Student Performance Objectives - for the lecture
1. Explain the difference between an endocrine and an exocrine gland.
2. Describe the relationship between a hormone and its target organ.
3. Explain how hormones are transported in the blood
4. Explain why hormones secreted yesterday or even a few hours ago have little or no effect on metabolic processes going on now.
5. List the 6 classes of hormones and give one example of each class.
6. Describe the difference in the interaction of the hydrophilic and hydrophobic hormones with their target cells.
7. Describe the process of negative feedback and give 2 examples.
8. Describe the process of positive feedback and give 2 examples.
9. Describe the interactions occurring within each of the following endocrine gland axes: hypothalamic-pituitary-thyroid axis, hypothalamic-pituitary-adrenal axis, and the hypothalamic-pituitary-gonadal axis.
10. For each of the following body regions, describe 2 endocrine glands, their hormonal secretions, and the hormone's actions: the head, the neck, the thoracic cavity, and the abdominopelvic cavity.
A. In General
1. Ductless glands produce hormones- While exocrine glands have a duct through which the gland's product reaches its destination (e.g., the parotid salivary gland secretes saliva through Stensen's duct into the mouth; the lacrimal gland secretes tears through the lacrimal duct onto the eye's surface), endocrine glands have no ducts - they are ductless glands. They secrete their products, called hormones, into the interstitial fluid surrounding the gland. From there the hormone diffuses into the blood and is carried throughout the body.
2. Hormones and target organs- when the product from an exocrine gland reaches its local destination, it carries out its intended effect: saliva lubricates and begins digestion of food, tears lubricate the eye's surface. When the hormone from an endocrine gland circulates in the blood, it travels far from the endocrine gland that secreted it. It comes into contact with all cells of the body but only influences cells that have receptors for that specific hormone. We say that the hormone only affects its target organ, or target structure. Receptors can be at the cell surface in which case the hormone never enters the target cell. Or, the hormone's receptors can be located inside the cell requiring that the hormone enter the cell through the cell membrane. In either case, the hormone exerts its effects after attaching to chemical receptors designed for the hormone to fit into.
3. Hormonal transport - some hormones can travel freely in the blood because they are water soluble (hydrophilic). The steroid hormones, being fat soluble (hydrophobic), must be bound to and carried around by transport proteins (like albumin and globulins which are blood proteins produced by the liver). Free (unbound) hormones are able to enter target organs. Bound hormones must be released from the transport proteins to get into their target organs.
4. Hormonal breakdown - hormones are broken down in target cells, in the liver and in the kidneys. The hormonal breakdown products are excreted from the body in urine and feces. Hormones that are freely dissolved in blood and other body fluids are broken down rapidly - they are said to have a short half-life. Examples are epinephrine and norepinephrine that have half-lives measured in minutes. Hormones that are bound to transport proteins can circulate longer and break down only after they disassociate from their transport proteins and are free in the body fluids. Hormonal breakdown is important in that at any given moment, the hormones circulating within us are mostly freshly made in response to recent environmental conditions. We are adapted hormonally to current conditions. The hormones from yesterday or several hours ago are mostly gone.
B. Hormonal Classification - most hormones fall into the following six classes. Some specific hormonal examples are given with a brief idea of the wide-range of activities of these hormones.
1. Polypeptides (small proteins: 14 - 199 amino acids)-
a. Growth hormone - stimulates growth of epiphyseal plates in the long bones.
b. Insulin - lowers the blood sugar after a meal.
c. Glucagon - raises the blood sugar if it falls when you are not eating.
2. Oligopeptides (very small proteins: 3 - 10 amino acids)
a. Anti-diuretic hormone - ADH - helps the body hold on to its water preventing dehydration.
b. Oxytocin - helps the uterus to contract during childbirth and causes the breasts to pump out milk.
c.Angiotensin II - a vasoconstrictor that raises blood pressure.
