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July 31, 2023 / 03:48 PM IST

HUMAN ENDOCRINE SYSTEM

The study of ductless glands (the endocrine glands) & their secretions (hormones) is called endocrinology.
Thomas Addison is known as the father of endocrinology (he discovered Addison’s disease).
Endocrine glands do not possess ducts to carry secretions out hence these glands are called as ductless glands. Secretions of these glands (hormones) released directly into the blood/lymph.
These secretions (hormones) are circulated in the body to reach their target cells/tissues/organs.
The main endocrine glands are… The hypothalamus, Pituitary gland, Pineal gland, Thyroid gland, Parathyroid glands, Thymus gland, Adrenal gland, Pancreas,Testes, Ovary etc.
The hypothalamus is the part of brain and thus comprises nervous tissue.
The pituitary and pineal glands are pure endocrine glands that are closely associated with the hypothalamus. Thus, these constitute the neuro-endocrine system.
The hypothalamus acts as the master control facility and controls all other endocrine glands through the pituitary gland.
The pituitary is the master gland of endocrine system, which controls other endocrine glands through its tropic hormones.
In addition to the main endocrine glands, few tissues in digestive tract, kidney, liver & heart also perform endocrine functions.
In this chapter we will discuss the chemical coordination brought about by the hormones of main endocrine glands in the human body.

Hormones

Starling was coined the term ‘hormone.’
First detected hormone is “Secretin’
Gastrin, somatostatin are the local hormones.
Hormones coordinate, accelerate & inhibit one or more phy siological functions.
Many of the hormones are highly specific regarding their targets (TSH, ACTH), but few hormones such as thyroxin & somatotropin hormones act on almost all somatic cells.
Hormones are short lived.
Hormones degraded by tissues & excreted through bile (by the liver) and urine (by the kidneys).
Hypo & hyper secretion of hormones produce certain disorders.
Based on chemical nature hormones are classified into following different groups…
Amino hormones
Peptide hormones
Protein hormones
Glycol-protein hormones
Steroid hormones
Fatty acid hormones etc.
Amino hormones:These are deriv atives of a single amino acids.
Catecholamines (Epinephrine & nor-epinephrin) & Thyroxine, derivatives of tyrosine.
Melatonin, a derivative of tryptophan.
Peptide hormones:These are polymers of 3 to 49 amino acids.
Vasopressin/ADH
Oxytocin
Protein hormones:These are poly mers of 50 to 200 amino acids
Hormones of anterior pituitary,
Hormones of pancreas,
Hormones of parathyroids.
Glyco-protein hormones:Complex protein hormones with carbohydrate side chains.
TSH l FSH l LH.
Steroid hormones:These are derivatives of cholesterol.
Aldosterone l Testosterone
Estrogen l Progesterone
Fatty acid hormones:These are derivatives of eicosanoids. Produced by all tissues.
Prostaglandins
Eicosanoids: a large group of molecules derived from the essential fatty acids (omega-3-fatty acids & omega-6-fatty acids).

The Hypothalamus

The hypothalamus is the region of the forebrain below the thalamus.
The hypothalamus acts as the master control facility and controls all other endocrine glands through the pituitary gland.
It performs neural and endocrine functions.
It helps in the maintenance of…
Osmoregulation
Body temperature &
Th key functions of homeostasis.
It also regulates…
Hunger,Thirst & Sexual functions.
The neurosecretory cells (nuclei) present in the hypothalamus act as endocrine cells.
These neurosecretory cells secrete two types of nine hormones.
a. The releasing hormones- stimulate secretion of pituitary hormones.
b. The inhibiting hormones- inhibit secretions of pituitary hormones.
Totally, above two types of nine hormones control the endocrine functions of the anterior pituitary gland, as detailed below.
1. Thyrotropin-releasing hormone (TRH): It stimulates adenohypophysis of pituitary to release TSH & Prolactin
hormones.
2. Gonadotropin-releasing hormone (GnRH): It stimulates pituitary to release FSH and LH. Through these two hormones it triggers sexual development and regulates normal sexual functions.
3. Growth hormone-releasing hormone (GHRH) It stimu lates pituitary to release GH.
4. Carticotropin-releasing hormone (CRH): It stimulates pituitary to releaseACTH.CRH also synthesized by the placenta. It determines the duration of pregnancy and protects the foetus from immune attack by T cells of mother.
5. Growth hormone releasing inhibiting hormone (GHRIH) It inhibits the release of growth hormone by adenohypophysis of pituitary.
6. Prolactin releasing hormone (PRH): It stimulates anterior pituitary to release prolactin hormone.
7. Prolactin releasing inhibiting hormone (PRIH): It inhibits the release of prolactin hormone by anterior pituitary.
8. Melanocyte stimulating hormone releasing hormone (MSHRH): It stimulates the release of MSH by pituitary.
9. Melanocyte stimulating hormo ne releasing inhibiting hormo ne (MSHRIH): It inhibits release of MSH from pituitary.
In addition to above hormones, the hypothalamus also produces two peptide hormones…
1. Vasopressin,
2. Oxytocin.

