Answer of the Question of Endocrine System and Chemical Messengers

Answer of the Question of  Endocrins System and Chemical Messengers

Q.27. What hormones does adenohypophysis secrete? Describe the functions of: (a) Growth hormone (b) Thyrotropin (c) Adrenocorticotropic hormone

Ans. The anterior pituitary (Adenohypophysis) produces many different hormones, four of these are tropic (=trophic) hormones that stimulate the synthesis and release of hormones from other endocrine glanos. Fig. 3.18, 3.18a.

  1. Thyroid – stimulating hormone (TSH) or Thyrotropin           v. Prolactin (PRL)

Adrenocorticotropic hormone (ACTH)                    vi. Growth hormone (OH)

  1. Follicle – stimulating hormone (FSH)                            vii. Endorphins, and
  2. Luteinizing hormone (LH)           viii. Melanocyte simulating hormone (MSH)

Hormones produced by the anterior pituitary, FSH and LH, are also called gonadotropins:

Hormones of the Vertebrate Pituitary

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Hormones of the hypothalamus and pituitary glands. The pituitary gland, located at the base of the brain and surrounded by bone, consists of the posterior pituitary (neurohypophysis) and the anterior pituitary (adenohy­pophysis) (a) The posterior pituitary. Neurosecretory cells in the hypothalamus synthesize antidiuretic homone (ADH) and oxytocin, peptide hormones that are transported down the axons to the posterior pituitary, where releases the hormones into the blood, where they circulated and bind to target cells in the kidneys (ADH) and mammary glands and uterus (oxylocin). (b) The anterior pituitary. Endocrine cells in the anterior pituitary manufacture a number of homones and secrete them into the circulation, but the release of these hormones is controlled by the hypothalamus. Neurosecretory cells in the hypothalamus secrete releasing hormones and inhibfing hormones into a capillary network located above the stalk of the pituitary. Blood containing the

(a)   Growth Hormone (GH)

Growth hormone (also called somatotropin) performs a vital role in governing body growth through its stimulatory effect on cellular mitosis, on synthesis of messenger RNA and protein, and on metabolism, especially in new tissue of young vertebrates. Growth hormone acts directly on growth and metabolism, as well as indirectly through a polypeptide hormone, insulin — like growth factor (IGF) or somatomedin produced by the liver and circulate in blood plasma and directly stimulate bone and cartilage growth. In the absence of GH. skeletal !.irowth of an immature animal will stop. If GH is injected into an animal that has been deprived of its own OH, growth tial be partially restored.

(b)   Thyrotropin

Thyrotropin, or thyroid—stimulating hormone (TSH) are glycoproteins, protein molecules with carbohydrates attached to them. It stimulates the thyroid gland’s synthesis and secretion of thyroxine, the main thyroid hormone.

(c)   Adrenocorticotropic hormone (ACTH):

Adrenocorticotropic hormone (ACTH) stimulates the adrenal gland to produce

and secrete steroid hormones called glucocorticoids (cortisol)         Secretion of
ACTH is regulated by the secretion of corticotropin releasing factor from the hypothalamus, which in turn, is regulated by a feedback system that invoives such factors as stress, insulin, ADH, and other hormones.

Q.28. What are the effect of gonadotropins in males and females?

Ms. The adenohypophysis produces two tropic hormones commonly called the gonadotropins because they stimulate the gonads (ovaries of females, testes of maes). These are

i. Leuteinizing hormone (LH), and.

ii. Follicle stimulating hormone

(FSH).TSH and LH were first named for their functions in females, but their molecular structure is exactly the same in males.

Effects of gonadotropins in males:

In males, the target cells of LH are the interstitial cells in the testes that secrete the male hormone testosterone. It once was called interstitial cell stimulating hormone (JOSH) in males, before it was discovered to be identical to LH in females.

FSI i causes the spermatogenic cells in the seminiferous tubules to initiate spermatogenesis. For details see Chapter 7, Question 14.

Effects of gonadotropins in Females:

If females, an increase of LH in the blood induces ovulation (the release of mature egg(s) from an ovary), corpus luteum production, and secretion of the female sex sterioids, progesterone and estrogen.

FSH stimulates the follicular cells in the ovaries to develop into mature eggs and to produce estrogen. See also chapter 7, Question 22.

Q.29. What is the function of pineal gland and melatonin?

