The word hormone is derived from a Greek term meaning “to excite” or “to spur on”. Hormones are chemical messengers that alter the activity of or “excite” targeted cells. They are produced in various specialized glands and cells throughout the body and are transported in the bloodstream to their specific sites of action. Hormones regulate many important body processes and functions, including growth, reproduction, and metabolism, the collective term for all the chemical reactions that occur in the body.
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Hormones are produced by a number of different glands and cells that are described collectively as the endocrine system. The major endocrine glands – the pituitary, thyroid, parathyroid, and adrenal glands and the pancreas, ovaries, and testes – are all primarily dedicated to the production of various hormones. However, other organs in the body with major functions of their own also contain cells that produce hormones. For example, the main function of the kidneys is to filter the blood, but they also contain endocrine cells that secrete hormones.
Hormones are transported in the bloodstream to their target tissues, which may then be stimulated to produce other hormones in a chain reaction. For example, the pituitary gland produces thyroid-stimulating hormone (TSH), which travels to the thyroid gland where it stimulates the production of thyroid hormones.
Hormones also regulate the balance of certain substances in the blood. If there is too much or too little of a substance, a feedback mechanism restores the correct levels. The pituitary, a pea-sized gland regulated by a part of the brain called the hypothalamus, has overall control of most hormone production.
Some hormones work on cells throughout the body. For example, certain thyroid hormones influence metabolic rate. Others have specific target cells, such as the hormone vasopressin, which acts on specific cells in the kidneys to regulate urine concentration.
When we are under stress or excited, the two adrenal glands increase production of hormones that affect our blood pressure, circulation, and breathing, creating the heightened states that form part of our “fight or flight” response to danger.
Throughout life, hormone levels rise and fall. For example, during puberty sex hormones rise sharply, and in women levels fall after the menopause.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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