- The term hormone was coined by “Starling“.
- The first discovered hormone is Secretin. It was discovered by “Bayliss & Starling in 1902″.
- Hormones are also called “Primary messengers” or “Chemical messengers“.
Classical definition – Hormones are chemicals produced by endocrine glands and released into the blood and transported to distantly placed target organs.
Modern scientific definition – Hormones are non-nutrient chemicals that act as an intercellular messenger and are produced in trace amounts.
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Facts about hormones
- Produced in very small quantities because these are very reactive substances.
- Hormone molecules are small, and their molecular weight is low.
- Most of the hormones are water-soluble but some are lipid-soluble and are easily diffusible in tissues.
- They can not be stored in the body (Except thyroxine).
- Hormones are destroyed after use i.e, hormones can not be reused in the body.
- The liver and kidneys separate hormones from blood and decompose them.
- The product formed after decomposition is excreted with urine.
- Hormones are non-antigenic & non-species-specific substances.
- Usually, hormones do not participate in the metabolic activities of target cells but they affect and control the activity level of these target cells.
Mechanism of hormone action
- Once hormone enters the bloodstream it can reach almost up to any cell in the body. However, each hormone affects only a certain kind of cells, which are called target cells.
- All hormones do not act in the same way due to the different locations of their receptors. Fat-soluble hormones have receptors inside the cell but water-soluble hormones have receptors on the cell because the permeability of the plasma membrane is different for water and fat.
- Hormone receptors present on the cell membrane of the target cells are called membrane-bound receptors and the receptors present inside the target cell are called intracellular receptors, mostly nuclear receptors (present in the nucleus).
- The binding of a hormone to its receptor leads to the formation of a hormone-receptor complex.
- Each receptor is specific to one hormone only and hence receptors are specific. We can also say hormones are receptor-specific.
- Hormone-Receptor complex formation leads to certain biochemical changes in the target tissue. Thus, target tissue metabolism and physiological functions are regulated by hormones.
Nature of hormone
- Water-soluble hormones
- Lipid or fat-soluble hormones
Water-soluble hormones
Water-soluble hormones interact with membrane-bound receptors and normally do not enter into the target cell, but generate secondary messenger e.g. cyclic AMP, IP3, Ca+2, etc.) which in turn regulate cellular metabolism.
Example: Proteinaceous hormones
Lipid or fat-soluble hormones
Lipid or fat-soluble hormones interact with intracellular receptors (mostly nuclear).
- Mostly regulate gene expression or chromosome function by the interaction of the hormone-receptor complex with the genome. Cumulative biochemical action results in physiological and developmental effects.
- The action of lipid-soluble hormones is slower and long-lasting than the action of water-soluble hormones.
Example: Steroidal hormones, Iodothyronine, etc.
List of hormones with their nature and source gland
| Nature of Hormone | Hormone | Source Gland |
| 1. Proteinaceous (i) Amino acid derivatives | Thyroxine or Iodothyronine Epinephrine Norepinephrine (NE) Melatonin | Thyroid gland Adrenal medulla Adrenal medulla Pineal gland |
| (ii) Short Peptides | Vasopressin Oxytocin Melanocyte Stimulating Hormone (MSH) | Hypothalamus Hypothalamus Adenohypophysis |
| (iii) Long Peptides | Parathyroid hormone Insulin Thyrocalcitonin or Calcitonin ACTH | Parathyroid Pancreas Thyroid Adenohypophysis |
| (iv) Glycoprotein | Thyroglobulin FSH LH TSH | Thyroid Adenohypophysis Adenohypophysis Adenohypophysis |
| 2. Steroids | Mineralocorticoids (Aldosterone) Glucocorticoid (Cortisol) Testosterone Progesterone Oestrogen or estrogen | Adrenal cortex Adrenal cortex Testes Ovary Ovary |
| 3. Fatty – Acid Derivatives | Prostaglandin (PG) | Local cells |
Types of hormone
Hormones mainly are of two types-
- Synergistic hormones
- Antagonistic hormones
Synergistic hormones
When two or more hormones complement the function of each other and both are needed for the full expression of hormonal effects is called synergistic hormones. These effects cannot be observed by a single hormone alone. Thus, the synergistic effect produces greater effects than the sum of the individual effects of the hormones.
Example: Insulin and growth hormone/thyroxin have a synergistic effect on the physical growth of the mammary gland.
Antagonistic hormones
Antagonistic hormones are a pair of hormones in which two hormones oppose the actions of each other.
Example: Insulin and glucagon (insulin functions to decrease the blood glucose levels, whereas glucagon functions to increase blood glucose levels), parathyroid hormone and calcitonin, MSH, and melatonin