Copyright EMAP Publishing 2021 This article is not for distribution except for journal club use Clinical Practice Keywords Endocrine system/ Hormones/Homoeostasis Systems of life This article has been Endocrine system double-blind peer reviewed In this article... ● The difference between endocrine glands and exocrine glands ● Major endocrine glands and their hormones ● The role of hormones in physiological processes and homoeostatis Endocrine system 1: overview of the endocrine system and hormones Key points Author John Knight is associate professor in biomedical science, College of Human The endocrine and Health Sciences, Swansea University. system comprises glands and tissues Abstract The endocrine system is made up of glands and tissues that produce and that secrete secrete hormones to regulate and coordinate vital bodily functions. This article – the NT SELF- hormones to first in an eight-part series on the anatomy and physiology of the endocrine system ASSESSMENT regulate and – explores the nature of endocrine glands and tissues, and the role of hormones as coordinate vital chemical signals that are carried in the blood. It also highlights the varying roles of functions in the body hormones in regulating and coordinating physiological processes, as well as maintaining homoeostasis in the body. Endocrine glands differ from exocrine Citation Knight J (2021) Endocrine system I: overview of the endocrine system and glands by releasing hormones. Nursing Times [online]; 117: 5, 38-42. their secretions directly into the bloodstream, rather he endocrine system is a series of organ”, secreting a cocktail of chemical sig- than a central duct glands and tissues that produce nals that further influences human physi- and secrete hormones, which are ology (O’Callaghan et al, 2016). Endocrine glands’ Tused by the body to regulate and highly vascular coordinate vital bodily functions, including Endocrine and exocrine glands nature allows growth and development, metabolism, By definition, all glandular tissues pro- variables in the blood sexual function and reproduction, sleep and duce secretions. Most glandular structures to be monitored mood. This article – the first in an eight-part are epithelial in origin, and many are continuously and series on the anatomy and physiology of the folded and organised into recognisable appropriate endocrine system – provides an overview of glands with a central duct. Glands pos- hormones to be the system, focusing on endocrine glands sessing a duct are exocrine glands (Fig 2); rapidly released into and tissues, and the role of hormones as the duct acts as a conduit into which secre- the circulation chemical signals that are bloodborne. It also tions are released before being carried explains the diverse roles of hormones in away to their sites of action. Exocrine Hormones exert their regulating and coordinating physiological glands include many of the digestive physiological effects processes, and maintaining homoeostatic glands in the gut, sweat glands in the skin by binding to balance in the body. and mucus-producing glands in the specific receptors The endocrine system (Fig 1) is incred- mucous membranes of the mouth and associated with their ibly complex: it consists of dedicated, spe- reproductive tracts. target cells cialised endocrine glands – such as the thy- In contrast, endocrine glands have no roid, parathyroids and adrenal glands duct, but release their secretions, called Hormones regulate – together with tissues such as fat (adipose hormones, directly into the blood (Fig 2). physiological tissue) and bone that have a secondary For this reason, most endocrine glands are processes and are endocrine function and also secrete a range highly vascularised, and many of their key to maintaining of hormones. It has been suggested that the component cells are in direct contact with homoeostatic microbial biome (the diverse plethora of blood capillaries. This close association balance in the body micro-organisms colonising the human with blood vessels facilitates the direct body) also functions as a “virtual endocrine release of hormones into the blood and Nursing Times [online] May 2021 / Vol 117 Issue 5 38 www.nursingtimes.net Copyright EMAP Publishing 2021 This article is not for distribution except for journal club use Clinical Practice Systems of life Fig 1. Major endocrine glands and tissues, and their hormones glands. As an example of this, adrenocorticotropic hormone regulates Pineal gland the release of the long-term stress Hypothalamus Melatonin hormone, cortisol, from the adrenal GHRH, TRH, CRH, cortex. Pituitary gland somatostatin, dopamine, As the pituitary gland regulates hor- vasopressin TSH, LH, GH, ATCH, MSH, vasopressin, prolactin, oxytocin mone release from other endocrine