Study the Endocrine System, its Organs and its Functions

2. What are the components of the endocrine system?

The endocrine system is composed of the endocrine glands and the hormones they secrete.

• The Endocrine System Review - Image Diversity: endocrine glands

3. What is the histological nature of glands? How are they formed?

Glands are epithelial tissue. They are made of epithelium that during the embryonic development invaginated into other tissues during embryonic development..

In exocrine glands, the invagination contains preserved secretion ducts. In endocrine glands, the invagination is complete and there are no secretion ducts.

• The Endocrine System Review - Image Diversity: gland formation

4. Why is the endocrine system considered one of the integrative systems of the body? What other physiological system also has this function?

The endocrine system is considered to be of an integrative nature, since the hormones produced by endocrine glands are substances that act at a distance and many of them act in different organs of the body. therefore, endocrine glands receive information from certain regions of the body and can produce effects in other regions, providing functional integration for the body.

In addition to 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 circulation.

5. What are hormones?

Hormones are substances secreted by endocrine glands and collected by circulation. They produce effects on specific organs and tissues.

Hormones are the effectors of the endocrine system.

6. What are the target organs of hormones?

Target organs, target tissues and target cells are the specific organs, tissues and cells on which each hormone acts and produces its effects. Hormones selectively act on their targets due to the specific receptor proteins present in these targets.

7. How does the circulatory system participate in the function of the endocrine system?

The circulatory system is fundamental for the functioning of the endocrine system. Blood collects hormones produced by endocrine glands and these hormones reach their targets through circulation. Without the circulatory system, the "action at distance" feature of the endocrine system would not be possible.

8. Are hormones only proteins?

Some hormones are proteins, such as insulin, glucagon and ADH, others are derived from proteins (modified amino acids), such as adrenaline and noradrenaline.  Others are steroids, such as 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 gland), the thyroid, the parathyroids, the endocrine part of the pancreas, the adrenal glands and the gonads (the testicles or ovaries).

Other organs such as the kidneys, the heart and the placenta also play a role in the endocrine system.

10. What is the pineal gland?

The pineal gland, also known as the pineal body or epiphysis, is located in the center of the head. It secretes the hormone melatonin, a hormone produced at night and related to the regulation of circadian rhythm (or the circadian cycle, the wakefulness-sleep cycle). Melatonin may also regulate many body functions related to the night-day cycle.

• The Endocrine System Review - Image Diversity: pineal gland

11. In which bone cavity is the pituitary gland located?

The pituitary gland, or hypophysis, is located in the sella turcica of the sphenoid bone (one of the bones at the base of the skull). Therefore, this gland is located within the head.

• The Endocrine System Review - Image Diversity: the 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.

The adenohypophysis produces two hormones that act directly, growth hormone (GH) and prolactin. It also produces four tropic hormones, that is, hormones that regulate other endocrine glands: adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), luteinizing hormone (LH) and follicle-stimulating hormone (FSH).

The neurohypophysis stores and releases two hormones produced in the hypothalamus, oxytocin and antidiuretic hormone (ADH, or vasopressin).

13. What is the relationship between the hypothalamus and the hypophysis?

The hypothalamus is a part of the brain located just above the hypophysis. The hypothalamus receives peripheral and central neural impulses that trigger the response of its neurosecretory cells. The axons of these cells descend into the adenohypophysis to regulate hypophyseal secretions by means of negative feedback. When the levels of adenohypophyseal hormones in the plasma are too high, the hypothalamus detects this information and commands the stoppage of the production of the hormone. When the blood level of an adenohypophyseal hormone is low, the hypothalamus stimulates the secretion of the hormone.

Hypothalamic cells produce the hormones released by the neurohypophysis. These hormones are transported by their axons to the hypophysis and are then released into the circulation.