Ans. The pineal gland (or pineal body) is a small mass of tissue near the center of the

mammaliam brain (closer to the brain surface in some other vertebrates). The pineal gland secretes the hormone melatonin, a modified amino acid. The pineal body contains light — sensitive cells or has nervous connections from the eyes,

and Melatonin repulatps functions related to light and to seasons marked by cnanges in day ienytt 1. rut eAaiiEtAt, melatonin, like MSH, affects skin pigmentation in many vertebrates. Most of the pineal’s functions, are related to biological rhythms associated with reproduction. Since melatonin is secreted at night, the amount secreted depends on the length of the night. In winter, for example, the days are short and the nights are long, so more melatonin is secreted. Thus, melatonin production is a link between a biological clock and daily or seasonal activities, such as reproduction. However, the precise role of melatonin in mediating rhythms is not yet clear.

Q.30. Describe the hormones of thyroid glands and their functions in mammals.

Ans. Thyroid Gland:

In humans and other mammals, the thyroid gland consists of two lobes located on the ventral surface of the trachea. The thyroid glard produces two very similar hormones derived from the amino acid tyrosine: Fig. 3.18a.

i. Triiodothyronine (T3), it contains three iodine atoms.

Tetraiodothyronine, Or

thyroxine (T4), it contains four iodine atoms.

Calcitonin. Functions:

In mammals, T3 is usually more active than T4, although both have the same effects on their target cells. The thyroid is important  in  human

  1. An  inherited condition of thyroid deficiency known as cretinism results in markedly retarded skeletal growth and poor mental development. Deficiency of thyroid hormone    thyroxin (hypothyroidism) brings about, apart from cretinism, conditions

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goiter, while over production of the hormone results in the condition known as exophthalmic goiter.

The thyroid gland also plays a vital role in homeostasis. In adult mammals, for instance, thyroid hormones help maintain normal blood pressure, heart rate, muscle tone, digestion, and reproductive functions. Throughout the body, T3 and T4 are important in bioenergetics, generally increasing the rate of oxygen

consumption and cellular metabolism. Too much or too little of these hormones in the blood can result in serious metabolic disorders.

The secretion of thyroid hormones is controlled by the hypothalamus and pituitary in a complex negative feed back system. The mammalian thyroid gland also contains endocrine cells that secrete calcitonin. This peptide lowers calcium levels in the blood as part of calcium homeostasis.

Q.31. What effect does parathormone have on blood calcium concentration?

Ans. The parathyroid glands are tiny, pea — sized glands embedded in the thyroid lobes, usually two glands in each lobe. The parathyroids secrete parathormone (PTH), which raises blood levels of calcium and thus has an

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Figure 3.19

Hormonal control of calcium homeostasis in mammals. A negative feedback system involving two antagonistic hormones, calcitonin and parathyroid hormone (PTH), maintains the concentration of calcium in blood within a very narrow range ot about 10mg/100 mt.. A rise in blood Ca2+ induces the thyroid gland to secrete calcitonin, which lowers the Ca2+ concentration by increasing bond deposition, reducing Ca2+ uptake in the intestines, and reducing reabsorpfion In the kidneys. These effect are reversed by PTH, which is secreted from the parathyroid glands when the concentration of blood Ca2+ falls below the set piont. Blood calcium levels begin to increase as target cells in the kidneys, intestines, and bone respond to PTH. Blood Ca2+ will only rise so far before the thyroid counters by secreting more calcitonin. In classic feedback fashion, these two hormones balance each othe’r’s effects,thereby minimizing fluctuations in the concentration of blood Ca2-, an ion essential to the normal functioning of all cells. Synthesized in an inactive form by skin exposed to sunlight, vitamin D plays an important role in calcium homeostasis. Vitamin D is carried in the blood and convened to its active form in many tissues such as the liver and kidneys. The active form enables PTH to increase Ca2+ uptake by the intestines.

effect opposite that of the thyroid hormone calcitonin. Parathormone elevates blood Ca2+ by stimulating Ca2+ reabsorpfion in the kidneys, and by inducing specialized bone cells called osteoclasts to decompose the mineralized matrix of bone and release Ca2+ to the blood. Calcitonin has just the opposite effects on

the kidneys and bone, thus decreasing blood Ca2+. Vitamin D, synthesized in the skin and converted to its active form in many tissues, is essential to PTH function, so it is also required for complete calcium balance. A lack of PTH causes blood levels of calcium to drop dramatically, leading to convulsive contractions of the skeletal muscles. If unchecked, this condition, known as tetany, is fatal. The control of blood calcium level is an example of how homeostasis is often maintained by the balancing of two antagonistic hormones in this case, PTH and calcitonin. Fig. 3.19.

