{"id":635,"date":"2015-04-10T20:32:37","date_gmt":"2015-04-10T20:32:37","guid":{"rendered":"https:\/\/courses.candelalearning.com\/ap2x2master\/?post_type=chapter&#038;p=635"},"modified":"2017-02-20T07:44:15","modified_gmt":"2017-02-20T07:44:15","slug":"hormones","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/nemcc-ap2\/chapter\/hormones\/","title":{"raw":"Hormones","rendered":"Hormones"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\nBy the end of this section, you will be able to:\r\n<ul>\r\n \t<li>Describe the functions of hormones<\/li>\r\n \t<li>Identify the mechanisms of hormone action<\/li>\r\n \t<li>Discuss how regulation of hormone levels is maintained through feedback systems<\/li>\r\n \t<li>Identify the nature of hormones<\/li>\r\n \t<li>Discuss the mechanisms of hormone action<\/li>\r\n \t<li>Discuss the role of feedback loops and humoral, hormonal, and neural stimuli in hormone control<\/li>\r\n<\/ul>\r\n<\/div>\r\nAlthough a given hormone may travel throughout the body in the bloodstream, it will affect the activity only of its target cells; that is, cells with receptors for that particular hormone. Once the hormone binds to the receptor, a chain of events is initiated that leads to the target cell\u2019s response. Hormones play a critical role in the regulation of physiological processes because of the target cell responses they regulate. These responses contribute to human reproduction, growth and development of body tissues, metabolism, fluid, and electrolyte balance, sleep, and many other body functions.\r\n<h2>Functions of Hormones<\/h2>\r\nThe endocrine system and the nervous system work hand in hand to coordinate the activity of our hormones. \u00a0The nervous system consist of neurons, which allow signals to move between the brain and the body. \u00a0The endocrine system also plays a vital role in communication throughout our body. \u00a0The endocrine system consist of glands located all through the body, which secrete hormones that regulate things such as metabolism. digestion, blood pressure, and growth. \u00a0These two systems are not directly linked, however the two interact in many ways.\r\n\r\nHormones are chemical substances that are secreted by cells into extracellular fluids and regulate a variety of activities throughout the body.\r\n<h2>Nature of Hormones<\/h2>\r\nThe hormones of the human body can be divided into two major groups on the basis of their chemical structure. Hormones derived from amino acids include amines, peptides, and proteins. Hormones can be non steroids( composed of amino acids), steroids(made from cholesterol), and eicosanoids (lipid molecules that cause actions in cells, but are not considered hormones and include prostaglandins and leukotrienes). Damaged tissues release the prostaglandins and leukotrienes to function in vasoconstriction, constriction of respiratory passages, sensitize pain receptors, and inflammation. \u00a0These chemical groups affect a hormone\u2019s distribution, the type of receptors it binds to, and other aspects of its function.\r\n<h2>Amine Hormones<\/h2>\r\nHormones derived from the modification of amino acids are referred to as amine hormones.\u00a0Amine hormones are synthesized from the amino acids tryptophan or tyrosine. An example of a hormone derived from tryptophan is melatonin, which is secreted by the pineal gland and helps regulate circadian rhythm. Tyrosine derivatives include the metabolism-regulating thyroid hormones, as well as the catecholamines, such as epinephrine, norepinephrine, and dopamine. Epinephrine and norepinephrine are secreted by the adrenal medulla and play a role in the fight-or-flight response, whereas dopamine is secreted by the hypothalamus and inhibits the release of certain anterior pituitary hormones.\r\n<h2>Peptide and Protein Hormones<\/h2>\r\nWhereas the amine hormones are derived from a single amino acid, peptide and protein hormones consist of multiple amino acids that link to form an amino acid chain. Peptide hormones consist of short chains of amino acids, whereas protein hormones are longer polypeptides. Both types are synthesized like other body proteins: DNA is transcribed into mRNA, which is translated into an amino acid chain.\r\n\r\nExamples of peptide hormones include antidiuretic hormone (ADH), a pituitary hormone important in fluid balance, and atrial-natriuretic peptide, which is produced by the heart and helps to decrease blood pressure. Some examples of protein hormones include growth hormone, which is produced by the pituitary gland, and follicle-stimulating hormone (FSH), which has an attached carbohydrate group and is thus classified as a glycoprotein. FSH helps stimulate the maturation of eggs in the ovaries and sperm in the testes.