• The Endocrine System Review - Image Diversity: hypothalamus

14. What hormones are secreted by the adenohypophysis? What are their respective functions?

The adenohypophysis 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 on bones, cartilage and muscles to promote the growth of these tissues. Prolactin is the hormone that stimulates the production and secretion of milk by the mammary glands in women. ACTH is the hormone that stimulates the cortical portion of the adrenal gland to produce and secrete cortical hormones (glucocorticoids). TSH is the hormone that stimulates the activity of the thyroid gland, increasing the production and secretion of its hormones T3 and T4. FSH is a gonadotropic hormone, meaning that it stimulates the gonads and, in women, it acts on the ovaries to induce the growth of follicles and, in men, it stimulates spermatogenesis. LH is also a gonadotropic hormone; it acts upon the ovaries of women to stimulate ovulation and the formation of the corpus luteum (which secretes estrogen); in men, it acts on the testicles to stimulate the production of testosterone.

15. What is the relationship between the thyroid and the hypophysis?

The hypophysis secretes TSH, thyroid-stimulating hormone. This hormone stimulates 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 the hypophysis, and the latter reduces the TSH secretion. When thyroid hormone levels are low, TSH secretion increases. This is therefore an example of negative feedback.

Injuries to the hypophysis that cause TSH hyposecretion (for example, in the case of tissue destruction) or hypersecretion (for example, 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?

During childhood, GH secretion deficiencies may lead to delayed growth and in severe cases to nanism (dwarfism). Excessive production of GH in children may cause exaggerated bone growth and gigantism. In adults, excess GH (for example, in hypophyseal cancer or in people that wrongly mistakenly ingest GH as a nutritional supplement) may lead to acromegaly, which is excessive and disproportional growth of bone extremities, such as the skull, the maxillaries, the hands and the feet.

17. What are the target tissues and target organs of each adenohypophyseal hormone?

GH: bones, cartilage and muscles. Prolactin: the mammary glands. ACTH: the cortical portion of the adrenal glands. TSH: the thyroid gland. FSH and LH: the ovaries and testicles.

18. What hormones are secreted by the neurohypophysis? What are their respective functions?

The neurohypophysis secretes oxytocin and antidiuretic hormone (ADH).

Oxytocin is secreted in women during delivery to increase the strength and frequency of uterine contractions and therefore 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, which then increases the secretion of milk by the mammary glands.

Vasopressin, or ADH, participates in the regulation of water in the body and therefore in the control of blood pressure, since it allows the reabsorption of free water through the renal tubules. As water goes back into circulation, the volume of blood 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 the impaired capture of this hormone by cells. Diabetes insipidus is the disease caused by deficient ADH secretion by the pituitary gland (hypophysis) or also by an impaired sensitivity to this hormone in the kidneys.

In diabetes insipidus, blood lacks ADH and, as a result, the reabsorption of water by the tubules in the kidneys is reduced, and a large volume of urine is produced. The patient urinates in large volumes and many times a day, a symptom which is also accompanied by polydipsia (increased thirst and an exaggerated ingestion of water) and sometimes by dehydration.

20. Why does the volume of urine increase when alcoholic beverages are ingested?

Alcohol inhibits ADH (antidiuretic hormone) secretion by the hypophysis. Low ADH reduces the tubular reabsorption of water in the kidneys and therefore urinary volume increases.

21. What 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 bilobed mass below the Adam’s apple.

• The Endocrine System Review - Image Diversity: the thyroid

23. What hormones are 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 bones, thus controlling the level of calcium in the blood.

• The Endocrine System Review - Image Diversity: thyroxine molecule

24. Why is the ingestion of dietary iodine so important for thyroid function?

Obtaining iodine from your diet is important for the thyroid because this chemical element is necessary for the synthesis of the thyroid hormones T3 and T4. Iodine supply often comes from the diet.

25. What is goiter? What is endemic goiter? How is this problem socially solved?

Goiter is the abnormal enlargement of the thyroid gland. Goiter appears as a tumor in the anterior neck. It may or may not be visible but is often palpable. Goiter can occur as a result of hypothyroidism or hyperthyroidism.

