Endocrine system is responsible for making hormones which target specific cells to carry out a particular action. Endo means within and crine means to secrete. The endocrine glands secrete hormones into surrounding tissue fluids. The endocrine system utilises hormones, has no ducts and acts over long distances. Hormones are chemicals that regulate metabolism and function of cells.6
The hypothalamus, the pituitary, the thyroid, the parathyroid glands, the adrenal glands, the pancreas, the gonads (testes and ovaries), and the pineal gland are all endocrine glands.
The hypothalamus secretes releasing hormones for the pituitary, ADH and oxytocin. The releasing hormones/factors stimulates the pituitary to release its hormone. Gonadotropin Releasing Hormone stimulates the pituitary to release follicle stimulating hormone (FSH ) and luteinizing hormone (LH). It also releases corticotropin releasing factor, a peptide hormone involved in the stress response. Antidiuretic hormone is a vasopressin , which means it increases water reabsorption in the kidney. This mechanism conserves water and increases blood pressure. Oxytocin stimulates uterine contractions during labor and also milk secretion during suckling.6
The thyroid is controlled by thyroid-stimulating hormone from the anterior pituitary. The thyroid is on the front surface of the trachea; it can be palpated (felt) as an organ near the base of the neck that moves up and down with swallowing. The thyroid has two major functions: setting basal metabolic rate and calcium homeostasis. It mediates the first effect by releasing triiodothyronine (T3) and thyroxine (T4), whereas calcium levels are controlled by calcitonin.
The parathyroids are four small pea-sized structures that sit on the posterior surface of the thyroid. The hormone produced by the parathyroid glands is aptly named parathyroid hormone (PTH). PTH serves as an antagonistic hormone to calcitonin, raising blood calcium levels; specifically, it decreases excretion of calcium by the kidneys, increases absorption of calcium in the gut (via vitamin D), and increases bone resorption, thereby freeing up calcium. PTH is also subject to feedback inhibition. As levels of plasma calcium rise, PTH secretion is decreased. Parathyroid hormone also affects phosphorus homeostasis by resorbing phosphate from bone and reducing reabsorption of phosphate in the kidney (thus promoting its excretion in the urine). PTH also activates vitamin D, which is required for the absorption of calcium and phosphate in the gut.
The adrenal glands are located on top of the kidneys. Adrenal actually translates to near or next to the kidney. Each adrenal gland consists of a cortex and a medulla. This distinction is more than anatomical. Each part of the gland is responsible for the secretion of different hormones. The adrenal cortex secretes corticosteroids. These are steroid hormones that can be divided into three functional classes: glucocorticoids, mineralocorticoids, and cortical sex hormones. Glucocorticoids are steroid hormones that regulate glucose levels. In addition, these hormones also affect protein metabolism. Mineralocorticoids are used in salt and water homeostasis; their most profound effects are on the kidneys.6 The most noteworthy mineralocorticoid is aldosterone, which increases sodium reabsorption in the distal convoluted tubule and collecting duct of the nephron. The adrenal glands also make cortical sex hormones (androgens and estrogens). Because males already secrete large quantities of androgens in the testes, adrenal testosterone plays a small role in male physiology. However, females are much more sensitive to disorders of cortical sex hormone production.
From an endocrine standpoint, small groups of hormone-producing cells are grouped together into islets of Langerhans throughout the pancreas. Islets contain three distinct types of cells: alpha (α), beta (β), and delta (δ) cells. Each cell type secretes a different hormone: α-cells secrete glucagon, β-cells secrete insulin, and δ-cells secrete somatostatin.6 Glucagon is secreted during times of fasting. When glucose levels run low, the secretion of glucagon stimulates degradation of protein and fat, conversion of glycogen to glucose, and production of new glucose via gluconeogenesis. Insulin is antagonistic to glucagon and is therefore secreted when blood glucose levels are high. Insulin induces muscle and liver cells to take up glucose and store it as glycogen for later use. Somatostatin is an inhibitor of both insulin and glucagon secretion. High blood glucose and amino acid concentrations stimulate its secretion. Somatostatin is also produced by the hypothalamus, where it functions to decrease growth hormone secretion in addition to its effects on insulin and glucagon.
The testes secrete testosterone in response to stimulation by gonadotropins (LH and FSH). Testosterone causes sexual differentiation of the male during gestation and also promotes the development and maintenance of secondary sex characteristics in males, such as axillary and pubic hair, deepening of the voice, and muscle growth. The ovaries secrete estrogen and progesterone in response to gonadotropins. Estrogen is involved in development of the female reproductive system during gestation and also promotes the development and maintenance of secondary sex characteristics in females, such as axillary and pubic hair, breast growth, and body fat redistribution.
Pineal gland secretes makes melatonin, which makes you sleepy at night.
1) Dorland’s (2012). Dorland’s Illustrated Medical Dictionary (32nd ed.). Elsevier Saunders.
2) Black, J.A., Sontheimer, H., Oh, Y., and Waxman, S.G. (1995). In The Axon, S. Waxman, J. Kocsis, and P. Stys, eds. Oxford University Press, New York, pp. 116–143.
3) Jessen KR, Mirsky R (August 1980). “Glial cells in the enteric nervous system contain glial fibrillary acidic protein”. Nature. 736–7
4) Hille, Bertil (2001) . Ion Channels of Excitable Membranes (3rd ed.). Sunderland,
Mass: Sinauer Associates, Inc. p. 5.
5) NV, B. (2002). Medical Biochemistry. San Diego: Harcourt/Academic Press. (lipid)
6) Vander, Arthur (2008). Vander’s Human Physiology: the mechanisms of body function. Boston: McGraw-Hill Higher Education. pp. 345-347