Endocrine System

Endocrine System is one of the most important system of the body which along the nervous system controls and coordinate all the system of the body. An endocrine gland is ductless gland that synthesized one or more hormone + secrete them directly into the blood.

Endocrine glands though present in different region of the body, work in coordination as a combine system. They secretes wide variety of chemical compound known hormones also called informational molecules. These hormones thus come into the blood circulation + ultimately reaches the target organ on which they act upon.

The study of endocrine gland is called endocrinology.

Types of Gland:

Gland are the organ of secretion. It is of 2 types.

1. Exocrine Gland: Exocrine gland are the gland secretion are carried duct into site of action.
   Eg: Pancreas, Salivary gland
2. Endocrine Gland: This gland are also called ductless gland. As they pour their secretion directly into the blood. Ex Pituitary gland.

Hormones

Hormones are non-nutrients chemical which acts as intercellular messenger + are produce in little amount. These are secretion of endocrine gland that do not act like enzyme but influences the synthesis activation or inhibition of certain metabolic process in the body.

First hormone secretion in 1903 AD was discovered by William M. Bayliss + Ernest H. Starling as Secretin. The term 'Hormone' was coined by Starling 1905. Banting + McLeod isolated insulin as first hormone in 1922 AD for which they received Nobel prize in 1923 AD. Thomas Addison is the father of endocrinology.

The properties of hormones are as follows:

1. They have low molecular weight.
2. They are the chemical messenger of the body.
3. Chemically, hormones are either steroids or peptides.
4. They are effective even in minute quantities.
5. Hormones are transported to the target organ through blood not by ducts.
6. Hormones controlled reaction aren't reversible.
7. Hormones are water soluble. Therefore, they can be transported in the blood.
8. They can easily diffuse through cell membrane.
9. Hormones cannot be stored as they can be readily destroyed.
10. Excess or deficiency of any of the hormones can lead to serious physiological dysfunction.

Functions of hormones:

1. Homeostasis
2. Metabolism
3. Antistress action
4. Water + mineral regulation
5. Growth
6. Combating emergency
7. Reproduction

Human Endocrine glands are:

1) Hypothalamus:

Types of hormone:

It is classified as:

1. Amine hormone: They are derived from amino acid Tyrosin. They have amino group. Ex: Thyroxine, Adrenaline.
2. Steroid hormone: They are derived from the cholesterol + have steroid group. Ex Cortisol, Aldosterone.
3. Proteinous/Peptide hormones: They have peptide bonds + have 3-200 amino acids. It includes:
   • a) Proteinous hormone: TSH, FSH, GH
   • b) Long Peptide: Insulin, Glucagon
   • c) Small: ADH, Oxytocin

Hypothalamus:

Hypothalamus develop from the ectoderm of the embryo. Hypothalamus is located at the base of the diencephalon, a part of the forebrain just below the thalamus region + above the pituitary gland.

Structure: The hypothalamus is composed of nervous tissue or nerve certain centre N1 + N2. It is connected with the anterior lobe of pituitary gland by hypophysial portal vein + with the posterior lobe of pituitary gland by axon of its neurons. The portal vein + axon pass through a infundibulum to the pituitary lobes.

Hormones: The neurosecretory cells of hypothalamus secrete hormone called neurohormones. These hormones are carried by the portal lobe to the anterior lobe of the pituitary gland + stimulates the pituitary glands to release hormone. They are releasing hormone or factor + inhibitory hormones.

1. Corticotrophic-releasing hormone (CRH): It stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH) which stimulates adrenal cortex to secrete cortisol + aldosterone.
2. Thyroid releasing hormone (TRH): It stimulates anterior pituitary gland to release thyroid stimulating hormone (TSH) + causes release of thyroid hormones from thyroid gland.
3. Gonadotropin releasing hormone (GnRH): It stimulates anterior pituitary gland to release follicle stimulating hormones (FSH) + luteinizing hormone (LH) which stimulates sex glands to release testosterone, oestrogen + progesterone.
4. Growth hormone releasing hormone + Somatostatin: It releases growth hormone + somatostatin that inhibit anterior pituitary from releasing growth hormones.
5. Prolactin-releasing hormone (PRH) + Prolactin inhibiting hormone (PIH): PRH stimulates release of prolactin from the anterior pituitary + causes secretion of milk + PIH inhibits the release of prolactin from anterior pituitary & therefore inhibit milk secretion.