Endemic goiter is goiter caused by a deficiency in iodine consumption (a deficiency of iodine in the diet). The endemic character of the disease is explained because dietary iodine is often a social or cultural condition affecting many people in certain geographical regions. The hypothyroidism caused by deficient iodine ingestion is more frequent in regions far from the coast (since sea food is rich in iodine).

Nowadays, the problem is often solved by the obligatory addition of iodine to table salt. As table salt is a widely used condiment, the supply of iodine in the diet is almost always assured by this method.

• The Endocrine System Review - Image Diversity: goiter

26. What happens to the level of TSH (thyroid-stimulating hormone) in the blood during hypothyroidism? Why is the thyroid enlarged in the endemic goiter?

When there is a low level of T3 and T4 secretion by the thyroid, TSH secretion by the hypophysis is very stimulated and the level of TSH in the blood level. The increase in the availability of TSH promotes the enlargement of the thyroid gland.

Thyroid enlargement is the reaction of a tissue that tries to compensate for the functional deficiency by making the gland increase in 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 an abnormally high production and secretion of T3 and T4 and, as a result, the basal metabolic rate is increased. The signs of this condition may be tachycardia (an 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 Review - 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, fatty acid and protein metabolism, etc.), a patient with hypothyroidism may present bradycardia (a low heart rate), a low respiratory rate, excessive tiredness, depression, cold intolerance and weight gain. Hypothyroidism is normally accompanied by goiter (the enlargement of the thyroid in the neck).

29. What is the physiological cause of the syndrome known as cretinism?

Cretinism is caused by a chronic deficiency of thyroid hormones (T3 and T4) during childhood. Chronic hypothyroidism during childhood may cause retardation and a low stature due to the low basal metabolic rate during a period of life when growth and the development of mental faculty occur.

30. What are the parathyroids? Where are they located and what hormones are secreted by these glands?

The parathyroids are four small glands, two of which are embedded in each posterior face of one lobe of the thyroid. The parathyroids secrete parathormone, a hormone that, along with calcitonin and vitamin D, regulates calcium levels in the blood.

• The Endocrine System Review - Image Diversity: parathyroids

31. What is the relationship between the secretion of parathormone and the level of calcium in the blood?

Parathormone increases the level of calcium in the blood, since it stimulates the reabsorption (remodeling) of the bone tissue. When osteoclasts remodel bones, calcium is released in the circulation.

Parathormone is also involved in increasing calcium absorption in the intestines via vitamin D activation. It also plays a role in the kidneys, promoting the tubular reabsorption of calcium.

32. What is a mixed gland? Why is the pancreas considered a mixed gland?

A mixed gland is a gland that produces endocrine and exocrine secretions.

The pancreas is an example of a mixed gland because it secretes hormones into circulation, such as insulin and glucagon, while also releasing an exocrine secretion, pancreatic juice.

• The Endocrine System Review - Image Diversity: endocrine pancreas

33. What pancreatic tissues are involved in exocrine and endocrine secretions? What are their respective hormones and enzymes?

Exocrine secretions of the pancreas are produced in the pancreatic acini, aggregates of secretory cells that surround small exocrine ducts. The exocrine pancreas secretes the digestive enzymes of pancreatic juice: amylase, lipase, trypsin, chymotrypsin, carboxypeptidase, ribonuclease, deoxyribonuclease, elastase and gelatinase.

Endocrine secretions of the pancreas are 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 Review - Image Diversity: islets of Langerhans

34. What is the importance of blood glucose levels for human health?

Blood glucose levels (glycemia) must be maintained normal. If they are abnormally low, there will not be enough glucose to supply the energy metabolism of cells. If they are abnormally and chronically high, it causes severe harm to peripheral nerves, the skin, the retina, the kidneys and other important organs, and may predispose the person to cardiovascular diseases (acute myocardial infarction, strokes, thrombosis, etc). If they are acutely in excess, medical emergencies such as diabetic ketoacidosis and a hyperglycemic hyperosmolar state may occur.