Q.32. Write a note on adrenal glands of mammals.

Ans. Adrenal Glands:


----------------------------


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The mammalian adrenal glands are adjacent to each kidney and each is a double gland
composed of two unrelated types of glandular tissues:
Fig. 3.20, 3.21.

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  1. an outer region of adrenocortical cells, or cortex, and
  2. an inner region

of specialized cells, the medulla Adrenal Cortex:

The adrenal cortex secretes three classes of steroid hormones:

  1. Glucocorticoids (cortisol),

Mineralocorticoids (aldosterone), and Sex hormones (androgens, estrogens).

  1. Glucocorticoids, such as cortisol and corticosterone, are concerned with food metabolism, inflammation, and stress They promote synthesis of glucose from compounds other than carbohydrates, particularly amino acids and fats. The overall effect of this process, called gluconeogenesis, is to increase the level of glucose in the blood, thus providing a quick energy source for muscle and nervous tissue. Glucocorticoids are also important in diminishing the immune response to various inflammatory conditions; (several diseases of humans are inflammatory diseases e.g., allergies, hypersensitivity, and rheumatoid arthritis). These corticosteroids have important medical applications.

Mineralocorticoids, the second group of corticosteroids, are those that regulate salt balance. Aldosterone is by far the most important steroid of this group. Aldosterone promotes tubular reabsorpfion of sodium and tubular

secretion of potassium by the kidneys, since sodium usually is in short supply in diet of many animals and potassium is in excess, the mtneralocorticoids play vital roles in preserving the correct balance of blood electrolytes; hence, it plays a major role in maintaining the homeostasis of extracellular fluid.

3. A third group of corticosteroids are sex hormones, mainly androgens (male hormones) similar to testosterone, and small amounts of estrogens and progesterone (female hormones). Adrenal androgens promote some developmental changes that occur just before puberty in human males and females.

Adrenal Medulla:

Adrenal medullary cells secrete two structurally similar hormones:

  1. epinephrine (adrenaline), and,
  2. norepinephrine (noradrenaline).

The adrenal medulla is derived embryologically from the same tissue that gives rise to the postganglionic sympathetic neurons of the autonomic nervous system Norepinephrine serves as a neurotransmitter at the endings of sympathetic nerve fibers. Thus the adrenal medullary hormones and the sympathelic nervous system have the same general effects on the body. These effects centre on responses to emergencies, such as fear and strong emotional states, flight from danger, fighting, lack of oxygen, blood loss, and exposure to pain. The increased heart beat rate, tightening of the stomach, dry mouth, trembling muscles, general feeling of anxiety, and increased awareness that attends sudden fright or other strong emotional states, These effects are attributable to increased activity of the sympathetic nervous system and to rapid release into the blood of epinephrine from the adrenal medulla.

Epinephrine and norepinephrine have many other effects of which we are not as aware, including constriction of arterioles (which, together with increased heart rate, increases blood pressure), mobilization of liver glycogen and fat stores to release glucose and fatty ,acids for energy, increased oxygen consumption and heat production, hastening of blood coagulation, and inhibition of the gastrointestinal tract. These changes prepare the body for emergencies and are activated in stressful conditions.

Q.33. How do the adrenal hormones help animal respond to stress?

Ans. The adrenal hormones, epinephrine, norepinephrine, and other catecholamines

are secreted in response to positive or negative stress. Their release into the blood gives the body a rapid bioenergetic boost, increasing the basal metabolic rate and having dramatic effects on several targets. Epinephrine and norepinephrine increase the rate of glycogen breakdown in the liver and skeletal muscles and glucose release into the blood by liver cells. They also stimulate the release of fatty acids from fat cells. The fatty acids may be used by cells for energy. In addition to increasing the avaiiability of energy sources. epinephrine and norepinephrine have profound effects on the cardiovascular and respiratory systems. For example, they increase both the rate and the stroke volume of the heart beat and dilate the bronchioles in the lungs, effects the increase in the rate of oxygen delivery to body cells. (This is why doctors prescribe epinephrine as a heart stimulant and to open breathing tubes during asthma attacks).

