A hormone is a chemical substance originating in an organ, gland or a part and which is conveyed through the blood to another part of the body, stimulating it by chemical action in order to increase the functional activity of that part.
Hormones are classified into three major classes, according to their chemical properties.
- Protein and polypeptides.
- Amino acid derivatives.
Hormones are also classified depending upon its solubility:
- Lipid soluble hormone.
- Water-soluble hormone.
A. Lipid Soluble Hormone
|Hormones||Site of secretion|
|(a) Steroid||Aldosterone, Cortisol||Adrenal cortex|
|(b) Thyroid||T3 (Triiodothyronine)||Thyroid gland|
B. Water Soluble Hormone
|Hormones||Site of secretion|
|(a) Amines||Epinephrine and norepinephrine |
|Adrenal medulla |
|(b) Peptides & Proteins||Hypothalamic releasing and inhibitory hormones |
Thyroid stimulating hormone
and small intestine
|(c) Eicosanoids||Prostaglandins Leucotrienes||All body cells except red blood cells.|
MECHANISM OF ACTION OF HORMONES
The hormones circulate to all parts of the body but acts only at a few specific sites. This is because of special features called ‘receptors’ in the target cells. The receptors can attract and interact with the hormones. The receptors may be present either on the plasma membrane, in the cytoplasm or in the nucleus. The receptors molecules are protein in nature and may contain carbohydrate or phospholipid moieties. After combining with the receptor, the hormone exerts its effect in more than one way.
The two important general mechanism by which the functions of hormones are regulated are:
- Activation of the cyclic AMP system of cells which in turn elicits the specific cellular function eg.T.S.H., Vasopressin.
- Activation of genes of the cells which causes the formation of intracellular proteins that initiates specific cellular functions eg., the action of steroid hormone.
REGULATION OF HORMONE SECRETION
A. Feed Back Mechanism
The quantity of hormones secreted is regulated by their requirement. Regulation according to the requirement is adjusted by the feedback mechanism.
- Negative feedback mechanism: In this mechanism, the substance synthesize inhibits the further secretion of the hormone.
- Positive feedback mechanism: In this mechanism, the substance synthesize stimulates the further secretion of hormones.
B. Nervous Control
Hormonal secretion from the endocrine gland is largely controlled by the Central Nervous System (CNS). The CNS contains neurons which synthesize and release peptides called neurosecretions. The neural control of endocrine glands occur by two mechanisms:
I. Direct innervation via Autonomic nervous system (ANS)
II. Neurosecretory neuron control of anterior and posterior lobe of the pituitary.
III. Cowper’s gland: This gland secretes alkaline fluid that increases mobility by protecting from acid.
IV. Penis: It is the organ of copulation and excretion. The urethra runs into it which is the last connecting link from the testes to the exterior.
Functions of Hormones
– Via antistress action
– Via water and mineral regulation
Testicular/ ovarian steroid
4. Combating emergency
Reference: Textbook of Practice of Medicine with Homoeopathic Therapeutics by Kamal Kansal & Rakesh Kaushal