Structure and Functions (Magill’s Medical Guide, Sixth Edition)
The definition of the term “hormone” has continued to change over the years. The classic definition is that of an endocrine hormone—that is, one secreted by ductless glands directly into the blood and acting at a distant site. The definition can be expanded to include any chemical substance secreted by any cell of the body that has a specific effect on another cell. A hormone can affect a nearby cell (paracrine action) or the cell that secretes it (autocrine action). Certain hormones are produced by the brain and kidneys, which are not thought of as classic endocrine glands. In fact, the largest producer of hormones is the gastrointestinal tract, which is not usually thought of as an endocrine gland.
Hormones fall into two major categories: peptide hormones, which are derived from amino acids, and steroid hormones, which are derived from cholesterol. The different classes of hormones have different mechanisms of action. Peptide hormones work by interacting with a specific receptor located in the plasma membrane of the target cell. Receptors have different regions, or domains, that perform specialized functions. One part of the receptor has a specific three-dimensional structure similar to a keyhole into which a certain hormone can fit. This design allows a specific action of a hormone despite the fact that the hormone is often circulating in minute quantities in the bloodstream along with myriad other...
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The Medical Use of Hormones (Magill’s Medical Guide, Sixth Edition)
The biological roles of hormones are numerous and critical to the normal function of important organ systems in human beings, and there are many examples of medical uses for hormones. In general, any derangements in the amount of hormones made, or in the timing of their production, can result in significant human disease or discomfort. For example, in women who reach the menopause, declining levels of estrogen and progesterone from the ovaries can lead to undesirable consequences such as hot flashes, vaginal dryness, and bone mineral density loss. Taking exogenous estrogen and progesterone, in the form of hormone therapy, can reduce or stop these consequences. Another example of exogenous hormone use in human disease is thyroid hormone. People with thyroid disease, such as Hashimoto’s thyroiditis, do not produce adequate levels of thyroid hormone. This condition can lead to intense fatigue and weight gain. These symptoms may be relieved by taking a synthetic thyroid hormone called levothyroxine.
Another example of the medical use of hormones is the role of synthetic erythropoietin in treating and preventing anemia. Normally, erythropoietin is made by the kidneys. It is essential for the differentiation and development of stem cells from the bone marrow into red blood cells. Most patients who develop kidney failure also suffer from severe anemia because the ability to synthesize erythropoietin is lost as the...
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Perspective and Prospects (Magill’s Medical Guide, Sixth Edition)
The study of hormones has been instrumental in understanding how human beings adapt to and live in their environment. Hormones are involved in the regulation of body homeostasis and all critical aspects of the life cycle. The study of hormones has expanded as scientists have produced large amounts of synthetic hormones in the laboratory for use in research.
The history of insulin discovery and production is an example of the rapid scientific progress made in the field of hormone research. In 1889, Joseph von Mering and Oskar Minkowski demonstrated that dogs whose pancreases had been removed exhibited abnormalities in glucose metabolism that were similar to those seen in human diabetes mellitus patients. This fact suggested that some factor made by the pancreas lowered the blood glucose. The search for this factor led to the discovery of insulin in 1921 by Frederick C. Banting and Charles H. Best. They were able to extract the active substance from the pancreas and to demonstrate its therapeutic effects in dogs and humans. The chemistry of insulin progressed with the establishment of the amino acid sequence and three-dimensional structure in the 1960’s. In 1960, insulin became the first hormone to be measured by radioimmunoassay. With advances in laboratory techniques in the 1970’s, it became the first hormone to be commercially available via recombinant DNA technology, thus ensuring the availability of pure...
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For Further Information: (Magill’s Medical Guide, Sixth Edition)
Barinaga, Marcia. “Obesity: Leptin Receptor Weighs In.” Science 271 (January 5, 1996): 29. This article presents a summary of leptin receptor research accessible to the nonspecialist, as well as a discussion of the prospects for obesity drug research.
Bliss, Michael. The Discovery of Insulin. 25th anniversary ed. Chicago: University of Chicago Press, 2007. Very stimulating reading that re-creates the excitement surrounding the discovery of insulin. This book gives the reader a feeling for the circumstances leading up to that momentous event.
Griffin, James E., and Sergio R. Ojeda, eds. Textbook of Endocrine Physiology. 5th ed. New York: Oxford University Press, 2004. A well-written book that presents an excellent summary of how hormones work and affect the body.
Kronenberg, Henry M., et al., eds. Williams Textbook of Endocrinology. 11th ed. Philadelphia: Saunders/Elsevier, 2008. This extensive book on endocrinology covers all different aspects of the field. It is written by recognized experts in endocrinology in a very readable style.
Marieb, Elaine N. Essentials of Human Anatomy and Physiology. 9th ed. San Francisco: Pearson/Benjamin Cummings, 2009. This introductory anatomy and physiology textbook, easily accessible to those with little science background, is richly illustrated with diagrams and photographs that help to illuminate body systems...
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Hormones (Encyclopedia of Science)
Hormones are chemicals produced by one kind of tissue in an organism and then transported to other tissues in the organism, where they produce some kind of response. Because of the way they operate, hormones are sometimes called "chemical messengers." Hormones are very different from each otherepending on the functions they performnd they occur in both plants and animals.
An example of hormone action is the chemical known as vasopressin. Vasopressin is produced in the pituitary gland (at the base of the brain) of animals and then excreted into the bloodstream. The hormone travels to the kidneys, where it causes an increase in water retention. Greater water retention produces, in turn, an increase in blood pressure.
Some of the earliest research on hormones involved plants. In the 1870s, English naturalist Charles Darwin (1809882) and his son Francis (1848925) studied the effect of light on plant growth. They discovered that plants tend to grow towards a source of light. They called the process phototropism. The reason for this effect was not discovered for another half century. Then, in the 1920s, Dutch-American botanist Frits Went (1863935) discovered the presence of certain compounds that control the growth of plant tips toward light. Went named those compounds auxins. Auxins are formed in the green tips of growing plants, in...
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Hormones (Encyclopedia of Psychology)
Biochemical agents that transmit messages between components of living organisms.
Hormones are biochemical messengers that regulate physiological events in living organisms. More than 100 hormones have been identified in humans. Hormones are secreted by endocrine (ductless) glands such as the hypothalamus, the pituitary gland, the pineal gland, the thyroid, the parathyroid, the thymus, the adrenals, the pancreas, the ovaries, and the testes. Hormones are secreted directly into the blood stream, where they travel to target tissues and modulate digestion, growth, maturation, reproduction, and homeostasis. Hormones do not fall into any one chemical category, but most are either protein molecules or steroid molecules. These biological managers keep the body systems functioning over the long term and help maintain health. The study of hormones is called endocrinology.
Most hormones are released into the bloodstream by a single gland. Testosterone is an exception, because it is secreted by both the adrenal glands and by the testes. The major site that keeps track of hormone levels is the hypothalamus. A number of hormones are secreted by the hypothalamus, and they stimulate or inhibit the secretion of hormones at other sites. When the hypothalamus detects high levels of a hormone, it reacts...
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