\r\n<h2>Steroid Hormones<\/h2>\r\nThe primary hormones derived from lipids are steroids. Stroid horns diffuse through the plasma membrane of target cells and enter the nucleus. \u00a0They bind to a receptor within the nucleus forming a hormone-receptor complex. \u00a0This complex interacts with DNA beginning transcription of specific genes. \u00a0mRNA travels to ribosomes where translation takes place. \u00a0Steroid hormones are derived from the lipid cholesterol. For example, the reproductive hormones testosterone and the estrogens\u2014which are produced by the gonads (testes and ovaries)\u2014are steroid hormones. The adrenal glands produce the steroid hormone aldosterone, which is involved in osmoregulation, and cortisol, which plays a role in metabolism.\r\n\r\nLike cholesterol, steroid hormones are not soluble in water (they are hydrophobic). Because blood is water-based, lipid-derived hormones must travel to their target cell bound to a transport protein. This more complex structure extends the half-life of steroid hormones much longer than that of hormones derived from amino acids. A hormone\u2019s half-life is the time required for half the concentration of the hormone to be degraded. For example, the lipid-derived hormone cortisol has a half-life of approximately 60 to 90 minutes. In contrast, the amino acid\u2013derived hormone epinephrine has a half-life of approximately one minute.\r\n<h2><strong>Non steroid Hormone Action<\/strong><\/h2>\r\nThe hormone will bind to a membrane receptor, but the hormone does not enter the cell. \u00a0The binding of the first messenger (hormone) catalyzes a reaction that produces a second messenger molecule- usually cyclic adenosine monophosphate (cAMP). The second messenger oversees additional intracellular changes to promote a specific response to the hormone.\r\n<h2>Pathways of Hormone Action<\/h2>\r\nThe message a hormone sends is received by a hormone receptor, a protein located either inside the cell or within the cell membrane. The receptor will process the message by initiating other signaling events or cellular mechanisms that result in the target cell\u2019s response. Hormone receptors recognize molecules with specific shapes and side groups, and respond only to those hormones that are recognized. The same type of receptor may be located on cells in different body tissues, and trigger somewhat different responses. Thus, the response triggered by a hormone depends not only on the hormone, but also on the target cell.\r\n\r\nOnce the target cell receives the hormone signal, it can respond in a variety of ways. The response may include the stimulation of protein synthesis, activation or deactivation of enzymes, alteration in the permeability of the cell membrane, altered rates of mitosis and cell growth, and stimulation of the secretion of products. Moreover, a single hormone may be capable of inducing different responses in a given cell.\r\n<h3><\/h3>\r\n<h3>Factors Affecting Target Cell Response<\/h3>\r\nTarget cells\/Target organs- are certain tissue cells or organs that a hormone will affect. Target cells must have receptors specific to a given hormone if that hormone is to trigger a response. Target cells must have a specific protein receptor for the hormone to bind or have an effect.\r\n<h2>Regulation of Hormone Secretion<\/h2>\r\nTo prevent abnormal hormone levels and a potential disease state, hormone levels must be tightly controlled. The body maintains this control by balancing hormone production and degradation. Feedback loops govern the initiation and maintenance of most hormone secretion in response to various stimuli. Hormone levels must maintain homeostasis to prevent disease or disorder.\r\n<h3>Role of Feedback Loops<\/h3>\r\nThe contribution of feedback loops to homeostasis will only be briefly reviewed here. Positive feedback loops are characterized by the release of additional hormone in response to an original hormone release. The release of oxytocin during childbirth is a positive feedback loop. The initial release of oxytocin begins to signal the uterine muscles to contract, which pushes the fetus toward the cervix, causing it to stretch. This, in turn, signals the pituitary gland to release more oxytocin, causing labor contractions to intensify. The release of oxytocin decreases after the birth of the child.\r\n\r\nThe more common method of hormone regulation is the negative feedback loop. Negative feedback is characterized by the inhibition of further secretion of a hormone in response to adequate levels of that hormone. This allows blood levels of the hormone to be regulated within a narrow range. An example of a negative feedback loop is the release of glucocorticoid hormones from the adrenal glands, as directed by the hypothalamus and pituitary gland. As glucocorticoid concentrations in the blood rise, the hypothalamus and pituitary gland reduce their signaling to the adrenal glands to prevent additional glucocorticoid secretion (Figure 4).