Chapter 10 - The Endocrine System PDF

Chapter 10 The Endocrine System Copyright © 2020, 2017, and 2013 Pearson Education, Inc. All Rights Reserved  Intercellular Com...

Chapter 10 The Endocrine System Copyright © 2020, 2017, and 2013 Pearson Education, Inc. All Rights Reserved  Intercellular Communication Maintains homeostasis Mostly accomplished through chemical messages Distant communication is coordinated by endocrine and nervous systems Nervous system is faster Specific destination and short-lived effects Endocrine system is slower Releases hormones into bloodstream that bind to target cells with longer-lasting effects Nervous and Endocrine Systems Comparison Both rely on release of chemicals that bind to specific receptors on target cells Both share chemical messengers Epinephrine (E) and norepinephrine (NE) Called hormones when released from adrenal medulla Called neurotransmitters when released across synapses Both are regulated by negative feedback mechanisms Both coordinate and regulate activities of other cells, tissues, organs, and systems to maintain homeostasis  The Endocrine System Includes all endocrine cells and tissues Cells are glandular, secretory cells Secretions enter the extracellular fluid Local hormones, such as prostaglandins, affect adjacent cells Hormones are chemical messengers released in one tissue and transported by the bloodstream to target cells in other tissues Copyright © 2020, 2017, and 2013 Pearson Education, Inc. All Rights Reserved  Three Groups of Hormones Based on their chemical structure: 1. Amino acid derivatives Structurally similar to amino acids Include epinephrine, norepinephrine, thyroid hormones, and melatonin 2. Peptide hormones Consist of chains of amino acids Largest class of hormones Include ADH, oxytocin, hypothalamic, pituitary, and pancreatic hormones  Three Groups of Hormones Cont. 3. Lipid derivatives Steroid hormones Structurally similar to cholesterol Bound to transport proteins in blood Released by reproductive organs and adrenal glands Eicosanoids Fatty acid–based, derived from arachidonic acid Coordinate local cellular functions and affect enzymatic processes Include prostaglandins  Hormone Action Hormones alter operations of the target cells Change types, activities, locations, or quantities of structural proteins and enzymes Sensitivity of target cell to hormone depends on presence or absence of receptors for that hormone Receptors are located either on plasma membrane or inside the cell Copyright © 2020, 2017, and 2013 Pearson Education, Inc. All Rights Reserved  Plasma Membrane Receptors Required for hormones that are not lipid soluble Examples: epinephrine, norepinephrine, peptide hormones, and eicosanoids Hormones cannot diffuse through the plasma membrane Must use receptors on outside of membrane Effect is not direct These hormones are first messengers that activate second messengers in the cytoplasm Action is linked by a G protein, an enzyme complex  Second Messenger Systems Small number of hormone molecules can activate thousands of second messengers Process called amplification Magnifies effect of hormone on the cell Examples of second messengers include: Cyclic-AMP Calcium ions Cyclic-GMP  Cyclic-AMP Second Messenger System First messenger activates a G protein Which activates enzyme adenylate cyclase Which converts ATP to second messenger, cAMP Which activates kinase enzymes inside cell Which attaches a phosphate group to another molecule in a process called phosphorylation Effect on target cell depends on nature of protein affected  Intracellular Receptors Receptors inside cytoplasm or nucleus Used for lipid-soluble hormones Examples: thyroid and steroid hormones Hormones cross plasma membrane Form hormone-receptor complex Activates or inactivates specific genes Alters rate of mRNA transcription Changes structure or function of cell  Hormone Secretion and Distribution Rapidly enter blood and distributed throughout body Freely circulating hormones are short-lived and inactivated when: 1. They diffuse to target cells and bind to receptors 2. They are absorbed and broken down in liver and kidney 3. They are broken down by enzymes in plasma or interstitial fluid Hormones bound to transport proteins stay in circulation longer (steroid