Chapter 9- The- Endocrine- System.docx
- 1. Chapter 9 The Endocrine System When insulin molecules, carried passively along in the blood, leave the blood and bind tightly to protein receptors of nearby cells, the response is dramatic: Bloodborne glucose molecules begin to disappear into the cells, and cellular activity accelerates. Such is the power of the second great controlling system of the body, the endocrine system. Along with the nervous system, it coordinates and directs the activity of the body's cells. However, the speed of control in these two great regulating systems is very different. The nervous system is "built for speed." It uses nerve impulses to prod the muscles and glands into immediate action so that rapid adjustments can be made in response to changes occurring both inside and outside the body. On the other hand, the more slowly acting endocrine system uses chemical messengers called hormones, which are released into the blood to be transported leisurely throughout the body. Although hormones have widespread and varied effects, the major processes controlled by hormones are reproduction; growth and development; mobilizing body defenses against stressors; maintaining electrolyte, water, and nutrient balance of the blood; and regulating cellular metabolism and energy balance. As you can see, the endocrine system regulates processes that go on for relatively long periods and, in some cases, continuously. The Endocrine System and Hormone Function: An Overview Define hormone and target organ. o Hormone – chemical messengers that are released into the blood to be transported leisurely throughout the body – the “arouse” or bring about their effects on the body’s cells primarily by altering cellular activity either by increasing or decreasing the rate of a normal metabolic process rather than stimulating a new one Amino acid – based molecules – proteins, peptides and amines Steroids – made from cholesterol including sex hormones made by gonads and the hormones produced by the adrenal cortex Prostaglandins – made from highly active lipids found in the cell’s plasma membrane o Target organ – target cells – although hormones circulate to all the organs of the body, a given hormone affects only certain tissue cells or organs – in order for a target cell to respond to a hormone, specific protein receptors must be present on its plasma membrane or in its interior, to which that hormone can attach – only when binding occurs can the hormone influence the workings of the cell Describe how hormones bring about their effects in the body. o The precise changes that follow hormone binding depends on the specific hormone and the target cell type, but one or more of the following occurs Changes in plasma membrane permeability or electrical state Synthesis of proteins or certain regulatory molecules (such as enzymes) in the cell Activation or inactivation of enzymes Stimulation of mitosis o Two mechanisms by which hormones trigger changes in cells Steroidal hormones – lipid soluble molecules Diffuse through the plasma membranes of their target cells Enter the nucleus Bind specific receptor proteins there
- 2. The hormone-receptor complex then binds to specific sites on the cell’s DNA This activates certain genes to transcribe messenger DNA The mRNA then is translated in the cytoplasm resulting in the synthesis of new proteins Nonsteroidal hormones – protein and peptide hormones – unable to enter the target cells and instead bind to receptors situated on the target cells’ plasma membrane The hormone binds to the membrane receptor Setting off a series of reactions that activates an enzyme That enzyme in turn catalyzes a reaction that produces a second messenger molecule The second messenger molecule oversees additional intracellular changes that promote the typical response of the target cell to the hormone o There are a variety of possible second messengers and many possible target cell responses to the same hormone depending on the tissue type stimulated Explain how various endocrine glands are stimulated to release their hormonal products. o The stimuli that activate the endocrine organs fall into three major categories Hormonal stimulus – endocrine organs are prodded into action by other hormones – hormones release promoted by this mechanism tends to be rhythmic, with hormone blood levels rising and falling again and again Humoral stimulus – changing blood levels of certain ions and nutrients stimulates hormone release – term humoral refers to the ancient use of humor to indicates the various body fluids Neural stimulus – nerve fibers stimulate hormone release Define negative feedback and describe its role in regulating blood levels of the various hormones. o Negative feedback – feedback that causes the stimulus to decline or end o Hormone secretion is triggered by some internal or external stimulus, then rising hormone levels inhibit further hormone release even while promoting responses in their target organs resulting in stable levels of many hormones staying only within a very narrow range The Major Endocrine Organs