Chapter 15: THE ENDOCRINE SYSTEM

15 The Endocrine System Chapter Outline The Endocrine System: Hormones Hormones are signaling molecules carried in the bloodstream Hormones are produced in small amounts and often interact Types of Hormones and Their Signals Hormones come in several chemical forms Steroid hormones interact with cell DNA Nonsteroid hormones act indirectly, by way of second messengers The Hypothalamus and Pituitary Gland The posterior pituitary lobe stores and releases hormones from the hypothalamus The anterior pituitary lobe makes hormones. Hormones as Long-Term Controllers GH Functions and Disorders The Thyroid and Parathyroid Glands Thyroid hormones affect metabolism and growth PTH from the parathyroids is the main calcium regulator Adrenal Glands and Stress Responses The adrenal cortex produces glucocorticoids and mineralocorticoids Hormones from the adrenal medulla help regulate blood circulation Long-term stress can damage health The Pancreas: Regulating Blood Sugar Blood Sugar Disorders Type 2 diabetes is a global health crisis ”Prediabetes” is a warning sign Low blood sugar threatens the brain Other Hormone Sources The gonads produce sex hormones The pineal gland makes melatonin, Skeletal muscle, the thymus, the heart, and the GI tract also produce hormones Connections: The Endocrine System in Homeostasis Summary Review questions self-quiz critical thinking explore on your own your future Objectives Know how hormones operate in the human body. Learn the chemical categories of hormones and how they produce an effect. Understand the structure and hormones produced by the hypothalamus and pituitary gland. Be able to describe the abnormalities associated with growth hormone. Know how hormones act as long-term controllers in the body. Understand the structure, hormones produced, and abnormalities associated with the thyroid and parathyroid glands. Learn the hormones produced by the adrenal glands and their relationship to stress. Be able to describe how the pancreas regulates your blood sugar. Understand disorders relating to the level of sugar in the bloodstream. Know the hormones and their functions for the following glands: gonads, pineal gland, thymus, heart, and GI tract. Be able to describe how the endocrine system helps to maintain homeostasis in various organ systems. Key Terms target cell hormones endocrine system opposing interaction synergistic interaction permissive interaction steroid hormones nonsteroid hormones second messenger hypothalamus pituitary gland gigantism acromegaly pituitary dwarfism thyroid gland simple goiter Grave’s disease parathyroid glands rickets adrenal cortex glucocorticoids gluconeogenesis hypoglycemia mineralocorticoids adrenal medulla pancreatic islet diabetes mellitus metabolic acidosis type 1 diabetes type 2 diabetes hypoglycemia gonads pineal gland biological clock seasonal affective disorder (SAD) Lecture Outline A. In diabetes, an individual’s cells lack access to enough insulin to effectively uptake glucose. Diabetes may be genetic or developed. With careful monitoring, diabetes can be managed. The Endocrine System: Hormones Hormones are signaling molecules carried in the bloodstream. 1. Hormones affect particular target cells in the body. a. The bloodstream is the route hormones take to travel to specific target cells in the body. b. The endocrine system is the hormone source. B. Hormones are produced in small amounts and often interact. 1. There are three types of hormone interactions. a. Opposing interactions occur when one hormone’s effect is opposed by another hormone. i. The relationship between insulin and glucagons is an example. b. A synergistic interaction occurs when two or more hormones are required to produce the desired effect. i. Prolactin, oxytocin, and estrogen are all necessary for milk production in females. c. In a permissive interaction, one hormone prepares a target cell for the effect of another hormone. i. The uterus must be exposed to estrogens and then progesterone in order for proper implantation of a fertilized egg. Types of Hormones and Their Signals A. Hormones come in several chemical forms. 1. Steroid hormones are lipids derived from cholesterol. a. Examples are estrogens, progesterone, testosterone, aldosterone, and cortisol. 2. Amine hormones are modified amino acids. a. Examples are melatonin, epinephrine, norepinephrine, and thyroid hormones. 3. Peptide hormones are made of a few amino acids. a. Examples are oxytocin, antidiuretic hormone, calcitonin, and parathyroid hormone. 