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The chapter discusses the physiology of the endocrine system, which consists of various glands that produce hormones to regulate the body's functions. Hormones affect growth, metabolism, emotions, sleep patterns, and sexual functioning. The endocrine system has a feedback system to maintain homeostasis. The major glands discussed include the ovaries, testes, pancreas, pineal gland, adrenal glands, parathyroid glands, thyroid gland, pituitary gland, and hypothalamus. The hypothalamus controls many organs and functions, while the pituitary gland produces hormones that affect other glands. The thyroid gland regulates metabolism, the parathyroid glands control calcium levels, and the adrenal glands produce hormones for the fight-or-flight response. The pineal gland regulates the sleep-wake cycle, and the reproductive glands produce sex hormones. Chapter 3, Endocrine System Physiology. This chapter is all about the physiology of the endocrine system. The endocrine system in general is very busy physiologically. There are multiple glands functioning all the time in the body. They produce hormones that act on other organ systems in ways that affect practically every cell in the body all the time. There is a unique feedback system in many endocrinological systems that make sure the hormone levels are such that homeostasis is maintained always. Endocrine Basics The human endocrine system consists of numerous glands located throughout the body. The key feature that distinguishes glands from other organs of the body is that they secrete hormones. Hormones are molecular substances, usually proteins, that are secreted and used to regulate the activity of various organs and cells of the body. Hormones can affect the growth of the body, the body's metabolic processes, the emotions a person experiences, their sleep patterns, their sexual functioning, and more. Hormones do not act directly on a connected organ but are instead secreted into the bloodstream to be acted upon by distant organs or cells. Hormones are chemical messengers that travel to distant body parts and interact with receptors on different types of cells to alter how the cell functions. Together, hormones coordinate the function of the body. Without hormones, the body would be significantly dysregulated and could not sustain life. The major glands that will be discussed in full later include the ovaries, the testes, the pancreas, the pineal gland, the adrenal glands, the parathyroid glands, the thyroid gland, the pituitary gland, and the hypothalamus. The pancreas is a unique gland in that it has both endocrine and exocrine functions because, besides secreting hormones, it excretes digestive enzymes that are not part of the endocrine system. The endocrine system has a unique feedback system that regulates the way and time in which the various hormones are secreted. The feedback system acts much like a thermostat in a home. When the thermostat needs heat, it sends a signal to the furnace to put out more heat. When the room is warm, the thermostat shuts off the furnace to cool down the room. The pituitary gland and the hypothalamus have a similar mechanism going on. There are many different feedback loops going on in the hypothalamus and the pituitary gland that regulate most of the other glands in the body. As hormone levels rise, the feedback loop turns on in distal organs, telling these glands to shut down production. When hormone levels go down, these glands are stimulated to put out more hormone. The hypothalamus is one of the master glands of the body because it controls a great many organs and cellular functions. It is in the lower middle part of the brain. It regulates a person's urge to stop eating, metabolism, and body temperature. It has a great deal of influence over the pituitary gland because much of what it does is to secrete hormones that directly stimulate parts of the pituitary gland. Many of the hormones released by the hypothalamus are releasing hormones that are not released into the bloodstream but are released into the hypophysial portal system that sends the hormones directly to the pituitary gland, causing it to be stimulated. Also, the hypothalamus secretes a hormone that blocks some pituitary action. It releases somatostatin, which carries force to the pituitary gland to tell it not to make gross hormone. There are many hormones that interact directly with the hypothalamic pituitary system. Some of these include the thyrotropin-releasing hormone, which signals the pituitary gland to secrete thyroid-stimulating hormone, or TSH. This, in turn, stimulates the thyroid gland to secrete thyroxin, or T4, and triiodothyronine, or T3. The hypothalamus also secretes corticotropin-releasing factor, which results in the secretion of adrenocorticotropic hormone, or ACTH. This causes the adrenal cortex to release cortisol. Luteinizing hormone-releasing hormone, or LHRH, is also called gonadotropin-releasing hormone, or GNRH. It is released by the hypothalamus into the hypophysial portal system to cause the pituitary gland to release follicle-stimulating hormone, or FSH, and luteinizing hormone, or LH. This, in turn, results in the release of estrogen or testosterone from the reproductive organs of the male or female reproductive tract. Growth hormone-releasing hormone is another hormone released by the hypothalamus that causes the pituitary gland to release growth hormone. This causes cells of the body to release insulin-like growth factor, or IGF-1, which causes body growth. The pituitary gland is a tiny gland located at the base of the brain just beneath the hypothalamus. Some consider this to be the master gland of the body instead of the hypothalamus. This is because it produces most of the blood-circulating hormones that have a direct effect on the other glands of the