Regulating the Reproductive Cycle

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The eight hormones known to be released from the hypophysis gland are vasopressin (the "anti-diuretic hormone"), oxytocin, prolactin, growth hormone, thyrotropin, adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH). Vasopressin is released in response to low water levels in the blood; it stimulates the kidneys to retain water, reduce urine output, and increase blood pressure until blood water levels return to normal, upon which it will no longer be produced. Oxytocin causes muscular contractions in the uterus during childbirth and in the breast for the secretion of milk for an infant. Prolactin is present in males and females, but it is functional only in females. It stimulates milk production from fat deposits in the breast. Growth hormone causes growth in children; it is present in adults, but contributes only to the control of metabolic rate. Thyrotropin stimulates the thyroid gland to produce and secrete various hormones that control metabolism (examples: thy-roxine and triiodothyronine). Adrenocorticotropic hormone stimulates the adrenal cortex, located above each kidney, to release its metabolismcontrolling steroid hormones. At puberty, follicle-stimulating hormone stimulates the female ovarian follicle to mature and begin producing the steroid hormone estrogen; it directs the male testes to begin producing sperm. Also at puberty, luteinizing hormone stimulates the ovary to begin producing eggs and the steroid hormone progesterone; it directs the testes to begin producing the steroid hormone testosterone.

The hypophyseal hormones, all proteins encoded by genes, have a major impact upon metabolism and development in mammals. This is especially true for the sexual-cycle hypophyseal hormones FSH and LH. In females, puberty begins with the first menstrual cycle. Each menstrual cycle is the female body's way of preparing for a possible pregnancy. At the beginning of the cycle, the hypophysis produces high concentrations of FSH, which stimulates the ovarian follicle to develop and produce estrogen, a steroid hormone that increases body fat in regions such as the buttocks and breasts. Simultaneously, increased LH production matures the egg in the ovarian follicle and stimulates progesterone production. Progesterone causes the endometrium (the lining of the uterus) to increase its blood vessel content and thickness for receiving and maintaining a fertilized egg and for the subsequent long gestation period (nine months in humans). If the egg is fertilized by sperm, it will adhere to the endo-metrium, and progesterone will continue to be secreted to maintain the endometrium and the pregnancy. If the egg is not fertilized, progesterone levels will drop, estrogen levels will rise, the endometrium will be sloughed away (menstrual bleeding will occur), and the cycle will start all over again.

The primate female menstrual cycle is only one very complex example of how hormones are intricately involved in mammalian developmental processes. There are many subtler aspects of the menstrual cycle that still are not well understood, such as the identity of the hormonal signal from the fertilized egg that stimulates the female ovary to continue progesterone production for continuation of the pregnancy. All mammalian hormones are interconnected by cause-and-effect relationships. Tremendous research remains before a clear and complete picture of hormonally controlled mammalian development will emerge.

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Boost Your Metabolism and Burn Fat

Boost Your Metabolism and Burn Fat

Metabolism. There isn’t perhaps a more frequently used word in the weight loss (and weight gain) vocabulary than this. Indeed, it’s not uncommon to overhear people talking about their struggles or triumphs over the holiday bulge or love handles in terms of whether their metabolism is working, or not.

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