\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"520\"]<img src=\"http:\/\/cnx.org\/resources\/5679c569435d196d3ff8e8f6ae9390fc\/1805_Negative_Feedback_Loop.jpg\" alt=\"This diagram shows a negative feedback loop using the example of glucocorticoid regulation in the blood. Step 1 in the cycle is when an imbalance occurs. The hypothalamus perceives low blood concentrations of glucocorticoids in the blood. This is illustrated by there being only 5 glucocorticoids floating in a cross section of an artery. Step 2 in the cycle is hormone release, where the hypothalamus releases corticotropin-releasing hormone (CRH). Step 3 is labeled correction. Here, the CRH release starts a hormone cascade that triggers the adrenal gland to release glucocorticoid into the blood. This allows the blood concentration of glucocorticoid to increase, as illustrated by 8 glucocorticoid molecules now being present in the cross section of the artery. Step 4 is labeled negative feedback. Here, the hypothalamus perceives normal concentrations of glucocorticoids in the blood and stops releasing CRH. This brings blood glucocorticoid levels back to homeostasis.\" width=\"520\" height=\"1094\" \/> Figure 4. Negative Feedback Loop[\/caption]\r\n\r\nThe release of adrenal glucocorticoids is stimulated by the release of hormones from the hypothalamus and pituitary gland. This signaling is inhibited when glucocorticoid levels become elevated by causing negative signals to the pituitary gland and hypothalamus.\r\n<h3>Role of Endocrine Gland Stimuli<\/h3>\r\nReflexes triggered by both chemical and neural stimuli control endocrine activity. These reflexes may be simple, involving only one hormone response, or they may be more complex and involve many hormones, as is the case with the hypothalamic control of various anterior pituitary\u2013controlled hormones. Endocrine gland stimuli consist of <em>humoral, hormonal, and neural.<\/em>\r\n\r\n<em>Humoral stimuli<\/em> are changes in blood levels of non-hormone chemicals, such as nutrients or ions, which cause the release or inhibition of a hormone to, in turn, maintain homeostasis. For example, osmoreceptors in the hypothalamus detect changes in blood osmolarity (the concentration of solutes in the blood plasma). If blood osmolarity is too high, meaning that the blood is not dilute enough, osmoreceptors signal the hypothalamus to release ADH. The hormone causes the kidneys to reabsorb more water and reduce the volume of urine produced. This reabsorption causes a reduction of the osmolarity of the blood, diluting the blood to the appropriate level. The regulation of blood glucose is another example. High blood glucose levels cause the release of insulin from the pancreas, which increases glucose uptake by cells and liver storage of glucose as glycogen.\r\n\r\nHormones may also be released in response to <em>hormonal stimuli<\/em>. \u00a0An endocrine gland may also secrete a hormone in response to the presence of another hormone produced by a different endocrine gland. Such hormonal stimuli often involve the hypothalamus, which produces releasing and inhibiting hormones that control the secretion of a variety of pituitary hormones. Glands are activated by other hormones which activates another hormone and so forth.\r\n\r\nIn addition to these chemical signals, hormones can also be released in response to <em>neural stimul<\/em>i. A common example of neural stimuli is the activation of the fight-or-flight response by the sympathetic nervous system. When an individual perceives danger, sympathetic neurons signal the adrenal glands to secrete norepinephrine and epinephrine. The two hormones dilate blood vessels, increase the heart and respiratory rate, and suppress the digestive and immune systems. These responses boost the body\u2019s transport of oxygen to the brain and muscles, thereby improving the body\u2019s ability to fight or flee.\r\n<div class=\"textbox examples\">\r\n<h3>Everyday Connections<\/h3>\r\nBisphenol A and Endocrine DisruptionYou may have heard news reports about the effects of a chemical called bisphenol A (BPA) in various types of food packaging. BPA is used in the manufacturing of hard plastics and epoxy resins. Common food-related items that may contain BPA include the lining of aluminum cans, plastic food-storage containers, drinking cups, as well as baby bottles and \u201csippy\u201d cups. Other uses of BPA include medical equipment, dental fillings, and the lining of water pipes.\r\n\r\nResearch suggests that BPA is an endocrine disruptor, meaning that it negatively interferes with the endocrine system, particularly during the prenatal and postnatal development period. In particular, BPA mimics the hormonal effects of estrogens and has the opposite effect\u2014that of androgens. The U.S. Food and Drug Administration (FDA) notes in their statement about BPA safety that although traditional toxicology studies have supported the safety of low levels of exposure to BPA, recent studies using novel approaches to test for subtle effects have led to some concern about the potential effects of BPA on the brain, behavior, and prostate gland in fetuses, infants, and young children. The FDA is currently facilitating decreased use of BPA in food-related materials. Many US companies have voluntarily removed BPA from baby bottles, \u201csippy\u201d cups, and the linings of infant formula cans, and most plastic reusable water bottles sold today boast that they are \u201cBPA free.