3. Modified amino acids
a. Thyroxine - regulates the basal metabolic rate. Thyroxine consists of two attached amino acids (both are tyrosine) with attached iodine atoms.
b. Epinephrine - a "fight or flight" hormone affecting heart and breathing rate.
c. Norepinephrine - works much like epinephrine but as a "local" neurotransmitter. Both epinephrine and nor epinephrine are modifications of a single amino acid, tyrosine, and are in a class frequently called monoamines.
4. Steroids (derivatives of cholesterol)
a. Androgens, like testosterone, that stimulate male sexual characteristics.
b. Estrogens, like estradiol, that stimulate female sexual characteristics.
c. Aldosterone (an adrenal steroid) regulates blood sodium and potassium levels.
5. Glycoproteins (combinations of protein and carbohydrate)
a. Follicle stimulating hormone - FSH - stimulates egg and sperm development.
b. Luteinizing hormone - LH - causes ovulation.
c. Thyroid stimulating hormone - TSH - stimulates the thyroid gland to release its hormone, thyroxine.
6. Paracrines - There are other substances that act as hormones, but more locally. These are called paracrines: e.g., neurotransmitters like acetylcholine, histamine (a mediator of inflammation), and the eicosanoids, which are fatty acid derivatives influencing metabolism (e.g., blood pressure, blood clotting, and inflammation). The neurotransmitters are discussed in the nervous system area. Eicosanoids are generally considered as a group in courses in nutrition, and also, often, in the cardiovascular system area with regard to their influence on cardiovascular health and inflammatory processes.
C. Hormonal-Cell interactions
1. Hydrophilic (water soluble) hormones like epinephrine, norepinephrine, dopamine, glucagon, and ADH, attach to receptors on the cell surface. These water soluble hormones cannot easily penetrate through the cell membrane which is mostly hydrophobic. Attachment of hydrophilic hormones to surface receptors activates second messenger systems on the cytoplasmic side of the cell membrane.
There are several different 2nd messenger systems, but they operate in the same general way. The second messenger system eventually produces the hormone's effects. It is an intracellular enzyme amplification system in that 5 or 6 sequential chemical reactions are triggered inside the cell by the initial attachment of a hormone to the surface receptor: the hormone's attachment stimulates thousands of molecules of GTP (a high energy molecule) to break down which causes thousands of cyclic AMP molecules to be produced from ATP; each cyclic AMP molecule stimulates production of thousands of enzymes called protein kinases; each protein kinase molecule stimulates formation of thousands of other enzymes, and so on. Taken as a whole, an initial stimulus from one molecule of hormone attaching to a receptor, results in thousands, times thousands, times thousands (at least 6 times) of "downstream" molecules being produced that carry out the hormone's work. So hormones are very potent chemicals and only tiny amounts are necessary to create powerful effects on cells and the body as a whole.
2. Hydrophobic (fat soluble) hormones like the steroid hormones do not attach to surface receptors. They break off from their blood transport proteins and then pass right through the cell and nuclear membranes attaching to receptors near or on specific regions of DNA (genes).
DNA is then stimulated to transcribe messenger RNA resulting in new cytoplasmic protein synthesis that redirects cell metabolism. The thyroid hormones, also lipid soluble, enter the cytoplasm and attach to receptors on mitochondria and ribosomes as well as in the nucleus. The effect of thyroid hormones is to increase mitochondrial oxidations, increase protein synthesis, and to increase production of membrane ionic (sodium and potassium) transporters, all of which result in increases in the body's metabolic rate and overall heat production.
D. Feedback, negative and positive - When the body receives a signal (input signal), there is a response - the output signal. We call any activity of the body a parameter. If we are measuring body temperature, then body temperature is the parameter. If we are measuring blood sodium concentration (level), then blood sodium level is the parameter. All body parameters have a normal range of values (e.g., body temperature is 37± 1° C). We will utilize this terminology to explain negative and positive feedback in the endocrine system.
