![\"Figure](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Glass_bottle_for_Progynon_pills_United_Kingdom_1928-1948_Wellcome_L0058274-scaled-3.jpg\")
Figure 9.3.1 Pills from pee?<\/em>[\/caption]\r\n\r\nPills from Pee<\/h1>\r\n<\/div>\r\nThe medication pictured in Figure 9.3.1 with the brand name Progynon was a drug used to control the effects of menopause in women. The pills first appeared in 1928 and contained the human sex hormone estrogen. Estrogen secretion declines in women around the time of menopause and may cause symptoms like mood swings and hot flashes. The pills were supposed to ease the symptoms by supplementing [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] in the body. The manufacturer of Progynon obtained estrogen for the pills from the urine of pregnant women, because it was a cheap source of the hormone. Progynon is still used today to treat menopausal symptoms. Although the drug has been improved over the years, it still contains estrogen, which is an example of an endocrine hormone.\r\n\r\nHow Do\u00a0Endocrine Hormones\u00a0Work?<\/h1>\r\n<\/div>\r\nEndocrine hormones\u00a0like estrogen are messenger molecules secreted by\u00a0endocrine glands\u00a0into the bloodstream. They travel throughout the body in the\u00a0circulation. Although they reach virtually every cell in the body in this way, each hormone affects only certain cells, called target cells. A\u00a0[pb_glossary id=\"3422\"]target cell[\/pb_glossary]<\/strong>\u00a0is the type of cell on which a hormone has an effect. A target cell is affected by a particular hormone because it has receptor proteins\u00a0\u2014\u00a0either on the cell surface or within the cell\u00a0\u2014\u00a0that are specific to that hormone. An endocrine hormone travels through the bloodstream until it finds a target cell with a matching receptor to which it can bind. When the hormone binds to the receptor, it causes changes within the cell. The manner in which it changes the cell depends on whether the hormone is a steroid hormone or a non-steroid hormone.\r\nSteroid Hormones<\/h2>\r\nA\u00a0[pb_glossary id=\"3423\"]steroid hormone[\/pb_glossary]<\/strong> (such as estrogen) is made of [pb_glossary id=\"5651\"]lipids[\/pb_glossary]. It is fat soluble, so it can diffuse across a target cell\u2019s [pb_glossary id=\"5489\"]plasma membrane[\/pb_glossary], which is also made of lipids. Once inside the cell, a steroid hormone binds with receptor proteins in the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary]. As you can see in Figure 9.3.2, the steroid hormone and its receptor form a complex \u2014 called a steroid complex \u2014 which moves into the [pb_glossary id=\"5797\"]nucleus[\/pb_glossary], where it influences the expression of genes. Examples of steroid hormones include [pb_glossary id=\"5959\"]cortisol[\/pb_glossary], which is secreted by the [pb_glossary id=\"5869\"]adrenal glands[\/pb_glossary], and sex hormones, which are secreted by the [pb_glossary id=\"3408\"]gonads[\/pb_glossary].\r\n\r\n \r\n\r\n[caption id=\"attachment_3424\" align=\"aligncenter\" width=\"653\"]![\"Steroid](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Regulation_of_gene_expression_by_steroid_hormone_receptor.svg_-2.png\")
Figure 9.3.2 A steroid hormone crosses the plasma membrane of a target cell, binds with a receptor protein within the cytoplasm, and forms a complex that moves to the nucleus, where it affects gene expression.<\/em>[\/caption]\r\nNon-Steroid Hormones<\/h2>\r\n[caption id=\"attachment_3426\" align=\"alignright\" width=\"381\"]![\"\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Non-steroid-hormone-pathway-2.png\")
