Chemical Endocrine System

Endocrine system formed of all endocrine glands of body. Though different endocrine glands are different in embryonic origin and are isolated from one another but these interact with one another so collectively form an endocrine system. Endocrine system along with nervous system, controls and coordinates the body functions and maintains a homeostais. So both are collectively form neuro-endocrine system. The study of these two systems is called neuro-endocrinology.

Brief History

  • Berthold (1849) first showed the loss of secondary sexual characters in cocks by removal of testes.
  • Schiff (1854) reported animals dying after removal of thyroid gland.
  • Claude Bernard (1855) established the control of endocrine glands by nervous system.
  • Addison (1855) demonstrated the symptoms of disease (i.e. the Addison’s disease) in human by removing the cortical part of adrenal gland, he is therefore, called as Father of Endocrinology.

Glands of body : A cell, a tissue or an organ which secretes certain useful chemical compounds is called a gland. Animals have three types of glands.

(i) Exocrine gland (Gr., ex = out + krinein = to secrete) : These glands have ducts for discharging their secretions. Therefore, they called duct glands. ex – Liver, Sweat gland, Sebaceous gland, Gastric glands and some intestinal glands.

(ii) Endocrine glands ( Gr., endo = within + krinein = to secrete) : These glands lack ducts and pass secretions into the surrounding blood directly. Therefore they called ductless glands. ex – Thyroid, parathyroid, adrenal, pituitary, pineal body and thymus.

(iii) Heterocrine glands : These glands consist of both exocrine and endocrine tissue. The exocrine discharge its secretion by a duct and the endocrine tissue discharge its secretion into the blood. Pancreas and gonads are heterocrine glands. These are also called mixed glands.

Hormone (Gr. hormaein = to excite)

Hormones are informational molecules secreted by the endocrine cells in one part of the body and carried by blood to another part where they stimulate or inhibit specific physiological process. In other words the hormones are chemical messengers or informational molecules that regulate the biological processes and metabolism. Hormones organs called target organs. Targate cells have receptor proteins for specific hormone.

Discovery : First hormone discovered was secretin. It was discovered by two English physiologists  William M Bayliss and Ernest H. Starling in 1903.

Nomenclature :  Term hormone was coined by starling (1905) from Greek work Hormaein means to excite. It is a mishomer because a number of hormones are known to have inhibitory effect (e.g. Somatostatin)

(i) Properties of hormones

(a) These are secreted by endocrine gland (biogenic in origin).

(b) Their secretions is released directly into blood (except local hormones e.g. gastrin).

(c) These are carried to distantly located specific organs, called target organ.

(d) These have specific physiological action (excitatory or inhibatory). These co-ordinate different physical, mental and metabolic activities and maintain homeostasis.

(e) The hormones have low molecular weight e.g. ADH has a molecular weight of 600–2000 daltons.

(f) These act in very low concentration e.g. around10–10 molar.

(g) Hormones are non antigenic.

(h) These are mostly short-lived. So have a no camulative effect.

(i) Some hormones are quick acting e.g. adrenalin, while some acting slowly e.g. ostrogen of ovary.

(j) Some hormones secreted in inactive form called Prohormone e.g. Pro-insulin.

(k) Hormones are specific. They are carriers of specific information to their specific target organ. Only those target cell respond to a particular hormone for which they have receptors.


Chemically of two types :

(1) Proteinoid and

(2) Steroid hormone

with different types of influence upon the cellular processes.

Proteinoid hormones are amino acid, polypeptides, proteins and glycoproteins. They do not enter the cell but accomplish their effects at the membrane level in two following ways:

It alters the permeability of membrane thereby allowing various substances to cross through.

It stimulates the enzyme cascade of the cell membrane, most common is adenyl cyclase enzyme which mediates the formation of cyclic-AMP (c-AMP) from ATP.

c-AMP acts as second messenger to affect the cellular metabolism in various ways (Sutherland).