\u201d In contrast, both Canada and the European Union have completely banned the use of BPA in baby products.\r\n\r\nThe potential harmful effects of BPA have been studied in both animal models and humans and include a large variety of health effects, such as developmental delay and disease. For example, prenatal exposure to BPA during the first trimester of human pregnancy may be associated with wheezing and aggressive behavior during childhood. Adults exposed to high levels of BPA may experience altered thyroid signaling and male sexual dysfunction. BPA exposure during the prenatal or postnatal period of development in animal models has been observed to cause neurological delays, changes in brain structure and function, sexual dysfunction, asthma, and increased risk for multiple cancers. In vitro studies have also shown that BPA exposure causes molecular changes that initiate the development of cancers of the breast, prostate, and brain. Although these studies have implicated BPA in numerous ill health effects, some experts caution that some of these studies may be flawed and that more research needs to be done. In the meantime, the FDA recommends that consumers take precautions to limit their exposure to BPA. In addition to purchasing foods in packaging free of BPA, consumers should avoid carrying or storing foods or liquids in bottles with the recycling code 3 or 7. Foods and liquids should not be microwave-heated in any form of plastic: use paper, glass, or ceramics instead.\r\n\r\n<\/div>\r\n<h2>Glossary<\/h2>\r\n<strong>adenylyl cyclase:\u00a0<\/strong>membrane-bound enzyme that converts ATP to cyclic AMP, creating cAMP, as a result of G-protein activation\r\n\r\n<strong>cyclic adenosine monophosphate (cAMP):\u00a0<\/strong>second messenger that, in response to adenylyl cyclase activation, triggers a phosphorylation cascade\r\n\r\n<strong>diacylglycerol (DAG):\u00a0<\/strong>molecule that, like cAMP, activates protein kinases, thereby initiating a phosphorylation cascade\r\n\r\n<strong>downregulation:\u00a0<\/strong>decrease in the number of hormone receptors, typically in response to chronically excessive levels of a hormone\r\n\r\n<strong>first messenger:\u00a0<\/strong>hormone that binds to a cell membrane hormone receptor and triggers activation of a second messenger system\r\n\r\n<strong>G protein:\u00a0<\/strong>protein associated with a cell membrane hormone receptor that initiates the next step in a second messenger system upon activation by hormone\u2013receptor binding\r\n\r\n<strong>hormone receptor:\u00a0<\/strong>protein within a cell or on the cell membrane that binds a hormone, initiating the target cell response\r\n\r\n<strong>inositol triphosphate (IP<sub>3<\/sub>):\u00a0<\/strong>molecule that initiates the release of calcium ions from intracellular stores\r\n\r\n<strong>phosphodiesterase (PDE):\u00a0<\/strong>cytosolic enzyme that deactivates and degrades cAMP\r\n\r\n<strong>phosphorylation cascade:\u00a0<\/strong>signaling event in which multiple protein kinases phosphorylate the next protein substrate by transferring a phosphate group from ATP to the protein\r\n\r\n<strong>protein kinase:\u00a0<\/strong>enzyme that initiates a phosphorylation cascade upon activation\r\n\r\n<strong>second messenger:\u00a0<\/strong>molecule that initiates a signaling cascade in response to hormone binding on a cell membrane receptor and activation of a G protein\r\n\r\n<strong>upregulation:\u00a0<\/strong>increase in the number of hormone receptors, typically in response to chronically reduced levels of a hormone","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<p>By the end of this section, you will be able to:<\/p>\n<ul>\n<li>Describe the functions of hormones<\/li>\n<li>Identify the mechanisms of hormone action<\/li>\n<li>Discuss how regulation of hormone levels is maintained through feedback systems<\/li>\n<li>Identify the nature of hormones<\/li>\n<li>Discuss the mechanisms of hormone action<\/li>\n<li>Discuss the role of feedback loops and humoral, hormonal, and neural stimuli in hormone control<\/li>\n<\/ul>\n<\/div>\n<p>Although a given hormone may travel throughout the body in the bloodstream, it will affect the activity only of its target cells; that is, cells with receptors for that particular hormone. Once the hormone binds to the receptor, a chain of events is initiated that leads to the target cell\u2019s response. Hormones play a critical role in the regulation of physiological processes because of the target cell responses they regulate. These responses contribute to human reproduction, growth and development of body tissues, metabolism, fluid, and electrolyte balance, sleep, and many other body functions.