Figure 9.3.3 A non-steroid hormone binds with a receptor on the plasma membrane of a target cell. Then, a secondary messenger affects cell processes.<\/em>[\/caption]\r\n\r\nA\u00a0[pb_glossary id=\"3425\"]non-steroid hormone[\/pb_glossary]<\/strong>is made of\u00a0[pb_glossary id=\"5707\"]amino acids[\/pb_glossary]. It is not fat soluble, so it cannot diffuse across the\u00a0plasma membrane\u00a0of a target cell. Instead, it binds to a receptor\u00a0protein\u00a0on the\u00a0cell membrane. In the Figure 9.3.3<\/span>\u00a0diagram, you can see that the binding of the hormone with the receptor activates an\u00a0[pb_glossary id=\"5757\"]enzyme[\/pb_glossary]\u00a0in the cell membrane. The enzyme then stimulates another molecule, called the second messenger, which influences processes inside the cell. Most endocrine hormones are non-steroid hormones. Examples include [pb_glossary id=\"6043\"]glucagon[\/pb_glossary] and [pb_glossary id=\"2590\"]insulin[\/pb_glossary], both produced by the\u00a0[pb_glossary id=\"3197\"]pancreas[\/pb_glossary].<\/span>\r\n\r\nRegulation of Endocrine Hormones<\/h1>\r\n<\/div>\r\nEndocrine hormones regulate many body processes, but what regulates the secretion of endocrine hormones? Most endocrine hormones are controlled by [pb_glossary id=\"6007\"]feedback mechanism[\/pb_glossary]s. A feedback mechanism is a loop in which a product feeds back to control its own production. Feedback loops may be either negative or positive.\r\n\r\n \t- Most endocrine hormones are regulated by [pb_glossary id=\"2955\"]negative feedback[\/pb_glossary] loops. Negative feedback keeps the\u00a0concentration\u00a0of a hormone within a relatively narrow range, and maintains\u00a0[pb_glossary id=\"5761\"]homeostasis[\/pb_glossary].<\/li>\r\n \t
- Very few endocrine hormones are regulated by [pb_glossary id=\"2962\"]positive feedback[\/pb_glossary] loops. Positive feedback causes the\u00a0concentration\u00a0of a hormone to become increasingly higher.<\/li>\r\n<\/ul>\r\n
Regulation by Negative Feedback<\/h2>\r\n[caption id=\"attachment_3430\" align=\"alignleft\" width=\"320\"]![\"\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Thyroid-Negative-Feedback-Loop-2.jpg\")
Figure 9.3.4 This diagram shows how the thyroid gland is regulated by a negative feedback loop that also involves the hypothalamus and pituitary gland.<\/em>[\/caption]\r\n\r\nA [pb_glossary id=\"2955\"]negative feedback[\/pb_glossary] loop controls the synthesis and secretion of hormones by the [pb_glossary id=\"2958\"]thyroid gland[\/pb_glossary]. This loop includes the [pb_glossary id=\"2937\"]hypothalamus[\/pb_glossary] and [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary], in addition to the thyroid, as shown in the diagram (Figure 9.3.4). When the levels of thyroid hormones circulating in the blood fall too low, the hypothalamus secretes [pb_glossary id=\"3428\"]thyrotropin releasing hormone[\/pb_glossary] (TRH). This hormone travels directly to the pituitary gland through the thin stalk connecting the two structures. In the pituitary gland, TRH stimulates the pituitary to secrete [pb_glossary id=\"3429\"]thyroid stimulating hormone[\/pb_glossary] (TSH). TSH, in turn, travels through the bloodstream to the thyroid gland, and stimulates it to secrete thyroid hormones. This continues until the blood levels of thyroid hormones are high enough. At that point, the thyroid hormones feed back to stop the hypothalamus from secreting TRH and the pituitary from secreting TSH. Without the stimulation of TSH, the thyroid gland stops secreting its hormones. Eventually, the levels of thyroid hormones in the blood start to fall too low again. When that happens, the hypothalamus releases TRH, and the loop repeats.\r\n\r\n \r\n\r\n\r\nRegulation by Positive Feedback<\/span>\r\n\r\n<\/div>\r\n[pb_glossary id=\"3431\"]Prolactin[\/pb_glossary] is a non-steroid endocrine hormone secreted by the\u00a0[pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. One of the functions of prolactin is to stimulate a nursing mother\u2019s [pb_glossary id=\"3432\"]mammary glands[\/pb_glossary]\u00a0to produce milk. The regulation of prolactin in the mother is controlled by a [pb_glossary id=\"2962\"]positive feedback loop[\/pb_glossary] that involves the [pb_glossary id=\"3433\"]nipples[\/pb_glossary], [pb_glossary id=\"2937\"]hypothalamus[\/pb_glossary], [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary], and [pb_glossary id=\"3432\"]mammary glands[\/pb_glossary]. Positive feedback begins when a baby suckles on the mother\u2019s nipple.\u00a0Nerve impulses\u00a0from the nipple reach the hypothalamus, which stimulates the pituitary gland to secrete prolactin. Prolactin travels in the\u00a0blood\u00a0to the mammary glands and stimulates them to produce milk. The release of milk causes the baby to continue suckling, which causes more prolactin to be secreted and more milk to be produced. The positive feedback loop continues until the baby stops suckling at the breast.\r\n\r\n \r\n\r\n[caption id=\"attachment_3437\" align=\"aligncenter\" width=\"679\"]![\"Lactation](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Lactation-Positive-Feedback-Loop-2.png\")