Steroid hormones are lipids or derivatives of fatty acids which with the help of some receptors, can directly enter the target cell and affect the gene activity inducing the synthesis of specific protein (or enzymes) or suppressing the activity of other genes. This involves the synthesis of enzymes or stopping it.

Thyroid hormones, though proteinoid type (amino acids), acts as steroid hormones.

Control Of Hormone Secretion

Secretion of many hormones are under direct control of hypothalamus which secretes the ‘releasing hormone’ (RH) or inhibiting hormone (IH) for the concerned hormones ; e.g. GHRH (Growth hormone releasing hormone) and GHIH (Growth hormone inhibiting hormone).

Differences between hormone and enzymes : Though both hormones and enzymes regulate the body functions, but they have following differences.







Always proteinaceous

May be proteinaceous, or amine or steroids.


Molecular weight

Macromolecules with high molecular weights.

Have low molecular weights.



Non-diffusible through cell membrane.

Diffusible through cell membrane.


Site of action

Either act intracellularly or carried by some duct to another site.

Generally carried by blood to a target organ.


Mode of action

Always act as biocatalysts and increase the rate of metabolic physiological process.

May be excitatory or inhibitatory in their physiological action.



These catalyze reversible reactions.

Hormone controlled reactions are not reversible.


Effect of concentration

Reaction rate increase with increase in their concentration upto a limit.

Deficiency or excess of hormone causes metabolic disorders and diseases.



Act quickly

Some are quick acting, while some are slow acting with a lag period.



Not used in metabolic functions.

Used up in metabolic functions.

Difference between hormone and vitamin :







Synthesized in the endocrine cells of body.

Taken along with food from outside.



Steroids or proteinous or amino acid derivatives.

Simple organic compounds like amines, esters, organic acids etc.



Either excitatory or inhibatory. Do not act as co-enzymes.

These generally act as co-enzymes for enzyme activity.


Cause of disorders

Both excess as well as deficiency of hormones.

Generally avitaminosis (deficiency of vitamins) leads to deficiency diseases.

Differences between nervous and hormonal informations :



Nervous control

Hormones control


Speed of action

Always quick acting.

May be quick acting or acting with a long period.


Mode of transmission of informations

As electrochemical nerve impulses.

As chemical messengers.


Path of transmission

Through nerve fibres.

Through blood.


Direction of the informations

Towards a specific direction (effector organ or CNS).

Released in general blood circulation from where taken by specific receptor.



For quick reactions like reflexes.

For long-term changes e.g. maintenance of pregnancy.



Short time effect.

Long lasting.

Formation of cAMP : Mechanism of formation of cAMP was discovered by E.W. Sutherland in 1950. The hormone receptor complex causes the release of an enzyme adenyl cyclase. From the receptor site. This enzyme hydrolise the ATP into c-AMP. The c-AMP activates the existing enzyme system of the cell. This accelerates the metabolic reactions in cell. The hormone is called first messenger and the c-AMP is termed the second messenger. e.g. Adrenaline causes the secretion of glucose from the liver cell from this mechanism.

chemical endocrine system

Functions of some important hormones

  • MSH controls skin colour.
  • Pituitary controls other endocrine glands.
  • Thymosine secreted by the thymus gland provides immunity to the infants.
  • Thyroid is the largest gland. Its hormone thyroxine controls oxidative metabolism.
  • Normally, family planning pills consists of estrogen and progesterone.
  • The Leydig cells secrete testosterone.
  • Steroid sex hormones are secreted by the gondas. The hormones control the process of reproduction and secondary sexual characters.
  • Adrenal gland is found attached to the kidney as cap. This gland secretes adrenalin and non-adrenalin hormones.
  • Oxytocin controls parturition.
  • Prolactin controls growth is mammary glands and secretion of milk in woman.
  • FSH controls spermatogenesis.
  • LH controls secretion of androgen from the Leydig cells in man and helps in the release of ovum from the ovary in woman.

