| Metabolism, the sum total of all the chemical processes which occur in the living body, is ultimately controlled by instructions contained in the chromosomes in the nucleus of every cell. But cells must coordinate their activity, and to do this they must communicate. The nervous system provides communication via nerves, delivering messages in micro-seconds, and the endocrine system provides communication via hormones, which travel around the body in the bloodstream.
Chemically, hormones are either proteins, amino acid derivatives, or cholesterol derivatives, and they are secreted in response to nervous stimuli - sight, touch, hearing, taste, smell, thoughts, emotions - and in response to various chemicals in the bloodstream, including other hormones. The pituitary gland at the base of the brain, for example, secretes a number of hormones designed to chivvy other hormone-producing glands into action. The hormones produced by these subservient glands circulate in the blood, but if their concentration in the blood falls below or rises above a certain level the pituitary adjusts its output of 'trophic' or stimulating hormones reciprocally. However, the pituitary is not the prime mover of the endocrine world. This role is played by the hypothalamus.
The hypothalamus is a distinct group of cells at the base of the forebrain, with extensive connections to the cortex, brain stem, and hind brain. Stimulating and inhibitory hormones secreted by the hypothalamus in response to nervous impulses from other parts of the brain travel the short distance to the pituitary beneath it, and the pituitary responds by increasing or decreasing its secretory activities.
The anterior lobe of the pituitary secretes growth hormone, prolactin (to stimulate production of breast-milk), and hormones which stimulate the thyroid and adrenal glands, and also the ovaries and testes; its posterior lobe produces antidiuretic hormone (to decrease excretion of water by the kidneys), and oxytocin (to stimulate milk flow and contraction of the womb after childbirth).
The thyroid gland, which lies in the neck at Adam's apple level, produces thyroxin, which controls metabolic rate, the pace of chemical activity in every cell in the body; iodine is an essential component of thyroxine. Directly behind the thyroid are the four parathyroid glands; these produce parathormone which, together with calcitonin produced by the thyroid, controls calcium metabolism.
The two adrenal glands, one perched on top of each kidney, secrete the 'fight or flight' hormones adrenalin and noradrenalin in response to stimulation of the sympathetic nervous system. They also secrete steroids, all of which are made from cholesterol. Two of the most important steroids are aldosterone and cortisone. Aldosterone controls the balance of potassium and sodium in the blood, and cortisone plays a crucial part in carbohydrate, protein, and fat metabolism, and many other processes. The adrenal glands also produce small amounts of androgens, masculinizing hormones, in both women and men.
However, the major androgen-producing role in the male body is assumed by the testes, which produce testosterone. In women, femaleness, fertility, and the menstrual cycle are controlled by oestrogens and progestins manufactured in the ovaries. The testes also produce small amounts of oestrogens. Another very important endocrine gland is the pancreas. Lying under and behind the stomach, it produces two hormones which regulate the level of glucose in the blood, insulin and glucagon; insulin encourages cells to absorb glucose and glucagon encourages stores of glucose to be released into the bloodstream. Both hormones are produced in response to levels of glucose in the blood. If the cells which produce insulin become diseased or unresponsive, the result is diabetes.
Many other parts of the body - the heart, kidneys, stomach, intestines, and placenta - also contain cells which produce hormones. |
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