2) Pituitary Glands

Pituitary gland originates from the ectoderm of the embryo. Pituitary gland is located just below the hypothalamus.

It is connected with the brain ie thalamus by means of short thin stalk infundibulum. Pituitary gland works under the influence of hypothalamus. The pituitary gland is also called hypophysis.

Structure: The pituitary is a very small, brownish pea sized gland about 1.3 cm in diameter + weight about 0.5 gm. The pituitary gland is composed of three lobes:

1. Anterior lobe or adenohypophysis or pars distalis
2. Posterior lobe or neurohypophysis or pars nervosa
3. Intermediate lobe or pars intermedia

Hormones of the anterior Pituitary:

1. Follicle Stimulating hormones (FSH): The FSH promotes oogenesis in female + spermatogenesis in male. FSH acts mainly on growth gonads. It stimulates the testes in man to produce sperm + ovaries produces ova thus releases also sex hormones.
2. Luteinizing hormone (LH): It stimulates testes + ovaries. LH is also called Interstitial cell stimulating hormone ie ICSH in male that stimulates interstitial cells of leydig to produce male sex hormone ie testosteron. In female LH stimulates ovulation, formation of corpus luteum + secretion of estrogen + Progesteron.
3. Prolactin: Prolactin stimulates lactation (milk production) in female by stimulating mammary gland during lactation.
4. Adrenal Corticotropic hormone (ACTH): ACTH stimulates the adrenal cortex to produce mineralocorticoids ex: aldosterone, glucocorticoids ex: cortisol + androgen ex: testosteron.
5. Thyroid stimulating hormone (TSH): TSH stimulates the thyroid gland to produce thyroid hormone. It increases the rate of absorption of iodine from blood to produce normal amount of thyroid hormone.
6. Somatotrophin hormone (Growth stimulating hormone) or Growth hormone (GH): GH controls the growth + physical development of body.
7. Melanocyte Stimulating hormone (MSH): MSH is secreted by the intermediate lobe of pituitary + stimulates melanocytes to produce melanin responsible for the colour of skin. It appears to be functional in cold blooded animals such as fishes, amphibians to make skin darker to protect from enemies.

Disorder of the anterior pituitary gland:

1. Prolactin: Hypersecretion is due to prolactinoma. It causes galactorrhoea in men + amenorrhoea in women.
2. Growth hormone: Hyposecretion causes Cretinism ie dwarfish in children with small body size with sexual immaturity. Hypersecretion causes Gigantism in children, abnormal growth (long height) whereas Acromegaly in adult result in the abnormal growth of feet, hand + face especially lower jaw + looks like Gorilla.

B) Hormones of Posterior Pituitary or Neurohypophysis:

The posterior pituitary is made up of nervous tissue extending down from the hypothalamus. It is of 2 types of hormones.

1. Oxytocin/Pitocin: Oxytocin is also called 'birth hormone' because it causes uterine contraction in female child birth. It also help in ejection of milk secreted by mammary gland so called "Milk ejection hormone".
2. Vasopressin or anti-diuretic hormone (ADH): ADH is responsible for tubular reabsorption of water in distal convoluted tubules (DCT) + collecting duct in kidney. ADH concentrates the urine to minimize water loss from the body.

Disorders of posterior pituitary hormones:

ADH: Hyposecretion causes Diuresis ie there is abnormally large volume of urine output the condition called diabetes insipidus.

* Addison's disease: It is caused by the deficiency of mineralocorticoids and glucocorticoids. The symptom includes low blood sugar, low plasma Na+, high potassium plasma, nausea, vomiting, diarrhoea.

Pineal gland:

It is a small reddish grey, cone shaped gland located in the midbrain area. It is attached to the roof of the 3rd ventricle of brain. It is called pineal gland because it resembles the shape of a pine cone. It has no direct connection with the CNS. It is richly supplied with the blood vessels. Two types of cells are present ie parenchymal cells + neuroglial cells. It produces melatonin.

Functions:

1. Melatonin stimulates melanin synthesis in melanocyte of skin in lower vertebrates like frog.
2. Causes pigmentation in skin + also regulates sexual cycle + behaviour.
3. Whereas in human melatonin is involved in dark-light a daily cycle called circadian rhythm. Thus it maintain the normal rhythm of sleep awake cycle, body temperature etc.

