Endocrinology
ü Endo = Internal , Crions = secretion
ü Branches of medical science which deals with the study of different endocrine glands of the body, are referred to as endocrinology.
What is gland?
Gland is an organ that consists of cells that secrete materials into other regions of the body.
Types of glands:
There are two types of glands. Those are-
1. Endocrine
2. Exocrine
Exocrine Glands
These glands typically secrete material into ducts that lead to the body surface or to one of the cavities that is continuous with the body surface, i.e., digestive, reproductive, respiratory tract.
• Exo = outside and crine = secrete.
Endocrine Glands
• Endo = within.
The glands those do not secrete material into ducts and secrete chemical signals called hormones into the bloodstream where they travel through the body and affect other cells, are known as endocrine glands.
Examples
– Thyroid, thymus, testes, ovaries, pituitary, pineal, adrenal, etc.
| Exocrine | Endocrine |
| Secrete nonhormonal chemicals into ducts | Ductless glands that are located throughout the body |
| transports the chemicals to a specific location inside and outside the body | secretes hormones into the bloodstream through the fluid that surrounds their cells |
| Example- Sweat Glands, Salivary Glands, and other Digestive Glands | Example- Thyroid, thymus, testes, ovaries, pituitary, pineal, adrenal, etc |
Importance of endocrine system
1. Transmit information from one place to another by the hormone
2. Maintains the internal environment of the body
3. Regulate the slow & gradual process of the body function such as growth.
Hormone
A hormone is a chemical substance secreted by a cell or group of cells into the blood for transport to a distant target, where it exerts its effect at very low concentrations.
HORMONE Vs ENZYME
| Hormone | Enzyme |
| Generally protein & steroid | Always protein |
| Secreted by endocrine glands | By tissue or cell |
| Acts locally or distally | Generally acts locally |
| Generally act by producing 3-5 cAMP or specific protein | Act upon biochemical reactions |
| Named according to the nature, source, & function | According to the chemical change |
| Specific receptor is required for action | Not required |
| It is a chemical substance that is secreted into the internal body fluid by one cell or a group of cells, exerting physiological control effect on other body cells. | It is an organic catalyst produced by the living cells. |
| It does not act as a catalyst | It acts as a catalyst |
Classification of endocrine glands
1. Hypothalamus (neuroendocrine gland)
2. Pituitary glands( master gland)
a. Anterior Pituitary glands
b. Posterior Pituitary glands
3. Thyroid gland
4. Parathyroid gland
5. Adrenal gland
6. Pancrease
7. Testes & Ovary
8. Placenta
Types of hormones
I. According to chemical nature
1. Peptide (at least 3 amino acids)
2. Steroid
3. Derivatives of amino acid tyrosine
II. According to action
1. General Hormone
2. Local Hormone
3. Trophic Hormone
Peptide Hormones
1. water soluble
2. stored in vesicles
3. fast acting
4. short half-life
5. e.g.-Hormones of anterior pituitary & pancreas ,ADH, oxytocin
Steroid Hormones
1. made from cholesterol
2. lipophilic
3. specific protein carrier molecules
4. slow acting
5. e.g.- Sex hormones, Adrenocortical hormone
Derivatives of amino acid tyrosine
1. derivatives of single amino acid residues
2. primarily TYROSIN
3. can behave either like peptides or steroids
Characteristics
- lipophilic
- carrier molecules required
- surface and intracellular receptors
- fast and slow acting
- e.g. Thyroid hormones, Adrenal medullary hormones
Mechanisms of Hormones Action
Hormones generally act in three different ways
1. By Activating the genes of the cell: steroid hormone generally acts in this way.
2. By forming cyclic AMP (cAMP): protein hormone generally act in this way.
3. Direct action e.g. growth hormone, insulin etc act directly by changing the permeability of cell membrane.
By forming cyclic AMP (cAMP)
Protein hormones at first binds with receptors at the surface of the cell membrane. This hormone receptor binding stimulate enzyme adenylcyclase This enzyme then converts cytoplasmic ATP into cyclic adenine monophosphate (cAMP).The cAMP then activates a cascade of enzyme action. That is it activates a second enzyme which activates a third and so forth. In this way hormone acts on the cell.
Cell may response in following ways
a. Activates enzyme
b. Alter cell permeability
c. Cause muscle contraction or relaxation
d. cause protein synthesis
e. cause secretion
Hormones act by cAMP system
1. Adrenocorticotropin
2. Thyroid stimulating hormone
3. Luteinizing hormone
4. Follicle stimulating hormone
5. Vasopressin
Regulation of Hormone Secretion
Most hormones are controlled by a FEEDBACK mechanism
Types of feedback mechanism:
There are two types of feedback control:
1. Negative Feedback
2. Positive Feedback
Negative Feedback
- Most hormone systems use negative feedback in which release of an initial hormone stimulates release or production of other hormones or substances that subsequently inhibits further release of the initial hormone.
- The hypothalamus, the anterior pituitary, and the other endocrine controlled by the anterior pituitary are all involved in a self-regulating negative feedback mechanism.
Positive Feedback
- in positive feedback, release of an initial hormone stimulates release or production of other hormones or substances, which stimulates further release of the initial hormone.
- One example of this is the surge of luteinizing hormone (LH) that occurs as a result of the stimulatory effect of estrogen on the anterior pituitary before ovulation.
- The secreted LH then acts on the ovaries to stimulate additional secretion of estrogen, which in turn causes more secretion of LH.
Structure of Pituitary Gland
The pituitary gland ,also called hypophysis, is a small gland –about 1 cm in diameter and 0.5 to 1 g in weight that lies in the sella turcica at the base of the brain and is connected with the hypothalamus by the pituitary stalk.
The pituitary gland is divided into two parts
a. Anterior pituitary
b. Posterior pituitary
Secretion of different parts of pituitary glands
1. Posterior Pituitary
i. Antidiuretic hormone
ii. Oxytocin
2. Anterior Pituitary
i. Growth Hormone (GH)
ii. Prolactin (PL)
iii. Follicle-stimulating Hormone (FSH)
iv. Luteinizing Hormone (LH)
v. Thyroid-stimulating hormone (TSH)
vi. Adrenocorticotropic hormone (ACTH)
3. Middle Pituitary
i. Melanocyte stimulating hormone (MSH)
Anterior pituitary
Functions of the secreted six major hormones are given below.
- Growth Hormone (GH)
i. regulates body growth
ii. increases protein synthesis, fat use, CHO storage
iii. bone growth
2. Prolactin (PL)
i. development of mammary glands during pregnancy
ii. milk production
3. Follicle-stimulating Hormone (FSH)
i. production of sperm, testosterone
ii. production of ovum,estrogen, progesterone
4. Luteinizing Hormone (LH)
i. production of sperm, testosterone
ii. production of ovum, estrogen, progesterone
5. Thyroid-stimulating hormone (TSH)
i. stimulates thyroid gland to produce thyroxine & tri-iodothyronine
6. Adrenocorticotropic hormone (ACTH)
i. stimulates adrenal gland to secrete adrenocortical hormone
Posterior Pituitary
Functions of the secreted two major hormones are given below.
1. Antidiuretic Hormone (ADH/vasopressin)
i. Cause the kidney to retain water, thus increasing the water content of the body
ii. High concentration causes constriction of the blood vessel throughout the body and elevates the blood pressure.
2. Oxytocin
I. Acts on smooth muscle
a) milk release from mammary glands
b) uterine contractions
Problems with GH
a) Too much GH in children leads to gigantism
b) Too little GH in children leads to dwarfism
c) Too much GH in adults leads to acromegaly
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