Sweat is made by sweat glands in the skin and carried to the skin's surface by ducts. Sweating helps keep the body cool. Thus, people sweat more when it is warm. They also sweat when they are nervous, under stress, or exercising.
Sweat is composed mostly of water, but it also contains salt (mostly sodium chloride) and other chemicals. When a person sweats a lot, the lost salt and water must be replaced.
Last full review/revision October 2007 by Daniel W. Collison, MD