Describe the difference between endocrine and exocrine glands. o Endocrine glands – ductless glands that produce hormones that they release into the blood or lymph o Exocrine glands – release their products at the body’s surface or into body cavities through ducts On an appropriate diagram, identify the major endocrine glands and tissues. o See figure 9.3 on page 283 List hormones produced by the endocrine glands and discuss their general functions. o See table 9.1 on pages 297-298 o Pituitary gland Anterior pituitary – master endocrine gland – all are proteins or peptides, act through second-messenger systems, and are regulated by hormonal stimuli and in most cases negative feedback Growth hormone – GH – growth of skeletal muscles and long bones of the body – plays a role in determining final body size – protein-sparing and anabolic hormone that causes amino acids to be built into proteins and stimulates most target cells to grow in size and divide and is causes fats to be
- 3. broken down and used for energy, and spares glucose, helping to maintain blood sugar homeostasis Prolactin – PRL – protein hormone – only known target is breast tissue where it stimulates and maintains milk production after childbirth Adrenocorticotropic hormone – ACTH – regulates the endocrine activity of the cortex portion of the adrenal gland Thyroid-stimulating hormone (TSH) also called thyrotropic hormone (TH) influences the growth and activity of the thyroid gland Godadotropic hormone – regulates the hormonal activity of the gonads Females – follicle-stimulating hormone (FSH) – stimulates follicle development in the ovaries and as they mature, they produce estrogen, and eggs are readied for ovulation – luteinizing hormone (LH) triggers ovulation of an egg from the ovary and causes the ruptured follicle to become a corpus luteum, it then stimulates the corpus luteum to produce progesterone and some estrogen Males – FSH stimulates sperm development by the testes – LH is also referred to as interstitial cell-stimulating hormone (ICSH) that stimulates testosterone production by the interstitial cells of the testes Posterior pituitary Oxytocin – released only during childbirth and in nursing women – stimulates contractions of the uterine muscles during labor, during sexual relations, and when breast-feeding infants (causes milk ejections/letdown reflex) Antidiuretic hormone – ADH – causes the kidneys to reabsorb more water from the forming urine and as a result, urine volume decreases and blood volume increases – also increases blood pressure by causing constriction of the arterioles therefore also called vasopressin o Thyroid gland Thyroid hormone – the body’s major metabolic hormone – two active iodine- containing hormones; thyroxine (T4) and triiodothyronine (T3) – controls the rate at which glucose is burned and converted to body heat and chemical energy – also important for normal tissue growth and development, especially in the reproductive and nervous systems Calcitonin – thyrocalcitonin – decreases blood calcium levels by causing calcium to be deposited in the bones – acts antagonistically to parathyroid hormone o Parathyroid glands Parathyroid hormone (PTH) – parathormone – stimulates bone destruction cells (osteoclasts) to break down bone matrix and release calcium into the blood when blood calcium levels drop – also stimulates the kidneys and intestines to absorb more calcium o Adrenal glands – two glands in one – glandular (cortex) and neural tissue (medulla) parts Adrenal cortex – produces three major groups of steroid hormones collectively called corticosteroids Mineralocorticoids – mainly aldosterone – stimulated by humoral factors – produced by the outermost adrenal cortex cell layer – regulate the mineral (or salt) content of the blood, particularly the concentrations of sodium and potassium ions - target the kidney tubules that selectively reabsorb the minerals or allow them to be flushed out in the urine – when sodium is reabsorbed, water follows so they also help regulate both water and electrolyte balance in body fluids
- 4. Renin – an enzyme produced by the kidneys when blood pressure drops, also causes the release of aldosterone by triggering a series of reactions that result in formation of angiostensin II, a potent stimulator of aldosterone release Atrial natriuretic peptide (ANP) – hormone released by the heart that prevents aldosterone release to reduce blood volume and blood pressure Glucocorticoids – including cortisone and cortisol – promote normal cell metabolism and help the body to resist long-term stressors, primarily by increasing blood glucose levels by breaking down fats and proteins into glucose, which is released into the blood – also reduce edema, reduce pain by inhibiting some pain-causing molecules called prostaglandins Sex hormones – both male and female – produced in small amounts throughout life – bulk of the sex hormones produced by innermost cortex layer are androgens (male sex hormones), some estrogens (female sex hormones) are also produced Adrenal Medulla – develops from a knot of nervous tissue