<\/p>\n<h2>Functions of Hormones<\/h2>\n<p>The endocrine system and the nervous system work hand in hand to coordinate the activity of our hormones. \u00a0The nervous system consist of neurons, which allow signals to move between the brain and the body. \u00a0The endocrine system also plays a vital role in communication throughout our body. \u00a0The endocrine system consist of glands located all through the body, which secrete hormones that regulate things such as metabolism. digestion, blood pressure, and growth. \u00a0These two systems are not directly linked, however the two interact in many ways.<\/p>\n<p>Hormones are chemical substances that are secreted by cells into extracellular fluids and regulate a variety of activities throughout the body.<\/p>\n<h2>Nature of Hormones<\/h2>\n<p>The hormones of the human body can be divided into two major groups on the basis of their chemical structure. Hormones derived from amino acids include amines, peptides, and proteins. Hormones can be non steroids( composed of amino acids), steroids(made from cholesterol), and eicosanoids (lipid molecules that cause actions in cells, but are not considered hormones and include prostaglandins and leukotrienes). Damaged tissues release the prostaglandins and leukotrienes to function in vasoconstriction, constriction of respiratory passages, sensitize pain receptors, and inflammation. \u00a0These chemical groups affect a hormone\u2019s distribution, the type of receptors it binds to, and other aspects of its function.<\/p>\n<h2>Amine Hormones<\/h2>\n<p>Hormones derived from the modification of amino acids are referred to as amine hormones.\u00a0Amine hormones are synthesized from the amino acids tryptophan or tyrosine. An example of a hormone derived from tryptophan is melatonin, which is secreted by the pineal gland and helps regulate circadian rhythm. Tyrosine derivatives include the metabolism-regulating thyroid hormones, as well as the catecholamines, such as epinephrine, norepinephrine, and dopamine. Epinephrine and norepinephrine are secreted by the adrenal medulla and play a role in the fight-or-flight response, whereas dopamine is secreted by the hypothalamus and inhibits the release of certain anterior pituitary hormones.<\/p>\n<h2>Peptide and Protein Hormones<\/h2>\n<p>Whereas the amine hormones are derived from a single amino acid, peptide and protein hormones consist of multiple amino acids that link to form an amino acid chain. Peptide hormones consist of short chains of amino acids, whereas protein hormones are longer polypeptides. Both types are synthesized like other body proteins: DNA is transcribed into mRNA, which is translated into an amino acid chain.<\/p>\n<p>Examples of peptide hormones include antidiuretic hormone (ADH), a pituitary hormone important in fluid balance, and atrial-natriuretic peptide, which is produced by the heart and helps to decrease blood pressure. Some examples of protein hormones include growth hormone, which is produced by the pituitary gland, and follicle-stimulating hormone (FSH), which has an attached carbohydrate group and is thus classified as a glycoprotein. FSH helps stimulate the maturation of eggs in the ovaries and sperm in the testes.<\/p>\n<h2>Steroid Hormones<\/h2>\n<p>The primary hormones derived from lipids are steroids. Stroid horns diffuse through the plasma membrane of target cells and enter the nucleus. \u00a0They bind to a receptor within the nucleus forming a hormone-receptor complex. \u00a0This complex interacts with DNA beginning transcription of specific genes. \u00a0mRNA travels to ribosomes where translation takes place. \u00a0Steroid hormones are derived from the lipid cholesterol. For example, the reproductive hormones testosterone and the estrogens\u2014which are produced by the gonads (testes and ovaries)\u2014are steroid hormones. The adrenal glands produce the steroid hormone aldosterone, which is involved in osmoregulation, and cortisol, which plays a role in metabolism.<\/p>\n<p>Like cholesterol, steroid hormones are not soluble in water (they are hydrophobic). Because blood is water-based, lipid-derived hormones must travel to their target cell bound to a transport protein. This more complex structure extends the half-life of steroid hormones much longer than that of hormones derived from amino acids. A hormone\u2019s half-life is the time required for half the concentration of the hormone to be degraded. For example, the lipid-derived hormone cortisol has a half-life of approximately 60 to 90 minutes. In contrast, the amino acid\u2013derived hormone epinephrine has a half-life of approximately one minute.