Figure 9.3.5 The positive feedback loop for lactation involves the suckling, the breast and the pituitary gland.<\/em>[\/caption]\r\n\r\n\r\nFeature: Myth vs. Reality<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3438\"]Anabolic steroids[\/pb_glossary] are synthetic versions of the naturally occurring male sex hormone [pb_glossary id=\"3409\"]testosterone[\/pb_glossary]. Male hormones have androgenic (or masculinizing) effects, but they also have anabolic (or muscle-building) effects. The anabolic effects are the reason that synthetic steroids are used by athletes. In addition to building\u00a0muscles, they also accelerate the\u00a0development\u00a0of\u00a0[pb_glossary id=\"5913\"]bones[\/pb_glossary]\u00a0and [pb_glossary id=\"3439\"]red blood cells[\/pb_glossary], increase endurance so athletes can train harder and longer, and\u00a0speed\u00a0up muscle recovery. Unfortunately, these benefits of steroid use come with costs. If you ever consider taking anabolic steroids to build muscles and improve athletic performance, consider the following myths and corresponding realities.\r\n
How Do\u00a0Endocrine Hormones\u00a0Work?<\/h1>\r\n<\/div>\r\nEndocrine hormones\u00a0like estrogen are messenger molecules secreted by\u00a0endocrine glands\u00a0into the bloodstream. They travel throughout the body in the\u00a0circulation. Although they reach virtually every cell in the body in this way, each hormone affects only certain cells, called target cells. A\u00a0[pb_glossary id=\"3422\"]target cell[\/pb_glossary]<\/strong>\u00a0is the type of cell on which a hormone has an effect. A target cell is affected by a particular hormone because it has receptor proteins\u00a0\u2014\u00a0either on the cell surface or within the cell\u00a0\u2014\u00a0that are specific to that hormone. An endocrine hormone travels through the bloodstream until it finds a target cell with a matching receptor to which it can bind. When the hormone binds to the receptor, it causes changes within the cell. The manner in which it changes the cell depends on whether the hormone is a steroid hormone or a non-steroid hormone.\r\nSteroid Hormones<\/h2>\r\nA\u00a0[pb_glossary id=\"3423\"]steroid hormone[\/pb_glossary]<\/strong> (such as estrogen) is made of [pb_glossary id=\"5651\"]lipids[\/pb_glossary]. It is fat soluble, so it can diffuse across a target cell\u2019s [pb_glossary id=\"5489\"]plasma membrane[\/pb_glossary], which is also made of lipids. Once inside the cell, a steroid hormone binds with receptor proteins in the [pb_glossary id=\"5465\"]cytoplasm[\/pb_glossary]. As you can see in Figure 9.3.2, the steroid hormone and its receptor form a complex \u2014 called a steroid complex \u2014 which moves into the [pb_glossary id=\"5797\"]nucleus[\/pb_glossary], where it influences the expression of genes. Examples of steroid hormones include [pb_glossary id=\"5959\"]cortisol[\/pb_glossary], which is secreted by the [pb_glossary id=\"5869\"]adrenal glands[\/pb_glossary], and sex hormones, which are secreted by the [pb_glossary id=\"3408\"]gonads[\/pb_glossary].\r\n\r\n \r\n\r\n[caption id=\"attachment_3424\" align=\"aligncenter\" width=\"653\"] Figure 9.3.2 A steroid hormone crosses the plasma membrane of a target cell, binds with a receptor protein within the cytoplasm, and forms a complex that moves to the nucleus, where it affects gene expression.<\/em>[\/caption]\r\nNon-Steroid Hormones<\/h2>\r\n[caption id=\"attachment_3426\" align=\"alignright\" width=\"381\"] Figure 9.3.3 A non-steroid hormone binds with a receptor on the plasma membrane of a target cell. Then, a secondary messenger affects cell processes.