The endocrine mechanism comprises glands and tissues that produce hormones for regulating and also coordinating vital bodily functions. This article, the very first in an eight-part series, is an introduction of the system


The endocrine system is consisted of of glands and also tissues that produce and also secrete hormones to regulate and also coordinate an important bodily functions. This post – the first in an eight-part collection on the anatomy and also physiology of the endocrine mechanism – explores the nature of endocrine glands and also tissues, and the role of hormones together chemical signals that are carried in the blood. It additionally highlights the varying functions of hormone in regulating and also coordinating physiological processes, and maintaining homoeostasis in the body.

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

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

This write-up has to be double-blind peer reviewed


The endocrine system is a series of glands and also tissues the produce and secrete hormones, which are offered by the human body to regulate and coordinate crucial bodily functions, including growth and also development, metabolism, sexual function and reproduction, sleep and also mood. This short article – the first in an eight-part series on the anatomy and also physiology of the endocrine device – provides summary of the system, concentrating on endocrine glands and tissues, and the duty of hormones together chemical signal that are bloodborne. It likewise 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 exceptionally complex: it is composed of dedicated, specialised endocrine glands – such together the thyroid, parathyroids and also adrenal glands – in addition to tissues such together fat (adipose tissue) and also bone that have actually a second endocrine duty and additionally secrete a range of hormones. It has actually been argued that the microbial biome (the diverse plethora of micro-organisms colonising the human body) likewise functions as a “virtual endocrine organ”, secreting a cocktail the chemical signals that more influences human being physiology (O’Callaghan et al, 2016).


Endocrine and also exocrine glands

By definition, all glandular tissues create secretions. Many glandular structures space epithelial in origin, and also many space folded and organised right into recognisable glands through a main duct. Glands possessing a duct space exocrine glands (Fig 2); the duct acts as a conduit right into which secretions space released prior to being brought away to their sites the action. Exocrine glands include many of the digestive glands in the gut, sweat glands in the skin and also mucus-producing glands in the mucous membranes of the mouth and also reproductive tracts.


In contrast, endocrine glands have actually no duct, however release your secretions, referred to as hormones, straight into the blood (Fig 2). Because that this reason, most endocrine glands are highly vascularised, and also many of their component cells are in direct contact with blood capillaries. This nearby association v blood ship facilitates the direct release of hormones right into the blood and enables the blood come be repetitively monitored for physiological alters that have the right to initiate hormone release. As an instance of this, the insulin-producing cells of the pancreas will release insulin once they detect boost in blood–glucose concentration after ~ the consumption of carbohydrate-rich food.

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

The significant endocrine glands

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

The hypothalamus

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

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

Indeed, the hypothalamus can be believed of as the vital crossover allude between the concerned system and the endocrine system.

The pituitary gland

The pituitary gland is a pea-sized structure, typically weighing roughly 500mg; it is situated at the basic of the brain, just behind the nasal cavity, where it is safeguarded by the sphenoid bone of the skull (Ganapathy and Tadi, 2020). It has two major regions:

The posterior (back portion) –essentially, an expansion of the hypothalamus, the posterior that the pituitary gland stores and concentrates 2 neuropeptide hormones dubbed anti-diuretic hormone (ADH) and oxytocin, i beg your pardon are produced by the neurons (nerve cells) that the hypothalamus. ADH helps regulate fluid balance and also blood pressure, if oxytocin – among other points – initiates parturition (childbirth).The anterior (front portion) – this establishes from the epithelial organization in the roof that the embryonic dental cavity, which bulges up into the skull, fusing v the posterior pituitary. That produces several an essential hormones such together somatotropin (growth hormone) and melanocyte-stimulating hormone, which helps to regulate skin pigmentation. The anterior pituitary additionally produces numerous stimulating hormones that manage the release of hormones from other endocrine glands. As an instance of this, adrenocorticotropic hormone regulates the release of the irreversible stress hormone, cortisol, from the adrenal cortex.

As the pituitary gland regulates hormone relax from other endocrine glands, the is often referred to as the ‘master’ gland. This is miscellaneous of a misnomer together the relax of stimulating hormones from the pituitary gland is, itself, under the control of hormones produced by the hypothalamus; this will be discover in part 2.