35. How are insulin and glucagon involved in blood glucose control?

Glucagon increases glycemia and insulin reduces it. They are antagonistic pancreatic hormones. Glucagon stimulates glycogenolysis, thus forming glucose from the breakdown of glycogen. Insulin is the hormone responsible for the entrance of glucose from blood into cells.

When glycemia is low, for example, during fasting, glucagon is secreted and insulin is inhibited. When glycemia is high, like after meals, glucagon is inhibited and insulin secretion is increased.

• The Endocrine System Review - Image Diversity: insulin molecule glucose uptake

36. What are the target organs of insulin and glucagon?

Glucagon mainly acts on the liver. In general, insulin acts on all cells. Both also act on the adipose tissue, stimulating (glucagon) and inhibiting (insulin) the use of fatty acids by the energy metabolism (an alternate path of energy metabolism is activated when there is a shortage of glucose).

37. What are the effects of somatostatin on pancreatic hormonal secretions?

Somatostatin inhibits both insulin and glucagon secretions.

38. What is diabetes mellitus?

Diabetes mellitus is the disease caused by the deficient production or action of insulin and, as a result, characterized by a low glucose uptake by cells and a 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 reabsorption in the renal tubules due to the increased osmolarity of glomerular filtrate (caused by excessive glucose). Polydipsia is the exaggerated ingestion of water; the thirst is due to excessive water loss in the urine. Polyphagia is the exaggerated ingestion of food caused by a deficiency in energy generation by glucose-deficient 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 broken down into glucose and this molecule is absorbed in the intestines. The goal of dietary sugar restriction is to control glycemia and to maintain it at normal levels.

The main complications of diabetes are tissue injuries that occur in various organs caused by chronic high 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; and in the skin, as a consequence of the neuropathy. Diabetes mellitus is also one of the major risk factors for cardiovascular diseases such as 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, since type II diabetes may become insulin-dependent), is the impaired production of insulin by the pancreas, and is believed to be caused by the destruction of the cells of the islets of Langerhans by autoantibodies (autoimmunity).

Type II diabetes occurs adults and it is often diagnosed in older people. In type II diabetes, the pancreas secretes normal or low levels of insulin, 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, Hippocrates, 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 renal glomeruli is almost entirely reabsorbed in the nephron tubules and is not excreted in urine. With elevated blood glucose levels, the renal tubules cannot reabsorb all the filtered glucose and a certain amount of the substance appears in the urine. This amount is enough to provide the sweet taste that helped Hippocrates diagnose diabetes and differentiate it from other diseases accompanied by polyuria. Nowadays,  this method is not used due to the danger of contaminating the tester with disease agents possibly present in the patient's urine.

• The Endocrine System - Image Diversity: Hippocrates

43. What are the main treatments for diabetes mellitus?

The general goal of diabetes treatment is to maintain normal glycemic levels.

Type I diabetes is treated with the parenteral administration of insulin. Insulin must be administered intravenously or intramuscularly because, as a protein, it will be digested if ingested orally. In type II diabetes, treatment is done with oral drugs that regulate glucose metabolism or, in more severe cases, with parenteral insulin administration. The moderation of carbohydrate ingestion is an important aid in diabetes treatment.

Diabetes treatment with the use of hypoglycemic agents, such as insulin or oral medicines, must be carefully and medically supervised, since if wrongly used, these drugs may abruptly decrease the blood glucose levels, causing hypoglycemia and even death.

Many other forms of diabetes treatment are being researched worldwide.

44. How can bacteria produce human insulin on an industrial scale? What are other forms of insulin are made available by the pharmaceutical industry?

Bacteria do not naturally synthesize insulin. However, it is possible to implant human genetic material containing the insulin gene into bacterial DNA. The mutant bacteria then multiply and produce human insulin. The insulin is isolated and purified for subsequent sale. This biotechnology is known as recombinant DNA technology.