Pituitary Gland (Hypophysis)

Pituitory is known as hypophysis cerebri, its name pituitary was given by vesalius. Muller,s gland of amphioxus and subneural gland of hardmania is homologous to pituitary of vertebrates. Weight to pituitary is 0.5 gm. Removal of pituitary is knows as hypophysectomy.

chemical endocrine

Structure of pituitary gland :

Pituitary gland is comprised of two main lobes – Adenohypophysis and Neurohypophysis. Adenohypophysis is arises as hypophysial or Rathke's pouch from dorsal wall of embyronic stomodeum. It is the anterior lobe of pituitary. The neurohypophysis (Pars nervosa or Posterior lobe) form as an outgrowth from the infundibulum of the floor of hypothalamus.

endocrine chemical system

Thyroid gland

The name "thyroid" was introduced by Thomas Wharton (1656). It is derived from Greek "Thyreos" a shield.

(i) Location : This is the largest endocrine gland of our body. It is located in our neck upon the ventral aspect of larynx (sound box or Adam's apple) and a few anteriomost tracheal rings. It is a dark brown and H-shaped bilobed gland.

(ii) Origin : It is endodermal in origin and arises in the embryo as a midventral process from the floor of the tongue in pharyngeal region between the first and second pharyngeal pouches. Later, the duct-like connection (thyroglossal duct) of the process degenerates, so that the process is separated from the tongue and becomes endocrine. Probably, the gland is homologous to the endostyle of lower chordates.

Structure of thyroid gland :

Structure of thyroid gland

Difference between Adrenal cortex and Adrenal medulla


Adrenal cortex

Adrenal medulla


It is external firm region of the adrenal gland.

It is central soft region of the adrenal gland.


It is pale yellowish-pink in colour.

It is dark reddish-brown in colour.


It is enclosed by a fibrous capsule.

It is not enclosed by a fibrous capsule.


It forms about 80% of the adrenal capsule.

It forms just 20% of the adrenal gland.


It develops from the mesoderm.

It develops from the ectoderm (neural crests).


It consists of 3 concentric regions : Outer zona glomerulosa, middle zona fasiculata and inner zona reticulars.

It is not differentiated into regions.


It is essential for life, its destruction causes death.

It is not essential for life, its destruction does not cause death.


It secretes 3 groups of hormones : mineralocorticoids, glucocorticoides and sexocortocoids.

It secretes 2 similar hormones nor adrenaline and adrenaline.


It is stimulated to release its hormones by the adrenocorticortrophic hormone from the anterior pituitray.

It is stimulated to secrete its hormones by nerve impulses reaching via sympathetic nerve fibres.


There is no cooperation between adrenal cortex and sympathetic nervous system.

Adrenal medulla and sympathetic nervous system function as an integrated system called sympatheticoadrenal system.


It causes many deficiency / excess disorders.

It is not known to cause any disorder.

Difference between diabetes mellitus and diabetes insipedus


Diabetes mellitus

Diabetes insipidus


It is due to deficiency of insulin.

It is due to deficiency of ADH.


The blood sugar becomes high and glucose appears in urine.

The blood glucose is normal and glucose does not appear in urine.


There is high blood cholesterol and ketone body formation.

There is no such phenomenon.

Thymus gland

Origin and position : The thymus gland is located in the upper part of the thorax near the heart. It is endodermal in origin, arising in the embryo from the epithelium of outer part of third branchial pauches.

Structure :

Thymus gland

  • Endocrine glands are also called ductless glands or glands of internal secretions.
  • Hormones are also called autocoids or chemical messengers or information molecules.
  • Parahormones : Hormone-like substances but are not produced by the endocrine glands e.g. pheromones and prostaglandins.
  • Feedback inhibition : In this, end product sends certain inhibitory signals (called negative feedback) when end product is at required level.

