12.3: Endocrine Hormones - Biology

12.3: Endocrine Hormones - Biology

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Pills from Pee

The medication pictured above with the brand name Progynon was a drug used to control the effects of menopause in women. The pills first appeared in 1928 and contained the human sex hormone estrogen. Estrogen secretion declines in women around the time of menopause and may cause symptoms such as mood swings and hot flashes. The pills were supposed to ease the symptoms by supplementing estrogen in the body. The manufacturer of Progynon obtained estrogen for the pills from the urine of pregnant women because it was a cheap source of the hormone. Progynon is still used today to treat menopausal symptoms. Although the drug has been improved over the years, it still contains estrogen. Estrogen is an example of an endocrine hormone.

How Do Endocrine Hormones Work?

Endocrine hormones like estrogen are messenger molecules that are secreted by endocrine glands into the bloodstream. They travel throughout the body in the circulation. Although they reach virtually every cell in the body in this way, each hormone affects only certain cells, called target cells. A target cell is the type of cell on which a hormone has an effect. A target cell is affected by a particular hormone because it has receptor proteins — either on the cell surface or within the cell — that are specific to that hormone. An endocrine hormone travels through the bloodstream until it finds a target cell with a matching receptor to which it can bind. When the hormone binds to the receptor, it causes changes within the cell. The manner in which it changes the cell depends on whether the hormone is a steroid hormone or a non-steroid hormone.

Steroid Hormones

A steroid hormone such as estrogen is made of lipids. It is fat soluble, so it can diffuse across a target cell’s plasma membrane, which is also made of lipids. Once inside the cell, a steroid hormone binds with receptor proteins in the cytoplasm. As you can see in the diagram below, the steroid hormone and its receptor form a complex, called a steroid complex, which moves into the nucleus where it influences the expression of genes. Examples of steroid hormones include cortisol, which is secreted by the adrenal glands, and sex hormones, which are secreted by the gonads.

Non-steroid Hormones

A non-steroid hormone is made of amino acids. It is not fat soluble, so it cannot diffuse across the plasma membrane of a target cell. Instead, it binds to a receptor protein on the cell membrane. In the following diagram, you can see that the binding of the hormone with the receptor activates an enzyme in the cell membrane. The enzyme then stimulates another molecule, called the second messenger, which influences processes inside the cell. Most endocrine hormones are non-steroid hormones. Examples include glucagon and insulin, both produced by the pancreas.

Regulation of Endocrine Hormones

Endocrine hormones regulate many body processes, but what regulates the secretion of endocrine hormones? Most endocrine hormones are controlled by feedback mechanisms. A feedback mechanism is a loop in which a product feeds back to control its own production. Feedback loops may be either negative or positive.

  • Most endocrine hormones are regulated by negative feedback loops. Negative feedback keeps the concentration of a hormone within a relatively narrow range and maintains homeostasis.
  • Very few endocrine hormones are regulated by positive feedback loops. Positive feedback causes the concentration of a hormone to become increasingly higher.

Regulation by Negative Feedback

A negative feedback loop controls the synthesis and secretion of hormones by the thyroid gland. This loop includes the hypothalamus and pituitary gland in addition to the thyroid, as shown in Figure (PageIndex{4}). When the levels of thyroid hormones circulating in the blood fall too low, the hypothalamus secretes thyrotropin releasing hormone (TRH). This hormone travels directly to the pituitary gland through the thin stalk connecting the two structures. In the pituitary gland, TRH stimulates the pituitary to secrete thyroid stimulating hormone (TSH). TSH, in turn, travels through the bloodstream to the thyroid gland and stimulates it to secrete thyroid hormones. This continues until the blood levels of thyroid hormones are high enough. At that point, the thyroid hormones feedback to stop the hypothalamus from secreting TRH and the pituitary from secreting TSH. Without the stimulation of TSH, the thyroid gland stops secreting its hormones. Eventually, the levels of thyroid hormones in the blood start to fall too low again. When that happens, the hypothalamus releases TRH, and the loop repeats.

Regulation by Positive Feedback

Prolactin is a non-steroid endocrine hormone secreted by the pituitary gland. One of the functions of prolactin is to stimulate a nursing mother’s mammary glands to produce milk. The regulation of prolactin in the mother is controlled by a positive feedback loop that involves the nipples, hypothalamus, pituitary gland, and mammary glands. Positive feedback begins when a baby suckles on the mother’s nipple. Nerve impulses from the nipple reach the hypothalamus, which stimulates the pituitary gland to secrete prolactin. Prolactin travels in the blood to the mammary glands and stimulates them to produce milk. The release of milk causes the baby to continue suckling, which causes more prolactin to be secreted and more milk to be produced. The positive feedback loop continues until the baby stops suckling at the breast.

