino acid-derived hormones

These room synthesised from amino acids, so are tiny molecules through low molecular weights. Examples include:

Adrenaline (epinephrine), obtained from tyrosine;Thyroid hormones thyroxine T4 and T3, obtained from tyrosine;Melatonin (which helps to control sleep), derived from tryptophan (Kleine and also Rossmanith, 2016).

Like peptide hormones, some amino acid-derived hormones, such as adrenaline, tie to receptor on the surface ar of target-cell plasma membranes. Others however, such together T3 native the thyroid, cross the plasma membrane of your target cells and also bind come receptors inside the cabinet in a similar manner to steroid hormones.

Locally exhilaration hormones: autocrine and paracrine

As well together the hormones secreted by the significant endocrine glands, there room various in your ar acting hormone-like substances. These room usually released right into the interstitial fluid (the thin film of tissue liquid surrounding many cells) and also exert their effects in the local vicinity.

Autocoids room chemical signals released by a cell the exert their results on that very same cell; paracrine signal act more widely, affecting adjoining cells in the prompt vicinity (Alberts et al, 2015). These in your ar acting hormone – both autocrine and paracrine – room usually rapidly broken down prior to they can enter the broader circulation. An excellent examples space the eicosanoids, a large family the lipid-derived molecules, which incorporate the prostaglandins, thromboxanes, leukotrienes and also lipoxins (O’Donnell et al, 2009).

Prostaglandins and the heat response

Fever (pyrexia) is commonly associated with infection. Once phagocytic leukocytes (white blood cells) such together monocytes go into sites of infection and begin to trap and also kill pathogens, they relax a cytokine (a signalling chemical the is created by immune cells) referred to as interleukin-1 (IL-1). IL-1 is a tiny peptide the circulates in the blood before binding to receptor on cells in the hypothalamus – the region of the brain containing the thermoregulatory centre that is responsible for controlling body temperature, i beg your pardon usually has actually a collection point of around 37°C (Knight et al, 2020).

Once IL-1 has binded come its receptor, the enzyme cyclooxygenase (COX) is activated, resulting in the manufacturing of the eicosanoid, prostaglandin E2 (PGE2); this in your ar acting signal move the set point of the hypothalamus upwards (typically to roughly 38°C-39°C), leading to fever (Eskilsson et al, 2017).

Fever is a useful response during epidemic as it have the right to slow the replication the pathogens, when simultaneously increasing and enhancing pathogen killing by leukocytes. However, fever likewise takes enzyme in the human body cells exterior of their common optimal temperature of 37°C, slow the biochemical reactions the are vital for life. This can reason people to experience malaise and feel typically unwell till the infection is handle with and also the temperature deserve to return come normal.

If fever becomes incredibly high (≥40°C), there is an raised risk of febrile convulsions. Antipyretic drugs – which include many common non-steroidal anti-inflammatory drugs (NSAIDs), such together aspirin – may be provided to alleviate the fever. NSAIDs work, primarily, through inhibiting the activity of the enzyme COX, thereby staying clear of the production of PGE2 and also shifting upwards the collection point of the thermoregulatory centre.

If a patient’s fever requirements to it is in reduced, it is common practice to incorporate the usage of antipyretic drugs and interventions such as reducing bed linen – because that example, air-circulating or water-circulating blankets or hydrogel-coated water-circulating pads can also be used. Over there is no evidence that fans help with temperature regulation and should it is in avoided as they can increase the hazard of shivering (Doyle and also Schortgen, 2016).

The endocrine system and homoeostasis

An typical adult human being with a weight of 70kg is thought to comprise roughly 30-40 trillion cells (Sender et al, 2016). For each cabinet to function effectively, it should be kept at the correct temperature and pH, and noted with a secure stream the nutrients and also oxygen. In ~ the same time, the local environment of each cell needs any kind of waste metabolites, such as carbon dioxide and urea, come be successfully removed.

Homoeostasis deserve to be generally defined together the ability to keep a reasonably stable interior environment; it is crucial to great health and survival (Modell et al, 2015). A multitude that variables in the body are susceptible to constant and significant fluctuation, and most that the significant organ solution of the human body are specialized to maintaining these variables in ~ their common physiological ranges.