Catecholamines – Epinephrine (adrenaline) and Norepinephrine (noradrenaline) – released during the “fight-or-flight response of the sympathetic nervous system to help the body cope with short-term stressful situations o Pancreatic Islets The pancreas is a mixed gland – participates in digestion as well as the endocrine system Pancreatic islets – masses of hormone-producing tissue scattered among the enzyme-producing tissue of the pancreas Insulin – released when there is a high level of glucose in the blood that stimulates beta cells of the islets to release the hormone – increases a cell’s ability to transport glucose across the membrane into the cell for use or storage Hypoglycemic – sweeps glucose into the cells and out of the bloodstream and falling blood-glucose levels stops the stimulus to release insulin – negative feedback Glucagon – antagonists of insulin – released by alpha cells of the islets stimulated by low blood-glucose levels Hyperglycemic – stimulated the liver to break down stored glycogen into glucose and to release it into the blood o Pineal Gland – pineal body Melatonin – sleep trigger – plays a role in establishing the body’s day-night cycle – also coordinates the hormones of fertility and to inhibit the reproductive system (especially of females) so that sexual maturation is prevented from occurring during childhood, before the adult body size has been reached o Thymus – large in infants and children – decreases in size throughout adulthood Thymosin – programs T cells - during childhood the thymus acts as an incubator for the maturation of a special group of white blood cells (T lymphocytes) that are important in the immune response o Gonads – produce the same hormones as the adrenal cortex cells – differences are the source and amounts Hormones of the ovaries – stimulated to be released in a cyclic way by the anterior gonadotropic hormones
- 5. Estrogens – estrone and estradiol – produced by the Graafian follicles of the ovaries – stimulate the development of the secondary sex characteristics in females – also work with progesterone to prepare the uterus to receive a fertilized egg or to stimulated the menstrual cycle Progesterone – with estrogen, helps maintain the menstrual cycle – during pregnancy, it quiets the muscles of the uterus to prevent the implanted embryo from being aborted and it helps prepare the breast tissue for lactation Hormones of the testes Androgens – testosterone is most important – produced by interstitial cells of the testes and cause the development of the adult male secondary sex characteristics and promotes the growth and maturation of the reproductive system organs – necessary for the continuous production of sperm Discuss ways in which hormones promote body homeostasis by giving examples of hormonal actions. o The parathyroids secrete parathyroid hormone (PTH) or parathormone – the most important regulator of calcium ion homeostasis of the blood – when blood calcium levels drop below a certain level, the parathyroids release PTH, which stimulates bone destruction cells (osteoclasts) to break down bone matrix and release calcium into the blood whereas calcitonin or thyrocalcitonin produced by the thyroid gland decreases blood calcium levels by causing calcium to be deposited in the bones o Rising blood sugar levels stimulate insulin-secreting cells of the pancreas causing cells to increase the uptake of glucose from the blood and lower the blood sugar levels while dropping blood sugar levels stimulates the glucagon-releasing cells of the pancreas to break down glycogen into glucose and release it into the blood stream to raise the blood sugar levels Describe the functional relationship between the hypothalamus and the pituitary gland. o The release of hormones from the anterior pituitary is controlled by releasing and inhibiting hormones produced by the hypothalamus o The hypothalamus releases these regulatory hormones into the blood of the portal circulation, which connects the blood supply of the hypothalamus with that of the anterior pituitary o The hypothalamus also produces oxytocin and antidiuretic hormones, which are transported along the axons of the hypothalamic neurosecretory cells to the posterior pituitary for storage to be released later in response to nerve impulses from the hypothalamus Describe major pathological consequences of hypersecretion and hyposecretion of the hormones considered in this chapter. o Hypersecretion – too much is produced Too much GH during childhood results in gigantism with heights up to 8-9 feet Too much FSH or LH given to promote fertility stimulate the release of the gonadotropic hormones and multiple births can occur Too much thyroid hormone leads to Grave’s disease – the thyroid gland enlarges and the eyes may bulge, or protrude anteriorly Too much parathyroid hormone causes massive bone destruction and the bones become very fragile and spontaneous fractures begin to occur Too much of the outer most adrenal cortex hormones result in hyperaldosteronism (excessive water and sodium are retained leading to high blood pressure and edema, and potassium is lost to such an extent that the activity of the heart and nervous system may be disrupted) Too much middle cortex hormones (glucocorticoids) leads to Cushing’s Syndrome – moon face, buffalo hump of fat on upper back, high blood pressure, hyperglycemia