<\/em>[\/caption]\r\n\r\nA\u00a0[pb_glossary id=\"3425\"]non-steroid hormone[\/pb_glossary]<\/strong>is made of\u00a0[pb_glossary id=\"5707\"]amino acids[\/pb_glossary]. It is not fat soluble, so it cannot diffuse across the\u00a0plasma membrane\u00a0of a target cell. Instead, it binds to a receptor\u00a0protein\u00a0on the\u00a0cell membrane. In the Figure 9.3.3<\/span>\u00a0diagram, you can see that the binding of the hormone with the receptor activates an\u00a0[pb_glossary id=\"5757\"]enzyme[\/pb_glossary]\u00a0in the cell membrane. The enzyme then stimulates another molecule, called the second messenger, which influences processes inside the cell. Most endocrine hormones are non-steroid hormones. Examples include [pb_glossary id=\"6043\"]glucagon[\/pb_glossary] and [pb_glossary id=\"2590\"]insulin[\/pb_glossary], both produced by the\u00a0[pb_glossary id=\"3197\"]pancreas[\/pb_glossary].<\/span>\r\n\r\nRegulation of Endocrine Hormones<\/h1>\r\n<\/div>\r\nEndocrine hormones regulate many body processes, but what regulates the secretion of endocrine hormones? Most endocrine hormones are controlled by [pb_glossary id=\"6007\"]feedback mechanism[\/pb_glossary]s. A feedback mechanism is a loop in which a product feeds back to control its own production. Feedback loops may be either negative or positive.\r\n\r\n \t- Most endocrine hormones are regulated by [pb_glossary id=\"2955\"]negative feedback[\/pb_glossary] loops. Negative feedback keeps the\u00a0concentration\u00a0of a hormone within a relatively narrow range, and maintains\u00a0[pb_glossary id=\"5761\"]homeostasis[\/pb_glossary].<\/li>\r\n \t
- Very few endocrine hormones are regulated by [pb_glossary id=\"2962\"]positive feedback[\/pb_glossary] loops. Positive feedback causes the\u00a0concentration\u00a0of a hormone to become increasingly higher.<\/li>\r\n<\/ul>\r\n
Regulation by Negative Feedback<\/h2>\r\n[caption id=\"attachment_3430\" align=\"alignleft\" width=\"320\"] Figure 9.3.4 This diagram shows how the thyroid gland is regulated by a negative feedback loop that also involves the hypothalamus and pituitary gland.<\/em>[\/caption]\r\n\r\nA [pb_glossary id=\"2955\"]negative feedback[\/pb_glossary] loop controls the synthesis and secretion of hormones by the [pb_glossary id=\"2958\"]thyroid gland[\/pb_glossary]. This loop includes the [pb_glossary id=\"2937\"]hypothalamus[\/pb_glossary] and [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary], in addition to the thyroid, as shown in the diagram (Figure 9.3.4). When the levels of thyroid hormones circulating in the blood fall too low, the hypothalamus secretes [pb_glossary id=\"3428\"]thyrotropin releasing hormone[\/pb_glossary] (TRH). This hormone travels directly to the pituitary gland through the thin stalk connecting the two structures. In the pituitary gland, TRH stimulates the pituitary to secrete [pb_glossary id=\"3429\"]thyroid stimulating hormone[\/pb_glossary] (TSH). TSH, in turn, travels through the bloodstream to the thyroid gland, and stimulates it to secrete thyroid hormones. This continues until the blood levels of thyroid hormones are high enough. At that point, the thyroid hormones feed back to stop the hypothalamus from secreting TRH and the pituitary from secreting TSH. Without the stimulation of TSH, the thyroid gland stops secreting its hormones. Eventually, the levels of thyroid hormones in the blood start to fall too low again. When that happens, the hypothalamus releases TRH, and the loop repeats.\r\n\r\n \r\n\r\n\r\nRegulation by Positive Feedback<\/span>\r\n\r\n<\/div>\r\n[pb_glossary id=\"3431\"]Prolactin[\/pb_glossary] is a non-steroid endocrine hormone secreted by the\u00a0[pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. One of the functions of prolactin is to stimulate a nursing mother\u2019s [pb_glossary id=\"3432\"]mammary glands[\/pb_glossary]\u00a0to produce milk. The regulation of prolactin in the mother is controlled by a [pb_glossary id=\"2962\"]positive feedback loop[\/pb_glossary] that involves the [pb_glossary id=\"3433\"]nipples[\/pb_glossary], [pb_glossary id=\"2937\"]hypothalamus[\/pb_glossary], [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary], and [pb_glossary id=\"3432\"]mammary glands[\/pb_glossary]. Positive feedback begins when a baby suckles on the mother\u2019s nipple.\u00a0Nerve impulses\u00a0from the nipple reach the hypothalamus, which stimulates the pituitary gland to secrete prolactin. Prolactin travels in the\u00a0blood\u00a0to the mammary glands and stimulates them to produce milk. The release of milk causes the baby to continue suckling, which causes more prolactin to be secreted and more milk to be produced. The positive feedback loop continues until the baby stops suckling at the breast.