The inner biochemical processes necessary for life are primarily driven by biological catalysts known as enzymes, i beg your pardon generally autumn into 2 categories:

Anabolic enzymes – these are responsible for building molecules in the body. For example, DNA polymerase builds brand-new molecules the DNA necessary for cell department and growth, when glycogen synthase takes single molecules the glucose and polymerises them (links castle together) to kind long, branching chain of glycogen (animal starch), which is save in big amounts in liver and muscle;Catabolic enzymes – these break down molecules and also include the enzymes of the cradle tract, i m sorry digest the macromolecules (large, complex molecules) that food into simple components that have the right to be absorbed and used by the body. Other crucial catabolic enzymes room those affiliated in moving respiration, in i m sorry sugars room metabolised (usually in the presence of oxygen) to relax the energy necessary because that life.

Anabolic and also catabolic enzymes deserve to only role efficiently in narrow ranges of temperature and pH; they likewise require a secure supply the the substrate molecules on i m sorry they action (Puri, 2018). As an example, because that aerobic to move metabolism come occur, the respiratory enzymes in cells need a stable stream that glucose and oxygen.

The homoeostatic mechanisms that ensure a stable environment in the body depend on a process called an unfavorable feedback, i beg your pardon is questioned below.

Set points, an unfavorable feedback and also the role of hormones

For every variable in the person body, over there is a hypothetical ideal worth – the collection point. As an example, the set point because that glucose is approximately 5mmol/L (Fig 4); at 5mmol/L, human being cells are offered with a stable supply the glucose, which can be offered to relax energy during cellular respiration.


The body strives to keep variables together close come their set points as feasible using negative-feedback mechanisms. During an adverse feedback, any type of deviations indigenous the set point are resisted and also minimised, allowing a change to be constrained within a narrow, common physiological range. If blood–glucose concentration is measured throughout the day, it would certainly be supposed to fluctuate roughly its set point. As an example, after ~ exercise, blood–glucose concentration commonly falls as glucose is offered to carry out energy because that muscle contraction; conversely, after a carbohydrate-rich meal or snack (such together a chocolate bar), the blood–glucose level rises.

Hormones commonly play major roles in an unfavorable feedback and also often occupational together in antagonistic pairs. Fig 4 reflects that once blood–glucose concentration rises, the hormone insulin is released; this promotes glucose uptake by the cell of the body and also the blood–glucose level drops. Whereas if blood–glucose concentration falls, the hormone glucagon is released; this stimulates the release of stored glucose from the liver, which causes blood glucose to increase again. The two pancreatic hormones, insulin and glucagon, work antagonistically to each other to successfully constrain the blood–glucose concentration in its normal physiological variety of 4-6mmol/L (Knight et al, 2020).

Effects that variables outside of their common range

One in 14 human being in the UK has actually the chronic metabolic disease, diabetes mellitus, which means they no longer develop insulin (type 1) or become resistant to its effects (type 2). There is no an reliable insulin response, blood–glucose concentration will increase markedly over its common physiological range. Some undiagnosed patients through diabetes can have seriously high blood–glucose concentrations of >33mmol/L requiring instant treatment. Elevated blood glucose is called hyperglycaemia and also is the vital clinical function of diabetes.

Many patients through diabetes inject insulin come manage and normalise your blood–glucose levels. Top top occasion, some might inject too much insulin or eat inadequate carbohydrate so their blood–glucose concentration drops far below its common physiological range; this is referred to as hypoglycaemia and can be exceptionally dangerous. Once pronounced, hypoglycaemia deserve to lead to mental impairment, behavioral changes, unconsciousness, coma and potentially fatality (Mukherjee et al, 2011).

The instance of hyperglycaemia and also hypoglycaemia shows how, when a variable is taken external of that normal selection for any type of protracted length of time, it is detrimental to health and leads come pathology (disease states); both hyperglycaemia and also hypoglycaemia are commonly encountered in poorly regulated diabetes.

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This short article has noted a general overview the the nature that hormones, together with the major endocrine glands and also their importance in regulating and coordinating an important bodily functions. Every of the significant endocrine glands and their hormonal secretions will certainly be be check in greater detail later in the series; component 2 focuses on the hypothalamus and pituitary gland.