Chemical Control and Coordination

In the previous post, we read about Neural Control and Coordination.Let us now talk about chemical control and coordination. Since, the cellular functions need to be continuously regulated. This cellular regulation is carried out by another coordinating system. They use chemical messengers called hormones, which are released by endocrine glands. The endocrine glands release their secretion directly into the blood and through blood they reach each and every cell of the body and regulate their functions.

In this post, we will study about different endocrine glands and the hormones release by them. This would help you to understand how different hormones regulate the cellular functions of the body and how these hormones help in coordination of different organs of the body.

ENDOCRINE SYSTEM

The endocrine system comprises endocrine glands which are also called ductless glands as they lack ducts and release their secretions directly into the venous blood or lymph. These hormones are taken up by specific organs from blood circulation to initiate a particular action.

It is slower than the neural system but its response is long lasting.

Hormones are defined as non-nutrient chemicals which acts as intercellular messengers that are produced in trace amounts, secreted by endocrine glands

PROPERTIES OF HORMONES:

• Secreted by living glandular endocrine cells.
• Released directly into the blood and transported to different cells of the body by circulatory system.
• Required in small quantitites.
• They don’t provide any energy.
• They are regulatory chemiclals that control and coordinate functions of different body organs.
• They are produced to a particular stimulus.
• They are highly specific in nature.

HUMAN ENDOCRINE SYSTEM

The endocrine glands and hormone producing diffused tissue/cellslocated in different parts of our body constitute the endocrine system.

It includes pituitary gland, pineal gland, thyroid gland, adrenal glands, pancreas, parathyroid glands, thymus and gonads(testes in males andovary in females). In addition to these glands, certain other organs, e.g. Gastrointestinal tract, liver, kidney, heart also produce hormones.

HYPOTHALAMUS

Hypothalamus is the basal part of diencephalon, forebrain. It isconnected to pituitary gland. It contains several groups ofneurosecretory cells called nuclei which produces hormones byregulating wide spectrum of body functions. There are severalhormones released by hypothalamus which regulate the synthesis andsecretion of anterior pituitary hormones. The hormones produced areof two types: releasing hormones(which stimulates secretion ofpituitary hormones) and inhibiting hormones(which inhibits thesecretion of pituitary hormones).

PITUITARY GLAND

• It is located in a bony cavity of the sphenoid bone of skull called sellatursica. The pituitary gland is divided into adenohypophysis and neurohypophysis.
• Pituitary gland is ectodermal in origin.
• It is small, pear shaped gland, red- grey in colour.

ADENOHYPOPHYSIS

It comprises of 75% part of Pituitary gland. It is highly cellular andvascular. It consists of two portions-pars distalis and pars intermedia.

Pars distalis is commonly known as anterior pituitary. It produces

• GROWTH HORMONE(GH),
• PROLACTIN(PRL),
• THYROID- STIMULATING HORMONE(TSH),
• ADRENOCORTICOTROPHIC HORMONE(ACTH),
• LUTEINIZING HORMONE(LH),
• FOLLICLE STIMULATING HORMONE(FSH)

The hormones synthesised and released from adenohypophysis (ant.pituitary) are :

1.GROWTH HORMONE: This is also known as somatotrophin or somatotophic hormone. It regulates the growth of the body bypromoting protein anabolism, intestinal absorption of calcium,conservation of glucose etc. It helps in synthesis and deposition ofproteins, growth of long bones and other parts of the skeleton.

Over and under secretion of growth hormone leads to many disorderssuch as:

• Dwarfism– the low secretion of growth hormone before puberty causes pituitary dwarfism. It leads to stunted growth due to pre mature stoppage of body growth and reduced development of skeleton. The dwarfism is also called Ateleiosis and the dwarfs are called midgets.
• Gigantism– Over secretion of growth hormone before puberty causes gigantism. The person suffering from gigantism has extra ordinary growth in height caused by abnormal elongation of long bones in childhood.
• Sometimes, a person develops hump due to bending of vertebralcolumn. This is called kyphosis.

2. PROLACTIN (PRL)

Previously, it was called as luteotrophic hormone.Prolactin regulates the growth of mammary glands and milkproduction in them.

It activates the growth of breasts during pregnancy and secretion ofmilk from mammary glands after child birth.

3. THYROID STIMULATING HORMONE (TSH)

TSH stimulates the synthesis and secretion of thyroid hormones likethyroxine from the thyroid gland.

It also controls the growth of thyroid gland.

4. ADRENOCORTICOTROPHIC HORMONE (ACTH)

ACTH controls the structure and functioning of adrenal cortex, especiallysecretions of glucocorticoids from it.

