Glands in Endocrine System and Mechanism of Hormone Action

Mechanism of hormone action
Glands in Endocrine System and Mechanism of Hormone Action

Glands in the Endocrine System and Mechanism of Hormone Action: Hormones with Cell Surface Receptors and Intracellular Receptors


The human body contains many hormones that regulate the various functions of the body. Hormones are known as body chemicals produced by the endocrine glands, which include pituitary glands, adrenal glands, thyroid glands, thymus, and pancreas.
Hormones are characterized by secretion at some particular place in the body, but they work far away from this place, but some hormones also affect nearby the place of secretion.

What are hormones?

Hormones can be defined as chemicals secreted by certain organs of the body known as glands, and are classified into a device known as the endocrine system. In fact, the principle in the presence of hormones is to achieve communication among membranes. These hormones are released into the bloodstream, and bloodstream is transferred to the organs or tissues involved. Once the hormone is reached, a series of reactions occur within the organ or tissue. It is important to note that the hormones are very powerful chemicals, they are not only released by the glands found in humans but also there are hormones in plants and animals and help each of them to perform the functions of the body correctly and accurately.

The function of the endocrine system

The hormones produced by the endocrine system in the body play a large and very important role in the body's functions, including the following:

Metabolism, which occurs within the body, including metabolic processes associated with food.
Regulate the speed of chemical reactions in various cells of the body.
Stimulate some substances to move or move across cellular membranes in the body.
Stimulate growth and development of cells and tissues,
Initiation, development, and maintenance of processes related to sexual growth.
Control of human thirst.
Maintain body temperature constant.

Control human temperament and cognitive abilities.

Types of hormones according to the gland

The most important hormones in the body, according to the gland that secret as follows:

Hypothalamus Hormones

There is a group of hormones produced by the hypothalamus, which are primarily responsible for stimulating the secretion of pituitary gland hormones preventing the secretion. The most important hormones under the hypothalamus are:

1.     Corticotropin-releasing hormone
2.     Gonadotropin-releasing hormone
3.     Growth hormone-releasing hormone

4.     Thyrotropin-releasing hormone

Hormones of the Pituitary Gland

The pituitary gland is located at the bottom of the brain and does not exceed the size of the pea. The hormones secreted by this gland include:

  • Growth hormone that stimulates bone and tissue growth.
  • Thyroid-stimulating hormone, which stimulates the thyroid gland to secrete hormones.
  • Adrenocorticotropic hormone, which stimulates the adrenal cortex to secrete steroids.
  • Luteinizing hormone and Follicle-stimulating hormone, which controls sexual functions in the body, including the secretion of progesterone hormones; Estrogen in women and testosterone in men.
  • Prolactin, which stimulates the secretion of milk in women.
  • Antidiuretic hormone, which controls the loss of water from the body through the kidneys.
  • Oxytocin which stimulates the contraction of the uterine smooth muscle during childbirth, and enhances milk secretion.

Hormones of the adrenal gland

In the human body, there are two adrenal glands, one on the right kidney and the other on the left kidney. In fact, each consists of two main parts: the first is the adrenal cortex, the second is the adrenal medulla.  Each part is responsible for the secretion of a group of hormones that can be described as follows:

Hormones of adrenal cortex: They are characterized by two main types of corticosteroids: Glucocorticoids and Mineralocorticoids. The following is their statement:

Glucocorticoids, namely hydrocortisone, and corticosterone. Hydrocortisone regulates the conversion of carbohydrates, fats, and proteins into energy, as well as its role in controlling blood pressure and cardiovascular functions. As for corticosterone, it is combined with hydrocortisone and controls the response of the immune system and inflammatory reactions in the body.

Mineralocorticoids: The most important of which is the aldosterone which controls the level of water and salts in the body, and thus regulates blood pressure.

Hormones of adrenal medulla: Although the importance of hormones secreted by the adrenal medulla is less important than that of the adrenal cortex, it has an important role in responding to emotional conditions and tension in various forms. These hormones include:

Adrenaline (also known as epinephrine), which responds to stress by increasing strokes and pumping blood to the brain and bones, as well as raise blood sugar levels.

Norepinephrine, which works alongside adrenaline in stressful conditions, in addition to narrowing blood vessels in such cases, causing an increase in blood pressure.

Hormones of the thyroid gland

The thyroid gland secretes two primary hormones, Triiodothyronine (20% of the total secretion of hormones) and Thyroxine-T4 (80% of secretion). It should be noted that triiodothyronine is the most powerful hormone.

Hormones of the parathyroid gland

 Parathyroid gland secrets a hormone is known as parathyroid hormone and works primarily on bone cells by stimulating calcium into the bloodstream.

Hormones of the pineal gland

The pineal gland is located in the center of the brain, it produces hormone melatonin. The primary function of this hormone is to regulate the Circadian Rhythm, Which is what happens during the twenty-four hours of the day, including waking up and sleeping. The hormone melatonin is also responsible for controlling the reproductive hormones and processes associated with the reproductive system. It should be noted that this hormone is very distinctive, as it is produced only by the presence of light.