Origin of endocrine glands :

  1. Endodermal—Thyroid, Parathyroids, Islets of Langerhans, Gastric glands, Intestinal glands, Thymus, etc.

  2. Ectodermal—Pituitary gland, Hypothalamus, Pineal body.

  3. Mesodermal—Gonads.

  4. Ectomesodermal—Adrenal gland.

  • Ecdysone : A steroid hormone secreted by prothoracic glands present in the prothorax of insects like cockroach and controls moulting or ecdysis.

  • Juvenile hormone : Secreted by a pair of rounded endocrine glands called corpora allata, present just behind corpora cardiaca.

  • Spleen does not secrete any hormone.

  • Thyroid is largest endocrine gland. It is larger in size in human female than male.

  • Gudernatch (1912) : Metamorphosis of tadpole into adult frog controlled by thyroxine.

  • Hashimoto disease : An auto-immune thyroiditis. Also called suicide of thyroid.

  • Parathyroids were reported by Raynard (1835) while its structure was given by Sandstrom. Endocrine cells of parathyroids are called chief cells.

  • Adrenal glands were first reported by Eustachius, while it was differentiated into adrenal cortex and adrenal medulla by Cuvier (1805).

  • Aldosterone is also called salt retaining hormone as increases Na+ reabsorption.

  • Nandrolone : Steroid used by sportsmen/sportswomen to improve their performance.

  • Chromaffin cells : Endocrine cells of adrenal medulla.

  • In human pituitary, the intermediate lobe is functional in embryo but is rudimentary in adult.

  • Muller’s gland of Amphioxus and sub-neural gland of Herdmania are homologous to pituitary gland of vertebrates.

  • Pituitary dwarf is called midget, while pituitary giant suffers from gigantism.

  • Hyposecretion of TSH leads to thyroid atrophy, while its hypersecretion may lead to Grave’s disease.

  • Oxytocin stimulates milk ejection so has a galactogogic effect.

    • Antidiuresis : Decreased urine output as more water is retained in body tissues. It occurs due to increased secretion of ADH.

  • Simmond’s syndrome is due to complete failure of anterior pituitary.

  • Alpha cells form 15-25% of islet cells and lie along the periphery while -cells form
    70-80% and -cells only 5% of islet cells.

  • Islets of Langerhans also contain delta cells which secrete somatostatin.

  • Leydig’s cells of testes form about 10% of testicular volume in adult man.

  • Progesterone : Also called anti-abortion hormone.

  • Prostglandins : These are lipid compounds first reported in seminal fluid of man and are produced by prostate gland. Term prostglandins was coined by von Euler (1937). These control either contraction/relaxation of smooth muscles so shrinking or dilation of uterus and fallopian tubes.

  • Insulin shock : Quick fall of sugar level upto 43 mg/100 ml of blood when insulin is injected after exercise or delayed meal. It causes unconsciousness and may lead to death.

  • Renin : An enzyme secreted by juxtaglomerular cells of kidney and helps in osmoregulation through renin-angiotensinogen system.

  • Erythropoeitin : A glycoproteinous hormone of kidney which regulates erythropoeisis in bone marrow.

  • Contraceptive pills contain oestrogens and progesterone so called combined pills. These check ovulation and pregnancy in female.

  • Alloxan treatment damages -cells of Islet of Langerhans which lead to hyperglycaemia and glycosuria.

List of hormones their chemical nature and functions

Name of endocrine gland

Name of hormone and

its chemical nature


(1)Neurosecretory cells of Hypothalamus (Supraoptic Nucleus and Paraventricular Nucleus)

  1. Oxytocin and vasopressin nanopeptide.


(i)Milk ejection and parturition (oxytocic effect).

(ii)Vasoconstriction and antidiuretic (vasotocin) effects.  

(2)Gonadotropin releasing hormones

Stimulates FSH and LH sysnthesis.

(3)Other releasing hormones e.g. TSHRH, MSHRH, ACTHRH, GHRH etc. Proteinaceous

Stimulate TSH, MSH, ACTH GH secretions from pituitary.



(Pass Nervosa)

(b) Adenohypehypsis

(contains diverse cell types)


Store and release Oxytocin and Vasopressin.