Feature: Myth vs. Reality

Anabolic steroids are synthetic versions of the naturally occurring male sex hormone testosterone. Male hormones have androgenic, or masculinizing, effects, but they also have anabolic, or muscle-building effects. The anabolic effects are the reason that synthetic steroids are used by athletes. In addition to building muscles, they also accelerate the development of bones and red blood cells, increase endurance so athletes can train harder and longer, and speed up muscle recovery. Unfortunately, these benefits of steroid use come with costs. If you ever consider taking anabolic steroids to build muscles and improve athletic performance, consider the following myths and corresponding realities.

Myth: Steroids are safe.

Reality: Steroid use may cause several serious side effects. Prolonged use may increase the risk of liver cancer, heart disease, and high blood pressure.

Myth: Steroids will not stunt your growth.

Reality: Teens who take steroids before they have finished growing in height may have their growth stunted so they remain shorter throughout life than they would otherwise have been. Such stunting occurs because steroids increase the rate at which skeletal maturity is reached. Once skeletal maturity occurs, additional growth in height is impossible.

Myth: Steroids do not cause drug dependency.

Reality: Steroid use may cause dependency as evidenced by the negative effects of stopping steroid use. These negative effects may include insomnia, fatigue, and depressed mood, among others.

Myth: There is no such thing as “roid rage.”

Reality: Steroid use has been shown to increase aggressiveness in some people. It has also been implicated in a number of violent acts committed by people who had not demonstrated violent tendencies until they started using steroids.

Myth: Only males use steroids.

Reality: Although steroid use is more common in males than females, some females also use steroids. They use them to build muscle and improve physical performance, generally either for athletic competition or for self-defense.


  1. What are endocrine hormones?
  2. Define the target cell in the context of endocrine hormones.
  3. Explain how steroid hormones influence target cells.
  4. How do non-steroid hormones affect target cells?
  5. Compare and contrast negative and positive feedback loops.
  6. Outline the way feedback controls the production of thyroid hormones.
  7. Describe the feedback mechanism that controls milk production by the mammary glands.
  8. Why do endocrine hormones only affect some of the cells in the body? Choose the best answer.
    1. They only reach certain cells.
    2. Many hormones cannot cross the plasma membrane of cells.
    3. Some cells feedback negatively in response to a hormone.
    4. Only some cells have receptor proteins that can bind to a given hormone.
  9. People with a condition called hyperthyroidism produce too much thyroid hormone. What do you think this does to the level of TSH? Explain your answer.
  10. Which is more likely to maintain homeostasis — negative feedback or positive feedback? Explain your answer.
  11. Does testosterone bind to receptors on the plasma membrane of target cells or in the cytoplasm of target cells? Explain your answer.
  12. True or False. Endocrine hormones can affect the expression of genes.
  13. True or False. Non-steroid hormones cannot affect intracellular processes.
  14. True or False. Insulin binds to receptors on the plasma membrane of cells.
  15. Which hormone is secreted by the pituitary gland?
    1. Prolactin
    2. Insulin
    3. Cortisol
    4. Thyrotropin releasing hormone

Explore More

For a funny and fast-paced lesson that covers endocrine hormones in greater detail, watch this CrashCourse video:

12.3: Endocrine Hormones - Biology

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12.3: Endocrine Hormones - Biology

What is the System?

  1. Made up of glands that produce and secrete hormones, _________________________
  2. Regulation of growth, metabolism, and ______________________________
  3. Responses to ________________________________
  4. Maintains _____________________________________

Major Structures & Location

Hypothalamus (part of the brain)

1. Pineal
2. Pituitary
3. Thyroid & Parathyroid

4. Thymus
5. Adrenals
6. Pancreas
7. Ovary
8. Testes

Types of Glands

Control of Hormonal Secretions - Negative versus Positive Feedback

Negative Feedback

When the levels go above or below a _______________________, the endocrine system secretes hormones to lower or raise the level.

Positive Feedback

Pituitary Gland

Why is it called the master gland?

What part of the brain controls it?

Anterior Pituitary Hormones

Prolactin or PRL –
Growth hormone or GH
Adrenocorticotropin or ACTH –
Thyroid-stimulating hormone or TSH -.
Luteinizing hormone or LH –

Follicle-stimulating hormone or FSH

Posterior Pituitary Hormones

Antidiuretic hormone or ADH

Thyroid Gland

The thyroid hormones control your _________________________, which is the body's ability to break down food and store it as energy

Thyroxin (T4) & Tri-iodothyronine (T3) - increase the rate at which cells release energy from carbohydrates
Calcitonin – regulates the blood concentration of calcium

Hypothyroidism (cretinism)
Hyperthyroidism (Grave&rsquos disease)

Parathyroid Gland

Located behind the thyroid, four tiny glands that help maintain calcium and phosphorous levels

Parathyroid Hormone (PTH) - takes calcium from the bones to make it available in the blood

Adrenal Glands Located above each kidney.

