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The Endocrine System - Questions and Answers
1. What is the difference between the endocrine gland and the exocrine gland?
Endocrine gland is a gland whose secretions (called hormones) are collected by the blood and reach the tissues through the circulation. The hypophysis (pituitary) and the adrenals are examples of endocrine glands. Exocrine gland is a gland whose secretions are released externally through ducts (into the skin, intestinal lumen, mouth, etc.). The sebaceous glands and the salivary glands are examples of exocrine glands.
The Endocrine System - Image Diversity: exocrine glands
2. What is the constitution of the endocrine system?
The endocrine system is constituted by the endocrine glands and the hormones they secrete.
The Endocrine System - Image Diversity: endocrine glands
3. What is the histological nature of the glands? How are they formed?
The glands are epithelial tissues. They are made of epithelium that during the embryonic development invaginated into other tissues.
In the exocrine glands the invagination has preserved secretion ducts. In the endocrine glands the invagination is complete and there are no secretion ducts.
The Endocrine System - Image Diversity: gland formation
4. Why is the endocrine system considered one of the integrative systems of the body? What is the other physiological system that also has this function?
The endocrine system is said to have integrative character since the hormones produced by the endocrine glands are substances that act at a distance and many of them act in different organs of the body. So the endocrine glands receive information from some regions of the body and they can produce effects in other regions providing functional integration for the body.
Besides the endocrine system, the other physiological system that also has integrative function is the nervous system. The nervous system integrates the body through a network of nerves connected to central and peripheral neurons. The endocrine system integrates the body through hormones that travel through the circulation and are produced by the endocrine glands.
5. What are hormones?
Hormones are substances secreted by the endocrine glands and collected by the circulation that act to produce effects upon specific organs and tissues.
Hormones are effectors of the endocrine system.
6. What are target organs of the hormones?
Target organs, target tissues and target cells are those specific organs, tissues and cells upon which each hormone acts and produces its effects. Hormones selectively act upon their targets due to specific receptor proteins present in these targets.
7. How does the circulatory system participate in the functioning of the endocrine system?
The circulatory system is fundamental for the functioning of the endocrine system. The blood collects the hormones made by the endocrine glands and through the circulation these hormones reach their targets. Without the circulatory system the 'action at distance' characteristic of the endocrine system would not be possible.
8. Are hormones only proteins?
Some hormones are proteins, like insulin, glucagon and ADH, others are derived from proteins (modified amino acids), like adrenaline and noradrenaline, other are steroids, like the corticosteroids and estrogen.
9. What are the main endocrine glands of the human body?
The main endocrine glands of the human body are the pineal gland (or pineal body), the hypophysis (or pituitary), the thyroid, the parathyroids, the endocrine part of the pancreas, the adrenals and the gonads (testicles or ovaries).
Other organs like the kidneys, the heart and the placenta also have endocrine functions.
10. What is the pineal gland?
The pineal gland, also known as pineal body or epiphysis, is situated centrally in the head. It secretes the hormone melatonin, a hormone produced at night and related to the regulation of the circadian rhythm (or circadian cycle, the wakefulness-sleep cycle). Melatonin possibly regulates many body functions related to the night-day alternation.
The Endocrine System - Image Diversity: pineal gland
11. What is the osseous cavity where the pituitary gland is located?
The pituitary gland, or hypophysis, is located in the sella turcica of the sphenoid bone (one of the bones in the base of the skull). So the gland is situated within the head.
The Endocrine System - Image Diversity: hypophysis
12. What are the main divisions of the hypophysis? What are their functions?
The hypophysis is divided into two portions: the adenohypophysis, or anterior hypophysis, and the neurohypophysis, or posterior hypophysis.
In the adenohypophysis two hormones that act directly, the growth hormone (GH) and the prolactin, and four tropic hormones, i.e., hormones that regulate other endocrine glands, the adrenocorticotropic hormone (ACTH), the thyroid-stimulating hormone (TSH), the luteinizing hormone (LH) and the follicle-stimulating hormone (FSH) are produced.