1. In a negative feedback mechanism, an input signal changes a body parameter from the normal range: it goes either above or below the normal range. This causes the body to respond (output signal) so as to return the parameter back to the normal range. Within the endocrine system, negative feedback mechanisms keep the level of circulating hormones within a "normal" range. The parameter being measured here is the level of a given circulating hormone. If the circulating level of hormone rises above the normal range (this is the input signal), then the negative feedback mechanism slows down further hormonal secretion: the hormonal level falls back to the normal range (output signal). If the circulating level of hormone falls below the normal range (new input signal), then the negative feedback mechanism increases hormonal secretion: the hormonal level rises up to the normal range (new output signal). Notice that whether the hormone level (parameter) goes above or below the normal range, the negative feedback mechanism brings the level back to the normal range. E.g., when TSH from the pituitary stimulates the thyroid gland to produce thyroxine (increased blood thyroxine is the input signal), thyroxine is said to feed back to the pituitary: further secretion of TSH is slowed down (output signal). As thyroxine does its work and is broken down (decreased blood thyroxine is the new input signal), less thyroxine is available to inhibit the pituitary: the pituitary secretes more TSH (new output signal).
2. In a positive feedback mechanism, an input signal changes a body parameter from the normal range: it goes either above or below the normal range. This causes the body to respond (output signal) so as to intensify the level of the parameter even further outside the normal range. In positive feedback mechanisms the body responds to an input signal by intensifying the direction of the signal, either above or below the normal range.
a. Birth of a baby: contractions of the body of the uterus stretches the cervix or uterine neck which is where the baby emerges from during birth. Stretching the cervix (input signal) results in even more intense contractions of the body of the uterus (output signal). This stretches the cervix even more (new input signal) resulting in even more intense contractions of the uterine body (new output signal), and so on until the baby is squeezed out. Then, with the baby out of the body, the cervix is stretched less (new input signal) and the uterine body contracts less (new output signal). Notice that this positive feedback system results in a baby being born and then the uterus slowing and eventually stopping its contractions.
b. Milk production: the more a baby nurses, the more milk the body produces. As the baby grows and becomes more hungry, it nurses longer (even if the breast is depleted of milk), and the result is that within a day or two the breast's milk production increases to meet the new increasing demand. Notice how a growing baby will always be a bit hungry as the breast cannot quite supply all the milk the baby demands, but the demand is met within a short time. For this reason, breast-fed babies, in general, weigh less than bottle-fed babies - that is the way nature intended for it to be. When the baby is weaned from human milk through the introduction of other food, the baby nurses less (new input signal). This results in the breast producing less milk (new output signal). When nursing stops completely, milk production stops completely.
c. Cardiac output (volume of blood the heart pumps per minute): at any given moment when you are not exercising, about 60% of your blood is in your veins. When you begin walking, running or engaging in some other exercise, the veins are signaled by the ANS to contract (vasoconstriction) and more blood is squeezed back to the heart (increased venous return) which stretches the heart muscle. The result of the stretching (input signal) is an increased force of contraction of the heart. This results in an increase in cardiac output (output signal). When you stop exercising, the veins receive less ANS signals and undergo vasodilation. So less blood returns to the heart - decreased venous return - (new input signal) - resulting in decreased cardiac output (new output signal). So you see that in both cases the output signal is in the same direction as the input signal.
d. Serious bleeding - This cardiac output mechanism in section c, working by itself, can result in shock after serious injury. If you sustain a major injury and are seriously bleeding, with each beat of the heart, less blood returns to the heart (input signal) because some of the blood actually leaves your body. The heart responds to reduced return of blood by pumping out less blood (the output signal). This leads to even less blood returning to the heart (new input signal) and the result is even less blood pumped out of the heart (new output signal). Eventually so little blood leaves the heart that it is not enough to maintain consciousness and the body lapses into a coma.
