Endocrine glands:
- Secrete chemicals called hormones (to excite) into the blood (have no ducts) which is transported to the site of action.
- Generally control long term activities of target organs as well as physiological processes such as digestion, metabolism, development and reproduction.
- Hormones are called chemical messengers; as they are chemical in nature, they are effective in minute amounts and their site of secretion is far from their site of action. Nervous actions are generally localized whereas hormones can act throughout the body.
FUNCTIONS (Regulatory and control activities) OF HORMONES
- Reproductive activities: Control gametogenesis, development sex ducts and secondary sexual characters, spawning etc.
- Growth, maturation, regeneration
- Metabolism and homeostasis
- Adaptation to external factors
On the basis of chemical composition hormones are of 3 types
- Steroids (androgens, estrogens, adrenocorticoids)
- Proteins (insulin, glucagon, Parathormone, oxytocin, vasopressin, hormones of pituitary gland, hormones secreted by intestinal mucosa etc.)
- Simple substances (neither protein nor steroids) – [adrenaline]
On the basis of function
- Regulatory hormones : control and regulate rate of secretion of other endocrine glands
E.g. Pituitary hormones
- Metabolic Hormones: stimulate metabolic activities. E.g. insulin
- Morphogenetic hormones: affect rate of development of body parts e.g. GSH, LH, Thyroxin etc.
Differences between : HORMONES AND ENZYMES
| HORMONES | ENZYMES |
| Peptides, proteins, amino acid derivatives or steroids in nature | All enzymes are complex proteins |
| They have low molecular weight. | They have high molecular weight. |
| Site of action and site of secretion are relatively far. | Site of action and secretion may be in the same cells or pass via ductsto act |
| Hormone controlled reactions are non-reversible. | Enzyme controlled reactions are reversible. |
| They may act slowly or quickly. | They act slowly. |
| They may accelerate or inhibit specific physiologic processes | Enzymes speed up biochemical reactions. |