Thyroid Gland:

The thyroid gland originates from the endoderm of the embryo. It is the largest endocrine gland (5 by 3cm) + 25gm on weight. It is a butterfly-shaped gland present just below the larynx (in front of neck) with its lobes on either side of trachea.

Structure: It is brownish red gland due to high number of blood vessels. The thyroid gland consist of 2 lobes connected to each by a thin strip of connective tissue called isthmus. Each lobe consist of numerous follicle inclose association with thin walled blood capillaries. The follicle are made up of single layer of secretory cuboidal cells. The cavity called lumen in centre is filled with gel-like fluid called colloid which is inactive thyroglobulin scattered in connective tissue + between thyroid follicle there are group of endocrine cells, called parafollicular cells. Thyroid gland produces three types of hormones. The hormone are stored in the colloid when necessary they are released in blood.

1. Thyroxine (T4)
2. Triidothyroxine (T3)
3. Calcitonin

Functions:

1. These hormones regulate basal metabolic rate (BMR) of the body as it control rate of cell respiration + energy production by mitochondria.
2. It helps in calcium + phosphorous sulphate.
3. They stimulate protein synthesis.
4. They maintain normal body temp in warm blooded animals (homeostasis).
5. They promote physical growth + are also responsible for the mental alertness + sexual growth.
6. They stimulate tissue differentiation + metamorphosis of tadpole of frog to adult.

Disorders due to thyroid hormones:

Hypothyroidism or deficiency of thyroid hormones:

1. Cretinism: It is a disorder in children due to hyposecretion of thyroid hormone from infant to childhood by:
   • Decreased BMR
   • Stunted growth
   • Retarded mental development
   • Delay delayed puberty
   • Decreased body temp
   • Decreased heart rate, pulse rate, blood pressure, reduced urine output
   • Decreased sugar level in blood + Pigeon's chest (chest bulging forward in sternal region)
2. Myxoedema: It occurs due to deficiency of thyroxine in adults. Like Cretinism it also has:
   • Low BMR
   • Low body temp
   • Reduced heart rate, pulse rate, blood pressure + cardiac output + low iodine in blood.
   • Peculiar feature of this includes swollen hand and face due to deposition of albuminous myxomatous tissue.
3. Endemic or Simple goitre: Enlargement of the thyroid gland occurs due to deficiency of iodine in diet. It is characterised by enlargement of thyroid gland due to increase in number + size of acinal cells of thyroid gland. It is more common in people of hilly region to prevent iodine salt is being iodized these days.
4. Hashimoto's disease: It is also called auto-immune thyroiditis + occurs due to age factor or injury to thyroid gland causing hyposecretion of thyroxine. When thyroxine secretion falls up to minimal limit the antibodies formed destroys the thyroid gland.

Hyperthyroidism or increased activity of thyroid gland: It leads to excess hormone in the body. It leads to the condition called exophthalmic goitre. It include:

• Enlargement of thyroid gland (2-3 times more than normal size)
• Increased BMR
• Increased heart rate, pulse rate + cardiac output
• Peculiar symptom is the bulging of eyeball with staring looks + less blinking
• Osteoporosis due to loss calcium + phosphorous from bones.

II. Thyrocalcitonin (TCT): It is long peptide hormone secreted by parafollicular cells of thyroid gland. Its secretion is regulated by increased plasma level of calcium. Its secretion is by feedback mechanism as the level of calcium is blood increased its secretion increases + causes lowering of calcium in blood depositing in bone thus there is less chances of osteoporosis.

4) Parathyroid gland:

The parathyroid gland originates from ectoderm of embryo. Parathyroid are found embedded on the dorsal surface of the thyroid gland ie two gland in each lobe of thyroid gland. They are in the form of four small oval shaped yellow gland two on each side of the thyroid gland. They are just 2-4mm long + weight about 0.05-0.03gm. These are totally independent of the thyroid both developmentally + functionally. The gland consist of polygonal cells called chief cells + oxyphil cell.

Hormone: They produce only one hormone called parathyroid or parathyroid hormone (PTH) or Collip's hormone. It is polypeptide hormone + is called hypercalcemic hormone.

Functions:

1. It increase the calcium level in the blood + decrease the phosphate level in the blood plasma by breakdown of osteocyte of bone.
2. It increases the uptake of Ca++ by the stomach (gut) from digested food + also reabsorption of Ca++ by renal tubules of kidney. Thus PTH is a hypercalcemic hormone (↑ blood Ca++ level).
3. It affects the growth of bone, membrane permeability, nerve functioning + the muscular activity of body.