It stimulates the synthesis and secretion of steroid hormones calledglucocorticoids from the adrenal cortex.

5. GONADOTROPHIC HORMONES

They are called as gonadotrophins as they stimulate gonadal activity.

A. Follicle stimulating hormone (FSH): In males, FSH regulatesspermatogenesis (formation of sperms) and in females, FSH stimulatesgrowth and development of ovarian follicles and secretion of estrogenhormone.

B. Luteinizing hormone (LH):

• In males, LH is called ICSH (interstitial cell stimulating hormone) which stimulates testes to synthesise and secrete androgens (testosterone). It activates Leydig cells of the testes to secrete testosterone.
• In females, LH stimulates the ovulation of the fully matured graafian follicles and also maintain the corpus luteum. The corpus luteum secretes progesterone and some estrogen under action of LH.

Only one hormone is released from pars intermedia i.e. MSH(melanocyte stimulating hormone).

MELANOCYTE STIMULATING HORMONE

It acts on Melanocytes( which contain melanin pigment) and regulatespigmentation of skin. It stimulates synthesis, secretion and dispersion of melanin pigment inskin.

NEUROHYPOPHYSIS

It comprises 25% portion of pituitary. It is also called pars nervosa orposterior pituitary.

It is under neural regulation of hypothalamus.

The two neurohormones synthesized in hypothalamus and secreted byneurohypophysis are:

OXYTOCIN

• Also called pitocin.
• It acts on smooth mucles and stimulates their contraction.
• In females, it stimulates vigourous contraction of the uterus at the time of child birth. It also causes contraction of mammary glands leading to milk ejection.
• The oxytocin induce contractions in mammary glands which helps in flow of stored milk from mammary gland to mouth of infant.
• Therefore, oxytocin is called ‘milk ejection hormone’ and ‘birth hormone’.

VASOPRESSIN OR ANTIDIURETIC HORMONE(ADH)

• It mainly acts on kidneys and stimulates reabsorption of water fromdistal tubules. Thereby, it reduces water loss from the body.
• It prevents diuresis hence it is called antidiuretic hormone(ADH).
• DIABETES INSPIDUS- Failure of secretion of vasopressin leads to reducedrenal reabsorption of water and elimination of large amount of verydilute urine.
• Vasopressin also enhances arterial blood pressure by causingconstriction or narrowing of arterioles.

THYROID GLAND

• It is the largest endocrine gland.
• It is composed of two lateral lobes which are located on either side of trachea. These lobes are interconnected with a thin flap of connective tissue called isthmus.
• The cells present in gland synthesise two hormones, namely
• tetraiodothyronine or thyroxine (T4)
• tri-iodothyronine (T3)
• Both the hormones are iodonated form of amino acid- Tyrosine. T3 is more active and several time more potent than T4.
• It is the only gland which is stores its secretory product in large quantity. when necessary hormones are released.
• These hormones are released under the influence of TSH, released by anterior pituitary.

THYROCALCITONIN

C cells are present in between thyroid follicles which release calcitonin(TCT). It is involved in calcium homeostasis. It is hypocalcemic. Both theparathormone and calcitonin helps in maintenance of calcium ions inblood plasma. This hormone is proteinaceous in nature.

Function of thyroxine:

• They control metabolism of proteins, fats and carbohydrates. Hence, regulates growth of body tissue.
• It is responsible for regulation of BMR(Basal metabolic rate). It greatly increase metabolic rate of the body.
• They stimulate tissue differentiation.
• Maintenance of water and electrolyte balance.
• They support erythropoiesis.

DISORDERS

• HYPOTHYROIDISM:

Deficiency or under secretion results in hypothyroidism.

• GOITRE:

Iodine is essential for synthesis of thyroid hormones. Deficiency of iodine results in low secretion of thyroid hormones and enlargement of thyroid gland.

• CRETINISM:

Failure of thyroid secretion from infancy or childhood slows the body growth and mental development and thus, reduces metabolic rate. The child remains physically stunted and mentally retarded low intelligence. Low resistance to diseases.Low blood sugar and high cholesterol.

• MYXEDEMA (GULL’S DISEASE):

Deficiency of thyroid hormones produces Myxedema in adults. The patient has puffy appearance and lack alertness, intelligence. It occurs more commonly in females.

• HYPERTHYROIDISM:

 The rate of synthesis and secretion of thyroid hormones is increased toabnormal high levels leading to a condition called hyperthyroidismwhich adversely affects the body physiology.

