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What is the endocrine system?

The endocrine system is a complex network of glands, hormones and receptors. It provides the key communication and control link between the nervous system and bodily functions such as reproduction, immunity, metabolism and behaviour.

In nearly all complex multicellular animals, there are two main systems controlling and coordinating the processes within the body:

  • The nervous system, which exerts rapid point-to-point control by means of electrical signals passing down the nerves to particular organs or tissues.
  • The endocrine system, which is a slower system based on chemical messengers, the hormones, which are secreted into the blood (or other extracellular fluids) and can reach all parts of the body.

The nervous system works in tandem with the endocrine system to control all bodily functions and processes. The endocrine system has three main components:

Endocrine glands, situated at various sites around the body, and in specialised areas of the brain. The cells in these glands secrete specific chemicals called hormones.

Hormones circulate around the body via the blood stream and modulate cellular or organ functions by binding with receptors in the target cells. Hormones that stimulate and control the activity of other endocrine glands are called trophic hormones.

Receptors in the target cells, once activated by binding of the hormone, regulate the functions and processes in the tissue through interactions with the cell's DNA or other complex intracellular signalling processes.

The main human hormones and their functions are shown below :

Gland Hormones Functions
Hypothalamus Releasing hormones Stimulate pituitary activity
Pituitary Trophic (stimulating) hormones Stimulate thyroid, adrenal, gonadal and pancreatic activity
Thyroid Thyroid hormones Regulate metabolism, growth and development, behaviour and puberty
Adrenal Corticosteroid hormones Catecholamines Regulate metabolism Regulate behaviour
Pancreas Insulin and glucagon Regulate blood sugar levels
Gonads Sex steroid hormones (androgens and oestrogens) Regulate development & growth, reproduction, immunity, onset of puberty and behaviour

The production and circulating levels of hormones are controlled by means of negative feedback processes. For example, synthesis of thyroid hormone is stimulated by thyroid stimulating hormone (TSH) produced by the pituitary gland. If blood levels of thyroid hormone fall, a part of the brain, the hypothalamus, responds to the change and releases thyroid hormone releasing hormone (TRH), which stimulates a particular cell type in the pituitary to increase TSH synthesis. As thyroid hormone levels in blood again rise in response to TSH, TRH production is reduced and, in turn, TSH secretion is suppressed. Such feedback systems maintain the balance of various body systems (operating in a fashion analogous to the system that controls a domestic central heating system) - a process known as homeostasis.

Thyroid hormone negative feedback loop