4. Protein hormones are composed of long chains of amino acids. a. Examples are growth hormone, insulin, prolactin, follicle-stimulating hormone, and luteinizing hormone. 5. Several factors involve how a target cell reacts to a hormone. a. Different types of mechanisms are activated in target cells as a response to specific hormones. b. Not all cells can respond to a particular hormone signal. B. Steroid hormones interact with cell DNA. 1. Steroids are lipid-soluble, so they can easily pass through the phospholipids of the plasma membrane. 2. The hormone can then enter the nucleus and interact with the DNA. 3. By this method the hormone can control the synthesis of proteins in the cell. C. Nonsteroid hormones act indirectly, by way of second messengers. 1. Nonsteroid hormones are water-soluble, so they cannot pass through the plasma membrane. 2. These hormones bind to receptors on the surface of the plasma membrane. 3. A second messenger, such as cyclic AMP, is required to bring about the desired effect within the cell. The Hypothalamus and Pituitary Gland A. The posterior pituitary lobe stores and releases hormones from the hypothalamus. 1. Axons from neurons in the hypothalamus dip into the posterior pituitary and make two hormones. 2. Antidiuretic hormone helps the kidneys to conserve water and dissolved substances. 3. Oxytocin produces uterine contractions and the milk let-down reflex. B. The anterior pituitary lobe makes hormones. 1. Corticotropin stimulates the adrenal glands to release hormones. 2. Thyrotropin stimulates the thyroid gland to release hormones. 3. Growth hormone stimulates growth and also is involved in basic metabolism. 4. Follicle-stimulating hormone and luteinizing hormone are classified as gonadotropins and involve reproduction. 5. Prolactin involves the production of milk. 6. The hypothalamus produces hormone releasers, such as GnRH and TRH, which control secretion from the anterior pituitary. 7. The hypothalamus also releases inhibitors that stop secretions from the anterior pituitary. Hormones as Long-Term Controllers A. Pancreatic islets produce insulin (lowers blood sugar), glucagons (raises blood sugar), and somatostatin (involves carbohydrate metabolism). B. The adrenal cortex produces glucocorticoids (breakdown of proteins to glucose) and mineralocorticoids (control the reabsorption of water and sodium). C. The adrenal medulla produces epinephrine (raises blood sugar, increases heart rate) and norepinephrine (affects ability to control diameter of blood vessels). D. The thyroid makes thyroid hormones (control of metabolic rate) and calcitonin (lowers blood levels of calcium). E. The parathyroid glands make parathyroid hormone (increases calcium and phosphate levels in the bloodstream). F. The thymus produces thymosin (undertakes immune functions). G. The testes secrete androgens (production of sperm, sexual traits). H. The ovaries produce estrogens (necessary for egg maturation, sexual traits) and progesterone (breast development and implantation of the fetus). I. The pineal gland produces melatonin (regulates internal time clock). GH Functions and Disorders A. Growth hormone stimulates the growth of muscles, cartilage, and bone. B. Several disorders result from a release of an inappropriate amount of growth hormone. 1. Gigantism results from a release of an excess of growth hormone during the growing years. 2. Acromegaly results from an excess of growth hormone in adulthood. a. This results in inappropriate bone growth in the jaws, forehead, and hands. 3. Pituitary dwarfism results from too little growth hormone during the growing years. a. Patients can be given a genetically engineered form of growth hormone, though this treatment is expensive and controversial. The Thyroid and Parathyroid Glands A. Thyroid hormones affect metabolism and growth. 1. The thyroid gland is located in the neck with a lobe on either side of the trachea. 2. The thyroid gland produces thyroid hormones: thyroxine (T4) and triiodothyronine (T3). a. These hormones regulate the body’s metabolic rate, growth, and development. b. A simple goiter occurs when there is too little iodine in the system, so that the thyroid hormones cannot be properly produced. c. Hypothyroidism results in sluggishness, weight gain, depression, and intolerance of cold. d. Graves disease (hyperthyroidism) results in heavy sweating, and increased heart rate and blood pressure. 3. Calcitonin, which is also produced by the thyroid gland, reduces the level of calcium in the bloodstream. 