Adrenal Cortex = ______________ area Medulla = ______________

Adrenal glands produce _______________________________

Epinephrine & Norepinephrine – increased heart rate, breathing rate, elevated blood pressure (fight or flight, response to stress)
Aldosterone –helps kidneys conserve sodium and excrete potassium, maintaining ___________________
Cortisol – glucocortoid, keeps blood glucose levels stable response to ___________
Adrenal Sex Hormones - androgens (male) and estrogens (female)

Adrenal Gland Disorders

Large gland behind stomach, maintains healthy blood sugar (glucose) levels.
Contains islands of cells called the Islets of Langerhans which secrete glucagon and insulin

Glucagon – stimulates the liver to break down glycogen, Raises ______________________________________

Insulin – decreases blood sugar concentrations, affects the ____________________ of glucose by cells
Diabetes Mellitus –insulin deficiency, blood sugar rises (hyperglycemia) and excess is excreted in the urine

What is a diabetic neuropathy?

Other Endocrine Glands

Pineal Gland – secretes melatonin which maintains _____________________________

Thymus Gland – large in young children, gradually shrinks with age, secretes thymosins, important to ______________________

Reproductive Glands – testes and ovaries – testosterone, progesterone, estrogen
What is gonadotropin?

/>This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

Amino Acid-Derived Hormones

The amino acid-derived hormones are relatively small molecules that are derived from the amino acids tyrosine and tryptophan, shown in Figure 2. If a hormone is amino acid-derived, its chemical name will end in –ine. Examples of amino acid-derived hormones include epinephrine and norepinephrine, which are synthesized in the medulla of the adrenal glands, and thyroxine, which is produced by the thyroid gland. The pineal gland in the brain makes and secretes melatonin which regulates sleep cycles.

Figure 2. (a) The hormone epinephrine, which triggers the fight-or-flight response, is derived from the amino acid tyrosine. (b) The hormone melatonin, which regulates circadian rhythms, is derived from the amino acid tryptophan.

Study the Endocrine System, its Organs and its Functions

Endocrine glands are glands whose secretions (called hormones) are collected by the blood and reach tissues through circulation. The hypophysis (pituitary gland) and the adrenal glands are examples of endocrine glands. Exocrine glands are a glands whose secretions are released externally through ducts (into the skin, the intestinal lumen, the mouth, etc.). The sebaceous glands and the salivary glands are examples of exocrine glands.

Endocrine Glands and Hormones

More Bite-Sized Q&As Below

2. What are the components of the endocrine system?

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

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.

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.

The Pineal Gland

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.

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The Hypophysis

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.

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 Adenohypophysis

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.

The Thyroid Gland

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਋ilobed mass below the Adam’sਊpple.

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.

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.

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.

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.

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.

The Pancreas

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.

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.

Hormonal Glucose Regulation

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.

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.

Diabetes Mellitus Explained

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,਋ut 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򠫌ompanied 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.

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.

The Adrenal Glands

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.

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�t 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.

Reproductive Hormones

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.

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.

Now that you have finished studying Endocrine System, these are your options:

Parts of the endocrine system

The endocrine system consists of

Many other organs, such as the liver, skin, kidney, and parts of the digestive and circulatory systems, produce hormones in addition to their other physiological functions.

Endocrine vs. exocrine glands

Endocrine glands are ductless glands that secrete hormones directly into the bloodstream, whereas exocrine glands release their secretions through ducts or tubes.

Examples of exocrine glands are sweat glands, salivary glands, and tear (lacrimal) glands.

Video: Endocrine glands and hormones review

Fire retardants

What do breast milk and polar bears have in common? In 1999, some Swedish scientists studying women’s breast milk discovered something totally unexpected: The milk contained an endocrine-disrupting chemical found in fire retardants, and the levels had been doubling every five years since 1972! These incredibly persistent chemicals, known as polybrominated diphenyl ethers or PBDEs, have since been found to contaminate the bodies of people and wildlife around the globe – even polar bears. These chemicals can imitate thyroid hormones in our bodies and disrupt their activity. That can lead to lower IQ, among other significant health effects. While several kinds of PBDEs have now been phased out, this doesn’t mean that toxic fire retardants have gone away. PBDEs are incredibly persistent, so they’re going to be contaminating people and wildlife for decades to come.