Thyroid gland and also associated parathyroids

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

Follicular cells – these create 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 helps to control blood–calcium concentration.

The parathyroid glands are found embedded in the posterior section of the thyroid gland. Most people have four parathyroid glands (explored in component 3); these develop parathyroid hormone, which works antagonistically to calcitonin during calcium homoeostasis.


The pancreas is a crucial organ in both the digestive and also the endocrine systems; resident in the U-shaped loop that the duodenum, that is commonly 14-23cm in length and weighs around 100g (Longnecker, 2021).

The endocrine portions of the kidnize are recognized as the islets that Langerhans, i beg your pardon are tiny islands that glandular tissue found throughout the structure of the pancreas. The pancreatic islets save on computer several species of endocrine cells, including:

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

These 2 hormones – glucagon and also insulin – play a key role in regulation blood–glucose concentration, which will be debated in the section on homoeostasis later in this article.

Adrenal glands

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

Adrenal cortex (outer region) – this produce steroid hormones, consisting of the irreversible stress hormone cortisol, aldosterone (which regulates the level of sodium and potassium in the blood) and a group of testosterone-like hormones called androgens;Adrenal medulla (inner region) – this produces adrenaline (epinephrine) and noradrenaline (norepinephrine). These ‘fight-or-flight’ hormones – that are usually produced when a human is under threat, afraid or excited – function primarily to activate the sorry branch of the autonomic worried system and prepare the body for immediate action.

Ovaries and also testes

The ovaries are the main reproductive organs in females, responsible for producing ova. Tires ovaries are relatively irregular, lumpy and also almond shaped, generally 3-5cm long and weigh 5-8g, although they tend to decrease in size in later on life (Wallace and Kelsey, 2004). Ova construct in fluid-filled sacs referred to as follicles; as follicles enlarge, they release oestrogen, the female sex hormone that promotes the thickening of the uterine lining (endometrium).

Once a follicle ruptures and releases its tires ovum right into the fallopian tube throughout ovulation, the remnants the the follicle collapse right into a structure called the corpus luteum (yellow body). This to produce the second significant female sex hormone, progesterone, which prepares the endometrium for implantation the 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 shaped and, in adult males, are generally 4.5-5.1cm long and weigh 15-19g (Silber, 2018). Each testis has a specialised group of endocrine cells dubbed interstitial cells, which create the masculine sex hormone testosterone. This is an anabolic steroid produced in greater amounts during puberty, as soon as it promotes muscle development, expansion of facial and body hair, and expansion of the larynx, leading to a deepening that the voice.

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

Hormones together chemical signals

Hormones are traditionally identified as chemistry signals, transported to their target tissues in the blood; today, however, that meaning is often broadened to incorporate all chemical messengers that tie to target cells with high affinity. For this reason far, more than 100 hormones have actually been established in the person body, and also this rises to much more than 200 if hormone-like substances are had (Silver and Kriegsfeld, 2001).

Hormones exert your physiological results by binding to details receptors linked with your target cells (Fig 3). Many drugs have actually been designed to target this receptor sites, one of two people to mimic the actions of hormone (for example, in the case of a hormone deficiency such as hypothyroidism, i m sorry is treated with levothyroxine) or to act together competitive antagonists come physically block the receptor, staying clear of the natural hormone native binding and exerting its effect. Hormones have the right to be generally divided into three major classes:

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

These space the biggest hormones, with relatively high molecule weights. They space proteinaceous chemistry signals, making up chains of amino acids of differing lengths. Instances include:

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

Some peptide hormones are initially created as inactive forms called prohormones; a an excellent example is insulin, i m sorry is an initial synthesised together a much larger molecule, dubbed proinsulin, and also then cleaved into its active, shorter type before gift released into the blood.

Peptide hormones have tendency to exert their effects by binding to receptor on the surface of the plasma membrane of target cells, as presented in Fig 3. This cause a selection of transmembrane events, resulting in the production of 2nd messengers (such as cyclic adenosene monophosphate), which, subsequently, begin the wanted effect the the hormone in the target cell (Foster et al, 2019).