The Pituitary Gland

The pituitary is the master gland of endocrine system.
It is also called as hypophysis.
This is a pea-sized, stalked gland present at the lower part of the hypothalamus. It lies in a bony depression called sella turcica of the sphenoid bone.
Anatomically, it is divided into
a. Anterior pituitary/ adenohypophysis
b. Posterior pituitary / neurohypophysis.
Anterior pituitary/adenohypophysis:
Adenohypophysis consists of two portions…
Pars distalis and
Pars intermedia.
The pars distalis: the cells of this portion produce…
Growth hormone (GH),
Prolactin (PRL),
Thyroid stimulating hormone (TSH),
Adrenocorticotrophic hormone (ACTH),
Gonadotropins.
Growth hormone (GH)
It is also called somatotropin.
It promotes growth of bones of fore and hind limbs.
Over-secretion of GH stimulates abnormal growth of the body leading to gigantism (pituitary giants) and low secretion of GH results in stunted growth resulting in pituitary dwarfism (midgets).
Excess secretion of growth hormone in adults especially in middle age can result in severe disfigurement (especially of the face) called Acromegaly.
Prolactin(PRL)
this regulates the growth of the mammary glands and formation of milk in them.
Thyroid stimulating hormone (TSH)
TSH stimulates the synthesis and secretion of thyroid hormones from the thyroid gland.
Adrenocorticotrophic hormone (ACTH)
ACTH stimulates the synthesis and secretion of steroid hormones called glucocorticoids from the adrenal cortex.
Follicle stimulating hormone (FSH) and Luteinizing hormone (LH) or Interstitial cell stimul ating hormone (ICSH) are the two principal gonadotropins.
In males, LH stimulates the synthesis and secretion of hormo nes called androgens from testis.
In males, FSH and androgens regulate spermatogenesis.
In females, LH induces ovulation of fully mature follicles (graafian follicles) and maintains the corpus luteum, formed from the remnants of the graafian follicles after ovulation.
In females,FSH stimulates growth and development of the ovarian follicles in females.
The Pars intermedia:
Pars intermedia secretes only one hormone called melanocyte stimulating hormone (MSH).
MSH acts on the melanocytes (melanin containing cells) and regulates pigmentation of the skin.
Posterior pituitary/ neurohy pophysis:
This part of pituitary stores, and releases two hormones…
oxytocin and vasopressin.
These hormones are synthesized by the hypothalamus and are transported axonally to neurohypophysis.
Oxytocin:
Oxytocin acts on the smooth muscles of our body and stimulates their contraction.
In females, it stimulates a vigorous contraction of uterus at the time of child birth, and milk ejection from the mammary gland.
Vasopressin:
Vasopressin acts mainly at the kidney and stimulates resorption of water and electrolytes by the distal tubules and thereby reduces loss of water through urine (diuresis). Hence, it is also called as anti-diuretic hormone (ADH).
An impairment affecting synthesis or release of ADH results in a diminished ability of the kidney to conserve water leading to water loss and dehydration. This condition is known as Diabetes Insipidus.
The Pineal Gland
The pineal gland is also known as the pituitary body and the epiphysis
It is located on the dorsal side of forebrain
This glandproduces a hormone called melatonin which is derived from serotonin.
Melatonin regulatescircadian rhythms (a 24-hour (diurnal) rhythm of our body).
It helps in maintaining of our sleep-wake cycle, body temperature.
It influences metabolism, pigme ntation, the menstrual cycle, and defense capability.
Melatonin levels are higher in children but decline with increase in age.