What are the major glands of the endocrine system?

Endocrine glands are groups of cells that secrete hormones (chemical substances) in the bloodstream to regulate the function of cells and tissues in the body [1]. The major glands of the body are:

• Pineal Gland
• Pituitary Gland
• Pancreas
• Ovaries
• Testes
• Thyroid Gland
• Parathyroid Gland
• Hypothalamus
• Adrenal Glands

1. What Is Pineal Gland?

The pineal gland is a small endocrine gland which shape looks like a pinecone and is located in the middle of the brain.

The pineal gland is made of cells known as pinealocytes that secrete the hormone melatonin involved in the control of the wake-sleep pattern [2].

2. What Is Pituitary Gland?

The pituitary gland or hypophysis is a gland a size of a pea that sits on a saddle-shaped bony structure known as sella turcica in the middle of the brain.

The pituitary gland is divided into the anterior lobe, intermediate lobe, and posterior lobe.

The anterior lobe secretes several hormones and polypeptides involved in growth, reproduction, stress, pain, and metabolism:

• Adrenocorticotropic hormone (ACTH)

This hormone stimulates the adrenal gland to secrete glucocorticoids, mineralocorticoids, and sex corticoids.

• Thyroid-Stimulating Hormone (TSH)

This hormone stimulates the thyroid gland to secrete thyroid hormones, thyroxine (T4), and triiodothyronine (T3).

• Follicle-Stimulating Hormone (FSH)

This hormone is involved in regulating the growth of the reproductive system by targeting the gonads (ovaries and testes).

• Luteinizing Hormone (LH)

This hormone promotes the production of sex hormones by the gonads.

• Growth Hormone (GH)

This hormone is involved in the metabolism of carbohydrates and lipids by regulating the function of the liver adipose tissue.

• Prolactin

This hormone stimulates the ovaries to secrete estrogen and progesterone and the mammary glands to produce milk.

• Beta-endorphin

Beta-endorphin is an endogenous opioid that binds and blocks opioid receptors resulting in the inhibition of pain perception.

The intermediate lobe of the pituitary gland secretes the melanocyte-stimulating hormone (MSH) also known as melanotropin.

• Melanocyte-Stimulating Hormone (MSH)

MSH stimulates the production of the skin and her pigment, melanin through a process known as melanogenesis.

The posterior lobe of the pituitary gland does not produce hormones but rather stores and secrete the hypothalamus hormones oxytocin and vasopressin.

• Oxytocin is a hormone that acts on other parts of the brain to regulate social bonding, on the mammary gland for milk ejection, and on the uterus for cervical dilatation during labor.

• Vasopressin is a hormone that acts on the kidney to reabsorb free water which is recirculated in the blood.

3. What Is Pancreas?

The pancreas is an organ of the digestive system situated behind the stomach and has both endocrine and exocrine functions.

As an endocrine gland, the pancreas secretes inulin, glucagon, somatostatin, and pancreatic polypeptide.

• Insulin is a hormone produced by the beta cells of the pancreatic islets and is involved in the absorption of blood glucose be the liver, adipose tissue, and muscles.

• Glucagon is a hormone produced by the alpha cells of the pancreas and is involved in converting stored glycogen into glucose that is released in the bloodstream.

• Somatostatin is an inhibitor of the secretion of insulin, glucagon secretion, prolactin, growth hormone, thyroid stimulating hormone, and adenyl cyclase (enzyme).

• Pancreatic polypeptide is involved in the storage of glycogen by the liver and in the regulation of pancreatic secretion in the gastrointestinal system.

The exocrine function of the pancreas is associated with the secretion of the pancreatic juice that helps with the digestive process.

4. What Are Ovaries?

The ovaries are organs of the female reproduction system involved in the regulation of the menstrual cycle and fertility.

The ovaries secrete the hormones estrogen, progesterone, androgen, and inhibin.

• Estrogen are sex hormones that promote the development of the female reproductive system and secondary sexual characteristics such as breast and thickening of the inner layer of the uterus.

• Progesterone is a sex hormone involved in the regulation of female reproductive systems, such menstrual cycle, pregnancy, breastfeeding, and male reproductive system through its role in spermatogenesis and testosterone production.

• Androgens are involved in libido and sexual arousal in female and are precursors of estrogen.

5. What Are Testes?

Testes are organs of the male reproductive system which produce sperm, androgen (testosterone)

Androgen is a hormone involved in the development and maintenance of male characteristics (testosterone).

The release of testosterone is regulated by the pituitary gland hormones, follicle-stimulating hormone (FSH), and luteinizing hormone (LH).

6. What Is Thyroid Gland?

The thyroid gland is situated in the neck and produces the thyroid hormones, thyroxine (T4), triiodothyronine (T3), and calcitonin.

Thyroxine (T4), triiodothyronine (T3), and calcitonin are involved in the regulation of cellular metabolism, growth, and heartbeat rate.

7. What Is Parathyroid Gland?

The parathyroid gland is situated in the neck at the back of the thyroid gland and produces the parathyroid hormone involved in the maintenance of calcium and phosphate levels in the body through its effect on the bone, gut, and kidneys.

8. What Is Hypothalamus?

The hypothalamus is a gland at the interface between the nervous system and the endocrine system, represented by the pituitary gland.

The hypothalamus secretes hormones known as releasing hormones that regulate the secretions of hormones by the pituitary gland.

It is divided into three regions, the anterior region (supraoptic), the middle region (tuberal), and the posterior region (mammillary). Each region is involved in the secretion of hormones.

• Gonadotropin-Releasing Hormone (GnRH)

GnRH is secreted by the supraoptic region to stimulate the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) by the pituitary gland.

• Vasopressin

• Oxytocin

• Thyrotropin-Releasing Hormone (TRH)

TRH is secreted by the supraoptic region to stimulate the release of thyroid-stimulating hormone (TSH) and prolactin by the pituitary gland.

• Corticotropin-Releasing Hormone (CRH)

CRH is secreted by the supraoptic region to stimulate the release of adrenocorticotropic hormone (ACTH) and beta-endorphin by the pituitary gland.

• Somatostatin

• Growth Hormone-releasing Hormone (GHRH)

GHRH is secreted by the tuberal region to stimulate the release of growth hormone (GH) by the pituitary gland.

9. What Are Adrenal Glands?

Adrenal glands are found on the top of the kidneys and secrete the hormones, epinephrine, (adrenaline), norepinephrine, cortisol, and aldosterone.

• Epinephrine

Epinephrine or adrenaline is involved in regulating blood pressure and smooth muscle constriction.

• Cortisol

Cortisol is a steroid hormone involved in many functions in the human body, the inflammatory response, regulating metabolism, and immune function.

Cortisol mediates the stress response as part of the “fight-or-flight” response mediated by the hypothalamus-pituitary gland-adrenal gland (HPA) axis.

• Aldosterone

Aldosterone is a steroid hormone involved in the regulation of blood pressure and the homeostasis of sodium and potassium plasma levels.

• Norepinephrine

Norepinephrine is a hormone and chemical messenger associated with the “fight-or-flight” response mediated by the hypothalamus-pituitary gland-adrenal gland (HPA) axis.

Conclusion

The endocrine system involves communication and coordination between different glands situated in different tissues and organs. It can be regarded as the body messenger that delivers instructions to control essential functions such as growth, reproduction, metabolism, homeostasis, immune response, and stress response.