E. Endocrine glands in the head and their secretions
1. Pituitary gland
a. Location - this pea-sized gland is nestled in the sella turcica of the sphenoid bone and is connected to the hypothalamus by a stalk called the infundibulum. The pituitary consists of 2 parts: the adenohypophysis (anterior pituitary), and the neurohypophysis (posterior pituitary). The infundibulum conducts a portal vein from the hypothalamus to the adenohypophysis which permits hypothalamic influence on the adenohypophysis through hypothalamic hormones (see below) released into the blood. The infundibulum conducts axons from the hypothalamus to the neurohypophysis which permits hormones made in the hypothalamus to travel down the axons (axon flow) to the neurohypophysis to be stored there and secreted when required (see below).
(1) There are 6 adenohypophyseal hormones (hormones from the anterior pituitary):
(a) GH (growth hormone, somatotropin) - directly stimulates mitosis and protein synthesis in most body cells, especially bone, cartilage and muscle leading to growth in height in children, and thickening of bone and hypertrophy of skeletal muscles in exercising adults. It also stimulates fat breakdown from adipose tissue. GH also stimulates the liver to release somatomedins that do the same thing as GH only with a longer half-life. The major stimulus for the release of GH into the blood is vigorous exercise. Since baseline levels of the hormone drop with age, to prevent muscle atrophy and fat accumulation with age, one must exercise more. The typical American high carbohydrate fast-food diet suppresses GH secretion. This stimulates fat accumulation and muscular atrophy - the typical flabby-muscled, overweight, middle-aged adult (on a diet, only between meals). Higher protein diets supply amino acids which act as a stimulus for GH secretion (particularly the amino acid arginine).
(b) TSH (thyroid stimulating hormone, thyrotropin) - stimulates the thyroid gland to produce thyroid hormones that help to regulate the metabolic rate.
(c) ACTH (adrenocorticotropic hormone, corticotropin) - secreted in response to stress, ACTH stimulates the adrenal cortex to secrete a group of hormones called glucocorticoids, of which the major one is cortisol. In general, the glucocorticoids help the body resist and overcome stress.
(d) FSH (follicle stimulating hormone) - stimulates egg (follicle) development in ovaries, and stimulates sperm development in testes.
(e) LH (luteinizing hormone) - stimulates an egg to ovulate from the ovaries each month during a female's ovulatory-menstrual cycle. Stimulates the secretion of testosterone from the testes.
(f) Prolactin - its main effect is in females where it stimulates the breasts to produce milk after a pregnancy.
(2) There are 2 neurohypophyseal hormones (hormones from the posterior pituitary):
(a) ADH (anti-diuretic hormone) - secreted from the neurohypophysis due to a signal from the hypothalamus. The hypothalamus sends this signal when the body is dehydrated. The action of ADH is to stimulate water retention from fluid flowing in the kidneys. The body is trying to conserve its water content. The result is urine with minimal amounts of water - a concentrated urine often deeply colored and with an odor.
(b) Oxytocin - the main roles for oxytocin are in females: it stimulates smooth muscle contractions of the uterus during childbirth and smooth muscle contractions of the mammary glands expelling milk during nursing.
a. Location - the hypothalamus, part of the diencephalon, extends from the optic chiasma to the mammillary bodies and forms part of the walls of the 3rd ventricle. It is connected to the pituitary by the infundibulum.
b. Function - The hypothalamus sends 7 hormones by blood to the anterior pituitary, and 2 hormones by axonal flow to the posterior pituitary. It is clear that the CNS regulates many of the endocrine secretions, working through the hypothalamus.
(1) Hormones to the anterior pituitary - their names indicate their functions except for #7. These hormones reach the anterior pituitary through the hypothalmic-hypophyseal portal system (through the blood).