<\/p>\n<h2><strong>Non steroid Hormone Action<\/strong><\/h2>\n<p>The hormone will bind to a membrane receptor, but the hormone does not enter the cell. \u00a0The binding of the first messenger (hormone) catalyzes a reaction that produces a second messenger molecule- usually cyclic adenosine monophosphate (cAMP). The second messenger oversees additional intracellular changes to promote a specific response to the hormone.<\/p>\n<h2>Pathways of Hormone Action<\/h2>\n<p>The message a hormone sends is received by a hormone receptor, a protein located either inside the cell or within the cell membrane. The receptor will process the message by initiating other signaling events or cellular mechanisms that result in the target cell\u2019s response. Hormone receptors recognize molecules with specific shapes and side groups, and respond only to those hormones that are recognized. The same type of receptor may be located on cells in different body tissues, and trigger somewhat different responses. Thus, the response triggered by a hormone depends not only on the hormone, but also on the target cell.<\/p>\n<p>Once the target cell receives the hormone signal, it can respond in a variety of ways. The response may include the stimulation of protein synthesis, activation or deactivation of enzymes, alteration in the permeability of the cell membrane, altered rates of mitosis and cell growth, and stimulation of the secretion of products. Moreover, a single hormone may be capable of inducing different responses in a given cell.<\/p>\n<h3><\/h3>\n<h3>Factors Affecting Target Cell Response<\/h3>\n<p>Target cells\/Target organs- are certain tissue cells or organs that a hormone will affect. Target cells must have receptors specific to a given hormone if that hormone is to trigger a response. Target cells must have a specific protein receptor for the hormone to bind or have an effect.<\/p>\n<h2>Regulation of Hormone Secretion<\/h2>\n<p>To prevent abnormal hormone levels and a potential disease state, hormone levels must be tightly controlled. The body maintains this control by balancing hormone production and degradation. Feedback loops govern the initiation and maintenance of most hormone secretion in response to various stimuli. Hormone levels must maintain homeostasis to prevent disease or disorder.<\/p>\n<h3>Role of Feedback Loops<\/h3>\n<p>The contribution of feedback loops to homeostasis will only be briefly reviewed here. Positive feedback loops are characterized by the release of additional hormone in response to an original hormone release. The release of oxytocin during childbirth is a positive feedback loop. The initial release of oxytocin begins to signal the uterine muscles to contract, which pushes the fetus toward the cervix, causing it to stretch. This, in turn, signals the pituitary gland to release more oxytocin, causing labor contractions to intensify. The release of oxytocin decreases after the birth of the child.<\/p>\n<p>The more common method of hormone regulation is the negative feedback loop. Negative feedback is characterized by the inhibition of further secretion of a hormone in response to adequate levels of that hormone. This allows blood levels of the hormone to be regulated within a narrow range. An example of a negative feedback loop is the release of glucocorticoid hormones from the adrenal glands, as directed by the hypothalamus and pituitary gland. As glucocorticoid concentrations in the blood rise, the hypothalamus and pituitary gland reduce their signaling to the adrenal glands to prevent additional glucocorticoid secretion (Figure 4).<\/p>\n<div style=\"width: 530px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"http:\/\/cnx.org\/resources\/5679c569435d196d3ff8e8f6ae9390fc\/1805_Negative_Feedback_Loop.jpg\" alt=\"This diagram shows a negative feedback loop using the example of glucocorticoid regulation in the blood. Step 1 in the cycle is when an imbalance occurs. The hypothalamus perceives low blood concentrations of glucocorticoids in the blood. This is illustrated by there being only 5 glucocorticoids floating in a cross section of an artery. Step 2 in the cycle is hormone release, where the hypothalamus releases corticotropin-releasing hormone (CRH). Step 3 is labeled correction. Here, the CRH release starts a hormone cascade that triggers the adrenal gland to release glucocorticoid into the blood. This allows the blood concentration of glucocorticoid to increase, as illustrated by 8 glucocorticoid molecules now being present in the cross section of the artery. Step 4 is labeled negative feedback. Here, the hypothalamus perceives normal concentrations of glucocorticoids in the blood and stops releasing CRH. This brings blood glucocorticoid levels back to homeostasis.