PARATHYROID GLAND

• The parathyroid glands are present on the back side of thyroid gland.parathyroid glands are present on the surface of thyroid gland, these are functionally and developmentally independent from the thyroidgland.
• The parathyroid glands are small, oval-shaped, yellow-coloured,compact, solid, vascular mass of cells.
• The parathyroid glands secrete a peptide hormone called parathyroidhormonecollips hormone or parathormone(PTH).
• The release ofparathormone is regulated by circulating levels of calcium (Ca2+) ion inthe blood.
• when the calcium level in the blood decreases, parathyroid hormone is released in order to increase calcium level in the blood.
• PTH acts on bones and other skeletal structures and mobilises theCalcium (Ca2+) from bones into the blood.
• It is essential to maintain the optimum calcium level in the bloodbecause calcium is essential for proper functioning of muscles,heart, nerves and for growth of bones and teeth.

DISORDERS

HYPOPARATHYROIDISM:

• PARATHYROID TETANY-

If Parathyroid fails to secrete sufficient amount ofparathormone, the concentration of calcium ions falls abnormally in the plasma. This increases excitability of nerves and muscles due todeficiency of Ca2+ which causes depolarisation. Thecontractions(tetany) of muscles, larynx, face, hands and feet produced. This disease is called parathyroid tetany.

HYPERPARATHYROIDISM-

The over secretion of parathormone which caused increasedmobilisation of bone minerals into the blood, softening of bones, rise inthe concentration of calcium ions in the plasma, and deposition ofcalcium in kidney tubules and other soft tissues.

HYPERCALCEMIA:

It may cause osteitisfibrosacystica. Due to high blood Ca2+ level, itsdeposition in gall bladder and kidney leads to stone formation.Osteoporosis may occur due to excessive loss of calcium andphosphorus ions from bones.

ADRENAL GLAND

• Also called as ‘glands of emergency’. These paired glands located at the
• anterior part of the kidneys.
• Each adrenal gland is composed of two types of tissues, adrenal medulla
• andadrenal cortex.
• The adrenal medulla is located at the centre and the adrenal cortex lies outside the adrenal medulla.

ADRENAL CORTEX:

It is derived from mesoderm of embryo. It is divided into 3 layers:

1. zonareticularis- inner most layer

2. zonafasciculata- middle layer

3.zonaglormerulosa- outermost layer

The adrenal cortex secretes over 20 steroid hormones, commonlycalled as cortical steroids and corticoids. These are grouped into majorcategories called mineralocorticoids, glucocorticoids and sexcorticoids.

MINERALOCORTICOIDS:

• It is Released by zonaglomerulosa.
• These regulate the water and electrolyte balance in the body eg.aldosterone.
• When the concentration of K+ rises or Na+ falls, the aldosterone is released which stimulates the renal tubules to reabsorb Na+ and water and excrete K+ and phosphate ions from the body.
• Aldosterone causes the uptake of Na+ ions from filtrate.

GLUCOCORTICOIDS:

It is released by Zonafasciculata.

They are responsible for carbohydrate metabolism.

Glucocorticoids includes 3 hormones namely- Cortisol, cortisone andcorticosterone. Cortisol is the main Glucocorticoid.

A.Glucocorticoids stimulates:

•GLUCONEOGENESISi.e synthesis of glucose from amino acids andglycerol.

•LIPOLYSISi.e breakdown of fats stores in adipose tissue to release fattyacids in the blood.

•PROTEOLYSIS i.e. degradation of proteins within the cell and hence,increases the level of amino acids in the blood and inhibit cellularuptake and utilization of amino acids.

B. It is involved in maintaining cardiovascular system as well as kidneyfunctions.

C. It produces anti-inflammatory reactions and suppresses the immuneresponses.

D. The cortisol stimulates formation of RBCs and thereby Increase RBC count.

GONADOCORTICOIDS (SEX CORTICOIDS)-

These are steroids hormones mainly androgens and some estrogenwhich are produced by zonareticularis. These are produced in small amounts. These steroids play in growth of axial hair, pubic hair and facial hair during puberty. They are also connected with development of external sexual characters.

examples: androstenedione, dehydroepiandrostrone and estrogens.

DISORDERS

• ADDISON’S DISEASE-

A destruction of adrenal cortex by diseases liketuberculosis due to deficiency of both glucocorticoids andmineralocorticoids.

• CUSHING’S SYNDROME-

a tumor of adrenal cortex may secrete toomuch cortisol to produce cusing’s syndrome. In males excessive hairgrowth (hirsutism).In females, Maculinization with beard, moustacheetc.