- 6. and possible diabetes, weakening of the bones, and severe depression of the immune system Too much sex hormones leads to masculinization, regardless of sex Too much catecholamines leads to symptoms typical of excessive sympathetic nervous system activity o Hyposecretion – too little is produced Too little GH during childhood leads to pituitary dwarfism – max height of 4 feet Too little GH after long-bone growth has ended, acromegaly results – facial bones, particularly the lower jaw and bony ridges underlying the eyebrows, enlarge tremendously, as do the hands and feet Too little FSH or LH leads to sterility in both sexes Too little ADH leads to diabetes insipidus – excessive urine output due to being continuously thirsty and drinking huge amounts of water Too little thyroid hormone in early childhood results in cretinism – dwarfism in which adult body proportions remain childlike along with mental retardation Too little thyroid hormone in adulthood results in myxedema – physical and mental sluggishness, puffiness of the face, fatigue, poor muscle tone, low body temperature, obesity, and dry skin Too little adrenal cortex hormones leads to Addison’s disease – bronze tone of the skin caused by low levels of sodium and water and thus an electrolyte and water balance causing muscles to become weak – a complete lack of glucocorticoids will result in death Too little sex hormones causes a person to become sterile Other Hormone-Producing Tissues and Organs Indicate the endocrine role of the kidneys, the stomach and intestine, the heart, and the placenta. o See table 9.2 on page 301 o Kidneys – produces renin when blood pressure drops and causes the release of aldosterone causing the kidneys to reabsorb sodium and potassium ions – produces erythropoietin that promotes the production of red blood cells in the bone marrow – also activates vitamin D made by epidermal cells of skin that causes intestine cells to actively transport dietary calcium across the intestinal cell membranes o Stomach – produces gastrin – stimulates glands to release hydrochloric acid (HCL) o Duodenum (small intestine) – produces intestinal gastrin that inhibits HCL secretion by the stomach and gastrointestinal tract mobility – produces secretin that stimulates the release of bicarbonate-rich juice by the pancreas and increases release of bile from the liver and inhibits secretory activity of the stomach – produces cholecystokinin (CCK) that stimulates the pancreas to release a enzyme-rich juice, stimulates the gallbladder to release stored bile, causes the duodenal papilla sphincter to relax, allowing bile and pancreatic juice to enter the duodenum o Heart – atrial natriuretic peptide (ANP) – prevents aldosterone release by the adrenal cortex to reduce blood volume and blood pressure – inhibits the kidneys from reabsorbing sodium and from releasing renin o Placenta – produces human chorionic gonadotropin (hCG) – initially produced by the conceptus and then by the fetal part of the placenta – stimulates the corpus luteum of the ovary to continue producing estrogen and progesterone so that the lining of the uterus is not sloughed off in menses until the third month of pregnancy and then the placenta takes over the role of producing the hormones and the ovaries become inactive for the rest of the pregnancy – also helps prepare the breasts for producing milk – human placental lactogen (hPL) – works with estrogen and progesterone in preparing the breasts for
- 7. lactation – produces relaxin – causes the mother’s pelvic ligaments and pubic symphysis to relax and become more flexible, which eases birth passage Developmental Aspects of the Endocrine System Describe the effect of aging on the endocrine system and body homeostasis. o Most endocrine organs seem to operate smoothly until old age In late middle age, the efficiency of the ovaries begins to decline causing the onset of menopause – during this time, a woman’s reproductive organs begin to atrophy, and the ability to bear children ends Problems associated with estrogen deficiency begin to occur – arteriosclerosis, osteoporosis, decreased skin elasticity, fatigue, nervousness, mood changes, and changes in the operation of the sympathetic nervous system that result in hot flashes The efficiency of the endocrine system as a whole gradually declines in old age The growth hormone output by the anterior pituitary declines, partially explaining muscle atrophy in old age Since the anterior pituitary affects so many other endocrine glands, it is assumed that its target organs also become less productive – older people are often mildly hypothyroid, have some decline in insulin production leading to adult-onset diabetes, and are less able to resist stress and infection partly due to waning melatonin that plays a role in promoting the immune response