and thyroid hormones) Each hormone has an equilibrium between bound and free forms  Control of Endocrine Activity Hormonal secretion under negative feedback control is based on three types of stimuli 1. Humoral (“liquid”) stimuli Changes in composition of the extracellular fluid Example: hormonal control of blood calcium levels 2. Hormonal stimuli Changes in circulating hormone levels 3. Neural stimuli Neural stimulation of a neuroglandular junction through neurotransmitters Copyright © 2020, 2017, and 2013 Pearson Education, Inc. All Rights Reserved  Hypothalamus and Endocrine Control Hypothalamus provides highest level of endocrine control Coordinating centers in hypothalamus regulate nervous and endocrine systems in three ways  The Pituitary Gland Also called the hypophysis Protected by the sella turcica Hangs from hypothalamus by infundibulum Anterior and posterior have very different structure Secretes nine hormones All are unique peptides or small proteins All use cAMP second messenger mechanism  The Anterior Lobe of the Pituitary Gland Contains endocrine cells Cells are surrounded by a complex capillary bed The capillaries are part of the hypophyseal portal system Two capillary networks and the blood vessels that link the networks are called a portal system Named after their destinations (in this case the hypophysis or pituitary gland) The Hypophyseal Portal System Regulatory hormones diffuse onto target cells in anterior lobe Many of these hormones are tropic hormones meaning they target other endocrine glands  The Seven Anterior Lobe Hormones 1. Thyroid-stimulating hormone (TSH) 2. Adrenocorticotropic hormone (ACTH) 3. Follicle-stimulating hormone (FSH) 4. Luteinizing hormone (LH) 5. Prolactin (PRL) 6. Growth hormone (GH) 7. Melanocyte-stimulating hormone (MSH)  Thyroid-stimulating Hormone Also called thyrotropin Released in response to thyrotropin-releasing hormone (TRH) from hypothalamus Targets thyroid gland Triggers release of thyroid hormones Increases in thyroid hormones cause decrease in TRH and TSH secretion  Adrenocorticotropic Hormone (ACTH) Also called corticotropin Released in response to corticotropin- releasing hormone (CRH) from the hypothalamus Targets adrenal cortex Stimulates secretion of glucocorticoids Increases in glucocorticoids cause decrease in ACTH and CRH secretion  Gonadotropins Released in response to gonadotropin-releasing hormone (GnRH) from hypothalamus Target the male and female gonads Testes in males and ovaries in females Include two hormones Follicle-stimulating hormone (FSH) Luteinizing hormone (LH)  Follicle-stimulating Hormone (FSH) Promotes follicle development in females Promotes sperm production in males Cells of ovaries and testes produce inhibin Inhibits FSH production through negative feedback  Luteinizing Hormone (LH) Induces ovulation and secretion of estrogens and progesterone in females In males, also called interstitial cell-stimulating hormone (ICSH) Stimulates interstitial cells endocrine cells of the testes to produce androgens such as testosterone Estrogens, progesterone, and androgens inhibit GnRH (which in turn decreases levels of LH)  Prolactin (PRL) Released in response to prolactin-releasing factor (PRF) from the hypothalamus Targets mammary glands in females With other hormones, stimulates mammary gland development In pregnancy and nursing, stimulates production of milk Circulating PRL stimulates prolactin-inhibiting hormone (PIH) and inhibits PRF  Growth Hormone Also called human growth hormone (hGH) and somatotropin Regulated by: Growth hormone-releasing hormone (GH–RH) and growth hormone-inhibiting hormone (GH–IH) from the hypothalamus Stimulates cell growth and replication of all cells, but especially skeletal muscle and chondrocytes Indirect action Stimulates liver to release somatomedins or insulin-like growth factors (IGFs), triggering an increase in amino acid uptake and protein synthesis by cells following a meal Somatomedins stimulate GH–IH and inhibit GH–RH  Direct Actions of Growth Hormone In epithelial and connective tissues Stimulates stem cell division and differentiation In adipose tissue Stimulates breakdown of stored fats and release of fatty acids, a glucose-sparing effect In the liver Stimulates breakdown of glycogen