\r\n\r\n \r\n\r\n[caption id=\"attachment_3437\" align=\"aligncenter\" width=\"679\"] Figure 9.3.5 The positive feedback loop for lactation involves the suckling, the breast and the pituitary gland.<\/em>[\/caption]\r\n\r\n\r\nFeature: Myth vs. Reality<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3438\"]Anabolic steroids[\/pb_glossary] are synthetic versions of the naturally occurring male sex hormone [pb_glossary id=\"3409\"]testosterone[\/pb_glossary]. Male hormones have androgenic (or masculinizing) effects, but they also have anabolic (or muscle-building) effects. The anabolic effects are the reason that synthetic steroids are used by athletes. In addition to building\u00a0muscles, they also accelerate the\u00a0development\u00a0of\u00a0[pb_glossary id=\"5913\"]bones[\/pb_glossary]\u00a0and [pb_glossary id=\"3439\"]red blood cells[\/pb_glossary], increase endurance so athletes can train harder and longer, and\u00a0speed\u00a0up muscle recovery. Unfortunately, these benefits of steroid use come with costs. If you ever consider taking anabolic steroids to build muscles and improve athletic performance, consider the following myths and corresponding realities.\r\n
Figure 9.3.2 A steroid hormone crosses the plasma membrane of a target cell, binds with a receptor protein within the cytoplasm, and forms a complex that moves to the nucleus, where it affects gene expression.<\/em>[\/caption]\r\nNon-Steroid Hormones<\/h2>\r\n[caption id=\"attachment_3426\" align=\"alignright\" width=\"381\"] Figure 9.3.3 A non-steroid hormone binds with a receptor on the plasma membrane of a target cell. Then, a secondary messenger affects cell processes.<\/em>[\/caption]\r\n\r\nA\u00a0[pb_glossary id=\"3425\"]non-steroid hormone[\/pb_glossary]<\/strong>is made of\u00a0[pb_glossary id=\"5707\"]amino acids[\/pb_glossary]. It is not fat soluble, so it cannot diffuse across the\u00a0plasma membrane\u00a0of a target cell. Instead, it binds to a receptor\u00a0protein\u00a0on the\u00a0cell membrane. In the Figure 9.3.3<\/span>\u00a0diagram, you can see that the binding of the hormone with the receptor activates an\u00a0[pb_glossary id=\"5757\"]enzyme[\/pb_glossary]\u00a0in the cell membrane. The enzyme then stimulates another molecule, called the second messenger, which influences processes inside the cell. Most endocrine hormones are non-steroid hormones. Examples include [pb_glossary id=\"6043\"]glucagon[\/pb_glossary] and [pb_glossary id=\"2590\"]insulin[\/pb_glossary], both produced by the\u00a0[pb_glossary id=\"3197\"]pancreas[\/pb_glossary].<\/span>\r\n\r\nRegulation of Endocrine Hormones<\/h1>\r\n<\/div>\r\nEndocrine hormones regulate many body processes, but what regulates the secretion of endocrine hormones? Most endocrine hormones are controlled by [pb_glossary id=\"6007\"]feedback mechanism[\/pb_glossary]s. A feedback mechanism is a loop in which a product feeds back to control its own production. Feedback loops may be either negative or positive.\r\n\r\n \t- Most endocrine hormones are regulated by [pb_glossary id=\"2955\"]negative feedback[\/pb_glossary] loops. Negative feedback keeps the\u00a0concentration\u00a0of a hormone within a relatively narrow range, and maintains\u00a0[pb_glossary id=\"5761\"]homeostasis[\/pb_glossary].<\/li>\r\n \t
- Very few endocrine hormones are regulated by [pb_glossary id=\"2962\"]positive feedback[\/pb_glossary] loops. Positive feedback causes the\u00a0concentration\u00a0of a hormone to become increasingly higher.<\/li>\r\n<\/ul>\r\n