The neurohypohysis stores and releases two hormones produced in the hypothalamus, oxytocin and the antidiuretic hormone (ADH, or vasopressin).
13. What is the relation between the hypothalamus and the hypophysis?
The hypothalamus is a part of the brain situated just above the hypophysis. The hypothalamus gets peripheral and central neural impulses that trigger response of its neurosecretory cells. The axons of these cells go down to the adenohypophysis to regulate the hipophyseal secretions by means of negative feedback. When the plasma levels of adenohypophyseal hormones are too high the hypothalamus detects this information and commands the interruption of the production of the hormone. When the blood level of an adenohypophyseal hormone is low the hypothalamus stimulates the secretion of the hormone.
The hypothalamic cells produce the hormones released by the neurohypophysis. These hormones are transported by their axons to the hypophysis and then released in the circulation.
The Endocrine System - Image Diversity: hypothalamus
14. What are the hormones secreted by the adenohypophysis? What are their respective functions?
The adenohypophisys secretes GH (growth hormone), prolactin, ACTH (adrenocorticotropic hormone), TSH (thyroid-stimulating hormone), FSH (follicle-stimulating hormone) and LH (luteinizing hormone).
GH, also known as somatotropic hormone (STH), acts upon bones, cartilages and muscles promoting the growth of these tissues. Prolactin is the hormone that in women stimulates the production and secretion of milk by the mammary glands. The ACTH is the hormone that stimulates the cortical portion of the adrenal gland to produce and secrete the cortical hormones (glucocorticoids). The TSH is the hormone that stimulates the activity of the thyroid gland increasing the production and secretion of its hormones T3 and T4. The FSH is a gonadotropic hormone, i.e., it stimulates the gonads and in women it acts upon the ovaries inducing the growth of follicles, in men it stimulates spermatogenesis. The LH is also a gonadotropic hormone that acts upon the ovaries of women to stimulate ovulation and the formation of the corpus luteum (that secretes estrogen) and in men upon the testicles to stimulate the production of testosterone.
15. What is the relation between the thyroid and the hypophysis?
The hypophysis secretes TSH, thyroid-stimulating hormone. This hormone hastens the secretion of thyroid hormones (triiodothyronine and thyroxine, or T3 and T4).
When the plasma concentration of thyroid hormones is high this information is detected by the hypothalamus and by the hypophysis and this gland reduces the TSH secretion. When the thyroid hormone levels are low the TSH secretion increases. It is thus a negative feedback.
Injuries of the hypophysis that cause TSH hyposecretion (for example, in case of tissue destruction) or hypersecretion (in case, e,g., of excessive cell proliferation or cancer) can change the functioning of the thyroid gland completely.
16. What are some diseases caused by abnormal GH secretion by the hypophysis?
In childhood deficient GH secretion may lead to delayed growth and in severe cases to nanism (dwarfism). Excessive production of GH in children may cause exaggerated osseous growth and gigantism. In adults GH excess (for example, in hypophiseal cancer or in people that wrongly ingest GH as a nutritional supplement) may lead to acromegaly, excessive and disproportional growth of the bone extremities, like the skull, the maxillaries, the hands and the feet.
17. What are the target tissues and target organs of each adenohypophyseal hormone?
GH: bones, cartilages and muscles. Prolactin: mammary glands. ACTH: the cortical portion of the adrenals. TSH: thyroid gland. FSH and LH: ovaries and testicles.
18. What are the hormones secreted by the neurohypophysis? What are their respective functions?
The neurohypophysis secretes oxytocin and the antidiuretic hormone (ADH).
Oxytocin is secreted in women during delivery to increase the strength and frequency of the uterine contractions and thus to help the baby’s birth. During the lactation period the infant’s sucking action on the mother’s nipples stimulates the production of oxytocin that then increases the secretion of milk by the mammary glands.
Vasopressin, or ADH, participates in the water regulation of the body and thus in the control of the blood pressure since it allows the resorption of free water through the renal tubules. As water goes back to the circulation the blood volume increases.
19. What is the difference between diabetes mellitus and diabetes insipidus? What are the characteristic signs of diabetes insipidus?
