Hormones are the substance, usually a peptide or steroid, produced by one tissue and conveyed by the bloodstream to another to effect physiological activity, such as growth or metabolism.
The role of hormones
The major endocrine glands are the pituitary, the thyroid, the four parathyroids, the pancreas, the two adrenals, and the paired testes or ovaries (See endocrine). Hormones are also produced by organs or tissues whose function is not primarily an endocrine one: the digestive tract, the heart, and the kidneys all produce hormones. Even nerve cells produce them. For example, the hormones controlling secretion from the anterior lobe of the pituitary gland are synthesized in the hypothalamus, but they are released into the local blood supply to the anterior pituitary, rather than entering the general circulation. These cells are said to have a neuroendocrine function. Furthermore, it is now recognized that hormones need not even be released into blood vessels. The hormonal products of some nerve cells stimulate adjacent neurones and thus act as neuromodulators, while in the digestive tract hormones act on surrounding cells and are said to have a paracrine function (para: Greek for beside). Finally, some hormones, such as growth factors, can act on the originating cell itself; in this case they are described as exhibiting autocrine control. The classical definition has therefore been extended to include chemical messengers which are secreted by certain cells, and which reach and act upon cells which are receptive to them, whether local or distant.
Chemical nature of hormones
Chemically, most hormones belong to one of three major groups: proteins and peptides, steroids (fat-soluble molecules whose basic structure is a skeleton of four carbon rings), or derivatives of the amino acid tyrosine, characterized by a 6-carbon, or benzene, ring. There are some hormones, such as melatonin from the pineal gland and the locally actingprostaglandins, which cannot be included in any of these groups, but may share a number of their characteristics. The glands which produce protein and peptide hormones are the pituitary, certain cells of the thyroid, the parathyroids, and the pancreas. Steroids are produced by the cortex or outer layer of the adrenal gland and by the ovaries and testes. The tyrosine derivatives are the thyroid hormones, and the catecholamines (adrenaline and noradrenaline) which are produced in the medulla of the adrenal glands.
Knowledge of the chemical nature of a hormone is important as it enables one to predict how the hormone is produced, how rapidly it can be released in response to a stimulus, in what form it circulates in the blood, how it acts, the time course of its effect, and the route of administration therapeutically.
The chemical nature of the hormone also affects the mechanism of action. All hormones act on cells by way of their ‘receptors’. Each hormone has its own receptor to which it binds, matching rather like a lock and key. This is why hormones circulating throughout the body in the blood may leave capillaries to enter the extracellular fluid of many tissues, but act only on those cells which possess the appropriate receptor. Proteins and peptides cannot enter the cell and so act via cell membrane receptors, producing their effects by ‘second messengers’, which are activated in the cell as soon as the hormone binds to the receptor. Thus peptide hormones can produce quite rapid responses. Steroid and thyroid hormones, by contrast, can enter the cell and bind to intracellular receptors, producing their effects by stimulating the production of new proteins. There is therefore a relatively long lag period before the response to these hormones is seen.
Hormones produce a variety of responses throughout the body and may be grouped according to their actions, although there is overlap between the groups.
First there are the metabolic hormones which control the digestion of food, its storage and use. Such hormones include those produced by the digestive tract, which control secretion of digestive juices and activity of the muscle in the wall of the tract; also the hormones which regulate blood glucose, namely insulin, (which lowers it), and glucagon, growth hormone, the thyroid hormones, and cortisol, which all raise it.
Second are the hormones which regulate the composition of the blood, and hence of all the body fluids. Excluding those that regulate the glucose content, these are: aldosterone and atrial natriuretic hormone (produced in the heart), which control the amount of sodium in the blood; vasopressin or antidiuretic hormone, which controls the amount of water; parathyroid hormone and vitamin D, which raise blood calcium; and calcitonin, which lowers blood calcium. It is perhaps surprising to learn that a vitamin can also be a hormone, but it is similar in many ways to the steroid hormones, and the active form is produced in one part of the body for action an another. The vitamin D taken in the diet or formed in the skin under the action of UV light is not the active form: this is produced after modification takes place first in the liver and then the kidney.
Next are the stress hormones, primarily adrenaline and noradrenaline, which are under the control of the autonomic nervous system: cortisol and a number of the pituitary hormones are also involved in the response to stress.
A further group are those responsible for growth, development, and reproduction. These include growth hormone itself, and the hormones controlling ovarian and testicular function (luteinizing hormone, LH, and follicular stimulating hormone, FSH) — all of which come from the pituitary — and the hypothalamic hormones, which in turn control these pituitary secretions. Included also are the steroid hormones, produced by the ovaries (oestrogens and progesterone) and testes (testosterone), and those hormones involved in birth and lactation, chiefly oxytocin and prolactin.
The final major group includes those hormones that control other endocrine systems, and therefore interact with the other groups. The pituitary hormones adrenocorticotrophic hormone(ACTH), thyroid stimulating hormone (TSH), and the gonadotrophic hormones LH and FSH control the release of some of the metabolic and stress hormones and of the reproductive hormones, whilst hypothalamic hormones in turn control pituitary function.
Hormone effects vary widely, but can include:
- stimulation or inhibition of growth,
- In puberty hormones can affect mood and mind
- induction or suppression of apoptosis (programmed cell death)
- activation or inhibition of the immune system
- regulating metabolism
- preparation for a new activity (e.g., fighting, fleeing, mating)
- preparation for a new phase of life (e.g., puberty, caring for offspring, menopause)
- controlling the reproductive cycle
In many cases, one hormone may regulate the production and release of other hormones
Many of the responses to hormone signals can be described as serving to regulate metabolic activity of an organ or tissue.