\" width=\"520\" height=\"1094\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 4. Negative Feedback Loop<\/p>\n<\/div>\n<p>The release of adrenal glucocorticoids is stimulated by the release of hormones from the hypothalamus and pituitary gland. This signaling is inhibited when glucocorticoid levels become elevated by causing negative signals to the pituitary gland and hypothalamus.<\/p>\n<h3>Role of Endocrine Gland Stimuli<\/h3>\n<p>Reflexes triggered by both chemical and neural stimuli control endocrine activity. These reflexes may be simple, involving only one hormone response, or they may be more complex and involve many hormones, as is the case with the hypothalamic control of various anterior pituitary\u2013controlled hormones. Endocrine gland stimuli consist of <em>humoral, hormonal, and neural.<\/em><\/p>\n<p><em>Humoral stimuli<\/em> are changes in blood levels of non-hormone chemicals, such as nutrients or ions, which cause the release or inhibition of a hormone to, in turn, maintain homeostasis. For example, osmoreceptors in the hypothalamus detect changes in blood osmolarity (the concentration of solutes in the blood plasma). If blood osmolarity is too high, meaning that the blood is not dilute enough, osmoreceptors signal the hypothalamus to release ADH. The hormone causes the kidneys to reabsorb more water and reduce the volume of urine produced. This reabsorption causes a reduction of the osmolarity of the blood, diluting the blood to the appropriate level. The regulation of blood glucose is another example. High blood glucose levels cause the release of insulin from the pancreas, which increases glucose uptake by cells and liver storage of glucose as glycogen.<\/p>\n<p>Hormones may also be released in response to <em>hormonal stimuli<\/em>. \u00a0An endocrine gland may also secrete a hormone in response to the presence of another hormone produced by a different endocrine gland. Such hormonal stimuli often involve the hypothalamus, which produces releasing and inhibiting hormones that control the secretion of a variety of pituitary hormones. Glands are activated by other hormones which activates another hormone and so forth.<\/p>\n<p>In addition to these chemical signals, hormones can also be released in response to <em>neural stimul<\/em>i. A common example of neural stimuli is the activation of the fight-or-flight response by the sympathetic nervous system. When an individual perceives danger, sympathetic neurons signal the adrenal glands to secrete norepinephrine and epinephrine. The two hormones dilate blood vessels, increase the heart and respiratory rate, and suppress the digestive and immune systems. These responses boost the body\u2019s transport of oxygen to the brain and muscles, thereby improving the body\u2019s ability to fight or flee.<\/p>\n<div class=\"textbox examples\">\n<h3>Everyday Connections<\/h3>\n<p>Bisphenol A and Endocrine DisruptionYou may have heard news reports about the effects of a chemical called bisphenol A (BPA) in various types of food packaging. BPA is used in the manufacturing of hard plastics and epoxy resins. Common food-related items that may contain BPA include the lining of aluminum cans, plastic food-storage containers, drinking cups, as well as baby bottles and \u201csippy\u201d cups. Other uses of BPA include medical equipment, dental fillings, and the lining of water pipes.<\/p>\n<p>Research suggests that BPA is an endocrine disruptor, meaning that it negatively interferes with the endocrine system, particularly during the prenatal and postnatal development period. In particular, BPA mimics the hormonal effects of estrogens and has the opposite effect\u2014that of androgens. The U.S. Food and Drug Administration (FDA) notes in their statement about BPA safety that although traditional toxicology studies have supported the safety of low levels of exposure to BPA, recent studies using novel approaches to test for subtle effects have led to some concern about the potential effects of BPA on the brain, behavior, and prostate gland in fetuses, infants, and young children. The FDA is currently facilitating decreased use of BPA in food-related materials. Many US companies have voluntarily removed BPA from baby bottles, \u201csippy\u201d cups, and the linings of infant formula cans, and most plastic reusable water bottles sold today boast that they are \u201cBPA free.\u201d In contrast, both Canada and the European Union have completely banned the use of BPA in baby products.