How to avoid it? It’s virtually impossible, but passing better toxic chemical laws that require chemicals to be tested before they go on the market would help reduce our exposure. A few things that can you can do in the meantime include: use a vacuum cleaner with a HEPA filter, which can cut down on toxic-laden house dust avoid reupholstering foam furniture take care when replacing old carpet (the padding underneath may contain PBDEs). Find more tips at:

You may or may not like heavy metal music, but lead is one heavy metal you want to avoid. It’s well known that lead is toxic, especially to children. Lead harms almost every organ system in the body and has been linked to a staggering array of health effects, including permanent brain damage, lowered IQ, hearing loss, miscarriage, premature birth, increased blood pressure, kidney damage and nervous system problems. But few people realize that one other way that lead may affect your body is by disrupting your hormones. In animals, lead has been found to lower sex hormone levels. Research has also shown that lead can disrupt the hormone signaling that regulates the body’s major stress system (called the HPA axis). You probably have more stress in your life than you want, so the last thing you need is something making it harder for your body to deal with it – especially when this stress system is implicated in high blood pressure, diabetes, anxiety and depression.

How to avoid it? Keep your home clean and well maintained. Crumbling old paint is a major source of lead exposure, so get rid of it carefully. A good water filter can also reduce your exposure to lead in drinking water. (Check out for help finding a filter.) And if you need another reason to eat better, studies have also shown that children with healthy diets absorb less lead.

Endocrine Disorders

The endocrine system consists of a group of glands and organs that regulate and control various body functions by producing and secreting hormones. Hormones are chemical substances that affect the activity of another part of the body. In essence, hormones serve as messengers, controlling and coordinating activities throughout the body.

Endocrine disorders involve either

Too much hormone secretion (called "hyper" function)

Too little hormone secretion (called "hypo" function)

Disorders may result from a problem in the gland itself, or because the hypothalamic-pituitary axis (interplay of hormonal signals between the hypothalamus, and the pituitary gland) provides too much or too little stimulation. Depending on the type of cell they originate in, tumors can produce excess hormones or destroy normal glandular tissue, decreasing hormone production. Sometimes the body's immune system attacks an endocrine gland (an autoimmune disorder), decreasing hormone production.

Examples of endocrine disorders include

Disorders of puberty and reproductive function

Doctors usually measure levels of hormones in the blood to tell how an endocrine gland is functioning. Sometimes blood levels alone do not give enough information about endocrine gland function, so doctors measure hormone levels.

At certain times of the day or more than once or at different times of the day (such as cortisol )

After giving a stimulus or suppressor (such as a sugar-containing drink, a drug, or a hormone that can trigger or block hormone release)

After having the person take an action (such as fasting)

Endocrine disorders are often treated by replacing a hormone that is deficient or decreasing levels of a hormone that are excessive. However, sometimes the cause of the disorder can be treated. For example, a tumor involving an endocrine gland may be removed.

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The Central Nervous System and the Peripheral Nervous System

The nervous system can be divided into two major subdivisions: the central nervous system (CNS) and the peripheral nervous system (PNS) , shown in Figure 1. The CNS is comprised of the brain and spinal cord the PNS connects the CNS to the rest of the body. In this section, we focus on the peripheral nervous system later, we look at the brain and spinal cord.

Figure 1. The nervous system is divided into two major parts: (a) the Central Nervous System and (b) the Peripheral Nervous System.

Multiple Choice Questions on Endocrine System

2. Toxic agents present in food which interfere with thyroxine synthesis lead to the development of
a) toxic goitre
b) cretinism
c) simple goitre
d) thyrotoxicosis

3. Low Ca++ in the body fluid may be the cause of
a) tetany
b) gout
c) anaemia
d) angina pectoris

4. Which one of the following pair of organs includes only the endocrine glands?
a) thymus and testes
b) adrenal and ovary
c) pancreas and parathyroid
d) adrenal and parathyroid

5. The contraction of gall bladder is due to
a) gastrin
b) secretin
c) cholecystokinin
d) enterogastrone

6. Gastric secretion is stopped by hormone
a) gastrin
b) enterogastrone
c) cholecystokinin

7. The blood calcium level is lowered by the deficiency of
a) thyroxine
b) calcitonin
c) parathormone
d) both calcitonin and parathormone

8. A health disorder that results from the deficiency of thyroxine in adults and characterized by
i) a low metabolic rate ii) increase in body and iii) tendency to retain water in tissues is
a) cretinism
b) myxodema
c) simple goitre
d) hypothyroidism