Hormone release is related to physiological state and requirements.

Proteincaceous or glycoprotein

Affect growth, development differential pubertal changes and other metabolic mechanism.

(3) Pineal

Melatonin-derived from the amino acid tyrosine

  1. Antagonist to FSH / LH

(2) Regulates biological / circadian rhythms.

(4) Thyroid gland ((amine hormone) having – NH2 group)

(a) Thyroxine, iodinated amino acid called tyrosine (T2, T3, T4).

(a) Controls basal metabolic rate (BMR). All organ / system of body responds to thyroxine.

(b) Thyrocalcitonin (Peptide)

(b) Facilitates Ca+2 absorption

(5) Parathyroid gland

Parathormane, Peptide

Ca+2 and PO4 metabolism.

(6) Thymus

Thymosine (polypeptide)

Anti-FSH and LH; delays puberty

(7) Islets of lengerhans

(= Endocrine pancrease)




GlucagonIsolated by



(i)Gluconeogenesis / Glycogenolys

(ii) Glycogenesis

(iii)Gastric functions

(8)Adrenal gland

(a)Adrenal medulla

(Amine hormone have – NH2)


(a)Catecholamines (epinephrine = adrenaline, and norepinephrine = noradrenaline (derived from tyrosine)

  1. Stresses = emergency = Fright, Fight and Flight Hormone (3F) acclerates cardiac functions muscle activity etc.

(b)Adrenal cortex

(b)Mineralcorticoids and glucocorticoids and traces of androgen and estrogen steroids derived from cholesterol

(b)Electrolyte and carbohydrate metabolism.

(9) Ovary

(a)Ganulosa cells steroid, fat soluble have sterol group derived from cholesterol


Estrogen (Steroid)

Estrone, estradiol

(a)Secondary sex character primary action on uterine endometrium mitogenic.

(b)Corpus luteum

Estrogen and Progesterone  (Steroid)

(a)Secreted during luetal phase of menstrual cycle in human female and oestrous cycle of other mammals. Prepares uterine endometrium for receiving blastocytes for implantation. Progesterone is also called pregnancy hormone and is anti-FSH and anti-LH/anti-LTH.

(c)Placenta temporary endocrine gland formed during pregnancy

(a)Steroid secreted are estrogen and progesterone

(b) Relaxin-Polypeptide

(a)Maintenance of pregnant state, prevents lactogenesis folliculogenesis, and Ovulation.

(b)Act on pubic symphysis and enlarges the birth canal to facilitate birth. Acts synergestically with oxytocin during this process (parturition)

(10) Testis

(i)Sertoli cells

(=sustentacular cells)

  1. Inhibin – Polypeptide

Inhibits FHS action and attenuates spermatogenesis decrementally

(ii)Leydig cells

(=Interstitial cells)

(ii)Estradiol-Steroid Androgens (e.g. Testosterone) Steroid androstenedione)


(i)Pubertal changes in male

(ii) Secy. sex characters in male

(iii)Sex drives


(11) Gastro-intestinal hormones

(secreted by cells of mucosa of stomach and intestine) also called hormones


Stimulates gastric juices secretion from gastric gland, movement of sphincters of stomach and increased movement of stomach

(a)Pyloric stomach (Argentophil cells) Intestine



(ii)Cholecystokinin (CCK)



(v) Enterokinin


(i) Stimulates secretion of succus entericus

(ii) Bile released from gall bladder

(iii) Inhibits gastric secretin

(iv)Stimulates secretion of mucous from Brunner's gland

(v) Stimulate intestinal gland

(vi) Stimulate villi movement

Disease caused by hormonal irregularities

















Simmond's disease




Diabetes incipedus








Simple goitre








Exophthalamic goitre








Plummer's disease




Addison's disease



Adrenal cortex

Conn's disease



Adrenal cortex

Cushing's disease



Adrenal cortex

Diabetes mellitus




Myasthenia gravis





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