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Unit 4: Homeostasis

Unit 4: Homeostasis

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Unit 4: Homeostasis

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  1. Unit 4: Homeostasis Chapter 9: The Endocrine System

  2. Section 9.1: The Glands and Hormones of the Endocrine System • The functioning of the over 100 trillion diverse cells making up the tissues and organs in your body must be regulated and controlled • In order for this to occur, the cells must be able to communicate with each other • The body systems that facilitate cellular communication and control are the nervous and endocrine systems

  3. Section 9.1: The Glands and Hormones of the Endocrine System • Recall from Chap 8 that nervous system messages are transmitted rapidly to precise locations in the body through neurons • The body also secretes chemical messages from glands • Endocrine glands secrete chemical messengers called hormones directly into the bloodstream, which transports the hormones throughout the body • Original Greek meaning of the word hormone is to “excite” or “set in motion” • The endocrine glands and the hormones they secrete make up the endocrine system • Compared to the rapid actions of the nervous system, the endocrine system typically has slower and longer acting effects, and affects a broader range of cell types

  4. The Endocrine Glands • There are over 200 hormones or hormone-like chemicals in the human body • They have a wide variety of functions, such as: • Regulating growth and development • Speeding up or slowing down the metabolism • Regulating blood pressure or immune response

  5. The Endocrine Glands • Glands that function exclusively as endocrine glands include the: • Pituitary • Pineal • Thyroid • Parathyroid • Adrenal • Tissues and organs that secrete hormones (but don’t function exclusively as endocrine glands) include the: • Hypothalamus • Thymus • Pancreas • Testes • Ovaries

  6. Hormone Activity on Target Cells • When hormones are released, they act on target cells • Cells whose activity is affected by a particular hormone • Target cells contain receptor proteins • Circulating hormones bind to their specific receptor proteins, like a key fits into a lock • Human growth hormone (hGH) can be used as a specific example • hGH circulates in the bloodstream and interacts with the liver, muscle, and bone cells • Each of these cell types contains receptor proteins specifically shaped to bind with hGH • When hGH binds to its receptor, this triggers other reactions in the target cell • In other word, the target cell receives and responds to the chemical message sent by the hormone

  7. Steroid Hormones and Water-Soluble Hormones • Steroid hormones, such as testosterone, estrogen, and cortisol, are lipid-based • They can easily diffuse through the lipid bilayer of cell membranes • Inside the target cell, steroid hormones bind to their receptor proteins • This interaction activates specific genes, causing changes in the cell • Ex: Estrogen can trigger cell growth

  8. Steroid Hormones and Water-Soluble Hormones • Epinephrine, human growth hormone (hGH), thyroxine (T4), and insulin are water-soluble hormones • Can’t diffuse across the cell membrane • Water-soluble hormones bind to a receptor protein on the surface of the target cell • This starts a cascade of reactions inside the target cell • Each reaction that occurs triggers many other reactions • The impact of the hormone is greatly amplified

  9. Steroid Hormones and Water-Soluble Hormones • For example, a single molecule of epinephrine in the liver can trigger the conversion of glycogen into about 1 million molecules of glucose • When epinephrine reaches the liver, it stimulates the conversion of ATP to cyclic adenosine monophosphate (cAMP) • cAMP triggers an enzyme cascade that results in many molecules of glycogen being broken down into glucose • The glucose enters the bloodstream and will eventually be used by cells for energy • Once a hormone’s message has been delivered, enzymes inactivate the hormone • Any lingering effect could potentially be very disruptive

  10. Regulating the Regulators • For many years, scientists referred to the pituitary gland as the “master gland” • Many of the hormones it secretes stimulate other endocrine glands • Further research has shown that the pituitary gland is actually controlled by the hypothalamus • After receiving signals from various sensors in the body, the hypothalamus secretes releasing hormones, which often travel to the pituitary gland • Releasing hormones stimulate the pituitary gland to secrete hormones that act on other endocrine glands

  11. Regulating the Regulators • Hormones that stimulate endocrine glands to release other hormones are called tropic hormones • Many of the hormones released from the hypothalamus and anterior pituitary are tropic hormones • The hypothalamus and the pituitary gland control many physiological processes that maintain homeostasis

  12. Regulating the Regulators • Figure 9.5A shows the general mechanism of action of tropic hormones • The hypothalamus secretes a releasing hormone into the anterior pituitary • Causes the anterior pituitary to release a second tropic hormone into the bloodstream • The second tropic hormone stimulates the target gland to release a third hormone into the blood • This hormone travels to another target tissue and produces an effect