body. A deficiency of any pituitary gland is known as hypopituitarism. The pituitary gland is divided into two separate halves called the anterior lobe and the posterior lobe. The anterior lobe is regulated by the hypothalamus and produces these hormones. The first is growth hormone, which causes stimulation of the bone and tissue, causing these tissues to grow. A growth hormone deficiency causes failure of a child to grow. If growth hormone is deficient in adulthood, it causes difficulty with bone mass and obtaining the proper amounts of body fat. Emotional well-being is also impaired. The anterior pituitary gland also secretes thyroid-stimulating hormone, or TSH. It stimulated the thyroid gland to make thyroxin and triiodothyronine, collectively called the thyroid hormones. If TSH is deficient or the thyroid hormone does not respond to the TSH, hypothyroidism results and the patient's metabolism becomes extremely slow. Adrenocorticotropic hormone, or ACTH, is released by the anterior pituitary gland. It stimulates the adrenal cortex to release cortisol and other steroid hormones. Luteinizing hormone, or LH, and follicle-stimulating hormone, or FSH, are released into the bloodstream to influence the sex glands of both the male and the female. In females, these hormones cause the release of estrogen and progesterone, while in males it causes the release of testosterone. Finally, the anterior pituitary gland releases prolactin that stimulates milk production in women. The posterior lobe of the pituitary gland produces hormones that aren't regulated by the hypothalamus. It produces antidiuretic hormone, which is also called vasopressin. It controls water loss in the kidneys. It also makes oxytocin that results in uterine wall contraction during labor and makes the breasts produce milk. These hormones aren't produced by the pituitary gland, but are made in the brain and are just stored in the pituitary gland. Thyroid gland. The thyroid gland is a butterfly-shaped gland located in the anterior part of the human neck. It isn't very big, but it has a significant role to play in metabolism. It secretes thyroxin, or T4, and triiodothyronine, or T3, which regulate all the cell metabolism, regulate bone growth, and brain development in children. It is under the control of the anterior pituitary gland that secretes thyroid-stimulating hormone, or TSH, which tells the thyroid gland to secrete more thyroid hormones. The thyroid hormones regulate blood pressure, reproductive functions, muscle tone, digestion, and heart rate. Parathyroid glands. There are four parathyroid glands located deep within the tissue of the thyroid gland. They release the parathyroid hormone, or PTH, which regulates calcium levels in the bloodstream and the metabolism of bone. The parathyroid glands cause an increase in calcium in the blood and cause strengthening of the bones. If the parathyroid hormones are accidentally removed in thyroid surgery, the calcium levels drop suddenly and the brain begins to feel confused and foggy. If there is an excess of parathyroid hormone for whatever reason, the individual has too high a calcium level, and this can result in lethargy, irritability, and sleep difficulties. The brain is the most commonly affected organ when the parathyroid glands are diseased. Adrenal glands. There are two adrenal glands, each sitting atop one of the body's kidneys. There are two parts to the adrenal gland, which are the deeper tissues, called the adrenal medulla, and the more superficial tissues, which are called the adrenal cortex. The adrenal medulla secretes epinephrine and norepinephrine, which are part of the human sympathetic system and make up the fight-or-flight response. A release of these hormones causes many changes in body function, including increased heart rate and increased respiratory rate. The adrenal cortex is the part of the adrenal glands that makes the corticosteroids, which help regulate body metabolism, blood sugar, sexual function, immune system function, and the balance of water and salt in the body. Cortisol is the main hormone produced by the adrenal cortex. It increases blood sugar, regulates the metabolism of fat, proteins, and carbohydrates, and is involved in the immune response. Pineal gland. The pineal gland, or pineal body, is a small gland located deep within the middle part of the brain. It is responsible for the secretion of melatonin. Melatonin is a hormone that regulates the circadian rhythm, or sleep-wake cycle. The pineal gland is rarely injured, damaged, or surgically removed. But if it does not function properly, the individual will have sleep difficulties secondary to a lack of signaling to tell the body when to sleep. Reproductive glands. The reproductive glands are the testes in males and the ovaries in females. They are responsible for making the sex hormones in the body. In men, the testes are located within the scrotum. It makes male sex hormones, in particular, testosterone. These hormones result in sperm production, as well as have an effect on the secondary sex characteristics of men, such as facial hair, pubic hair, and deepening of the voice. In females, the reproductive glands are the ovaries. There are two ovaries located on both sides of the uterus. They make the female eggs and produce both estrogen and progesterone. These hormones affect female sexual characteristics, such as breast growth and pubic hair. They are also involved in the cyclical events involved in the menstrual cycle and the hormones used in pregnancy. Pancreas. The pancreas is a long and thin gland located in the abdomen near the back part of the abdomen behind the stomach. It has both endocrine and exocrine functions. The exocrine function involves its release of digestive enzymes that aid in protein digestion. The endocrine function involves its secretion of insulin and glucagon. Insulin