reserves and release of glucose into bloodstream  Melanocyte-stimulating Hormone (MSH) Increases activity of melanocytes in skin to increase melanin production Appears to be nonfunctional in adults MSH is secreted During fetal development In very young children In pregnant women In certain diseases Even in these situations, function unknown  The Posterior Lobe of the Pituitary Gland Contains axons from two groups of hypothalamic neurons The two hormones produced are: 1. Antidiuretic hormone (ADH) 2. Oxytocin (OXT) Hormones transported within the axons and then into capillaries of the posterior pituitary lobe  Antidiuretic Hormone (ADH) Also called vasopressin Released when the body is low on water Stimulated by increase in concentration of solutes in the blood or decrease in blood volume and pressure Primary target is kidney to decrease water loss Triggers vasoconstriction to increase blood pressure  Oxytocin (OXT) In women, stimulates contraction of uterine muscles during labor and delivery Also stimulates contraction of cells surrounding milk secretory cells in mammary glands Plays an unclear role in sexual activity In men, stimulates smooth muscle in walls of sperm duct, potentially aiding in emission In women, may stimulate smooth muscle contractions in the uterus and vagina promoting sperm transport Copyright © 2020, 2017, and 2013 Pearson Education, Inc. All Rights Reserved The Thyroid Gland Found anterior to trachea and inferior to thyroid cartilage Has two lobes connected by narrow isthmus Contains many spherical thyroid follicles Lined by simple cuboidal epithelium Filled with viscous colloid containing many proteins and thyroid hormone molecules  The Thyroid Follicles Follicular cells make thyroid hormones that are then stored in colloid TSH causes release of thyroid hormones Majority are transported bound to plasma proteins Provides large reserves of thyroid hormones Derived from amino acid tyrosine, and iodine Thyroxine (T4) or tetraiodothyronine has four atoms of iodine Triiodothyronine (T3) has three iodine atoms More potent than T4  The Effects of Thyroid Hormones Affect nearly every cell in body Increase rate of ATP production in mitochondria Activate genes coding for enzymes involved in glycolysis and energy production Enzymes increase rate of metabolism Calorigenic effect is when cell uses more energy, measured in calories, and heat is produced Thyroid hormones essential to normal development of skeletal, muscular, and nervous systems  The C Cells of the Thyroid Gland Also called parafollicular cells, are found between follicles and produce calcitonin (CT) Stimulated by increases in plasma Ca2+ Inhibits osteoclasts in bone Stimulates calcium excretion by kidneys Essential for normal bone growth in children and last trimester of pregnancy  Calcium Balance and Calcitonin Hypercalcemia (high blood calcium level) causes: Decreased sodium permeability of excitable membranes Results in less responsive muscles and nerves  The Parathyroid Glands Two pairs of small glands embedded in posterior surface of thyroid gland  The Parathyroid Glands Cont. Parathyroid principal or chief cells produce parathyroid hormone (PTH) Stimulated by decrease in plasma Ca2+ Activates osteoclasts and inhibits osteoblasts in bone Reduces calcium excretion by kidney Stimulates kidney to secrete calcitriol, which increases Ca2+ and PO43- absorption in digestive tract  Calcium Balance and PTH Hypocalcemia (low blood calcium level) causes: Increased sodium permeability of excitable membranes Highly excitable, spasmodic muscles and neurons Convulsions or muscular spasms Parathyroid glands prevent hypocalcemia The Adrenal Gland Also called the suprarenal gland Yellow, pyramid-shaped Sits on superior border of each kidney Two portions: adrenal cortex and adrenal medulla  The Adrenal Cortex Contains high levels of cholesterol and fatty acids Produces more than 24 steroid hormones called corticosteroids Are essential for metabolic functions and vital to life Transported in plasma bound to proteins Three zones of cortex produce three types 1. Zona glomerulosa produces mineralocorticoids 2. Zona fasciculata produces glucocorticoids 3. Zona reticularis produces androgens  Mineralocorticoids Also called MCs Affect electrolyte balance