Hormones in Context: Introduction
The word 'steroid' is much in the news nowadays, with emotive phrases like 'steroid abuse'. In fact the word is a chemist's terminology for a large group of organic chemicals, not necessarily hormones, which are soluble in organic solvents. This distinguishes them from peptides, which are soluble in water. It is an important distinction, predicting their interactions in the body.
Hormones are those body chemicals used by the endocrine system, acting as the messengers of the body, just as nerves do. However while nerves act quickly, but only on the sites to which they are connected, hormones are carried by the bloodstream and therefore may act more slowly on a number of sites simultaneously.
Peptides affect those cells that have specific receptors for them, usually on the cell membrane, where the receptor produces other chemicals, second messengers, to effect changes within the cells. Many steroids, however, enter the cell, performing their action directly.
Steroids tend to be produced more or less continuously, and become bound to plasma proteins in the blood, where they are stored until needed. Peptides tend to be produced as needed, often in pulses, and may be only apparent for brief periods.
The effects of hormones, therefore, cannot be assessed without a consideration of their receptors or their targets, which may vary in sensitivity from one cell to another, and between one individual and another.
They are produced by various endocrine glands scattered around the body. Hormone concentration in the blood is regulated in two ways. Generally, the rate at which glands produce hormones decreases as the concentration increases.
However, in most cases they are also controlled by the hypothalamus, part of the brain, and the pituitary gland, which produces over fifty distinct hormones.
The hypothalamus and the pituitary are linked by blood vessels through which peptide hormones, called releasing factors flow. In this way the central nervous system has a controlling function on the endocrine system. In addition, some hormones can act as neurotransmitters, chemicals involved in the operation and control of the nervous system.
The endocrine glands with which we are concerned, are the gonads - the testes and the ovaries - which produce various androgens and estrogens, known collectively as the gonadotrophic hormones. The adrenal glands also produce some gonadotrophins, but their main output is the adrenocorticotrophic hormones, which include adrenalin and noradrenalin.
To begin with, since we are describing a complex set of control systems, we need a brief excursion into engineering theory.
Bland, J., (1998) About Gender: Hormones in Context Introduction
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Last amended 26.04.98