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The Endocrine System

What is the endocrine system?

Quick Definition:
The endocrine system is a collection of glands that produce hormones, which regulate the body's growth, metabolism, and sexual development and function. The hormones are released into the bloodstream and transported to other tissues and organs throughout the body.

The endocrine system is made up of three parts:

  • Endocrine glands
  • Hormones produced by the endocrine glands
  • Tissues in the body that respond to the hormones released by the endocrine glands

Endocrine Glands
Endocrine glands are specialized organs that produce, store, and secrete (release) chemical messengers into the blood stream. These chemical messengers are called hormones. The endocrine system includes the following glands: pituitary, thyroid, parathyroid, adrenal, ovary, testes, part of the pancreas, and, during pregnancy, the placenta.

The endocrine glands release more than 20 different hormones that influence the functions of all of the body's tissues and organ systems.

Hormones, as the chemical messengers of the body, transfer information and instructions from one part of the body to another. Hormones regulate the following:

  • Growth and development
  • Responses to surroundings, stress, and injury
  • Metabolism and body energy levels
  • Sexual function and reproductive processes
  • Function of tissues in various organs

Hormone-Responsive Tissues
Hormones circulate throughout the bloodstream, but each kind of hormone affects only certain cells, known as target cells.

When the hormone reaches its target cell, it locks onto specific receptors on the cell. These hormone-receptor combinations then transmit chemical instructions to the inner workings of the cell. This causes various changes within the cell that affects the cell’s functioning.

Connection Between the Nervous System and the Endocrine System

The endocrine system is directly connected to the nervous system through the hypothalamus. The hypothalamus is part of the brain that lies just above the pituitary gland. Nerve cells located in the hypothalamus control the pituitary gland by producing chemicals that either speed up or slow down the release of hormones from the pituitary gland. The hypothalamus directly controls the pituitary gland, which in turn controls the functions of many of the other glands in the body.


How the Body Gives Feedback to the Endocrine System

To function properly, the body requires that hormone levels are not too high or too low. For example, if growth hormone levels are too low, a child may not grow and will be abnormally short. If growth hormone levels are too high, a child may develop acromegaly, which typically involves overgrowth of bone and internal organs like the heart and liver.

Too much or too little hormone can be harmful to the body. To prevent this, the body has a feedback mechanism to regulate the levels of hormones in the blood and tissues. This feedback mechanism works something like a home thermostat that regulates heat. When the temperature in a house falls, the thermostat responds by switching on the furnace. When the temperature is too warm, the thermostat switches the furnace off.

The feedback mechanism can be fairly simple, or very complicated and involve several glands. For example:

  • Insulin controls the level of sugar in the blood—when the blood sugar goes up, the pancreas produces more insulin, which lets more sugar go into the cells. When the level of sugar in the blood drops, the pancreas releases less insulin, until the sugar goes back up.
  • The hypothalamus secretes Thyrotropin Releasing Hormone (TRH), which causes the pituitary gland to release Thyroid Stimulating Hormone (TSH). This then causes the thyroid gland to secrete thyroid hormone, which regulates the way the body uses energy. When there is enough thyroid hormone in the blood, the body "feeds back" this information to the hypothalamus. The hypothalamus decreases the secretion of TRH, which then causes a decrease in secretion of TSH from the pituitary gland.

Learn More About the Endocrine Glands
To learn more about the endocrine glands and conditions due to problems with hormones, select from the following list:

Last modified on: 30 June 2015