ABSTRACT
In any human thermal environment, particularly in heat and during exercise, the human body has water requirements, and a programme of drinking is required to preserve both heat balance and water balance. Any reduction from ‘normal’ body water content (euhydration) to a water deficit (hypohydration) is termed dehydration and can, within limits, be tolerated for short periods of time but it can also lead to dramatically reduced human performance and death. The evaporation of sweat from the skin cools the body and plays an active role in human thermoregulation. Together with water loss due to insensible perspiration, moisture from breathing and that in urine and faeces (as well as other avenues such as tears or loss through injury), in total there can be a significant water and electrolyte loss from the body. Water provides the medium in which biochemical reactions occur and for the transportation of materials. From a state of euhydration the body maintains water balance if water inputs (by drinking) equal water outputs (sweat, urine, etc.). This is a continuous and dynamic process and there is an active controlling system to stimulate water intake, retention, secretion and excretion, as well as its appropriate distribution around the body.
