Dehydration (”hypohydration”) is excessive loss of body fluid. It is literally the removal of water from an object, however in physiological terms, it entails a deficiency of fluid within an organism.
There are three main types of dehydration: hypotonic (primarily a loss of electrolytes, sodium in particular), hypertonic (primarily a loss of water), and isotonic (equal loss of water and electrolytes). In humans, the most commonly seen type of dehydration by far is isotonic (isonatraemic) dehydration which effectively equates with hypovolemia, but distinction of isotonic from hypotonic or hypertonic dehydration may be important when treating people who become dehydrated.
Hypovolemia is specifically a decrease in volume of blood plasma. Furthermore, hypovolemia defines water deficiency only in terms of volume rather than specifically water. Nevertheless, the conditions usually appear simultaneously.
A person’s body, during an average day in a temperate climate such as the United Kingdom, loses approximately 2.5 litres of water. This can be through the lungs as water vapor, through the skin as sweat, or through the kidneys as urine. Some water (a less significant amount, in the absence of diarrhea) is also lost through the bowels. In warm or humid weather or during heavy exertion, however, the water loss can increase by an order of magnitude or more through perspiration; all of which must be promptly replaced.
Dehydration is best avoided by drinking sufficient water. The greater the amount of water lost through perspiration, the more water must be consumed to replace it and avoid dehydration. Since the body cannot tolerate large deficits or excesses in total body water, consumption of water must be roughly concurrent with the loss (in other words, if one is perspiring, one should also be drinking some water frequently).