• CONN’S SYNDROME-

Excessive secretion of aldosterone from adrenalcortical tumour produces aldosteronism.

• ADRENAL VIRILISM-

An excessive secretion of sex corticoids producesthe male type external sex characters such as beard and moustachesand male voice in women.

ADRENAL MEDULLA

• It is the inner part of adrenal gland. It releases two hormones called
• epinephrine and norepinephrine. The epinephrine is called adrenaline
• and norepinephrine is called noradrenaline. These hormones are called
• ascatecholamines.
• These hormones are released during stress of anykind or emergency situations and are therefore called as emergency hormones and adrenal gland is called emergency gland.
• Thesehormones also prepare the body during the flight, fright and fight
• hence are also called 3F or fight, flight and fright hormones.
• These havedifferent effects on different organs of the body.
• These hormones increase the heartbeat, the strength of heartcontraction and the rate of respiration.
• They increase the sweating, piloerection (raising of hair), alertnessand pupillary dilation.
• catecholamine stimulates breakdown of glycogen resulting in anincreased concentration of glucose in blood.
• In addition, they also stimulate the breakdown of lipids and proteins.

PANCREAS:

It is the heterocrine or composite gland. It acts both as endocrine andexocrine.

The two main types of cells are alphacells (α cells) and Beta cells ( β cells) which producesendocrine secretion.

1. ALPHA CELLS-These are also called as A cells.
   They secrete a hormone called glucagon.
   It is a peptide hormone which helps in maintaining normal glucose level in the body.
   Glucagon acts mainly on liver cells and stimulate conversion of storedglycogen into glucose.
   If the blood level is low, glycogenolysis takes place which increases thelevel of glucose.
   Glucagon stimulates the process of gluconeogenesis.
   It reduces cellular glucose uptake and utilization. thus, glucagon ishyperglycemic hormone.
2. BETA CELLS-
   These are also called B cells.
   The beta cells produce a peptide hormone called insulin which regulates the glucose homeostasis.
   Insulin acts mainly on adipocytes, hepatocytes and muscle cells.
   It enhances cellular uptake and utilization of glucose.
3. The level of glucose in the blood increases, then beta cells gets stimulated and release insulin which increases:
   Uptake of glucose by cells for respiration.
   Uptake of glucose by liver and muscle cells for glycogenesis or conversion of glucose into glycogen.
   Uptake of amino acids by cells and synthesis of proteins.
   Fat synthesis in adipose tissue.

• This would decrease glucose concentration in blood and would increase
• rapid movement of glucose from blood to hepatocytes and adipocytesresulting in hypoglycemia.
• Therefore, it could be concluded that both glucagon and insulin help inhomeostasis of glucose where both maintain the blood glucose level.
• Hypo secretion of insulin causes hyperglycemia. Prolongedhyperglycemia leads to a complex disorder called diabetes mellitus. Thisis characterized by loss of excess glucose through urine and alsoformation of complex harmful compounds known as ketone bodies.Diabetic patients are successfully treated with insulin therapy.

PINEAL GLAND

• It is located on the dorsal side of forebrain. Pineal gland secrete a hormone called as melatonin. Melatonin plays very important role inregulation of 24 hour(diurnal) rhythm of our body.
• Melatonin influences metabolism,body temperature,pigmentation ofskin,menstrual cycle as well as defense capability of an individual.
• Itacts like a biological clock.

THYMUS

• It is the lobular structure located near the heart and the aorta. It playsmajor role in development of immune system.
• It acts as primary lymphoid organ where development and maturationof T lymphocytes occur.
• Thymus gland secretes thymosin which helps in differentiation of T cellsand promote production of antibodies from B lymphocytes to provideHumoral immunity.
• It helps in acceleration of cell division, developing resistance toinfection.
• As age increases, thymus begins to degenerate. Thymus is degenerated in old individuals resulting in a decreasedproduction of thymosins which eventually decreases immunity.
• Therefore, older people have weaker immune responses.
• It promotes proliferation and maturation Of T lymphocytes. It is calledthe throne of immunity, or training school of T lymphocytes.

