Hormones are chemical messengers in the form of protein molecules that are produced in endocrine glands. These enter the bloodstream and can therefore reach every cell in the body. Hormones cause effects to occur in certain cells, but not every cell reacts to every hormone.  In order for a hormone to have effects on cells, those cells must have the right receptor molecules on the surface of their cell membrane. The hormone (protein) molecules will only bind to those cells that have the correct receptor molecules. When this happens, the hormone affects enzymes within the cell, affecting its... Show more

Hormones are chemical messengers in the form of protein molecules that are produced in endocrine glands. These enter the bloodstream and can therefore reach every cell in the body. Hormones cause effects to occur in certain cells, but not every cell reacts to every hormone. 

In order for a hormone to have effects on cells, those cells must have the right receptor molecules on the surface of their cell membrane. The hormone (protein) molecules will only bind to those cells that have the correct receptor molecules. When this happens, the hormone affects enzymes within the cell, affecting its function.

Hormones have a widespread and long lasting effect on the body, unlike nerves, whose action is rapid. One of the most important of the endocrine glands is the pituitary gland. This is situated at the base of the brain and has an effect on many other organs, including other glands. As these other glands are stimulated, they in turn can affect other glands. For example, the pituitary gland releases FSH (follicle stimulation hormone) which causes eggs in the ovaries to mature. FSH stimulates the ovaries to start producing higher estrogen levels. As estrogen levels increase in the bloodstream, this causes the pituitary gland to stop producing FSH and to produce LH (luteinising hormone) instead. This causes the egg to be released into the uterus. 

There are many more examples of organs and glands working together like this, creating feedback to control how your body works.

Hormones can now be produced artificially. Insulin, for example, used to come only from the pancreas of animals but is now made on a grand scale by genetically engineered bacteria. There are also uses for hormones outside of the body - in garden nurseries for example, where they are used to stimulate root growth, or on farms where their ability to delay fruit from ripening can extend the shelf life of a crop.

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