<\/p>\n<p>The potential harmful effects of BPA have been studied in both animal models and humans and include a large variety of health effects, such as developmental delay and disease. For example, prenatal exposure to BPA during the first trimester of human pregnancy may be associated with wheezing and aggressive behavior during childhood. Adults exposed to high levels of BPA may experience altered thyroid signaling and male sexual dysfunction. BPA exposure during the prenatal or postnatal period of development in animal models has been observed to cause neurological delays, changes in brain structure and function, sexual dysfunction, asthma, and increased risk for multiple cancers. In vitro studies have also shown that BPA exposure causes molecular changes that initiate the development of cancers of the breast, prostate, and brain. Although these studies have implicated BPA in numerous ill health effects, some experts caution that some of these studies may be flawed and that more research needs to be done. In the meantime, the FDA recommends that consumers take precautions to limit their exposure to BPA. In addition to purchasing foods in packaging free of BPA, consumers should avoid carrying or storing foods or liquids in bottles with the recycling code 3 or 7. Foods and liquids should not be microwave-heated in any form of plastic: use paper, glass, or ceramics instead.<\/p>\n<\/div>\n<h2>Glossary<\/h2>\n<p><strong>adenylyl cyclase:\u00a0<\/strong>membrane-bound enzyme that converts ATP to cyclic AMP, creating cAMP, as a result of G-protein activation<\/p>\n<p><strong>cyclic adenosine monophosphate (cAMP):\u00a0<\/strong>second messenger that, in response to adenylyl cyclase activation, triggers a phosphorylation cascade<\/p>\n<p><strong>diacylglycerol (DAG):\u00a0<\/strong>molecule that, like cAMP, activates protein kinases, thereby initiating a phosphorylation cascade<\/p>\n<p><strong>downregulation:\u00a0<\/strong>decrease in the number of hormone receptors, typically in response to chronically excessive levels of a hormone<\/p>\n<p><strong>first messenger:\u00a0<\/strong>hormone that binds to a cell membrane hormone receptor and triggers activation of a second messenger system<\/p>\n<p><strong>G protein:\u00a0<\/strong>protein associated with a cell membrane hormone receptor that initiates the next step in a second messenger system upon activation by hormone\u2013receptor binding<\/p>\n<p><strong>hormone receptor:\u00a0<\/strong>protein within a cell or on the cell membrane that binds a hormone, initiating the target cell response<\/p>\n<p><strong>inositol triphosphate (IP<sub>3<\/sub>):\u00a0<\/strong>molecule that initiates the release of calcium ions from intracellular stores<\/p>\n<p><strong>phosphodiesterase (PDE):\u00a0<\/strong>cytosolic enzyme that deactivates and degrades cAMP<\/p>\n<p><strong>phosphorylation cascade:\u00a0<\/strong>signaling event in which multiple protein kinases phosphorylate the next protein substrate by transferring a phosphate group from ATP to the protein<\/p>\n<p><strong>protein kinase:\u00a0<\/strong>enzyme that initiates a phosphorylation cascade upon activation<\/p>\n<p><strong>second messenger:\u00a0<\/strong>molecule that initiates a signaling cascade in response to hormone binding on a cell membrane receptor and activation of a G protein<\/p>\n<p><strong>upregulation:\u00a0<\/strong>increase in the number of hormone receptors, typically in response to chronically reduced levels of a hormone<\/p>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-635\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Anatomy &amp; Physiology. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.25\">http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.25<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.25<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":12253,"menu_order":2,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Anatomy & Physiology\",\"author\":\"\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.25\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.25\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-635","chapter","type-chapter","status-publish","hentry"],"part":1150,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/nemcc-ap2\/wp-json\/pressbooks\/v2\/chapters\/635","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/nemcc-ap2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/nemcc-ap2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/nemcc-ap2\/wp-json\/wp\/v2\/users\/12253"}],"version-history":[{"count":15,"href":"https:\/\/courses.lumenlearning.com\/nemcc-ap2\/wp-json\/pressbooks\/v2\/chapters\/635\/revisions"}],"predecessor-version":[{"id":2182,"href":"https:\/\/courses.lumenlearning.com\/nemcc-ap2\/wp-json\/pressbooks\/v2\/chapters\/635\/revisions\/2182"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/nemcc-ap2\/wp-json\/pressbooks\/v2\/parts\/1150"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/nemcc-ap2\/wp-json\/pressbooks\/v2\/chapters\/635\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/nemcc-ap2\/wp-json\/wp\/v2\/media?parent=635"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/nemcc-ap2\/wp-json\/pressbooks\/v2\/chapter-type?post=635"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/nemcc-ap2\/wp-json\/wp\/v2\/contributor?post=635"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/nemcc-ap2\/wp-json\/wp\/v2\/license?post=635"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}