  13. Regulating the Regulators • Like many hormones, this system is controlled by a negative feedback loop • In this case, the third hormone prevents further release of the first two hormones in the pathway • A specific example is the feedback system that controls thyroid-stimulating hormone (TSH) • Low blood levels of the thyroid hormone T4 initiate the response from the hypothalamus • When blood levels of T4 increase, the release of TRH and TRH is inhibited

  14. Working Together to Maintain Homeostasis • Homeostasis depends on the close relationship between the nervous system and the endocrine system • The functions of these two systems often overlap: • Some nervous system structures, such as cells in the hypothalamus, secrete hormones • Several chemicals function as both neurotransmitters and hormones • Epinephrine acts as a neurotransmitter in the nervous system, and as a hormone in the fight-or-flight response • The endocrine and nervous systems are regulated by feedback loops • The regulation of several physicological processes involves the nervous and endocrine systems acting together • Ex: When a mother breastfeeds her baby, the baby’s suckling initiates a sensory message in the mother’s neurons that travels to the hypothalamus. This triggers the pituitary to release the hormone oxytocin. Oxytocin travels to the mammary glands of the breast, causing the secretion of milk

  15. Section 9.2: Hormonal Regulation of Growth, Development, and Metabolism • You many have heard the expression “growing like a weed” used to refer to an adolescent who has grown several centimeters in just a few months • You may have heard people say they have a “fast metabolism” meaning they can eat whatever they want and not gain weight • The growth and development of muscles and bones are controlled by hormones released by the pituitary gland • The rate of metabolism is controlled by hormones released by the thyroid gland

  16. The Pituitary Gland • The pituitary gland has two lobes and is about 1 cm in diameter (about the size of a pea) • It sits in a bony cavity attached by a thin stalk to the hypothalamus at the base of the brain • Despite its small size, it releases 6 main hormones involved in the body’s metabolism, growth, development, reproduction, and other critical life functions

  17. The Pituitary Gland • The anterior pituitary and posterior pituitary make up the two lobes of the pituitary gland • Each lobe is really a separate gland and they release different hormones • The posterior pituitary is considered part of the nervous system • Don’t produce hormones • It stores and releases the hormones ADH and oxytocin, which was produced by the hypothalamus and transferred to the posterior pituitary by neurons

  18. The Pituitary Gland • The anterior pituitary is a true hormone-synthesizing gland • Its cells produce and release 6 major hormones • Thyroid-stimulating hormone (TSH) • Adrenocorticotropic hormone (ACTH) • Prolactin (PRL) • Human growth hormone (hGH) • Follicle-stimulating hormone (FSH) • Luteinizing hormone (LH) • A series of blood vessels called a portal system carries releasing hormones from the hypothalamus to the anterior pituitary • These hormones either stimulate or inhibit release of hormones from this gland

  19. Human Growth Hormone • The anterior pituitary regulates growth, development, and metabolism through the production and secretion of human growth hormone (hGH) • This hormone ultimately affects almost every body tissue • It can affect some tissues by direct stimulation, but the majority of the effects are tropic • hGH stimulates the liver to secrete hormones called growth factors • hGH and the growth factors influence many physiological processes. For example, they increase: • Protein synthesis • Cell division and growth, especially the growth of cartilage, bone, and muscle • Metabolic breakdown and release of fats stored in adipose (fat) tissue

  20. Human Growth Hormone • hGH stimulates the growth of muscles, connective tissue, and the growth plates at the end of the long bones, which causes elongation of the bones • If the pituitary gland secrete excessive amounts of hGH during childhood, it can result in a condition called gigantism • Insufficient gGH production during childhood results in pituitary dwarfism • Will be of extremely small stature as an adult, but have typical body proportions

  21. Human Growth Hormone • When someone reaches adulthood and skeletal growth is completed, overproduction of hGH can lead to a condition called acromegaly • Excess hGH can no longer cause an increase in height, so the bones and soft tissues of the body widen • Over time the face widens, the ribs thicken, and the feet and hands enlarge • Can also cause debilitating headaches, an enlarged heart, liver, and kidneys, fatigue, breathing problems, cardiovascular diseases, sugar intolerance leading to diabetes, muscle weakness, and colon cancer

  22. The Thyroid Gland: A Metabolic Thermostat • The thyroid gland lies directly below the larynx (voice box) • It has two lobes, one on either side of the trachea (windpipe), which are joined by a narrow band of tissue • Millions of cells within the thyroid secrete immature thyroid hormones into the spaces between the cells • One of these hormones, thyroxine (T4) will become functional and be released into the bloodstream