TESTES

• In males, a pair of testes is present in the scrotal sac located outside the abdominal cavity.
• It acts as a primary sex organs as well as an endocrine gland.
• The seminiferous tubules in testis are responsible for the formation of sperms and hence fulfill the function of primary sex organ where as leydig cells or interstitial cells which are located in the spaces between the seminiferous tubules produces a group of steroid hormones called androgens mainly testosterone.
• They secrete under the influence of LH,secreted by anterior pituitary.
• Androgens regulates the following functions:
• Regulate the development,maturation and function of male accessory sex organs like epididymis, vas deferens, seminal vesicles etc.
• Regulate the development of male pattern of external genitalia before birth.
• Stimulate muscular growth.
• Stimulate growth of secondary sexual characters such as growth of facial and axillary hair.
• The acts on CNS and influence the male sexual behavior (libido).
• stimulate the process of spermatogenesis(formation of spermatozoa)

DISORDER:

EUNUCHOIDISM–

This results from the failure of testosterone secretion.For the disorder, secondary sex organs, such as prostate gland, seminalvesicle and penis, remains infantile and small in size and fail to function.

Spermatozoa fail to produce. External sexual characters like beards,moustaches and low pitch male voice fail to develop.

GYNAECOMASTIA-

It is development of breast in males and is usually dueto pertubation of estrogen to androgen ratio. It is due to temporaryincrease in circulating estrogen.

Removal of testes in male is called castration. It will lead to decline inandrogen level and secondary characters fail to appear.

OVARIES

• In females, a pair of ovary is located in abdominal cavity. it performsthe function of both primary sex organ and endocrine gland.
• ovary produces two steroid hormones called estrogens andprogesterones.
• Ovary is composed of ovarian follicles and stromal tissues. Theestrogen is synthesised and secreted mainly by growing ovarianfollicles.During ovulation, the graafian follicle ruptures and releasesegg. After ovulation, the graafian follicle is converted to a structurecalled corpus luteum, which secretes mainly progesterone.
• These hormones have following effects:
• Estrogens– the maximum concentration of estrogen in females is duringthe time of puberty. It stimulates the development of growing ovarianfollicles and differentiation of ova in ovary. It controls the appearanceof females secondary sexual characters such as high pitch of voice,developmemt of breast. It regulates female’s sexual behaviour.

• Progesterones– It is mainly secreted by corpus luteum. It supportspregnancy. It produces routine temporary changes in endometrial lining of uterus. It acts on mammary glands and stimulates formation of alveoli and milk secretion through mammary gland.

• Relaxin- secreted by corpus luteum at the end of pregnancy. Causes relaxation of pelvic ligaments and cervix during child birth.

HORMONES OF HEART ,  LIVER , KIDNEY AND GASTROINTESTINAL TRACT

Apart from the endocrine glands discussed above, there are certain other tissues or organs which are responsible for the secretion of certain other types of hormones such as:

1.Heart :

• The atrial walls of our heart secrete a very important peptide hormone called ANF or atrial natriuretic factor . This hormone decreases the blood pressure.
• In case of blood pressure rise , the ANF is released from the atrial walls of the heart which causes dilation of blood vessels and decreases the blood pressure .
• ANF is secreted in response to increased return of deoxygenated venous blood to heart.

2. Liver:

• Liver produces a protein called angiotensinogen. Angiotensinogen is converted into angiotensin-I by the action of renin  hormone.
• In lungs , the angiotensin-I is converted to angiotensin-II .
• The angiotensin-II , then stimulates the adrenal cortex to release aldosterone now increases the reabsorption of the sodium ions from the filtrate through  distal convoluted tubules .
• The renin required for this conversion is secreted by juxtaglomerular apparatus of nephron in the kidney .
• Kidneys :
• The juxtaglomerular cells (JG cells ) of kidney produce a hormone  called erythropoietin which simulates erythropoiesis (formation of RBC) .
• Angiotensin-II stimulates adrenal cortex to release aldosterone which increases reabsorption of sodium ions from the nephric filtrate which is absorbed by the reabsorption and thus , helps in water retention .
• Kidneys also secrete calcitrol, which promotes absorption of Ca2+ and phosphorus in small intestine and accelerates bone formation.

• GI TRACT:

Different types of endocrine cells are present in different parts of gastro intestinal tract. The hormones are as follows:

• GASTRIN: the mucosa of pyloric region of stomach synthesizes, stores and secretes the hormone gastrin. The gastrin acts on gastric glands and stimulates the release of gastric juice containing digestive enzymes for digestion of food.
• SECRETIN: it is secreted by intestinal mucosa of duodenum and jejunum. Secretin stimulates the release of water and bicarbonate ions from pancreas and release of bile juice from liver to neutralize the acidity of food coming from stomach. It was first hormone to be discovered.
• CHOLECYSTOKININ: it is released from duodenum. It acts on both pancreas and gall bladder and stimulates secretion of enzymes Inpancreatic juice from pancreas and release of bile juice from gall bladder.
• GIP: Gastric Inhibitory Peptide: this hormone inhibits gastric secretions and motility.