Diabetes mellitus is the disease caused by deficient insulin secretion by the pancreas or by impaired capturing of this hormone by the cells. Diabetes insipidus is the disease caused by deficient ADH secretion by the pituitary (hypophysis) or also by impaired sensitivity of the kidneys to this hormone.
In diabetes insipidus the blood lacks ADH and so tubular resorption of water in the kidneys is reduced and a great volume of urine is produced. The patient urinates a lot and many times a day, a sign also accompanied by polydipsia (increased thirst and exaggerated ingestion of water) and sometimes by dehydration.
20. Why does the urinary volume increase when alcoholic beverages are ingested?
Alcohol inhibits the ADH (antidiuretic hormone) secretion by the hypophysis. Low ADH reduces the tubular resorption of water in the kidneys and thus the urinary volume increases.
21. Which are the target organs and target tissues of the neurohypophysis?
The target organs of oxytocin are the uterus and the mammary glands. The target organs of ADH are the kidneys.
22. Where in the body is the thyroid gland located?
The thyroid is located in the anterior cervical region (frontal neck), in front of the trachea and just below the larynx. It is a bilobated mass below the Adam’s apple.
The Endocrine System - Image Diversity: thyroid
23. What are the hormones secreted by the thyroid gland? What are their functions?
The thyroid secretes the hormones thyroxine (T4), triiodothyronine (T3) and calcitonin.
T3 and T4 are iodinated substances derived from the amino acid tyrosine. They act to increase the cellular metabolic rate of the body (cellular respiration, metabolism of proteins and lipids, etc.). Calcitonin inhibits the release of calcium cations by the bones thus controlling the blood level of calcium.
The Endocrine System - Image Diversity: thyroxine molecule
24. Why is the dietary obtainment of iodine so important for thyroid functioning?
The obtainment of iodine from the diet is important for the thyroid because this chemical element is necessary for the synthesis of the thyroid hormones T3 and T4. The iodine supply often comes from the diet.
25. What is a goiter? What is endemic goiter? How is this problem socially solved?
Goiter is the abnormal enlargement of the thyroid gland. The goiter appears as a tumor in the anterior neck and it may be visible or sometimes not visible but palpable. Goiter can occur in hypothyroidism or in hyperthyroidism.
Endemic goiter is the goiter caused by deficient iodine ingestion (deficiency of iodine in the diet). The endemic character of the disease is explained because the iodine content of the diet is often a social or cultural condition affecting many people of some geographical regions. The hypothyroidism caused by deficient iodine ingestion is more frequent in regions far from the sea coast (since sea food is rich in iodine).
Nowadays the problem is often solved by obligatory addition of iodine in table salt. As table salt is a widely used condiment the supply of iodine in diet is almost assured by this method.
The Endocrine System - Image Diversity: goiter
26. What happens to the TSH (thyroid-stimulating hormone) blood level in hypothyroidism? Why is there enlargement of the thyroid in the endemic goiter disease?
When there is low T3 and T4 secretion by the thyroid the TSH secretion by the hypophysis is very stimulated and the TSH blood level increases. The increase in the TSH availability promotes the enlargement of the thyroid gland.
The thyroid enlargement is a reaction of a tissue that tries to compensate the functional deficiency by making the gland increase its size.
27. What are some signs and symptoms found in patients with hyperthyroidism?
The hormones made by the thyroid gland stimulate the basal metabolism of the body. In hyperthyroidism there is abnormally high production and secretion of T3 and T4 so the basal metabolic rate is increased. The signs of this condition may be tachycardia (abnormally high heart rate), weight loss, excessive heat sensation, excessive sweating, anxiety, etc. One of the typical signs of hyperthyroidism is exophthalmos (protrusion of the eyeballs). Generally the patient also presents goiter.
