HARMONES

A hormone is a specific molecule that acts as a chemical messenger in the endocrine system. Hormones are produced by specific organs and glands and are secreted into the blood or other body fluids. Most hormones are carried by the circulatory system to different areas of the body, where they influence specific cells and organs. Hormones regulate various biological activities including growth; development; reproduction; energy use and storage; and water and electrolyte balance.

Hormone Signaling

Hormones that are circulated in the blood come in contact with a number of cells. However, they only influence certain target cells. Target cells have specific receptors for a specific hormone. Target cell receptors can be located on the surface of the cell membrane or inside of the cell. When a hormone binds to a receptor, it causes changes within the cell that influence cellular function. This type of hormone signaling is described as endocrine signaling because the hormones influence target cells over a distance. Not only can hormones impact distant cells, but they can also influence neighboring cells. Hormones act on local cells by being secreted into the interstitial fluid that surrounds cells. These hormones then diffuse to nearby target cells. This type of signaling is called paracrine signaling. In autocrine signaling, hormones don't travel to other cells but cause changes in the very cell that releases them.

Types of Hormones

Hormones can be classified into two main types: peptide hormones and steroid hormones.

• Peptide Hormones
  
  These protein hormones are composed of amino acids. Peptide hormones are water-soluble and are unable to pass through the cell membrane. Cell membranes contain a phospholipid bilayer which prevents fat-insoluble molecules from diffusing into the cell. Peptide hormones must bind to receptors on the cell's surface, causing changes within the cell by affecting enzymes within the cell's cytoplasm. The binding of the hormone to the cell membrane receptor initiates the production of a second messenger molecule inside the cell, which carries the chemical signal within the cell. Human growth hormone is an example of a peptide hormone.
•  Steroid Hormones
  
  Steroid hormones are lipid-soluble and able to pass through the cell membrane to enter a cell. Steroid hormones bind to receptor cells in the cytoplasm and the receptor-bound steroid hormones are transported into the nucleus. The steroid hormone-receptor complex binds to another specific receptor on the chromatin within the nucleus. The complex calls for the production of certain RNA molecules called messenger RNA (mRNA) molecules, which code for the production of proteins. Steroid hormones cause certain genes to be expressed or suppressed by influencing gene transcription within a cell. Sex hormones (androgens, estrogens, and progesterone) produced by male and female gonads are examples of steroid hormones.

• Hormone Regulation

  Hormones may be regulated by other hormones, by glands and organs, and by a negative feedback mechanism. Hormones that regulate the release of other hormones are called tropic hormones. The majority of tropic hormones are secreted by the anterior pituitary in the brain. The hypothalamus and thyroid gland also secrete tropic hormones. The hypothalamus produces the tropic hormone thyrotropin-releasing hormone (TRH), which stimulates the pituitary to release thyroid stimulating hormone (TSH). TSH is a tropic hormone that stimulates the thyroid gland to produce and secrete more thyroid hormones.
  Organs and glands also aid in hormonal regulation by monitoring blood content. For example, the pancreas monitors glucose concentrations in the blood. If glucose levels are too low, the pancreas will secrete the hormone glucagon to raise glucose levels. If glucose levels are too high, the pancreas secretes insulin to lower glucose levels.
  In negative feedback regulation, the initial stimulus is reduced by the response it provokes. The response eliminates the initial stimulus and the pathway is halted. Negative feedback is demonstrated in the regulation of red blood cell production or erythropoiesis. The kidneys monitor oxygen levels in the blood. When oxygen levels are too low, the kidneys produce and release a hormone called erythropoietin (EPO). EPO stimulates red bone marrow to produce red blood cells. As blood oxygen levels return to normal, the kidneys slow the release of EPO resulting in decreased erythropoiesis.
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