in body fluids Aldosterone – major MC Secreted in response to low plasma Na+, low BP, high plasma K+, or presence of angiotensin II Triggers reabsorption of sodium ions and elimination of potassium ions Prevents loss of sodium in urine, sweat, saliva, and digestive secretions Secondarily triggers water reabsorption through osmosis  Glucocorticoids Also called GCs Affect glucose metabolism Most important are cortisol (hydrocortisone), corticosterone, and cortisone Secreted in response to ACTH Increase rates of glucose synthesis and glycogen formation, resulting in increase in blood glucose levels Also act as anti-inflammatory  The Androgens Small quantities produced in both males and females Some converted to estrogens in plasma Stimulate development of pubic hair in boys and girls before puberty Not important in adult men In adult women, produce muscle mass, blood cell formation, and support sex drive  The Adrenal Medulla Highly vascular Contains cells similar to sympathetic ganglia Innervated by preganglionic sympathetic fibers Two groups of secretory cells produce: Epinephrine (E, or adrenaline); 75–80 percent of secretions Norepinephrine (NE, or noradrenaline); 20–25 percent Hormones trigger metabolic changes to increase availability of energy molecules Support and prolong overall sympathetic response  The Pineal Gland Located on posterior portion of roof of third ventricle Contains neurons, glial cells, and secretory cells that produce melatonin Rate of secretion affected by light and day–night cycles May influence timing of sexual maturation May protect CNS neurons with antioxidant activity Plays role in maintaining circadian rhythms (day–night cycles)  The Endocrine Pancreas Pancreas lies between stomach and proximal small intestine Contains both exocrine and endocrine cells  The Endocrine Pancreas Cont. Endocrine cells located in pancreatic islets, or islets of Langerhans Account for only 1 percent of all pancreatic cells Alpha cells secrete the hormone glucagon Beta cells secrete the hormone insulin  Blood Glucose and Insulin Increases in blood glucose levels activate beta cells to release more insulin Stimulates glucose uptake by cells that have insulin receptors, all cells except: Neurons and red blood cells, epithelial cells of kidney tubules, epithelial cells of intestinal lining Increases rates of protein synthesis and fat storage Result is lower blood glucose level Copyright © 2020, 2017, and 2013 Pearson Education, Inc. All Rights Reserved  Blood Glucose and Glucagon Decreases in blood glucose levels activate alpha cells to release more glucagon Mobilizes energy reserves Glycogen in liver and muscles broken down to glucose Adipose tissue releases fatty acids Result is higher blood glucose level  Pancreatic Regulation of Blood Glucose Secretion of insulin and glucagon is independent of direct neural stimulus Indirectly affected by any hormone that also influences blood glucose levels Examples: cortisol and thyroid hormones Also affected by ANS activity Parasympathetic stimulation enhances insulin release Sympathetic stimulation inhibits insulin release  Diabetes Mellitus Faulty glucose metabolism causing buildup of glucose in the blood and urine Type 1 diabetes (juvenile diabetes) Inadequate insulin production Type 2 diabetes Display insulin resistance where tissues do not respond properly to insulin Symptoms: hyperglycemia, glycosuria, polyuria Diabetes Mellitus Cont.  Secondary Endocrine Organs Many organs release hormones, but their primary function is in another system Examples: intestines, kidneys, heart, thymus, gonads, and adipose tissue Copyright © 2020, 2017, and 2013 Pearson Education, Inc. All Rights Reserved  The Intestines Release local hormones that coordinate digestive activities Major control over rate of digestive processes Can be influenced by ANS  Hormones Released by the Kidneys Calcitriol Stimulated by PTH, derived from vitamin D3, increases absorption of calcium and phosphate ions from digestive tract Erythropoietin, or EPO Stimulated by low oxygen levels in kidney tissues, causes increase in RBC production Renin An enzyme that triggers hormonal chain reaction (renin- angiotensin-aldosterone system) to increase blood pressure and blood volume  The Heart Endocrine cells in