The Endocrine System - Image Diversity: exophthalmos
28. What are some signs and symptoms found in patients with hypothyroidism?
In hypothyroidism the production and secretion of T3 and T4 are impaired. Since these thyroid hormones stimulate the basal metabolism of the body (cellular respiration, fat acid and protein metabolism, etc.) the patient with hypothyroidism may present bradycardia (low heart rate), low respiratory rate, excessive tiredness, depression, cold intolerance and weight gain. Hypothyroidism is normally accompanied by goiter (enlargement of the thyroid in the neck).
29. What is the physiological cause of the syndrome known as cretinism?
Cretinism is caused by chronic deficiency of the thyroid hormones (T3 and T4) during childhood. The chronic hypothyroidism during childhood may cause retardation and low stature due to the low basal metabolic rate in a period of life when growth and development of mind faculties occur.
30. What are the parathyroids? Where are they located and what are the hormones secreted by these glands?
The parathyroids are four small glands embedded two in each posterior face of one thyroid lobe. The parathyroids secrete parathormone, a hormone that together with calcitonin and vitamin D regulates the calcium blood level.
The Endocrine System - Image Diversity: parathyroids
31. What is the relation between secretion of parathormone and the calcium blood level?
The parathormone increases the calcium blood level since it stimulates the resorption (remodelation) of the osseous tissue. When osteoclasts remodel bones calcium is released in the circulation.
Parathormone also acts increasing the calcium absorption in the intestines by vitamin D activation. It acts in the kidneys promoting tubular calcium resorption too.
32. What is a mixed gland? Why is the pancreas considered a mixed gland?
Mixed gland is a gland that produces endocrine and exocrine secretions.
The pancreas is an example of a mixed gland because it secretes hormones in the circulation, like insulin and glucagon, but it also releases an exocrine secretion, the pancreatic juice.
The Endocrine System - Image Diversity: endocrine pancreas
33. What are the pancreatic tissues involved respectively in the exocrine and endocrine secretions? What are their respective hormones and enzymes?
The exocrine secretion of the pancreas is produced in the pancreatic acini, aggregates of secretory cells that surround small exocrine ducts. The exocrine pancreas secretes digestive enzymes of the pancreatic juice: amylase, lipase, trypsin, chymotrypsin, carboxypeptidase, ribonuclease, deoxyribonuclease, elastase and gelatinase.
The endocrine secretion of the pancreas is produced and secreted by small groups of cells dispersed throughout the organ called islets of Langerhans. The pancreatic islets make insulin, glucagon and somatostatin.
The Endocrine System - Image Diversity: islets of Langerhans
34. What is the importance of the glucose blood level for human health?
The glucose blood level (glycemia) must be kept normal. If it is abnormally low there is not enough glucose to supply the energetic metabolism of the cells. If it is abnormally and chronically high it causes severe harm to the peripheral nerves, the skin, the retina, the kidneys and other important organs and it may predispose to cardiovascular diseases (acute myocardial infarction, strokes, thrombosis, etc). If it is acutely too high medical emergencies like diabetic ketoacidosis and the hyperglycemic hyperosmolar state may occur.
35. What are the functions of insulin and glucagon for the blood glucose control?
Glucagon increases glycemia and insulin reduces it. They are antagonistic pancreatic hormones. Glucagon acts stimulating glycogenolysis and thus forming glucose from glycogen breaking. Insulin is the hormone responsible for the entrance of glucose from the blood into the cells.
When glycemia is low, for example, during fasting, glucagon is secreted and insulin is inhibited. When glycemia is high, as after meals, there are inhibition of glucagon and more secretion of insulin.
The Endocrine System - Image Diversity: insulin molecule glucose uptake
36. What are the target organs upon which insulin and glucagon act?
Glucagon mainly acts upon the liver. Insulin acts in general upon all cells. Both also act upon the adipose tissue respectively stimulating (glucagon) and inhibiting (insulin) the use of fatty acids in the energetic metabolism (an alternate path of the energetic metabolism is activated when there is shortage of glucose).
37. What are the effects of somatostatin for the pancreatic hormonal secretion?
Somatostatin inhibits both insulin and glucagon secretions.
38. What is diabetes mellitus?
Diabetes mellitus is the disease caused by deficient production or action of insulin and the consequent low glucose uptake by the cells and high blood glucose level.