(a) GHRH (Growth hormone releasing hormone)
(b) TRH (Thyrotropin releasing hormone)
(c) CRH (Corticotropin, adrenocorticotropic hormone)
(d) GnRH (Gonadotropin releasing hormone)
(e) PRH (Prolactin releasing hormone)
(f) PIH (Prolactin inhibiting hormone)
(g) Somatostatin - inhibits GH and TSH secretion
(2) Hormones to the posterior pituitary. These hormones travel to the posterior pituitary through axon flow and are released from the pituitary upon nerve signal from neurons of the hypothalamus (the same neurons that send the hormones down their axons).
(a) ADH - anti-diuretic hormone - helps the body conserve its water (see above).
(b) Oxytocin - stimulates smooth muscle contractions in uterus and breasts (see above).
3. Pineal gland
a. Location - the pineal is easily observed in the sheep's brain by gently depressing the brainstem and cerebellum and observing the region inferior to the occipital lobes: the pineal is seen as a medial, pea-sized bulge just superior to the corpora quadrigemina. In humans, the pineal, once proposed to be the residence of the human soul, is just below the posterior portion of the corpus callosum, on the roof of the third ventricle, and not easily seen.
b. Function - The gland produces serotonin during the day and melatonin during the night. It is thought to be related to human biorhythms such as helping to determine the onset of puberty and also helping to regulate human cycles of sleep and wakefulness. It is largest in children and becomes smaller and more fibrous (or even calcified) in adults. Definitive, detailed knowledge of the gland's actions is still lacking.
F. Endocrine glands in the neck and their secretions
1. Thyroid gland
a. Location - this largest of endocrine glands consists of two large lobes connected by an isthmus and is located just above the soft spot in your neck (just anterior to the suprasternal notch). It is wrapped around the anterior and lateral portions of the trachea. b. Function - The thyroid's secretion of hormones is regulated by TSH from the pituitary gland. Since the pituitary secretion of TSH is regulated by TRH from the hypothalamus, physiologists speak of a hypothalamic-pituitary-thyroid axis. Three major hormones are secreted by this gland: T3, T4, and calcitonin .
(1) Tri-iodothyronine (T3) and thyroxine (T4) are mainly regulators of the metabolic rate of other tissues of the body.
(a) They enter the cytoplasm and nucleoplasm of target cells and increase the rate of mitochondrial oxidations, protein synthesis, and m-RNA production. These hormones increase the production of adrenal gland and pituitary hormones, raise blood pressure, respiratory rate and body heat production, stimulate bone teeth and nail growth in adults and during fetal development, and promotes fat breakdown for energy.
(b) T3 is the active form of the hormone intracellularly (T4 is converted to T3 in the cytoplasm of target cells).
(c) T4 feeds back to the pituitary and inhibits (slows down) TSH secretion. As T4 is broken down, TSH secretion increases. This is a negative feedback mechanism.
(2) Calcitonin - this hormone is most important for helping to regulate blood calcium levels when a woman is pregnant or lactating. During pregnancy and lactation, the reduction of blood calcium as it passes from the maternal blood into the fetus or the milk, results in parathyroid gland activity (see below) that increases blood calcium levels. Whenever calcium levels of the blood rise, calcitonin is then secreted by the thyroid: it activates osteoblasts resulting in an overall reduction of blood calcium levels as some calcium is deposited in the woman's bones as well as entering the milk and the fetus. This hormone is also important in controlling the blood calcium concentration of infants and children whose blood calcium levels are generally higher than those of adults due to the bone growth and remodeling. Calcitonin has been used therapeutically to help individuals with osteoporosis.
2. Parathyroid glands
a. Location - Although their number and location can vary, most people have 4 yellowish, rice-grain sized parathyroid glands located on the posterior surface of the thyroid gland. http://www.parathyroid.com/parathyroid.htm
b. Function - They secrete the hormone, parathormone, when the blood calcium level drops. Parathormone has the following effects, all of which attempt to raise the blood calcium level back to the normal range (a negative feedback mechanism):
(1) Stimulates osteoclast activity which releases calcium into the blood from osseus tissue.
(2) Inhibits secretion of calcium into the urine by the kidneys thus maintaining the calcium level of the blood.