Disorders:

1. Hypoparathyroidism: It leads to parathyroid tetany ie there is fall in the calcium level in blood + causes painful spasmodic contraction of muscles of face, hands, feet + increases neuroexcitation + may causes death because of closure of glottis.
2. Hyperparathyroidism: This order is related to excess level of PTH. It causes osteoporosis, softening + bending of bones + kidney stones + decreases urine output.

5) Adrenal Glands:

Adrenal glands have due origin: mesoderm + ectoderm. There are 2 small yellowish gland each present just above the kidney so they are called suprarenal glands.

Structure: The adrenal are small conical, yellowish gland. They are richly vascularized + are smaller in female while larger in male. It weights about 5gm each. It is composed of two distinct region: an outer cortex (80-90%) + an inner medullar (10-20%). The tissue in these two region differ in origin hence they function as two different glands.

A) Adrenal Cortex:

It is mesodermal in origin. Pink tissue. It is divided into 3 layers:

1. Zona glomerulosa [outer layer]
2. Zona fasiculata [middle layer]
3. Zona reticularis [inner layer]

The 3 layers produces hormones commonly called corticoids. The adrenal cortex is essential for life. Its destruction or removal causes death of animals.

Hormones: The adrenal cortex produces steroid hormones. Cortical steroid can be grouped into three function:

1. Mineralocorticoids: These hormone are secreted by the cells of zona glomerulosa. Aldosterone is most important mineralocorticoid (salt-retaining hormone).
   • They regulate mineral metabolism + control the sodium + potassium ration in extracellular + intracellular fluid + control BP.
   • Target cell: mineralocorticoids acts on cells of kidney.
   • Functions: It maintain high sodium level in blood (hypernatraemic) + maintain low potassium level in blood (hypokalemia) + collected in interstitial fluid that causes hypertonic condition + helps in water absorption ie water + electrolyte balance. It is also called salt retaining hormone. Decrease sodium loss from urine + sweat.
2. Glucocorticoids: These hormones are secreted by the cells of zona fasiculata. It secrets 3 main hormones Cortisol, cortisone + corticosterone. Cortisol is main that stimulate liver to synthesize carbohydrate by connecting protein + fats.
   • Target cell: It acts on cells of liver.
   • Functions: i) It convert glycogen to glucose that result in hyperglycemia. ii) It reduces utilization of glucose. iii) They acts as antiinflammatory + antiallergic ie reduces pain + inflammation so used in treatment of asthma, organ transplantation, autoimmune disease, rheumatoid arthritis.
3. Gonadocorticoids (Sex Corticoids): These hormones are secreted by zona reticularis. These hormones are called sex hormones.
   • Testosterone: Stimulates development of male secondary sexual character, distribution of body hair, deepening of voice.
   • Estrogen: Stimulates female secondary sexual character + onset of menstruation cycle.
   • Target cell: These hormones acts on gonads (testes, ovaries).

Disorders: Improper functioning of adrenal cortex produces many disorders.

1. Addison's disease: This disease is caused by deficiency of mineralocorticoids + glucocorticoids. The characteristic features are: low blood sugar, low Na+ ion conc in plasma + high K+ ion concentration in plasma, increase loss of Na+ ions + water in urine, weight loss, weakness, vomiting + diarrhoea.
2. Cushing Syndrome: This is caused by excess of cortisol. It is characterised by: high blood sugar, excretion of sugar in urine, high Na+ ion conc in plasma + low K+ conc in plasma, obesity.
3. Adrenal Virilism: Appearance of male secondary sexual character in female is virilism. It is caused by excess of gonadocorticoids in female. The female develop male secondary sexual character such as beard, moustaches + hoarse voice.

B) Adrenal Medulla:

It forms the central core of adrenal gland. It is an internal soft dark reddish brown tissue.

Hormones: It secretes two catecholamine hormone ie adrenaline (epinephrine) + noradrenaline (nor-epinephrine). It is also called emergency hormone or hormone of fight or flight. The function of the hormones are:

1. Increases blood sugar level
2. Increase the supply to brain + heart muscles
3. Rate of heartbeat increase
4. Dilate pupil
5. It causes contraction of spleen to pour the blood
6. Increase BP

Noradrenalin: It acts opposite to the adrenaline. It brings to normal condition ie slows the process except digestion + excretion.