Steroid hormones

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


As steroid hormones are lipids, they swiftly diffuse across the phospholipid double layer of your target cabinet membranes (Fig 3) and also exert their impacts by binding to receptors in the cytoplasm or cell nucleus (Ozawa, 2006). Steroid hormones often tend to precipitate their desired effects through modulating the activity of specific genes in cells.

Amino acid-derived hormones

These space synthesised native amino acids, for this reason are little molecules v low molecular weights. Examples include:

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

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

Locally exhilaration hormones: autocrine and also paracrine

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

Autocoids space chemical signal released through a cell that exert their impacts on that exact same cell; paracrine signals act an ext widely, affecting neighbouring cells in the immediate vicinity (Alberts et al, 2015). These locally acting hormone – both autocrine and also paracrine – room usually rapidly damaged down prior to they can enter the more comprehensive circulation. Great examples room the eicosanoids, a huge family of lipid-derived molecules, which incorporate the prostaglandins, thromboxanes, leukotrienes and also lipoxins (O’Donnell et al, 2009).

Prostaglandins and also the fever response

Fever (pyrexia) is commonly linked with infection. When phagocytic leukocytes (white blood cells) such as monocytes go into sites of infection and begin come trap and kill pathogens, they relax a cytokine (a signalling chemical that is developed by immune cells) called interleukin-1 (IL-1). IL-1 is a tiny peptide the circulates in the blood before binding to receptor on cell in the hypothalamus – the an ar of the mind containing the thermoregulatory centre that is responsible for controlling body temperature, i beg your pardon usually has actually a set point of approximately 37°C (Knight et al, 2020).

Once IL-1 has binded come its receptor, the enzyme cyclooxygenase (COX) is activated, causing the production of the eicosanoid, prostaglandin E2 (PGE2); this locally acting signal move the collection point that the hypothalamus upwards (typically to around 38°C-39°C), resulting in fever (Eskilsson et al, 2017).

Fever is a useful solution during infection as it have the right to slow the replication of pathogens, when simultaneously increasing and enhancing pathogen killing by leukocytes. However, fever also takes enzymes in the human body cells exterior of their typical optimal temperature the 37°C, slowly the biochemical reactions the are necessary for life. This can cause people to experience malaise and also feel usually unwell until the epidemic is dealt with and the temperature can return come normal.

If heat becomes exceptionally high (≥40°C), over there is an enhanced risk of febrile convulsions. Antipyretic medicine – which encompass many usual non-steroidal anti-inflammatory medicine (NSAIDs), such as aspirin – may be provided to minimize the fever. NSAIDs work, primarily, by inhibiting the task of the enzyme COX, thereby avoiding the production of PGE2 and also shifting upwards the collection point of the thermoregulatory centre.

If a patient’s fever needs to be reduced, it is usual practice to integrate the use of antipyretic drugs and also interventions such as reducing bed linen – because that example, air-circulating or water-circulating blankets or hydrogel-coated water-circulating pads can likewise be used. There is no proof that fans assist with temperature regulation and also should be avoided as they can increase the risk of shivering (Doyle and Schortgen, 2016).

The endocrine system and also homoeostasis

An mean adult person with a weight of 70kg is thought to comprise about 30-40 sunshine cells (Sender et al, 2016). For each cell to duty effectively, it need to be preserved at the exactly temperature and pH, and detailed with a secure stream that nutrients and oxygen. At the exact same time, the local atmosphere of each cell needs any kind of waste metabolites, such as carbon dioxide and urea, come be effectively removed.

Homoeostasis can be broadly defined as the capability to keep a reasonably stable internal environment; that is essential to an excellent health and survival (Modell et al, 2015). A multitude of variables in the body space susceptible to consistent and significant fluctuation, and most of the significant organ equipment of the human body are devoted to maintaining these variables within their normal physiological ranges.

