What is the Endocrine System?

Human body functions due to the collective work of the organ systems. One of them is the endocrine system. It is a chemical messenger system constituting the hormones directly released by the endocrine glands into the circulatory system. The study of this system is known as endocrinology. The word 'endon' means inside, and 'crine' means secrete, making the word "endocrine."

"Types of Endocrine Glands"
CC BY 3.0 | Image Credits: https://upload.wikimedia.org/

Structure of Endocrine System


Endocrine glands are the glands that will secrete the hormones directly in the circulatory system. The hormones are secreted in the interstitial spaces; the blood then absorbs the hormones to make them a part of the circulatory system. Some hormones belong to exocrine glands, which are secreted via special ducts to the target organ. Examples of endocrine glands are– adrenal glands, pancreas, ovaries, pineal gland, pituitary gland, thyroid gland, parathyroid gland, the hypothalamus. The hypothalamus and pituitary gland makes the neuroendocrine organs.

a. Hypothalamus: Hypothalamus is the endocrine gland and part of the brain that performs various functions; one of the major glands links the nervous system to the endocrine system through the pituitary gland to control hormone secretions in the body.

b. Pituitary gland: The pituitary gland is the small gland responsible for regulating body functions and well-being. This master gland controls the activity of hormone-secreting glands like the hypothalamus.

c. Adrenal glands: Adrenal glands are endocrine glands releasing hormones like adrenaline and steroids (like aldosterone). Adrenal glands are found above kidneys regulating the immune system, blood pressure, stress responses, etc.

d. Thyroid glands: Thyroid glands are placed in the neck having two lobes. This endocrine gland secretes two thyroid hormones – thyroxine and triiodothyronine. These thyroid hormones are made from iron-rich food.

e. Sex hormones: sex hormones include both male and female hormones like estrogen, progesterone, luteinizing hormone, testosterone, etc. ovaries produce the female sex hormones while the testis releases the male sex hormones. The eggs and sperms produced by ovaries and testes form non-endocrine functions. The endocrine glands – ovary and testes release sex hormones that control and maintain the sexual libido, desire, pre body preparation for pregnancy in females, primary and secondary characteristics.

f. Parathyroid gland: This gland releases parathyroid hormones. Parathyroid hormone controls the level of calcium in bones and blood.

g. Pancreas: Pancreas control blood glucose level by releasing glucagon, which increases the blood sugar level to normal range.

The nervous system and endocrine glands/system coordinate to control all physiological processes in the body. The inefficiency of one can directly affect the other's functioning.


The cells that are part of this system are follicular cells, epithelial cells, alpha cells, beta cells, delta cells, Leydig cells, granulose cells. They make the large tissues and organs in the body which will function both inside and outside it.

The follicular cells are part of the thyroid gland. They produce T3 and T4 to increase TRH, secreted by the hypothalamus, and TSH (by the anterior pituitary gland). This will regulate the metabolic rate of the cells, which function for growth and differentiation.

The epithelial cells of the parathyroid gland are supplied with a rich amount of blood by inferior and superior thyroid arteries. They will secrete parathyroid hormone. These epithelial cells will increase the reabsorption of the calcium and excretion of phosphate.

The alpha cells of the pancreas secrete hormones required to maintain blood sugar. This includes both insulin and glucagon. Insulin is secreted to reduce the blood sugar to normal levels, while glucagon is secreted to increase the blood sugar level to the normal range.

Leydig cells are present in the testis, while the granulose cells are present in the ovaries.


The endocrine system regulates a variety of body activities through the release of hormones.

Hormones are secreted by the endocrine system's glands and pass through the bloodstream to numerous organs and tissues throughout the body. Hormones then instruct these organs and tissues on what to perform and how to function. The endocrine system regulates biological activities such as metabolism, growth and development, sexual function and reproduction, heart rate, blood pressure, appetite, sleeping and waking cycles, and body temperature.

Cell Signaling

Endocrine - Endocrine signaling occurs when a cell stimulates a distant cell via the bloodstream. One cell releases a signaling chemical, which subsequently travels via the bloodstream to attach to receptors on a distant target cell elsewhere in the body.

Autocrine- this signaling will secrete a hormone that will bind to autocrine receptors in the same cell, affecting the same cell; in easy words, the source cell is the target cell itself. Sometimes they target close-by cells which are of the same type as the source cell. For example – immune cells like T cells, lymphocytes, monocytes, B cells, etc.

Paracrine – This signaling includes targeting other nearby cells in the same organ-like somatostatin released by some pancreatic cells will target the other pancreatic cells. Neurotransmitters are one of the appropriate examples of paracrine signaling. Other agents of paracrine signaling are clotting factors and growth factors.

"Forms of Cell Signaling"
CC BY 4.0 | Image credits https://openstax.org

Clinical Significance


When any chemical messenger starts to malfunction, it leads to diseases. Some diseases in it are diabetes mellitus, obesity, thyroid disease, Addison's disease, Grave's disease, etc. The factors causing disease can be lack of gland, misrelated hormone release, failure of response to signaling, etc. Both the hyper and hypofunction of hormones can cause disease.

Diabetes mellitus is a disorder in which the blood sugar level increases to an abnormal range; insulin lowers the blood sugar level to the normal range. Thyroid disease restricts the thyroid gland from producing hormones; excess production leads to excess energy consumption, leading to hyperthyroidism.

The parathyroid gland shows hyperparathyroidism when the amount of calcium in the body is way too high, which leads to bone wasting. Hyperparathyroidism brings respiratory paralysis due to less amount of calcium/hypocalcemia.

Cushing's and Addison's disease are due to non-functioning of the adrenal glands; this could result in hypocortisolism or hypercortisolism. Cushing's disease is caused due to hypersecretion of ACTH (adrenocorticotropic hormone) due to pituitary adenoma.

Common Mistakes

The endocrine system is usually confused with the exocrine system.

Hypersecretion will cause different diseases and symptoms than hyposecretion.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

  • Bachelor of Science in Biology
  • Master of Science in Zoology
  • Bachelor of Medicine
  1. Exocrine glands
  2. Hormones
  3. Steroids
  4. Cell signaling

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