39. What are the three main signs of diabetes?
The three main signs of diabetes mellitus are known as the diabetic triad: polyuria, polydipsia and polyphagia.
Polyuria is the excessive elimination of urine; in diabetes it is caused by reduced water resorption in the renal tubules due to increased osmolarity of the glomerular filtrate (caused by excessive glucose). Polydipsia is the exaggerated ingestion of water; the thirst is due to the excessive water loss in the urine. Polyphagia is the exaggerated ingestion of food caused by deficiency in energy generation by glucose-lacking cells.
40. Why do diabetic patients often undergo dietary sugar restriction? What are the main complications of diabetes mellitus?
Diabetic patients are often advised to ingest less carbohydrates since these substances are degraded into glucose and this molecule is absorbed in the intestines. The dietary sugar restriction goal is to control glycemia to maintain it at normal levels.
The main complications of diabetes are tissue injuries that occur in vaious organs caused by the chronic increased blood osmolarity: in the peripheral nerves (diabetic neuropathy), resulting in sensitivity loss, increased wounds (the person does not feel that the tissue is being wounded and the wound expands) and muscle fatigue; in the kidneys (diabetic nephropathy), causing glomerular lesions that may lead to renal failure; in the retina (diabetic retinopathy), leading to vision impairment and blindness; in the skin, as a consequence of the neuropathy. Diabetes mellitus also is one of the major risk factors for cardiovascular diseases like embolism, myocardial infarction and stroke.
41. What is the difference between type I diabetes mellitus and type II diabetes mellitus?
Type I diabetes, also known as juvenile diabetes, or insulin-dependent diabetes (this name is not adequate as type II diabetes may become insulin-dependent), is the impaired production of insulin by the pancreas believed to be caused by destruction of cells of the islets of Langerhans by autoantibodies (autoimmunity).
Type II diabetes occurs in the adult individual and it is often diagnosed in people of more advanced age. In type II diabetes there is normal or low secretion of insulin by the pancreas but the main cause of the high glycemia is the peripheral resistance of the cells to the action of the hormone.
42. In ancient Greece the father of Medicine, Hypocrates, described a method of diagnosing diabetes mellitus by tasting the patient's urine. What is the physiological explanation for this archaic method?
Under normal conditions the glucose filtered by the renal glomeruli is almost entirely resorbed in the nephron tubules and not excreted in urine. With the elevated glucose blood level the renal tubules cannot resorb all the filtered glucose and some amount of the substance appears in the urine. This amount is enough to provide the sweet taste that helped Hypocrates to diagnose diabetes and to differentiate it from other diseases accompanied by polyuria. Nowadays the method is inconceivable due to the danger of contamination of the tester by disease agents possibly present in the patient's urine.
The Endocrine System - Image Diversity: Hypocrates
43. What are the main treatments of diabetes mellitus?
The general goal of the diabetes treatment is to maintain normal glycemic levels.
Type I diabetes is treated with parenteral administration of insulin. Insulin must be administered intravenously or intramuscularly because as a protein it would be digested if ingested orally. In type II diabetes treatment is done with oral drugs that regulate the glucose metabolism or in more severe cases with parenteral administration of insulin. The moderation of carbohydrate ingestion is an important aid to diabetes treatment.
The diabetes treatment with the use of hypoglycemic agents, like insulin or oral medicines, must be carefully and medically supervised since if wrongly used these drugs may abruptly decrease the glucose blood level, cause hypoglycemia and even death.
Many other forms of diabetes treatment are under research worldwide.
44. How can bacteria produce human insulin on an industrial scale? What are the other forms of insulin made available by the pharmaceutical industry?
Bacteria do not naturally synthesize insulin. It is possible however to implant human genetic material containing the insulin gene into the bacterial DNA. The mutant bacteria then multiply and produce human insulin. The insulin is isolated and purified for later commercialization. This biotechnology is known as the recombinant DNA technology.
Besides human insulin the pharmaceutical industry also produces insulin to be used by humans made from the pancreas of pigs and cows.