(3) Stimulates active vitamin D (calcitriol) formation in the kidneys. Calcitriol then increases calcium, phosphate and magnesium absorption from digested food in the intestine.
F. Endocrine glands in the thoracic cavity and their secretions
1. Thymus gland
a. Location - The thymus is easily observed in the mediastinum of a pig or cat as a large cap of tissue covering the anterior surface of the heart (cranial portion of the heart). In humans the thymus is large in infants, gets larger during childhood and covers the superior and anterior portion of the heart. It is much smaller in adults and is located superior to the heart and medial to the upper (superior) lung lobes. It becomes very small and fibrous in old age.
b. Function - The thymus is central to the operation of the immune system: T-lymphocytes (T-cells) develop and mature in the thymus; different classes of T-cells are responsible for cellular immunity (the other type of immunity is humoral immunity and is carried out by proteins called antibodies). The thymus produces hormones (thymosin, thymulin, thymopoietin) that stimulate other parts of the immune system (e.g., lymph nodes) to function.
2. Heart - although not generally considered part of the endocrine system, the heart's upper chambers, the atria, when stretched, produce a hormone, atrial natriuretic peptide (ANP), that results in blood pressure decrease. ANP causes vasodilation (reducing blood pressure) and increased kidney excretion of sodium (which causes increased body water loss by osmosis that reduces blood pressure). ANP can be thought of as opposing the actions of both aldosterone (helps the body retain sodium) and angiotensin II (a vasoconstrictor).
G. Endocrine glands in the abdominopelvic cavity and their secretions.
a. Location - Technically the abdominal cavity is enclosed by the peritoneum. Since the pancreas is located behind the parietal portion of the peritoneum, its location is described as retroperitoneal. It is an elongated organ consisting of a head, neck, body and tail. The head is located next to the first part of the small intestine, the duodenum; the body passes anterior to the left kidney with the tail located next to the inferior portion of the spleen.
b. Function - About 99% of the mass of the pancreas is concerned with exocrine secretion: production of pancreatic enzymes for digestion of food. Pancreatic juice empties into the duodenum through the pancreatic duct. Pancreatic endocrine secretions come from 1 -2 million islets of Langerhans: groupings of cells scattered throughout the pancreas that secrete 3 hormones: insulin, glucagon and somatostatin.
(1) Insulin is produced from islet beta cells: it is the only hormone that lowers blood glucose concentrations and is secreted after meals when the blood glucose level rises. Insulin stimulates target cells to produce the receptors that bind and transport glucose into cells, thus lowering the blood glucose levels. Insulin also stimulates adipocytes to absorb and store fat, muscle fibers to absorb and utilize amino acids, and the liver to synthesize both glycogen and triglycerides.
(2) Glucagon is produced from islet alpha cells: it raises blood glucose concentrations as would be required when we fast between meals: glucagon stimulates the liver's breakdown of stored glycogen into glucose, a process called glycogenolysis. Glucagon also stimulates the liver to convert amino acids into glucose, a process called gluconeogenesis which also promotes a rise in blood glucose.
(3) Somatostatin is produced from the islet delta cells and has the ability to locally inhibit secretion of both insulin and glucagon and to inhibit overall digestive activity whenever blood glucose and amino levels are high.
2. Adrenal glands (suprarenal glands). Due to the interaction of the hypothalamus and the pituitary with the adrenal gland, endocrinologists speak of a hypothalmic-pituitary-adrenal axis.
a. Location - like the pancreas, the kidneys are also located behind the parietal peritoneum and are called retroperitoneal. The adrenal glands are positioned like caps on the superior surface of each kidney.
b. Function - the adrenal gland consists of an outer cortex and an inner medulla.
The cortex secretes three classes of hormones (mineralocorticoids, glucocorticoids, and sex hormones); the medulla secretes mainly 2 hormones - epinephrine and norepinephrine..
