Glands are the groups of cells which are specialised in structure and function. They produce chemical substances required for the body for various metabolic activities.
These glands are of two types:
A. Endocrine and
A. Endocrine Glands:
There are secretory glands found in various parts of the body which have no ducts of their own. Their secretions called hormones or internal secretions are carried by diffusion into blood vessels. These are chemical regulators of the body.
They function essentially as catalytic agents by stimulating other glands, regulating growth, controlling metabolic activity, and maintaining chemical equilibrium in the body without undergoing change themselves. Such hormone secreting these glands is known as endocrine glands or ductless glands. All endocrine glands are derived from embryonic epithelial tissues.
B. Exocrine Glands:
Those glands which liberate their secretions into ducts are called exocrine glands or glands of external secretion. They may discharge their secretions into an internal organ, such as ducts of salivary glands and liver discharge their secretion into alimentary canal. Besides these, sweat and mammary glands produce their secretion to the exterior, i.e., over the body surface.
A few glands like pancreas and gonads are exocrine as well as endocrine in nature. Pancreas secretes enzymes which are discharged into the duodenum through duct, whereas islets of Langerhans, endocrine part, secrete hormones which diffuses into the blood of the capillaries.
Hormones (Gr., hormon = arouse or exite). These are the secretions of endocrine glands. The term hormone was coined by E.H. Starling (British physiologist) in 1905.
1. The hormones of endocrine glands coordinate the function of the body. Some of them act on muscles and glands due to secretion of chemicals in the same way as nerve impulses. Thus, endocrine glands assist the nervous system in integrating body activities.
2. Some hormones regulate metabolic activities, maintaining chemical equilibrium of the body and controlling growth.
3. Some endocrine glands play an important part in the development processes.
4. Some endocrine glands are influenced by or they influence other endocrine glands.
5. Some exert a restraining influence over the others and their secretions are called chalones in distinction from hormones or autocoids.
6. There are some endocrine glands which are under the control of the nervous system.
Properties of Hormones:
1. The hormones are of a chemical nature which may influence the body generally or only certain specific organs.
2. Hormones diffuse rapidly through tissues and produce their effects quickly.
3. Their actions may be of long duration or they may induce immediate and short-term responses.
4. Hormones are destroyed rapidly after their release, so that they do not produce unsuitably long responses. If the balance of hormones in the body is upset, it results in various pathological manifestations.
Though the endocrine glands are widely separated, they are an integrated group linked together functionally by nervous and circulatory systems, so that collectively they constitute an endocrine system which coordinates and dominates various activities by the hormones, but the endocrine and nervous systems are not sharply separated from each other, and some of their activities overlap.
Classification of Hormones:
They may be classified on the basis of their biochemical nature as follows:
1. Phenolic hormones such as thyroxine and adrenaline.
2. Steroid hormones like estradiol, progesterone, testosterone.
3. Proteinous hormones like parathormone, insulin, prolactin.