When nerve cells are excited by some form of stressful stimulus, they release the neurotransmitter acetylcholine in the adrenal medulla. Acetylcholine combines with receptors on the cells, stimulating the release of epinephrine. Norepinephrine is released independently of epinephrine. Its functions are similar to those of epinephrine, but its primary role is in sustaining blood pressure, while epinephrine generally has a stronger effects on heart and metabolic rates. The adrenal cortex, like adrenal medulla, reacts to stress. But it responds to endocrine signals rather than to nervous input. Fig. 3.21.

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Figure 3-21

stress and the adrenal gland. Stressful stimuli cause the hypothalamus to activate the adrenal medulla via nerve impulses and the adrenal codex via hormonal signals The hormones epinepbrine and norepinephrine. The adrenal cortex controls more prolonged responess by secreting steroid hormones

Q.34. What is pancreas. What types of hormones do the pancreatic islets secrete?

Ans. Pancreas:

The pancreai is ah elongated, fleshy organ posterior to the stomach. It functions both as an exocrine (with ducts) gland and as endocrine (ductless) gland.

The exocrine portion produces pancreatic juice, a mixture of digestive enzymes and bicarbonate ions conveyed by a duct to the digestive tract.

The endocrine portion of the pancreas makes up only about 1% of the gland. This portion synthesizes, stores, and secretes hormones from clusters of cells called pancreatic islets or islets of Langerhans. Fig. 3.22.

Pancreatic islets:

Sftattered within the extensive excorine portion of the pancreas are numerous trnall islets of tissue, called pancreatic islets. This is the endocrine portion of the pancreas. The islets are witnout ducts and secrete their hormones directly into blood vessels that extend throughout the pancreas.

Hormones of Pancreatic islets:

The pancreas contains 200,000 to 2,000,000 pancreatic islets. Each islet contains four special groups of cells, called alpha (a), beta (j3), delta (8), and F cells.

The alpha cells produce the hormone glucagon,

  1. The beta cells produce insulin.
  2. The delta cells secrete somatostatin, the hypothalamic growth — hormone inhibiting factor that also inhibits glucagon and insulin secrtion.
  3. F cells secrete a pancreatic polypeptide that is released into the bloodstream after a meal and inhibits somatostatin secretion, gallbladder contraction, and the secretion of pancreatic digestive enzymes.

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Figure 3-22        cell Langerhans

The pancreas is composed of two kinds of glandular tissue: exocrine cells that

secrete digestive juices that enter the intestine through the pancreatic duct and endocrine islets of Langerhans. The islets of Langerhans secrete the hormones insulin and glucagon directly into the blood circulation.

Q.35. How does glucagon function in opposition to insulin?

Ans. •Insulin and glucagon are antagonistic hormones that regulate the concentration of glucose in the blood. This is a critical bioenergetic and homeostatic function, because glucose is a major fuel for cellular respiration and a key source of carbon skeletons for the synthesis of other organic compounds. Metabolic

balance depends on the maintenance of blood glucose at a concentration near a set point, which is about 90mg/100m1 in humans. When blood glucose exceeds this level, insulin is released and acts to lower the glucose concentration. When blood glucose drops below the set point, glucagon increases glucose concentration. By negative feedback, blood glucose concentration determines the relative a amounts of insulin and glucagon secreted by the islet cells.

Insulin and glucagon both influence blood glucose concentration by multiple mechanisms. Insulin lowers blood glucose levels by stimulating virtually all body cells except those of the brain to take up glucose from the blood. Insulin also decreases blood glucose by slowing glycogen breakdown in the liver and inhibiting the conversion of amino acids and fatty acids to sugar. Fig. 3.23.

Normally glucagon starts having an effect before blood glucose levels even drop below the set point. In fact, as soon as excess glucose is cleared from the blood, glucagon signals the liver cells to increase glycogen hydrolysis, convert amino acids and fatty acids to glucose, and start slowly releasing glucose back into the circulation.

The antagonistic effects of glucagbn and insulin are vital to glucose homeostasis, a mechanism that precisely manages both fuel storage and fuel use by body cells.

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Figure .23

Glucose homeostasis maintained by insulin and glucagon. A rise in blood glucose above the set point of about 50 mg/100m/. In human stimulates the Pancreas to secrete insulin, which triggers its target cells to take up the excess glucose from the blood. Once the excess is removed or when blood glucose concentration dips below the set point, the pancreas responds by secreting glucagon, which acts on the liver to raise the blood glucose level.

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