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The Endocrine System

Everything You Need to Know About the Endocrine System

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Updated December 21, 2009

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The Endocrine System
Photo © A.D.A.M.

The endocrine system consists of several glands located throughout the body. These glands secrete hormones -- chemical messengers that signal the body to perform essential functions, usually related to growth and metabolism.

There are two types of glands within the endocrine system.

Endocrine glands include the pancreas, thyroid, pituitary and adrenal glands. They secrete their hormones directly into the bloodstream, where they are carried to the site of action.

Exocrine glands secrete their hormones directly into ducts. Examples of exocrine glands include sebaceous, mammary, salivary and digestive glands.

How do Hormones Work?

Many endocrine glands are sensitive to the concentration of either the hormone they produce or the substance that activates them. If the concentration of the hormone or substance is lower then normal, it will typically activate the gland. If the concentration is high, it will stop production of the hormone. This is what is referred to as a negative feedback system. Endocrine glands can also be activated directly by nervous stimulation.

When receptors on the cell membranes of an endocrine gland are activated by a particular hormone, a cascade of chemical events is triggered within the cell. Receptors and hormones are very specific. Only one type of hormone will fit in a given receptor. If the incorrect hormone tries to fit into a receptor, no reaction will occur.

Endocrine Glands and the Hormones They Produce

Pituitary Gland – This is often called the “master gland” because of its large number of functions related to metabolism and maintenance of homeostasis. There are two lobes of the pituitary: the anterior and posterior.

The anterior lobe produces many hormones including:

The posterior lobe secretes: Hypothalmus – The hypothalmus is a small portion of the brain that is in very close proximity to the pituitary gland. It controls the pituitary hormones by releasing hormones that stimulate or inhibit their release. For example, the hypothalamus secretes gonadotropin releasing hormone, which causes the production of gonadotropins (follicle stimulating hormone and lutenizing hormone) by the pituitary. It also produces corticotrophin releasing hormone, thyrotropin releasing hormone, and growth hormone releasing hormone.

Thymus – A gland used primarily in childhood, the thymus secretes hormones that help the immune system develop. Around the time of puberty, its tissue becomes replaced with fat and is no longer necessary for normal immune function.

Pineal Gland – This is a small gland located within the brain that secretes melatonin. Melatonin has been found to regulate the wake-sleep cycle.

Thyroid – The thyroid is a gland found on the windpipe in the front of the throat. It produces thyroxin (T4) and tri-iodothyronine (T3), known to regulate metabolism. It also secretes calcitonin, which helps regulate calcium levels.

Parathyroid – Four tiny glands located on the thyroid make up the parathyroid. They produce parathyroid hormone. Its secretion controls levels of calcium and phosphorus in the body.

Adrenal Glands – There are two adrenal glands, one located on top of each kidney. Each of the glands is divided into two regions, the cortex and medulla, which have very different functions.

The hormones produced by the cortex are vital for life and include the glucocorticoids, mineralocorticoids and some of the sex hormones, like androgens and small amounts of estrogen.

The adrenal medulla secretes hormones that are not essential to life and include both epinephrine and norepinephrine.

Pancreas – The pancreas is a large gland in the abdomen that secretes insulin and glucagon. These two hormones are essential in the regulation and maintenance of normal blood sugar levels. Glucagon stimulates the liver to release more glucose into the body, while insulin causes the body cells to take more glucose.

Ovaries – Found only in women, these two small glands produce estrogen, progesterone and inhibin. Estrogen and progesterone are the primary sex hormones responsible for many of the female secondary sex characteristics. Inhibin is a hormone that controls levels of follicle stimulating hormone, which regulates egg development.

Testes – A pair of glands found only in men, the testicles secrete testosterone, the primary hormone responsible for the male secondary sex characteristics.

What Happens With Endocrine Disorders?

Any time one of these hormones is out of balance, many other systems, glands and hormones can be affected. Women with polycystic ovary syndrome, for example, may show alterations in follicle stimulating hormone, lutenizing hormone, androgens (testosterone) and insulin, which can in turn affect her estrogen levels. Alterations of any of these hormones can cause changes in weight, metabolism and energy levels.

Sources:

SEER Training Modules, The Endocrine System. U. S. National Institutes of Health, National Cancer Institute. Accessed Nov 28, 2009. http://training.seer.cancer.gov

The Endocrine System: Diseases, Types of Hormones & More. The Hormone Foundation. Accessed Nov 28, 2009. http://www.hormone.org/endo101.

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