Endocrine system

Endocrine system is a control system of ductless glands The that secrete chemical messengers called hormones that circulate within the body via the bloodstream to affect distant cells within specific organs. Endocrine glands secrete their products immediately into the blood or interstitial fluid, without storage of the chemical. Hormones act as ‘messengers‘ and are carried by the bloodstream to different cells in the body, which interpret these messages and act on them.

It seems like a far fetched notion or idea that a small chemical can enter the bloodstream and cause an action at a distant location in the body. Yet this occurs in our bodies everyday of our lives. The ability to maintain homeostasis and respond to stimuli is largely due to hormones secreted within the body. Without hormones, you could not grow, maintain a constant temperature, produce offspring or perform the basic actions and functions that are essential for life. The endocrine system provides an electrochemical connection from the hypothalamus of the brain to all the organs that control the body metabolism, growth and development and reproduction.

There are two types of hormones secreted in the endocrine system:

1. Steroidal

2. Non-steroidal or protein based hormones.

The endocrine system regulates its hormones through negative feedback, except in very specific cases like childbirth. Increases in hormone activity decrease the production of that hormone. The immune system and other factors contribute as control factors also, altogether maintaining constant levels of hormones.

Endocrine system

Types of Endrocrine Glands

Their are mainly two types of endocrine glands-

Exocrine Glands – Exocrine glands are those which release their cellular secretions through a duct which empties to the outside or into the lumen (empty internal space) of an organ. These are partly releaser endocrine and partly exocrine in function,i.e., hormones they are heterocrine . These include certain sweat glands, salivary glands, pancreatic glands and mammary glands.

Endocrine Glands – Endocrine glands are those glands which have no duct and release their secretions directly into the intercellular fluid or into the blood. The collection of endocrine glands make up the endocrine system. These are called devoted entirely to the secretion of hormones .

The main endocrine glands are the pituitary (anterior and posterior lobes), thyroid, parathyroids, adrenal (cortex and medulla), pancreas and gonads.

Parathyroid glands


The endocrine glands vary in embryonic origin and are isolated from one another. Thus, they do not form an organ system comparable to the other organ systems of the body. Still they are together referred to as an endocrine system because they interact. A hormone from one gland may stimulate or inhibit another endocrine gland. Hormones from the hypothalamus regulate the secretion of hormones from the pituitary gland; and the hormones from the pituitary gland, in turn, influence canal many other endocrine glands. The pituitary gland is called the primary target of the hypothalamic hormones. The endocrine glands influenced by the pituitary gland are termed the secondary targets of the hypothalamic hormones. The hormones of the secondary target glands affect the final target organs.

Important points – Endocrine glands are endogeneously located and lack ducts ( hence , called ductless glands). They secrete their secretion into blood for transportation to target organ.

Endocrine glands

Structure of endocrine glands

The endocrine glands vary in structure. Some are unicellular such as cells in the lining of alimentary canal, neurosecretory cells of hypothalamus. Others are multicellular such as thyroid, thymus, tissue etc. Some of these are compound organs, really consisting of two glands of different origin combined into one, e.g., adrenal, pituitary.

Function of endocrine glands

The main function of endocrine glands is to secrete hormones directly into the bloodstream. Increases in hormone activity decrease the production of that hormones. The endocrine hormones help control mood, growth and development , the way our organs work, metabolism and reproduction .

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