(1) Adrenal cortex - from superficial to deep regions, the adrenal cortex is divided into a zona glomerulosa, zona fasciculata, and a zona reticularis.
(a) Zona glomerulosa - secretes mainly mineralocorticoids, the main one being aldosterone. Aldosterone's function is to conserve body salt (sodium chloride) and water, and to excrete potassium.
(b) Zona fasciculata - secretes mainly glucocorticoids of which the main ones are cortisol and corticosterone. The glucocorticoids have two main functions: to reduce the body's inflammatory responses, and to promote gluconeogenesis, meaning that proteins and fats are stimulated to break down: the proteins are hydrolyzed into amino acids that are converted to glucose in the liver; the fats are converted to fatty acids. These actions give the body fuel - glucose and fatty acids - to combat stress by having energy sources readily available in the blood.
(c) Zona reticularis - secretes mainly sex hormones - androgens (sometimes called 17-ketosteroids) and estrogens. The adrenal's output of these sex hormones is not as great as that from the testes and ovaries in youth and middle age. However, at older ages, the adrenals become important sources of sex hormones that maintain energy levels and the sex drive.
(2) Adrenal medulla - The cells of the adrenal medulla are actually modified postganglionic neurons of the sympathetic nervous system that secrete epinephrine and norepinephrine in approximately a 75%:25% proportion when stimulated by sympathetic preganglionic neurons of the autonomic nervous system (ANS). The effect of epinephrine and norepinephrine is to prepare the body for "fight or flight" (see ANS section).
3. Gonads - ovaries and testes. Due to the interaction of the hypothalamus and the pituitary with the gonads, endocrinologists speak of a hypothalmic-pituitary-gonadal axis.
(1) Location - the ovaries, the female sex glands, are located in the pelvic cavity, being suspended and anchored in place by several ligaments - the ovarian ligament attaching to the uterus, the suspensory ligament attaching to the pelvic wall, and a broad ligament attaching to the uterus and uterine tubes.
(2) Function - The function of the ovaries is to ovulate eggs and to produce female sex hormones. The ovaries possess an outer cortex containing the germinal epithelium that produces eggs and female sex hormones. The inner medulla contains blood vessels and nerves.
(a) Eggs - Eggs, all of which were produced during the embryonic period, are enclosed in follicles consisting of the egg cell immersed in fluid surrounded by follicle cells arranged as a squamous epithelium. Hormonal signals from the pituitary gland and the ovary itself result in the maturation of a single follicle each month and the discharge of its egg from the surface of the ovary - a process called ovulation. If the egg is fertilized and pregnancy is successful, a baby will be born an average of 266 days (38 weeks) later, a little longer than 9 months. Other details of the maturation and ovulation of eggs are the subject matter of a section dealing with the reproductive system.
(b) Female sex hormones - estrogens are female sex hormones and there are three of them - estradiol (the most abundant), estriol, and estrone. They are responsible for female secondary sexual characteristics including maturation of the external genitalia and breasts, patterns of fat deposition under the skin, and patterns of brain development. Progesterone is another female sex hormone that prepares the uterine inner lining (the endometrium) and the breasts for pregnancy by stimulating the secretory capacity of these organs. Estrogens and progesterone are produced during a monthly cycle known as the ovulatory-menstrual cycle. This cycle is influenced by releasing hormones from the hypothalamus. Details of these hormonal interactions are provided elsewhere.
(1) Location - the testes are suspended outside the body in the scrotal sacs where the temperature is about 3° below the normal body temperature, which is optimal for sperm development.
(2) Function - The function of the testes is to form and mature sperm and to produce male sex hormones, mainly testosterone. The testes are divided by connective tissue partitions into about 300 lobules containing the seminiferous tubules whose walls consist of a germinal epithelium where sperm cells are formed. Male sex hormones are produced by interstitial cells of Leydig located between the seminiferous tubules. Testosterone and other androgens are responsible for male secondary sexual characteristics including maturation of the external genitalia, the development of the musculature, and patterns of brain development. Further details about male reproductive function are presented elsewhere.