Regulation by Negative Feedback<\/h2>\r\n[caption id=\"attachment_3430\" align=\"alignleft\" width=\"320\"] Figure 9.3.4 This diagram shows how the thyroid gland is regulated by a negative feedback loop that also involves the hypothalamus and pituitary gland.<\/em>[\/caption]\r\n\r\nA [pb_glossary id=\"2955\"]negative feedback[\/pb_glossary] loop controls the synthesis and secretion of hormones by the [pb_glossary id=\"2958\"]thyroid gland[\/pb_glossary]. This loop includes the [pb_glossary id=\"2937\"]hypothalamus[\/pb_glossary] and [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary], in addition to the thyroid, as shown in the diagram (Figure 9.3.4). When the levels of thyroid hormones circulating in the blood fall too low, the hypothalamus secretes [pb_glossary id=\"3428\"]thyrotropin releasing hormone[\/pb_glossary] (TRH). This hormone travels directly to the pituitary gland through the thin stalk connecting the two structures. In the pituitary gland, TRH stimulates the pituitary to secrete [pb_glossary id=\"3429\"]thyroid stimulating hormone[\/pb_glossary] (TSH). TSH, in turn, travels through the bloodstream to the thyroid gland, and stimulates it to secrete thyroid hormones. This continues until the blood levels of thyroid hormones are high enough. At that point, the thyroid hormones feed back to stop the hypothalamus from secreting TRH and the pituitary from secreting TSH. Without the stimulation of TSH, the thyroid gland stops secreting its hormones. Eventually, the levels of thyroid hormones in the blood start to fall too low again. When that happens, the hypothalamus releases TRH, and the loop repeats.\r\n\r\n \r\n\r\n\r\nRegulation by Positive Feedback<\/span>\r\n\r\n<\/div>\r\n[pb_glossary id=\"3431\"]Prolactin[\/pb_glossary] is a non-steroid endocrine hormone secreted by the\u00a0[pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. One of the functions of prolactin is to stimulate a nursing mother\u2019s [pb_glossary id=\"3432\"]mammary glands[\/pb_glossary]\u00a0to produce milk. The regulation of prolactin in the mother is controlled by a [pb_glossary id=\"2962\"]positive feedback loop[\/pb_glossary] that involves the [pb_glossary id=\"3433\"]nipples[\/pb_glossary], [pb_glossary id=\"2937\"]hypothalamus[\/pb_glossary], [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary], and [pb_glossary id=\"3432\"]mammary glands[\/pb_glossary]. Positive feedback begins when a baby suckles on the mother\u2019s nipple.\u00a0Nerve impulses\u00a0from the nipple reach the hypothalamus, which stimulates the pituitary gland to secrete prolactin. Prolactin travels in the\u00a0blood\u00a0to the mammary glands and stimulates them to produce milk. The release of milk causes the baby to continue suckling, which causes more prolactin to be secreted and more milk to be produced. The positive feedback loop continues until the baby stops suckling at the breast.\r\n\r\n \r\n\r\n[caption id=\"attachment_3437\" align=\"aligncenter\" width=\"679\"] Figure 9.3.5 The positive feedback loop for lactation involves the suckling, the breast and the pituitary gland.<\/em>[\/caption]\r\n\r\n\r\nFeature: Myth vs. Reality<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3438\"]Anabolic steroids[\/pb_glossary] are synthetic versions of the naturally occurring male sex hormone [pb_glossary id=\"3409\"]testosterone[\/pb_glossary]. Male hormones have androgenic (or masculinizing) effects, but they also have anabolic (or muscle-building) effects. The anabolic effects are the reason that synthetic steroids are used by athletes. In addition to building\u00a0muscles, they also accelerate the\u00a0development\u00a0of\u00a0[pb_glossary id=\"5913\"]bones[\/pb_glossary]\u00a0and [pb_glossary id=\"3439\"]red blood cells[\/pb_glossary], increase endurance so athletes can train harder and longer, and\u00a0speed\u00a0up muscle recovery. Unfortunately, these benefits of steroid use come with costs. If you ever consider taking anabolic steroids to build muscles and improve athletic performance, consider the following myths and corresponding realities.\r\n
Regulation of Endocrine Hormones<\/h1>\r\n<\/div>\r\nEndocrine hormones regulate many body processes, but what regulates the secretion of endocrine hormones? Most endocrine hormones are controlled by [pb_glossary id=\"6007\"]feedback mechanism[\/pb_glossary]s. A feedback mechanism is a loop in which a product feeds back to control its own production. Feedback loops may be either negative or positive.\r\n\r\n \t- Most endocrine hormones are regulated by [pb_glossary id=\"2955\"]negative feedback[\/pb_glossary] loops. Negative feedback keeps the\u00a0concentration\u00a0of a hormone within a relatively narrow range, and maintains\u00a0[pb_glossary id=\"5761\"]homeostasis[\/pb_glossary].<\/li>\r\n \t