4. The four parathyroid glands are located on the posterior side of the thyroid gland and secrete parathyroid hormone (PTH). a. More PTH is secreted when blood calcium levels drop below a certain point; less is secreted when calcium rises. b. Calcitonin contributes to processes that pull calcium out of the blood. 5. Rickets is a condition that occurs when children have a vitamin-D deficient diet, and therefore cannot absorb calcium properly. 6. Hyperparathyroidism results in too much calcium being withdrawn from the bones, resulting in dangerously weakened bones. Adrenal Glands and Stress Responses A. The adrenal cortex produces glucocorticoids and mineralocorticoids. 1. The outer portion of the adrenal glands, which are situated on top of each kidney, is called the cortex. 2. Glucocorticoids raise the level of glucose in the blood. a. The main glucocorticoid, cortisol, is secreted when the body is stressed and blood sugar levels drop; cortisol promotes gluconeogenesis, a mechanism for making glucose from amino acids derived from protein breakdown. b. Cortisol also dampens the uptake of glucose from the blood, stimulates the breakdown of fats for energy, and suppresses inflammation. c. Hypoglycemia can result when the adrenal cortex makes too little cortisol; this results in chronically low glucose levels in the blood. 3. Mineralocorticoids regulate the concentrations of mineral salts, such as potassium and sodium. a. Aldosterone is a mineralocorticoid that illustrates this effect in the kidneys. 4. The adrenal cortex also secretes sex hormones in a fetus and at puberty. B. Hormones from the adrenal medulla help regulate blood circulation. 1. The inner portion of the adrenal glands is called the medulla. 2. It secretes epinephrine and norepinephrine. a. Secretion by the adrenal medulla influences these molecules to behave like hormones to regulate blood circulation and carbohydrate use during stress. C. Long-term stress can damage health. 1. Stress can trigger the fight-or-flight response, which involves the release of cortisol, epinephrine, and norepinephrine. 2. Cortisol secretion can suppress the immune system and make an individual more prone to disease. 3. Positive social connections can reduce the amount of stress and its deleterious effects. The Pancreas: Regulating Blood Sugar A. The pancreatic islets contain three types of specialized cells that produce hormones. 1. Alpha cells secrete glucagon, which causes glycogen stored in the liver to be converted to glucose. 2. Beta cells secrete insulin, which brings about the uptake of glucose by the liver, muscle, and adipose cells to lower the blood sugar levels. 3. Delta cells secrete somatostatin, which can inhibit the secretion of insulin and glucagon. Blood Sugar Disorders A. Diabetes mellitus is a disease that produces a high blood sugar due to the hyposecretion of insulin. 1. Without insulin, cells cannot remove glucose effectively from the blood, so the water-solute concentrations become imbalanced. 2. This can lead to a pH imbalance known as metabolic acidosis. 3. Type 1 diabetes is an autoimmune disease where beta cells are destroyed. a. There is a genetic predisposition for this disease and potentially a viral trigger. b. Insulin injections can usually control the disease. B. Type 2 diabetes is a global health crisis. 1. In type 2 diabetes the insulin levels are near normal, but the target cells are unable to respond to the hormone. a. Eventually beta cells break down and produce less insulin. b. The excess sugar level can damage capillaries and affect many areas of the body. C. “Prediabetes” is a warning sign. 1. Over 20 million Americans have prediabetes, sometimes known as metabolic syndrome. 2. Features of this syndrome include: an apple body shape, elevated blood pressure, low levels of HDL, and elevated fasting glucose and triglycerides. D. Low blood sugar threatens the brain. 1. In hypoglycemia, the glucose levels in the bloodstream may reach dangerously low levels. a. Insulin shock may result, during which the patient may feel dizzy and confused. b. This can be caused by a miscalculated insulin injection of an insulin-secreting tumor. c. The blood sugar must be raised as a remedy. Other Hormone Sources A. The gonads produce sex hormones. 1. Testosterone is produced in the testes, and estrogens and progesterone are produced in the ovaries. a. Small amounts of hormones usually released from the opposite sex’s gonads are also produced and have reproductive functions. B. The pineal gland makes melatonin. 