Endocrine Glands and Hormones — General Introduction

Every multicellular organism needs a way to make its distant organs and tissues work together as a single coordinated whole, rather than as a loose collection of independently functioning parts. In humans, this coordination is achieved by two complementary systems working side by side. The nervous system provides fast, short-lived, precisely targeted signalling, carrying electrical impulses along nerve fibres to specific muscles or glands within a fraction of a second. The endocrine system provides a second, chemical form of coordination: it is comparatively slow to act, since its signals must travel in the bloodstream, but its effects, once triggered, are typically far more prolonged, and a single hormone can influence every tissue in the body that happens to carry the right receptor, rather than only the one organ a nerve fibre happens to be wired to.

The endocrine system is built around a set of specialised glands. An ordinary gland in the body is exocrine: it releases its secretion through a duct onto a body surface or into a cavity, as the salivary glands pour saliva into the mouth or the sweat glands release sweat onto the skin. An endocrine gland works quite differently. It has no duct at all; instead, it releases its secretion directly into the surrounding tissue fluid, from where it diffuses into nearby blood capillaries and is carried away in the general circulation. Because they lack a duct, endocrine glands are often called ductless glands, and their chemical secretions are called hormones.

A hormone can be defined as a chemical substance, secreted in small amounts by an endocrine gland (or by specialised endocrine cells) directly into the blood, which is carried to a distant site in the body and there produces a specific physiological effect on cells possessing a receptor able to recognise it. Several properties follow from this definition and distinguish hormonal signalling from nervous signalling. First, hormones are effective at extremely low concentrations, often in the nanogram or picogram range per millilitre of blood, so only a tiny amount needs to be secreted to produce a measurable effect. Second, a given hormone can reach essentially every cell in the body once it enters the general circulation, yet it will only actually influence those cells, called target cells, that carry a receptor protein specific to that hormone; cells lacking the matching receptor are simply unaffected, however much hormone flows past them, which is what gives hormonal signalling its precision despite being broadcast throughout the whole body rather than sent down a single wire. Third, because the effect depends on the hormone reaching its target through the blood rather than along a dedicated pathway, hormonal responses are generally slower to begin than nervous responses, but they also tend to persist for much longer once triggered, making the endocrine system particularly well suited to regulating ongoing processes such as growth, metabolic rate, blood glucose level and the reproductive cycle rather than the split-second reflexes that the nervous system handles.

The human endocrine system consists of a number of distinct glands and tissues distributed through the body, each secreting one or more hormones with its own specific targets and effects. The principal glands, taken up in turn in the sections that follow, are the hypothalamus (technically part of the nervous system but also a true endocrine structure and the master regulator of the pituitary), the pituitary gland (often itself called the 'master gland' because so many of its hormones regulate other endocrine glands), the pineal gland, the thyroid gland, the parathyroid glands, the adrenal glands, the endocrine portion of the pancreas, and the gonads (testis in the male, ovary in the female). Beyond these named glands, a number of other organs — including the heart, kidney, gastrointestinal tract and thymus — also secrete hormones as a secondary function alongside their main physiological role, though these lie outside the present chapter's scope, which is confined to the glands and disorders explicitly named in the syllabus. The chapter closes by taking up the two distinct molecular mechanisms by which hormones act on their target cells, and the range of disorders — dwarfism, acromegaly, cretinism, goitre, exophthalmic goitre, diabetes mellitus and Addison's disease — that arise when one of these glands secretes either too little or too much of its hormone.