In addition to 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 there and into which parts are they divided?

Each adrenal gland is located on the top of each kidney (forming a hat-like structure on the top of the kidneys); therefore, there are two glands. The adrenal parenchymal structure is divided into two parts: the most outlying part is the cortical portion, or the adrenal cortex, and the central part is the medullary portion, or the adrenal medulla.

The Endocrine System Review - Image Diversity: the adrenal glands

46. What hormones are secreted by the adrenal medulla? What are their respective functions?

The medullary portion of the adrenal glands secretes hormones of the catecholamine group: adrenaline (also known as epinephrine) and noradrenaline (also known as norepinephrine). Besides their hormonal function, adrenaline and noradrenaline also act as neurotransmitters. The neurons that use them as neurotransmitters are called adrenergic neurons.

Adrenaline increases the breakdown of glycogen into glucose (glycogenolysis), thus increasing glycemia and the basal metabolic rate of the body. Adrenaline and noradrenaline are released during situations of danger (fight or flight response) and they intensify the strength and rate of the heartbeat and selectively modulate blood irrigation in some tissues via selective vasodilation and vasoconstriction. Through vasodilation, they increase the supply of blood to the brain, the muscles and the heart and, through vasoconstriction, they reduce the supply of blood to the kidneys, the skin and the gastrointestinal tract.

Substances that promote vasodilation or vasoconstriction, such as adrenaline and noradrenaline, are called vasoactive substances.

47. What hormones are secreted by the adrenal cortex? What are their respective functions?

The cortical portion of the adrenal glands secretes hormones of the corticoid (or corticosteroid) group, which are derived from cholesterol: glucocorticoids, mineralocorticoids and cortical sex hormones.

The glucocorticoids secreted are cortisol and cortisone. Glucocorticoids stimulate the formation of glucose from the degradation of proteins of muscle tissue (gluconeogenesis) and, as a result, help to increase glycemia. These hormones play an important immunosuppressive role, meaning that they reduce the action of the immune system and for this reason are used as medicine to treat inflammatory and autoimmune diseases and the rejection of transplanted organs.

The mineralocorticoids aldosterone and deoxycorticosterone regulate the concentration of sodium and potassium in the blood and, as a result, control the water level in the extracellular space. Aldosterone increases sodium reabsorption and therefore water reabsorption in the renal tubules, and also stimulates the renal excretion of potassium and hydrogen.

The adrenal cortical sex hormones are androgens, male sex hormones present in both men and women. In men, their main site of production is the testicle and they promote the appearance of secondary male sex characteristics, such as body hair and a beard, a deep voice, the male pattern of fat distribution and the maturation of the genitalia. If abnormally high in women, they cause an inhibited maturation of the female genitalia and disturbances in the menstrual cycle.

• The Endocrine System Review - Image Diversity: glucocorticoid molecules

48. Why are glucocorticoids 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 it recognizes the grafted tissue as foreign material. In the prevention and treatment of this common problem, patients are given glucocorticoids or other immunosuppressants. Glucocorticoids have an immunosuppressant effect and, as a result, reduce the aggression of the immune system against the graft.

However, immune action is also very important for the individual. The immune system defends the body against invasion and infection by pathogenic agents (viruses, bacteria, toxins) in addition to being necessary for the elimination of modified cells that may proliferate and cause cancer. Patients receiving immunosuppressants such as glucocorticoids therefore have an increased risk of infectious and neoplastic diseases.

49. What hormones are produced by the testicles and the ovaries?

The testicles produce androgenic hormones, the main hormone of which is testosterone. The ovaries produce estrogen and progesterone.

• The Endocrine System Review - 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 nonetheless has an endocrine function. The placenta produces estrogen and progesterone. It also secretes human chorionic gonadotropin (HCG, which has a function similar to that of hypophyseal LH), human placental lactogen, similar to prolactin and a mammary gland stimulant, and a series of hormonal peptides similar to the hormones of the hypothalamus-hypophysis axis.