  23. The Thyroid Gland: A Metabolic Thermostat • The primary effect of thyroxine is to increase the rate at which the body metabolizes fats, proteins, and carbohydrates for energy • Doesn’t have one specific target organ’ • Stimulates the cells of the heart, skeletal muscles, liver, and kidneys to increase the rate of cellular respiration • Also plays an important role in the growth and development of children by influencing the organization of various cells into tissues and organs

  24. The Thyroid Gland: A Metabolic Thermostat • If the thyroid fails to develop properly during childhood, a condition called cretinism can result • The thyroid produces extremely low quantities of thyroxine and the person is said to have severe hypothyroidism • Individuals with this condition are stocky and shorter than average, and without hormonal injections early on in life they will have mental developmental delays

  25. The Thyroid Gland: A Metabolic Thermostat • Adults with hypothyroidism tend to: • Feel tired much of the time • Have a slow pulse rate and puffy skin • Experience hair loss and weight gain • Explains why someone with a slow metabolism due to an underactive thyroid may eat very little, but still gain weight

  26. The Thyroid Gland: A Metabolic Thermostat • Overproduction of thyroxine is called hyperthyroidism • Symptoms include: • Anxiety • Insomnia • Heat intolerance • Irregular heartbeat • Weight loss • Graves’ disease is a severe form of hyperthyroidism • Results when body’s immune system attacks the thyroid • Produces swelling of muscles around the eyes, causing them to protrude and interferes with vision

  27. The Thyroid Gland: A Metabolic Thermostat • Thyroxine secretion is controlled by negative feedback • The anterior pituitary releases a hormone called thyroid-stimulating hormone (TSH) • Causes thyroid to secrete thyroxine • As thyroxine levels rise in the blood, thyroxine itself feeds back to the hypothalamus and anterior pituitary • Suppresses secretion of TSH and, therefore, thyroxine • When the body is at homeostasis, the amount of thyroxine in the bloodstream stays relatively constant

  28. The Thyroid Gland: A Metabolic Thermostat • The thyroid requires iodine in order to make thyroid hormones • The short form of thyroxine, T4, refers to the four iodine molecules in the hormone • If there is insufficient iodine in the diet, thyroxine can’t be made, and there will be no signal to stop the secretion of TSH by the anterior pituitary • The continuous stimulation of the thyroid gland by TSH causes a goiter, an enlargement of the thyroid gland • Causes visible swelling in the neck • Also causes difficulty breathing and/or swallowing, and coughing

  29. The Thyroid Gland: A Metabolic Thermostat • In the Great Lakes region in Canada, iodine is lacking in the soil, and therefore in the drinking water • Why don’t we all have goiters? • Salt refiners add iodine to salt, making it iodized • Other dietary sources of iodine include: • Seafood • Fish (cod, haddock, and perch) • Kelp • Dairy products

  30. The Thyroid Gland and Calcitonin • Calcium (Ca2+) is essential for healthy teeth and skeletal development • Also plays crucial role in blood clotting, nerve conduction, and muscle contraction • Calcium levels in the body are regulated, in part, by the hormone calcitonin • When the concentration of calcium in the blood rises too high, calcitonin stimulates the uptake of calcium into bones • A different hormone, secreted by the parathyroid glands, is release if blood calcium levels get too low

  31. The Parathyroid Glands and Calcium Homeostasis • The parathyroid glands are four small glands attached to the thyroid • Produce the hormone called parathyroid hormone (PTH) • The body synthesizes and releases PTH in response to falling concentrations of calcium in the blood • PTH stimulates bone cells to break down bone material (calcium phosphate) and secrete calcium into the blood • PTH also stimulates the kidneys to reabsorb calcium from the urine, activating vitamin D in the process • Vitamin D, in turn, stimulates the absorption of calcium from food in the intestine • These effects bring the concentration of calcium in the blood back within a normal range so that the parathyroid glands no longer secrete PTH

  32. Section 9.3: Hormonal Regulation of the Stress Response and Blood Sugar • The stress response involves many interacting hormone pathways, including those that regulate: • Metabolism • Heart rate • Breathing • In this section we’ll focus on the hormones of the adrenal glands and their effects on the body

  33. Section 9.3: Hormonal Regulation of the Stress Response and Blood Sugar • The human body has two adrenal glands • Located on top of the kidneys • Named for two Latin words that mean “near the kidney” • Each gland is composed of: • An inner layer called the adrenal medulla • An outer layer called the adrenal cortex • The adrenal cortex produces hormones that are different in structure and function from the hormones produced by the adrenal medulla