right atrium of heart Respond to increased blood volume entering chamber Excessive stretch causes them to release atrial natriuretic peptide (ANP) Promotes loss of sodium, and therefore water Inhibits release of renin, ADH, and aldosterone Results in decrease in blood pressure and blood volume  The Thymus Located deep to sternum in mediastinum Very active in early childhood, atrophies in adults Releases thymosins Aid in development and maintenance of immune defenses  The Gonads: The Testes In males, interstitial endocrine cells of testes produce androgens Most important is testosterone Promotes sperm production Maintains secretory glands of male reproductive tract Determines secondary sex characteristics Stimulates protein synthesis in skeletal muscles Sperm production balance maintained by inhibin Secreted by nurse cells in the testes  The Gonads: The Ovaries In ovaries, the female sex cells (oocytes) are surrounded by follicles FSH triggers follicular cells to produce: Estrogens Support maturation of egg cell and growth of uterine lining Inhibin Provides negative feedback to FSH  The Gonads: The Ovaries cont. Once follicle releases an oocyte (ovulation) the corpus luteum is formed from remaining follicular cells Releases mixture of estrogens and progesterone Accelerates fertilized egg movement through uterine tube Prepares uterus for arrival of developing embryo Contributes to mammary gland enlargement All gonadal hormones regulated by hormones of the anterior pituitary gland  Adipose Tissue Produces peptide hormone leptin Provides negative feedback control of appetite Binds to neurons in hypothalamus to trigger satiation (fullness) and suppression of appetite Must be present for normal levels of GnRH and gonadotropin synthesis Low body fat can result in late puberty and cessation of menstrual cycles Increase in body fat can increase fertility  Hormonal Interactions Extracellular fluid contains mixture of hormones that may have the same target, resulting in four possibilities: 1. Antagonistic (opposing) effects Example: parathyroid hormone and calcitonin 2. Synergistic effects Net result of two is greater than the sum of their individual effects 3. Permissive effects Need for one hormone to be present for another to work 4. Integrative effects Hormones may have different but complementary effects Important in coordinating diverse physiological activities  Hormones and Growth Six key hormones required for normal growth 1. Growth hormone (GH) Supports muscular and skeletal development Undersecretion causes dwarfism Oversecretion causes gigantism 2. Thyroid hormones Required for normal nervous system development 3. Insulin Required for energy supply to growing cells  Hormones and Growth cont. 4. Parathyroid hormone (PTH) Promotes calcium availability for normal bone growth 5. Calcitriol Promotes absorption of calcium for building bone Lack of PTH and calcitriol can result in rickets 6. Reproductive hormones Can affect activity of osteoblasts and influence secondary sex characteristic development  Hormones and Stress Stress is triggered by: Physical injury or disease Emotional responses: anxiety or depression Environmental conditions: extreme cold or heat Metabolic conditions: acute starvation Stress triggers: The general adaptation syndrome (GAS) Also called the stress response Three phases: alarm, resistance, exhaustion  Hormones and Behavior Hypothalamus is key hormone regulator and monitor Behavior is also affected by hormonal abnormalities Precocious puberty can occur when sex hormones are released at an earlier than normal age Results in more aggressive and assertive behavior CNS intellectual functions like learning, memory, and emotions can be altered in the adult due to hormone imbalances  Hormones and Aging Usually most hormones remain the same throughout adulthood Exceptions are the reproductive hormones Changes to target organ receptors more likely to occur through reduced sensitivity  Endocrine System Integration with Other Systems Endocrine system provides long-term regulation and adjustment of homeostatic processes Adjusts metabolic rates of all systems Regulates growth and development Copyright © 2020, 2017, and 2013 Pearson Education, Inc. 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