45. Where are the adrenal glands located? How many are they and what are their portions?
Each adrenal gland is located on the top of each kidney (forming a hat-like structure for the kidneys), so there are two glands. The adrenal parenchymal structure is divided into two portions: the most peripheral is the cortical portion, or adrenal cortex, and the central is the medullary portion, or adrenal medulla.
The Endocrine System - Image Diversity: adrenals
46. What are the hormones secreted by the adrenal medulla? What are their respective functions?
The medullary portion of the adrenals secretes hormones of the catecholamine group: adrenaline (also known as epinephrine) and noradrenaline (also known as norepinephrine). Besides their hormonal function, adrenaline and noradrenaline act as neurotransmitters too. The neurons that use them as neurotransmitters are called adrenergic neurons.
Adrenaline increases the glycogen breaking into glucose (glycogenolysis) thus increasing glycemia and the basal metabolic rate of the body. Adrenaline and noradrenaline are released during situations of danger (fightfight or flight response) and they intensify the strength and rate of the heartbeat and selectively modulate the blood irrigation in some tissues by selective vasodilation and selective vasoconstriction. By vasodilation they increase the blood supply to the brain, the muscles and the heart and by vasoconstriction they reduce the blood supply to the kidneys, the skin and the gastrointestinal tract.
Substances like adrenaline and noradrenaline that promote vasodilation or vasoconstriction are called vasoactive substances.
47. What are the hormones secreted by the adrenal cortex? What are their respective functions?
The cortical portion of the adrenals secretes hormones of the corticoid (or corticosteroid) group, derived from cholesterol: glucocorticoid, mineralocorticoids and cortical sex hormones.
The glucocorticoids are cortisol and cortisone. The glucocorticoids stimulate the formation of glucose from the degradation of proteins of the muscle tissue (gluconeogenesis) and so they help to increase glycemia. These hormones play an important immunosuppressant role, i.e., they reduce the action of the immune system and for this reason they are used as medicine to treat inflammatory and autoimmune diseases and rejection of transplanted organs.
The mineralocorticoids aldosterone and deoxycorticosterone regulate the sodium and potassium blood concentration and thus they control the water level of the extracellular space. Aldosterone increases the sodium resorption and thus the water resorption in the renal tubules and it also stimulates the renal excretion of potassium and hydrogen.
The adrenal cortical sex hormones are androgens, male sex hormones present in men and women. In men their main site of production is the testicle and they promote the appearance of secondary male sex characteristics, like body hair and beard, deep voice, the male pattern of fat distribution and maturation of the genitalia. If abnormally high in women they cause inhibited maturation of the female genitalia and disturbances of the menstrual cycle.
The Endocrine System - Image Diversity: glucocorticoid molecules
48. Why are glucorticoids used in transplant patients?
Patients with transplanted organs are prone to host versus graft rejection since their own immune system tends to attack the grafted organ because of recognition of the grafted tissue as foreign matter. In the prevention and treatment of this common problem patients are given glucorticoids or other immunosupressants. Glucocorticoids have an immunosuppressant action and so they reduce the aggression of the immune system against the graft.
The immune action however is also very important for the individual. The immune system defends the body against invasion and infection by pathogenic agents (virus, bacteria, toxins) besides being fundamental for the elimination of modified cells that may proliferate and cause cancer. Patients receiving immunosuppressants like glucocorticoids are thus under increased risk of infectious and neoplastic diseases.
49. What are the hormones produced by the testicles and the ovaries?
The testicles make androgenic hormones, the main of them being testosterone. The ovaries produce estrogen and progesterone.
The Endocrine System - Image Diversity: testicles and ovaries
50. What is the endocrine function of the placenta?
The placenta is not a permanent gland of the endocrine system but it also has endocrinal function. The placenta produces estrogen and progesterone. It also secretes human chorionic gonadotropin (HCG, that acts similarly to the hypophyseal LH), human placental lactogen, similar to prolactin and stimulant of the mammary glands, and a series of hormonal peptides similar to the hormones of the hypothalamus-hypophysis axis.
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