Thymus Gland

Thymus gland is a lobular structure located on the dorsal side of heart + the aorta. Its outer part is called cortex + inner medulla. Production of antibody + development of immune response. It consist of two types of cells:

1. Thymocytes secrets thymotic hormones + releases thymosin
2. Hossal's cells has phagocytic nature

Function:

• It helps in stimulation of T-lymphocytes
• Thymosin helps in cell division
• It help in growth in early stage
• Promotes production of antibodies

Pancreas

Pancreas develop from endoderm of embryo. It is the second largest gland of the body. It lies below the stomach in a bend of the duodenum.

Structure: Pancreas is a elongated yellowish gland. It is about 12-15 cm long + weigh about 60 gm. It is both an exocrine + endocrine gland. The exocrine part secrete enzyme which drain into duodenum with the help of duct. The endocrine part consist of three types of cell which are arranged as islets of Langerhans. These cells are:

• α cell: Secrete glucagon
• β cell: Secrete insulin
• δ cell: Secrete somatostatin
• γ cell: Precursor of α-β cells (regenerate α-β cells)

1. Glucagon: If secreted by the alpha cells of the islets of Langerhans in response to fall on the blood glucose level. It stimulates liver to convert stored glycogen into glucose.
   • Target cell: glucagon acts on the cells of liver and adipose tissue.
2. Insulin: It is secreted by the beta cell of islets of Langerhans on the stimulation by the rise in blood glucose level such as after meal. It serve many function such as:
   • It enables liver muscle to store glucose as glycogen + enables the tissue to take up glucose as a source of energy thus lowers the blood glucose level.
   • It promotes synthesis of protein from amino acid in the tissue.
   • It reduces breakdown of protein to maintain the tissue.
   • It promotes the synthesis of fat from fatty acid + glucose in the adipose tissue.
   • It reduces breakdown of fat thus insulin is a anabolic process.
   • Target cells: Acts on the cell of liver, muscles and adipose tissue.
3. Somatostatin (SS): It is secreted by the delta cells. It inhibit the secretion of glucagon + insulin. It also decrease the secretion, motility + absorption in the digestive tract.

Disorders of Pancreas:

1. Diabetes mellitus (Hyperglycemia): It leads to the increase in the level of glucose in blood. This is due to under activity of beta cells in islets of Langerhans that result in reduction in secretion of insulin. Such condition is called insulin-dependent diabetes.
2. Hypoglycemia: It occurs when the blood glucose level falls below normal. It may be caused by an excess of insulin a deficiency of glucagon or a failure of secretion of both the hormone insulin + glucagon to completely regulate the blood glucose level symptoms include: weakness, profuse sweating, irritability, urgent intake of glucose, unconsciousness, confusion.

Sex Glands / Gonads / Sex Organ

Gonads are the reproductive organ that produce gamet they are also the endocrine gland that produce sex hormones.

1. Testes: Male have a pair of testes + produce male sex hormones testosterone. In the testis, there is exocrine (seminiferous tissue) + endocrine part ie interstitial leydig cells.
   1. Testosterone
   2. Androsterone
   3. Epiandrosterone
   4. Dehydroepiandrosterone

   Functions:

   1. It helps in development of secondary male sex organs ie epididymis, prostate gland, seminal vesicles.
   2. It also help in the development of secondary sex character: facial hair, deepening of voice, broadening of shoulder.
   3. Elongation of long bones.
   4. It also help in erythropoiesis.

   Disorders: Less androgen causes Eunuchoidism that causes under-development of secondary male sex organ.

2. Ovaries: Female have a pair of ovaries that produce ova as well as sex hormone (1) estrogen.
   1. Estrogen: It is steroid hormone secreted by Graafian follicles.

      Functions:

      • It helps in the development of secondary female sex organs.
      • Development of secondary sex character include oogenesis menstrual cycle, enlargement of breast, presence of hairs, female voice.
      • If decrease follicle stimulating hormone + increases luteinizing hormone in pituitary.
      • It brings about the repeated cyclic change in the uterus like thickening of uterine lining.
      • Hyposecretion: Cause Hypogonadism.
   2. Progesteron:
      • It secreted by corpus luteum.
      • It helps in thickening of endometrium.
      • It helps implantation of embryo.
      • It lowers the secretion of LH.
      • It promotes the proliferation of mammary gland + enlargement of breast.
   3. Relaxin: It is secreted by corpus albicans. It helps in parturition to soften the pubic symphysis + opening is wider that helps in child birth.