Hormones of the thymus gland

Thymosin is the only hormone produced by the thymus gland. This hormone stimulates the development of T-cells that are part of the body's immune system. When they are fully mature, they migrate to the lymph nodes scattered in parts of the body. It also helps fight infections and diseases that humans may experience in their lives.

Pancreatic hormones

Pancreatic hormones play an important role in regulating sugar levels and certain salts in the body. These hormones include:

1.     Gastrin: This hormone stimulates the secretion of acids from the stomach to help digestion.
2.     Insulin: It helps control blood sugar levels by stimulating cells to absorb glucose.
3.     Glucagon: It acts in an opposite way to insulin, to maintain glucose sugar in the blood.
4.     Somatostatin: It is produced in cases where insulin and glucagon levels are too high to regulate sugar levels and salts in the body.

5.     Vasoactive intestinal peptide: It is controlled by the secretion of water and the absorption of water from the intestines.

Sex hormones

It is also known as sex steroids, gonadocorticoids, and gonadal steroids. The ovaries secrete two essential hormones, estrogen and progesterone. These hormones are responsible for the development of sexual characteristics in the female in adulthood, as well as responsible for the organization of menstruation and pregnancy and so on. As for the testicles in men, they work on the secretion of testosterone and work on the normal development of male genitalia, the masculine qualities are generally observed in terms of sound, muscle, and so on.

Mechanism of Hormone Action

Hormones are divided in terms of their effect on the cell into two main types:

(A) Hormones with cell surface receptors: They work through receptors on the cell membrane (surface receptors) without entering the cell cytoplasm, such as peptide hormones, protein hormones, epinephrine hormones, prostaglandins.

(B) Hormones with intracellular receptors: They work through entering the center of the cell and its association with the receptors within the cytoplasm, for example, steroids, thyroid hormones, and vitamin D3.

A. Hormones with cell surface receptors:

A number of protein hormones and multiple bonds, as well as the hormone norepinephrine and prostaglandins function, and its direct impact on the surface receptors through the so-called second messenger system or cyclic adenosine monophosphate (C-AMP), after a series of enzymatic reactions produced at the end of which a special type of protein or enzyme is invaded by the body as a by-product due to the effect of the secreted hormone and by the type of receptor.

Therefore, the effect of the hormone on the cell depends mainly on the type of receptor located on the surface of the membrane where these receptors are strictly specialized and receive only the hormone. The connector usually consists of 3 protein components: the receptor, the helper and the vector. When the hormone reaches its receptor on the outer surface of the membrane, the action of the hormone ends and the catalyst is stimulated. In this case, the adenylate cyclase is on the inner end of the membrane. This enzyme activates the transformation of adenosine triphosphate (ATP).  Adenosine triphosphate into cyclic adenosine monophosphate ring (C-AMP) changes the state of protein from an inactive to active state.

These receptors originate from integral proteins and are usually located on one of three extracellular sites on the external membrane surface, transmembrane or on the tip of the membrane from the inside (Cytoplasmic).

The second connector theory of protein hormones can be summarized as follows:
The hormone (first connector) binds to its own receptor on the outer surface of the cell membrane. ATP is converted to C-AMP by the direct effect of adenylate cyclase in the presence of calcium ions and guanosine triphosphate (GTP). C-AMP as a second agent stimulates internal reactions that result in protein kinase synthesis (PKA).

The increase in CAMP results in the formation of more PKA and its receptor changes from an inactive state to an active state. When PKA becomes active it can add a phosphorylation group to a number of proteins within the cell that may include enzymes needed by the body in response to the effect of the secreted hormone or special protein needed in its growth.

B – Hormones with Intracellular Receptors

Hormones with cytoplasmic receptors inside the cellExamples of hormones that function in this way are fatty hormones as well as thyroid hormones and vitamin D3. They can be summarized as follows:

The cytoplasm enters through the cell membranes and is associated with special receptors within the cytoplasm or on the membrane of the nucleus, resulting in a chemical compound between the hormone and receptor (hormone receptor). The hormone moves in its receptors into the nucleus. This results in the transcription of special genes consisting of messenger-ribonucleic acid (m-RNA).

The mRNA synthesized into the cytoplasm moves to stimulate the formation of new proteins on the coarse endoplasmic network. These proteins may be enzymes needed by cells to perform a specific function, or new proteins that the cell may need in its growth and development.

In both cases, whether the hormone function through the receptor on the outer membrane of the cell (proteins) or inside the cell (lipids), the hormone may have a number of receptors of its own, but the response may vary depending on the target member and the type of receptor it carries.


There are many types of glands that secrete hormones such as the pituitary gland, hypothalamus, adrenal gland, thyroid gland, pineal gland, thymus gland, sex steroids, etc.
Hormones are divided in terms of their effect on the cell into two main types:
A. Hormones with cell surface receptors
B - Hormones with cytoplasmic receptors inside the cell
Hormone receptors play a vital role in the mechanism of hormone action. The hormone affects the target tissue by activating the target hormone receptors in the tissue cells; then it changes the function of the hormone receptors, so this receptor is the direct cause of the hormone effects.

Glands in Endocrine System and Mechanism of Hormone Action Glands in Endocrine System and Mechanism of Hormone Action Reviewed by The Scientific World on December 22, 2018 Rating: 5

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