The interior biochemical processes crucial for life are primarily driven by organic catalysts well-known as enzymes, i beg your pardon generally fall into 2 categories:

Anabolic enzymes – these are responsible for building molecules in the body. For example, DNA polymerase builds brand-new molecules the DNA essential for cell division and growth, if glycogen synthase takes solitary molecules that glucose and polymerises lock (links lock together) to form long, branching chain of glycogen (animal starch), i m sorry is save on computer in big amounts in liver and also muscle;Catabolic enzymes – these breakdown molecules and include the enzyme of the digestive tract, i beg your pardon digest the macromolecules (large, facility molecules) of food into basic components that deserve to be took in and provided by the body. Other vital catabolic enzymes room those affiliated in to move respiration, in i beg your pardon sugars space metabolised (usually in the visibility of oxygen) to relax the energy necessary because that life.

Anabolic and also catabolic enzymes deserve to only duty efficiently in narrow ranges of temperature and pH; they also require a secure supply of the substrate molecules on which they action (Puri, 2018). As an example, for aerobic to move metabolism to occur, the respiratory tract enzymes in cells require a stable stream the glucose and also oxygen.

The homoeostatic mechanisms the ensure a stable atmosphere in the body depend on a process called an adverse feedback, which is discussed below.

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

For each variable in the human being body, over there is a theoretical ideal worth – the set point. Together an example, the collection point for glucose is roughly 5mmol/L (Fig 4); in ~ 5mmol/L, human being cells are offered with a steady supply the glucose, which have the right to be offered to release energy throughout cellular respiration.


The human body strives to keep variables as close to their set points as feasible using negative-feedback mechanisms. During negative feedback, any type of deviations native the set point are resisted and minimised, enabling a variable to be constrained within a narrow, common physiological range. If blood–glucose concentration is measured throughout the day, it would certainly be intended to fluctuate approximately its set point. Together an example, ~ exercise, blood–glucose concentration generally falls as glucose is used to provide energy because that muscle contraction; conversely, ~ a carbohydrate-rich enjoy the meal or snack (such together a chocolate bar), the blood–glucose level rises.

Hormones frequently play major roles in negative feedback and often work-related together in antagonistic pairs. Fig 4 shows that as soon as blood–glucose concentration rises, the hormone insulin is released; this promotes glucose uptake by the cells of the body and the blood–glucose level drops. Vice versa, if blood–glucose concentration falls, the hormone glucagon is released; this stimulates the relax of save glucose from the liver, which causes blood glucose to climb again. The 2 pancreatic hormones, insulin and glucagon, work-related antagonistically to each other to properly constrain the blood–glucose concentration in its regular physiological variety of 4-6mmol/L (Knight et al, 2020).

Effects of variables exterior of their normal range

One in 14 civilization in the UK has the chronic metabolic disease, diabetes mellitus, which way they no longer produce insulin (type 1) or end up being resistant to its results (type 2). Without an effective insulin response, blood–glucose concentration will increase markedly over its regular physiological range. Part undiagnosed patients through diabetes have the right to have seriously high blood–glucose concentrations of >33mmol/L requiring immediate treatment. Elevated blood glucose is referred to as hyperglycaemia and is the key clinical function of diabetes.

Many patients v diabetes inject insulin come manage and normalise your blood–glucose levels. Top top occasion, some may inject too lot insulin or eat poor carbohydrate so their blood–glucose concentration falls far below its regular physiological range; this is dubbed hypoglycaemia and also can be exceptionally dangerous. Once pronounced, hypoglycaemia can lead to psychological impairment, behavioural changes, unconsciousness, coma and also potentially fatality (Mukherjee et al, 2011).

The instance of hyperglycaemia and also hypoglycaemia shows how, as soon as a change is taken outside of that normal selection for any kind of protracted size of time, it is detrimental to health and also leads come pathology (disease states); both hyperglycaemia and also hypoglycaemia are frequently encountered in poorly managed diabetes.

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This short article has provided a basic overview the the nature that hormones, along with the major endocrine glands and also their prestige in regulating and also coordinating an essential bodily functions. Each of the significant endocrine glands and their hormone secretions will be be check in greater information later in the series; component 2 focuses on the hypothalamus and pituitary gland.