- Very few endocrine hormones are regulated by [pb_glossary id=\"2962\"]positive feedback[\/pb_glossary] loops. Positive feedback causes the\u00a0concentration\u00a0of a hormone to become increasingly higher.<\/li>\r\n<\/ul>\r\n
Regulation by Negative Feedback<\/h2>\r\n[caption id=\"attachment_3430\" align=\"alignleft\" width=\"320\"] Figure 9.3.4 This diagram shows how the thyroid gland is regulated by a negative feedback loop that also involves the hypothalamus and pituitary gland.<\/em>[\/caption]\r\n\r\nA [pb_glossary id=\"2955\"]negative feedback[\/pb_glossary] loop controls the synthesis and secretion of hormones by the [pb_glossary id=\"2958\"]thyroid gland[\/pb_glossary]. This loop includes the [pb_glossary id=\"2937\"]hypothalamus[\/pb_glossary] and [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary], in addition to the thyroid, as shown in the diagram (Figure 9.3.4). When the levels of thyroid hormones circulating in the blood fall too low, the hypothalamus secretes [pb_glossary id=\"3428\"]thyrotropin releasing hormone[\/pb_glossary] (TRH). This hormone travels directly to the pituitary gland through the thin stalk connecting the two structures. In the pituitary gland, TRH stimulates the pituitary to secrete [pb_glossary id=\"3429\"]thyroid stimulating hormone[\/pb_glossary] (TSH). TSH, in turn, travels through the bloodstream to the thyroid gland, and stimulates it to secrete thyroid hormones. This continues until the blood levels of thyroid hormones are high enough. At that point, the thyroid hormones feed back to stop the hypothalamus from secreting TRH and the pituitary from secreting TSH. Without the stimulation of TSH, the thyroid gland stops secreting its hormones. Eventually, the levels of thyroid hormones in the blood start to fall too low again. When that happens, the hypothalamus releases TRH, and the loop repeats.\r\n\r\n \r\n\r\n\r\nRegulation by Positive Feedback<\/span>\r\n\r\n<\/div>\r\n[pb_glossary id=\"3431\"]Prolactin[\/pb_glossary] is a non-steroid endocrine hormone secreted by the\u00a0[pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. One of the functions of prolactin is to stimulate a nursing mother\u2019s [pb_glossary id=\"3432\"]mammary glands[\/pb_glossary]\u00a0to produce milk. The regulation of prolactin in the mother is controlled by a [pb_glossary id=\"2962\"]positive feedback loop[\/pb_glossary] that involves the [pb_glossary id=\"3433\"]nipples[\/pb_glossary], [pb_glossary id=\"2937\"]hypothalamus[\/pb_glossary], [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary], and [pb_glossary id=\"3432\"]mammary glands[\/pb_glossary]. Positive feedback begins when a baby suckles on the mother\u2019s nipple.\u00a0Nerve impulses\u00a0from the nipple reach the hypothalamus, which stimulates the pituitary gland to secrete prolactin. Prolactin travels in the\u00a0blood\u00a0to the mammary glands and stimulates them to produce milk. The release of milk causes the baby to continue suckling, which causes more prolactin to be secreted and more milk to be produced. The positive feedback loop continues until the baby stops suckling at the breast.\r\n\r\n \r\n\r\n[caption id=\"attachment_3437\" align=\"aligncenter\" width=\"679\"] Figure 9.3.5 The positive feedback loop for lactation involves the suckling, the breast and the pituitary gland.<\/em>[\/caption]\r\n\r\n\r\nFeature: Myth vs. Reality<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3438\"]Anabolic steroids[\/pb_glossary] are synthetic versions of the naturally occurring male sex hormone [pb_glossary id=\"3409\"]testosterone[\/pb_glossary]. Male hormones have androgenic (or masculinizing) effects, but they also have anabolic (or muscle-building) effects. The anabolic effects are the reason that synthetic steroids are used by athletes. In addition to building\u00a0muscles, they also accelerate the\u00a0development\u00a0of\u00a0[pb_glossary id=\"5913\"]bones[\/pb_glossary]\u00a0and [pb_glossary id=\"3439\"]red blood cells[\/pb_glossary], increase endurance so athletes can train harder and longer, and\u00a0speed\u00a0up muscle recovery. Unfortunately, these benefits of steroid use come with costs. If you ever consider taking anabolic steroids to build muscles and improve athletic performance, consider the following myths and corresponding realities.\r\n