1. Melatonin helps to regulate our internal time clock, particularly our sleep and waking cycles. a. Melatonin is secreted in the dark. 2. Seasonal Affective Disorder (SAD) is a depression during the winter months when the days are shorter. C. Skeletal muscle, the thymus, the heart, and the GI tract also produce hormones. 1. Irisin is produced in skeletal muscle and targets white adepose tissue. 2. Thymosin is released from the thymus gland to help infection-fighting T cells mature. Atrial natriuretic peptide (ANP) is released from the heart atria and regulates blood pressure. Gastrin and secretin are released from the GI tract and stimulate release of stomach and intestinal secretions. Suggestions for Presenting the Material The “core” of this chapter are the sections describing the various endocrine glands, their secretions and functions. The other important topic, located in sections 15.1 and 15.2, is that of signaling mechanisms. Keep an overhead or anatomical model handy to point out the location of each gland as it is discussed. Because the chapter contains excellent figures and tables, these can be used to great advantage. Ask students to bring their texts to class and invite them to follow along. Use transparencies or PowerPoint slides to help keep students on track. Make a simple poster showing the chemical nature of steroids (lipid-bilayer soluble) versus proteins (not lipid soluble) and the need for a second messenger, namely cyclic AMP. Having a visual representation may enable students to understand the concepts more easily. This chapter presents an array of complicated new words—hormone names that are long and unfamiliar. As an aid to learning, subdivide the name and give the literal meaning of each portion. For example, note the meaning of the word adrenocorticotropic; adreno- (adrenals), -cortico (cortex) and -tropic (stimulate). The term “antagonism” was previously mentioned in connection with the autonomic nervous system. Take the opportunity to point out antagonistic hormone pairs: calcitonin/parathyroid and insulin/glucagon. Emphasize the meaning of the prefixes hypo- (not enough) and hyper- (too much). Show how these prefixes are used in regular English usage (e.g., hyperactive) to reinforce their meanings. Point out that some organs function as both endocrine and exocrine glands. Mention plant hormones that measure length of day. What hormone does this resemble in humans? Emphasize the necessity of learning both the hormone name and the abbreviation, which is often more commonly used than the name itself. Classroom and Laboratory Enrichment Human nature is such that students are very interested in the abnormalities that hyper- and hyposecretion of human hormones cause. Stimulate interest in the total area of hormone control by showing photos of the physical manifestations of such imbalances. See if students can identify the conditions from the photos. Invite an endocrinologist to address the class concerning common endocrine abnormalities that he/she observes. Learn what types of tests are performed to diagnose these diseases. If a member of the class is willing to share his/her experiences as a diabetic, arrange for a presentation and allow time for questions. Seek evidence of a class member who has experienced or witnessed an epinephrine-mediated emergency response. Ask him/her to report. Stress the purpose of tropic hormones. Name and describe the function of each tropic hormone discussed in the chapter. If there is a deficiency of a hormone in the body, how can you determine if the abnormality actually began in its tropic “partner”? Survey local grocery stores to determine the relative stocks of iodized and non-iodized salt. Are there any implications for the unwary consumer? Use an anatomical model or a dissected fetal pig or cat to show the location of the various endocrine glands. Classroom Discussion Ideas Discuss the health impacts that diabetes can have on the human body. What was the fate of diabetic individuals before insulin was made available? Should health insurance differentiate between the Type 1 diabetes, which cannot be helped, and Type 2 diabetes, which is generally due to a lifetime of abuse? Beverage alcohol inhibits the action of ADH (antidiuretic hormone). How does a night of drinking effect your body at that time and the next day? Can this help you to develop a proactive approach to a “hangover”? Since the pituitary gland is often called the “master gland,” do you think it should be renamed based on the fact that the hypothalamus directs some of its functions? Why