glands, Thyroid and parathyroid it is often referred to as the ‘master’ gland. PTH, T3 and T4, Thymus This is something of a misnomer as the calcitonin Thymopoietin release of stimulating hormones from the Stomach Liver Ghrelin, gastrin, neuropeptide pituitary gland is, itself, under the control IGF, THPO Y, histamine, somatostatin, of hormones produced by the hypothal- Pancreas amus; this will be explored in Part 2. Adrenal gland Insulin, glucagon, somatostatin Adrenaline, noradrenaline, Thyroid gland and associated androgens, glucocorticoids Ovary and placenta parathyroids Oestrogens, progesterone The thyroid is a bilobed (two-lobed) organ Testes Oestradiol, androgens, Uterus that resembles a bow tie in shape; it typi- inhibin Relaxin, prolactin cally weighs 25-30g and is located just below the larynx (Dorion, 2017). The thy- TSH (Thyroid stimulating hormone); LH (luteinising hormone); GH (growth hormone); ATCH (adrenocorticotrophic hormone); MSH (melanocyte-stimulating hormone); roid itself has two major populations of GHRH (Growth hormone-releasing hormone); TRH (thyrotrophin-releasing hormone); endocrine cells: CRH (corticotrophin-releasing hormone); PTH (parathyroid hormone); ● IGF (insulin-like growth factor); THPO (thrombopoietin) Follicular cells – these produce the iodine-containing hormones triiodothyronine (T3) and allows the blood to be continuously moni- Indeed, the hypothalamus can be thought tetraiodothyronine (T4, also known as tored for physiological changes that can of as the key crossover point between the thyroxine), which regulate the body’s initiate hormone release. As an example of nervous system and the endocrine system. metabolism; this, the insulin-producing cells of the ● Parafollicular cells – these produce the pancreas will release insulin when they The pituitary gland hormone calcitonin, which helps to detect an increase in blood–glucose con- The pituitary gland is a pea-sized struc- regulate blood–calcium concentration. centration after the consumption of carbo- ture, typically weighing around 500mg; it The parathyroid glands are found hydrate-rich food. is located at the base of the brain, just embedded in the posterior portion of the The highly vascular nature of endocrine behind the nasal cavity, where it is pro- glands also allows for the delivery of sig- tected by the sphenoid bone of the skull Fig 2. Endocrine and exocrine nals (usually other hormones) from other (Ganapathy and Tadi, 2020). It has two glands glands to regulate release of their own hor- major regions: mones. For example, the thyroid gland ● The posterior (back portion) – releases hormones that regulate metabo- essentially, an extension of the lism, such as thyroxine, in response to the hypothalamus, the posterior of the Hormone thyroid-stimulating hormone, which is pituitary gland stores and concentrates secretion produced by the anterior pituitary gland. two neuropeptide hormones called Blood anti-diuretic hormone (ADH) and Thyroid flow The major endocrine glands oxytocin, which are produced by the (endocrine) Fig 1 shows the position of the major endo- neurons (nerve cells) of the gland crine glands in the body; however, it is hypothalamus. ADH helps regulate Endocrine important to be aware that many other fluid balance and blood pressure, while cell organs and tissues have a secondary endo- oxytocin – among other things – crine function, including the heart, kid- initiates parturition (childbirth). 2a. Thyroid (endocrine) gland neys, bone and adipose tissues (Knight et ● The anterior (front portion) – this Sweat (exocrine) gland al, 2020; Moser and van der Eerden, 2019). develops from the epithelial tissues in the roof of the embryonic oral cavity, The hypothalamus which bulges up into the skull, fusing The hypothalamus is a vital region of the with the posterior pituitary. It produces Duct brain, which plays an important role in: several key hormones such as ● Thermoregulation; somatotropin (growth hormone) and Skin Exocrine ● Behavioural and emotional responses; melanocyte-stimulating hormone, cells ● Regulation of appetite; which helps to regulate skin ● Coordination of the autonomic nervous pigmentation. The anterior pituitary system; also produces several stimulating ● Generating a range of hormones that hormones that control the release of 2b. Sweat (exocrine) gland JENNIFER N.R. SMITH regulate the activity of endocrine glands. hormones from other endocrine Nursing Times [online] May 2021 / Vol 117 Issue 5 39 www.nursingtimes.net Copyright EMAP Publishing 2021 This article is not for distribution except for journal club use Clinical Practice Systems of life thyroid gland. Most people have four para- Fig 3. Receptors for peptide and steroid hormones thyroid glands (explored in Part 3); these produce