4. Other abdominopelvic organs that have endocrine functions.
a. Liver - the liver produces several hormones and hormone precursors:
(1) Somatomedins - these are hormones whose synthesis is induced by GH and which stimulate growth throughout the body in a manner like GH itself. The main somatomedin is IGF-1 (insulin-like growth factor).
(2) Erythropoietin (EPO) is a hormone produced by both the liver and kidneys that stimulates erythrocyte production in the bone marrow.
(3) The liver produces an intermediate (called calcidiol) a precursor in the pathway to the production of active vitamin D.
(4) Angiotensinogen is a hormone precursor to the ultimate formation of angiotensin II, an important vasoconstrictor that raises blood pressure.
b. Kidneys - the kidneys produce erythropoietin, along with the liver, that stimulates erythrocyte formation, and calcitriol, the active form of vitamin D that stimulates the absorption of calcium, phosphorus and magnesium from the intestine and which inhibits the body's excretion of calcium through the kidneys - both actions making calcium more available for bone formation.
c. Organs of digestion - the stomach and small intestine produce many hormones that regulate digestive processes: e.g., the stomach produces gastrin, that stimulates the stomach's acid production, the duodenum produces secretin, that stimulates pancreatic secretion of sodium bicarbonate into the duodenum to neutralize gastric acid arriving mixed with food, and the duodenum also produces cholecystokinin (CCK), that stimulates the secretion of bile and pancreatic enzymes in response to the arrival of food from the stomach.
Biomedical Terminology: Define each term:
atrial natriuretic peptide
interstitial cells of Leydig
Endocrine System Problems
1. Choose one of the problems described below.
2. Prepare your solution as a word document.
3. Send it to your professor as an email attachment. You will receive an email response.
Problem #1: A nineteen year old college student regularly experiences anxiety, abdominal bloating, craving for sugary foods, and mild depression during the 2 weeks prior to menstruation. She also regularly experiences cramping during the first 2 days of menstruation. Her doctor recommends hormone pills to relieve the pre-menstrual symptoms as well as pain medication for the menstrual cramping. Utilize the Internet to research the pros and cons of hormonal therapy for PMS (pre-menstrual syndrome) and alternatives for such hormonal therapy.
Your report should include
1. A definition of PMS (sometimes referred to as PMT - pre-menstrual tension) and a description of its effects on the body.
2. A proposed physiological explanation of why these symptoms are occurring.
3. The benefits and side effects of taking hormones to relieve PMS symptoms.
4. The potential long-term side-effects of hormonal therapy.
5. A description of nutritional/herbal treatments for PMS and their side effects.
6. A decision, based on your reserach, of which therapy(ies) for PMS might be best to try.
Problem #2: A 45 yeaar old female, 5'4" and 220 lbs., experiencing low energy levels, periods of dizziness, and knee pain, decides to see her doctor. Examination reveals hypertension, elevated blood sugar, elevated total cholesterol levels, elevated triglycerides, and elevated blood insulin levels. Her doctor's diagnosis is diabetes mellitus Type II and she is initially placed on "oral insulin" and after a year with marginal sign and symptom relief, she begins injecting insulin. She receives some benefit but indicates she does not feel a sense of well-being. Utilize the Internet to answer the following questions:
1. What are the possible causes of type II diabetes mellitus?
2. How is diabetes type II different from diabetes type I?
3. What are the long-term consequences to health from diabetes mellitus? What is the underlying physiological reason for these consequences?
4. What is the medical/physiological reason for giving oral insulin and then injectable insulin to a type II diabetic who already has an abundance of insulin in their blood?
5. Are there any dietary (nutritional) measures that might help this situation? Are they practical- can they be accomplished?
6. If you were the son or daughter of the individual in this case, what treatment would you suggest to your parent for relief from the symptoms of this condition?