does insulin have to be administered by injection rather that orally to be most effective? Using the knowledge gained in an introductory biology class, an athlete decided he might be able to raise his blood sugar control by injecting glucagons. Why would this attempt be doomed? Which hormone can be purchased at a health food store without a prescription? Are there any studies to demonstrate that the distribution of this hormone without medical supervision is a safe practice? What do you think? What is the possible connection between the pineal gland and puberty? Some hormones seem to be doing another’s duties; for example, sex hormones from the adrenals, blood sugar control by epinephrine, and thyroxine regulation of growth. Why is this so? Do hormones occur only in vertebrates? Have you ever heard of ecdysone in insects? What is its function? Oxytocin has two functions: to initiate uterine contractions and to induce milk movement into appropriate ductwork. Mention how nursing a newborn can bring about mild uterine contractions. This action may help to shrink the enlarged, vascular uterus down to a more manageable size. On occasion, obstetricians even give oxytocin as an IV after delivery to decrease the size of the uterus. Why do hypoglycemics, who regularly ingest excessive amounts of sugar, frequently develop diabetes later in life? Untreated diabetes mellitus patients tend to be very thirsty and yet produce large amounts of urine. Why is this so? What other medical condition would present similar symptoms? What tests should be performed to distinguish between these two illnesses? How would each of these diseases be treated? Term Paper Topics, Library Activities, and Special Projects Discuss the ethical issues of administering somatotropin (growth hormone) to persons of normal stature who wish to be “super athletes.” What is the difference between such enhancement and the use of growth hormone therapeutically? What do you feel should be the criteria for administration of this hormone? Growth hormone is now given in a synthetic form. Previously it was extracted from the pituitaries of deceased individuals. What were the risks of this previous practice? Investigate and report on the fascinating discovery of the role of insulin by researchers Banting and Best. Make a chart showing all of the hormones that affect your blood sugar level. What effect does each of these hormones produce? Select a major hormone and prepare an in-depth report on the abnormalities that may result from hypo- and hypersecretion. How do calcitonin and parathyroid hormone affect the level of calcium in the bones and the bloodstream? What effects would be seen in hypo- and hypersecretion of either of these hormones. One of these two hormones, calcitonin and parathyroid hormone, is used to treat osteoporosis. Which one is it? The role of hormones in insect development has been elucidated in only the past 40 years. Check an entomology text and prepare a chronology of this research. Research the differences between diabetes mellitus and diabetes insipidus. What is the derivation of each of these diseases’ names and how does it relate to these conditions. What is the latest on insulin pumps? Which patient population would be good candidates for the pump? Are there any studies on attempts to produce new pancreatic islet cells? Investigate which human hormones are now being produced using genetic engineering methods. Report on current research designed to correct type 1 diabetes, which is the result of an autoimmune response. How does correcting type 1 diabetes compare to trying to correct type 2 diabetes? Investigate the role of the thyroid gland. What are some common disorders of the thyroid and how can they be treated? The thymus has an immune function since it helps mature T cells. Unfortunately the thymus decreases in size and function after puberty. Do you think that this explains an “immune advantage” for children? Since the HIV virus compromises the T cells, would it help with that disease if you could prevent the thymus from shriveling up? Look up any research projects on the thymus gland. Videos, Animations, and Websites Videos Films for the Humanities and Sciences The Endocrine System _http://ffh.films.com/id/16276/The_Endocrine_System.htm_ Discovery Health Overview of the endocrine system. _http://health.howstuffworks.com/human-body/systems/endocrine_ Animations ZeroBio Interactive animations of the endocrine glands and their hormones. _http://www.zerobio.com/drag_oa/endo.htm_ Association of the British