is secreted by the islet cells of the pancreas and is responsible for putting glucose into the cells of the body, reducing the blood sugar. Glucagon has the opposite effect. It causes an increase in blood sugar. Insulin has many functions in cell metabolism. It puts glucose, fatty acids, and amino acids into the cells and regulates aspects of glucose metabolism. Endocrine feedback loops. The endocrine system works by a complex series of endocrine feedback loops that together control sexual development, reproduction, growth and maintenance, metabolism, and the response to external stimuli. Most feedback loops involve both the pituitary gland and the hypothalamus. The hypothalamus produces stimulatory hormones that stimulate the pituitary gland to secrete a hormone. When the pituitary gland secretes the thyroid-stimulating hormone, for example, it causes an increase in the thyroid gland production of thyroxin and triiodothyronine, which act on most, if not all, of the cells of the body in the regulation of body metabolism. When the levels of thyroxin and triiodothyronine are too high, they feed back on the hypothalamus, resulting in a decrease in thyrotropin-releasing hormone in the hypothalamus. This shuts off the pituitary gland and no more thyroid hormone is released until the thyroid hormone levels drop. There are other, more complex feedback loops in the other glands of the body. The female ovaries have a particularly complex feedback loop. The female reproductive cycle is cyclical so that the hormone levels vary from day to day. The hormone levels of estrogen and progesterone vary throughout the cycle and are under the control of the luteinizing hormone and follicle-stimulating hormone. These are made by the pituitary gland. There is a complex mechanism in which the luteinizing hormone levels spike at the time of ovulation, stimulating the release of the egg. The levels of progesterone and estrogen feed back onto the hypothalamus, turning off luteinizing hormone production for the rest of the cycle. Key Takeaways The endocrine system is a collection of glands that produce hormones which regulate many cellular functions. The master gland is sometimes what people call the hypothalamus. However, others call the pituitary gland the master gland. Both are critical to the function of many other endocrine organs in the body. The pituitary gland is mostly under the regulatory control of the hypothalamus. The anterior pituitary gland is primarily under hypothalamic control while the posterior pituitary gland is autonomous. The endocrine system operates under feedback loops that control the release of other hormones in the body. Quiz Number 1 Which endocrine hormone is responsible for raising calcium levels in the bloodstream? A. Cyroxan B. Triiodocyranine C. Parasyroid hormone D. Cortisol Answer C. The calcium levels are under the control of the parasyroid hormone. Number 2 Which endocrine hormone triggers the release of thyroid-stimulating hormone? A. Gonadotropin releasing factor B. Thyrotropin releasing factor C. Corticotropin releasing factor D. Adrenocorticotropic stimulating hormone Answer B. Thyrotropin releasing factor is the hormone released by the hypothalamus that triggers the release of thyroid-stimulating hormone by the pituitary gland. Number 3 Which endocrine organ secretes oxytocin? A. The anterior pituitary gland B. The hypothalamus C. The posterior pituitary gland D. The pancreas Answer C. The posterior pituitary gland secretes oxytocin to cause uterine contractions and the release of milk from the female breast. Number 4 Which endocrine organ is responsible for the release of glucagon? A. Parathyroid gland B. Hypothalamus C. Adrenal gland D. Pancreas Answer D. The pancreas is responsible for the release of glucagon in the control of glucose metabolism. Number 5 Which organ or tissue of the endocrine system is responsible for the fight-or-flight response? A. The posterior pituitary gland B. The anterior pituitary gland C. The adrenal cortex D. The adrenal medulla Answer D. The adrenal medulla is responsible for the release of epinephrine and the release of norepinephrine in the fight-or-flight response. Number 6 Which organ of the endocrine system is turned off when thyroxin levels are too high? A. The posterior pituitary gland B. The anterior pituitary gland C. The hypothalamus D. The parathyroid gland Answer C. The hypothalamus is the gland that gets turned off when the thyroxin levels are too high. Number 7 Which organ of the endocrine system is intimately associated with the immune system? A. The thyroid gland B. The adrenal medulla C. The adrenal cortex D. The hypothalamus Answer C. The adrenal cortex secretes cortisol, which is intimately associated with the immune system. Number 8 Which hormone of the endocrine system is associated with cellular metabolism? A. Luteinizing hormone B. Triiodosirenine C. Glucagon D. Growth hormone Answer B. Triiodosirenine is associated with cellular metabolism. Number 9 Which organ of the endocrine system is most associated with uterine contractions? A. The hypothalamus B. The anterior pituitary gland C. The posterior pituitary gland D. The ovaries Answer C. The posterior pituitary gland is most associated with uterine contractions as it releases oxytocin, which stimulates the contraction of the uterus. Number 10 How do the hormones of the hypothalamus reach the pituitary gland? A. These two glands are next to one another, so the hormone diffuses from one gland to the other. B. Through the release of hormones into the bloodstream. C. Through the hypophysial portal system. D. The hypothalamus only affects the posterior pituitary gland. Answer C. Hormones interact with the anterior pituitary gland by being released by the hypothalamus and reaching the hypophysial portal system, where they finally reach the anterior pituitary gland.