  34. The Adrenal Medulla: Regulating the Short-Term Stress Response • The adrenal medulla produces two closely related hormones: • Epinephrine (also called adrenaline) • Norepinephrine (also called noradrenaline) • These hormones regulate a short-term stress response • Commonly called the flight-or-fight response • Effects are similar to those caused by stimulation of the sympathetic nervous system • In the developing embryo, sympathetic neurons and adrenal medulla cells are formed from nervous system tissue • Why the adrenal medulla is considered a neuroendocrine structure

  35. The Adrenal Medulla: Regulating the Short-Term Stress Response • In response to a stressor, neurons of the sympathetic nervous system carry a signal from the hypothalamus to the adrenal medulla • Stimulate adrenal medulla to secrete epinephrine and a small amount of norepinephrine • These hormones trigger an increase in: • Breathing rate • Heart rate • Blood pressure • Blood flow to the heart and muscles • Conversion of glycogen to glucose in the liver • In addition, pupils dilate and blood flow to extremities decreases

  36. The Adrenal Medulla: Regulating the Short-Term Stress Response • Epinephrine acts quickly • Epinephrine injections are used to treat life-threatening conditions • Can be used to stimulate the heart to start beating in someone with cardiac arrest • In cases of anaphylactic shock caused by severe allergies (such as nuts, bee stings, or certain medications), it will open up air passages and restore breathing • Release of epinephrine and norepinephrine is rapid because it is under nervous system control • But their effects lat 10X longer than the sympathetic nervous system’s effects

  37. The Adrenal Cortex: Regulating the Long-Term Stress Response • The adrenal cortex produce the stress hormones that trigger the sustained physiological responses that make up the long-term stress response • These hormones include: • Glucocorticoids • Increase blood sugar • Mineralcorticoids • Increase blood pressure • Gonadocorticoids • Supplement the hormones produced by the gonads (testes and overies)

  38. The Adrenal Cortex: Regulating the Long-Term Stress Response

  39. Cortisol • Cortisol is the most abundant glucocorticoid • A steroid hormone synthesized from cholesterol • When the brain detects danger, it directs the hypothalamus to secrete a releasing hormone • The releasing hormone stimulates the anterior pituitary gland to secrete adrenocorticotropic hormone (ACTH) • ACTH targets the adrenal cortex • Causes the release of the stress hormone cortisol

  40. Cortisol • Cortisol works in conjunction with epinephrine, but is longer lasting • Its main function is to raise blood glucose levels • Does this by promoting the breakdown of muscle protein into amino acids • Amino acids are taken out of the blood by the liver, where they are used to make glucose, which is then released back into the blood • Also prompts the breakdown of fat cells • Also releases glucose • Increased cortisol levels in the blood cause negative feedback on the hypothalamus and anterior pituitary • Suppresses ACTH production and stops the release of cortisol

  41. Cortisol • Sustained high levels of cortisol (such as chronic stress) can: • Impair thinking • Damage the heart • Cause high blood pressure • Lead to diabetes • Increase susceptibility to infection • Even cause early death • In Japan… • Long work hours and high-stress jobs are common • So many business people have died from heart attacks and strokes that the phenomenon has been called “karoshi”, which means “death from overwork”

  42. Cortisol • One of the ways the body fights disease is by inflammation • Cells of the immune system attack foreign material, such as invading bacteria • Cortisol is a natural anti-inflammatory • Suppresses the immune system • Probably why sustained high levels of cortisol makes people more susceptible to infections • Synthesized cortisol is commonly used as a medication to reduce inflammation associated with asthma, arthritis, or joint injuries

  43. Aldosterone • The main mineralcorticoid is the hormone aldosterone • Stimulates the kidneys to increase the absorption of sodium into the blood • Increases the concentration of solutes in the blood, which draws more water from the kidneys, raising blood pressure • If the adrenal cortex is damaged, Addison’s disease can result • The body secretes inadequate amounts of mineralcorticoids and glucocorticoids • Symptoms include: • Hypoglycemia (low blood sugar) • Sodium and potassium imbalances • Rapid weight loss

  44. Aldosterone • Low aldosterone results in a loss of sodium and water from the blood • Due to increase in urine output • As a result, blood pressure drops • A person with this condition needs to be treated within days, or the severe electrolyte imbalance will be fatal • Can be controlled with injections of glucocorticoids and mineralcorticoids

  45. The Hormones of the Pancreas • The pancreas is located behind the stomach and is connected to the small intestine by the pancreatic duct • Most of the pancreatic tissue secretes digestive enzymes into the small intestine • The pancreas also functions as an endocrine gland, secreting hormones directly into the bloodstream • Scattered throughout the pancreas are more than 2000 clusters of endocrine cells called the islets of Langerhans • Named for Paul Langerhans, the scientist who first described them in 1869

  46. The Hormones of the Pancreas