Pharmaceutical Industry Hormones and their effects (interactive). _http://www.abpischools.org.uk/page/modules/hormones/index.cfm_ Websites Endocrine Web A collection of articles on various endocrine disorders. _http://www.endocrineweb.com/_ PBS NOVA – Keeping the Weight Off Interview with a research scientist on obesity and weight loss. _http://www.pbs.org/wgbh/nova/body/weight-loss.html_ Possible Responses to Review Questions Hormones are secreted by endocrine cells, glands, or organs and generally have targets that are far away. Neurotransmitters are generally small molecules that specifically are involved in nervous system communication. Local signaling molecules may be several different types of molecules, all of which work only in tissues immediately surrounding the secreting cell. Pheromones are released outside of the body to affect another organism. The signaling molecule is indicative of a non-steroid hormone. Since these hormones are water soluble, they cannot pass through the lipid-rich plasma membrane, so they must make use of a second messenger, such as cyclic AMP. In the diagram illustrating the hormones secreted from the posterior lobe of the pituitary, the answers from left to right are: antidiuretic hormone (ADH), oxytocin, and oxytocin again. In the lower diagram illustrating the hormones secreted from the anterior lobe of the pituitary, the answers from left to right are: growth hormone, adrenocorticotropic hormone (ACTH), thyroid stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin. Below is a list of each of the major endocrine glands in the body and their locations: hypothalamus – in the brain pituitary gland – in the brain adrenal glands – on top of each kidney pineal gland – in the brain thyroid gland – in the neck parathyroid glands – in the neck thymus gland – behind the sternum pancreas – near the stomach ovaries – in the abdominopelvic body cavity testes – in the scrotum Parathyroid hormone and calcitonin are opposing hormones in a negative feedback mechanism. Parathyroid hormone facilitates the movement of calcium from the bones to the bloodstream, and calcitonin helps move calcium from the bloodstream to the bones. Together, in most situations, homeostasis is maintained. The secretion of oxytocin is an example of a positive feedback mechanism. There is nothing to stop the secretion of oxytocin (no opposing hormone). The secretion of oxytocin only reduces when receptors in the uterine wall indicate that the baby has left the uterus. Possible Responses to Critical Thinking Questions A patient with Addison’s disease, such as President John F. Kennedy, may lack the glucocorticoid cortisol. This results from an autoimmune condition where cells in the adrenal cortex are destroyed, thus leading to less cortisol production. A low cortical level will interfere with metabolism that results in weight loss. Since the cells of the adrenal cortex are destroyed, the aldosterone level will also drop, leading to lower blood pressure. Sluggishness, depression, and intolerance to cold are symptoms of hypothyroidism, which involves a lack of hormones from the thyroid gland. These symptoms could also result from a low level of thyrotropin from the anterior lobe of the pituitary gland, because it serves to stimulate the thyroid gland. Marianne caused the glucose level in her bloodstream to drop to a dangerous level by an overdose of insulin. The orange juice is a quick source of glucose that serves to provide enough sugar to resume normal activities in the body. The target cells for antidiuretic hormone are located in the kidneys. Diabetes insipidus involves hyposecretion of ADH, which results in excessive urination and subsequent thirst. Possible Responses to Explore on Your Own Questions It is interesting to see if a student’s perception of their own stress level is accurately represented by their score on the student stress scale. Most participants might not view pleasant events as being stressful. The stress scale helps to show that any major changes in your life can result in stress. An ancient Chinese curse states, “May God grant you an exciting life.” This reminds us that fewer changes in an individual’s life can result in a more stress-free lifestyle. 180 Chapter Fifteen 181 The Endocrine System 180 Chapter Fifteen 181 The Endocrine System 180 Chapter Fifteen 181 The Endocrine System 180 Chapter Fifteen 181 The Endocrine System 180 Chapter Fifteen 181 The Endocrine System 180 Chapter Fifteen 181 The Endocrine System 180 Chapter Fifteen 181 The Endocrine System