Regulation by Negative Feedback<\/h2>\r\n[caption id=\"attachment_3430\" align=\"alignleft\" width=\"320\"] Figure 9.3.4 This diagram shows how the thyroid gland is regulated by a negative feedback loop that also involves the hypothalamus and pituitary gland.<\/em>[\/caption]\r\n\r\nA [pb_glossary id=\"2955\"]negative feedback[\/pb_glossary] loop controls the synthesis and secretion of hormones by the [pb_glossary id=\"2958\"]thyroid gland[\/pb_glossary]. This loop includes the [pb_glossary id=\"2937\"]hypothalamus[\/pb_glossary] and [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary], in addition to the thyroid, as shown in the diagram (Figure 9.3.4). When the levels of thyroid hormones circulating in the blood fall too low, the hypothalamus secretes [pb_glossary id=\"3428\"]thyrotropin releasing hormone[\/pb_glossary] (TRH). This hormone travels directly to the pituitary gland through the thin stalk connecting the two structures. In the pituitary gland, TRH stimulates the pituitary to secrete [pb_glossary id=\"3429\"]thyroid stimulating hormone[\/pb_glossary] (TSH). TSH, in turn, travels through the bloodstream to the thyroid gland, and stimulates it to secrete thyroid hormones. This continues until the blood levels of thyroid hormones are high enough. At that point, the thyroid hormones feed back to stop the hypothalamus from secreting TRH and the pituitary from secreting TSH. Without the stimulation of TSH, the thyroid gland stops secreting its hormones. Eventually, the levels of thyroid hormones in the blood start to fall too low again. When that happens, the hypothalamus releases TRH, and the loop repeats.\r\n\r\n \r\n\r\n\r\nRegulation by Positive Feedback<\/span>\r\n\r\n<\/div>\r\n[pb_glossary id=\"3431\"]Prolactin[\/pb_glossary] is a non-steroid endocrine hormone secreted by the\u00a0[pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. One of the functions of prolactin is to stimulate a nursing mother\u2019s [pb_glossary id=\"3432\"]mammary glands[\/pb_glossary]\u00a0to produce milk. The regulation of prolactin in the mother is controlled by a [pb_glossary id=\"2962\"]positive feedback loop[\/pb_glossary] that involves the [pb_glossary id=\"3433\"]nipples[\/pb_glossary], [pb_glossary id=\"2937\"]hypothalamus[\/pb_glossary], [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary], and [pb_glossary id=\"3432\"]mammary glands[\/pb_glossary]. Positive feedback begins when a baby suckles on the mother\u2019s nipple.\u00a0Nerve impulses\u00a0from the nipple reach the hypothalamus, which stimulates the pituitary gland to secrete prolactin. Prolactin travels in the\u00a0blood\u00a0to the mammary glands and stimulates them to produce milk. The release of milk causes the baby to continue suckling, which causes more prolactin to be secreted and more milk to be produced. The positive feedback loop continues until the baby stops suckling at the breast.\r\n\r\n \r\n\r\n[caption id=\"attachment_3437\" align=\"aligncenter\" width=\"679\"] Figure 9.3.5 The positive feedback loop for lactation involves the suckling, the breast and the pituitary gland.<\/em>[\/caption]\r\n\r\n\r\nFeature: Myth vs. Reality<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3438\"]Anabolic steroids[\/pb_glossary] are synthetic versions of the naturally occurring male sex hormone [pb_glossary id=\"3409\"]testosterone[\/pb_glossary]. Male hormones have androgenic (or masculinizing) effects, but they also have anabolic (or muscle-building) effects. The anabolic effects are the reason that synthetic steroids are used by athletes. In addition to building\u00a0muscles, they also accelerate the\u00a0development\u00a0of\u00a0[pb_glossary id=\"5913\"]bones[\/pb_glossary]\u00a0and [pb_glossary id=\"3439\"]red blood cells[\/pb_glossary], increase endurance so athletes can train harder and longer, and\u00a0speed\u00a0up muscle recovery. Unfortunately, these benefits of steroid use come with costs. If you ever consider taking anabolic steroids to build muscles and improve athletic performance, consider the following myths and corresponding realities.\r\n
Figure 9.3.5 The positive feedback loop for lactation involves the suckling, the breast and the pituitary gland.<\/em>[\/caption]\r\n\r\n\r\n
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