12.4: Water and the Body’s Cooling System

Although as a whole, the body can survive in a very wide range of temperatures, most of our cells cannot. Instead, our cells are more like the single-celled animals of the seas and ponds, most of which can function only within a very limited temperature range. Our body’s chemical reactions work best at our typical human body temperature, about 98-99°F. Temperatures somewhat higher or lower can impair our body chemistry.

Moreover, many of life’s chemical reactions are heat-producing. Because these reactions may be more intense in some cells than in others—for example, in muscle cells during exercise—water becomes important to keep body heat rather evenly distributed. Otherwise, our bodies would continually develop “hot spots,” and cells in those areas might become inefficient or even be seriously damaged.

Because body cells can’t survive when temperatures rise more than a few degrees, the removal of added heat must be prompt and dependable. Water not only carries the heat away from the cells that produce the heat, but water is also a key factor in disposing of the heat.

The Cooling Breeze

Evaporation of water is a key mechanism for heat disposal. Although we think first of the cooling evaporation of water in sweat, the evaporation of watery secretions in the lungs and mouth also contributes to cooling. Dogs provide a good example, since they can’t sweat like we can. Dogs must rely on evaporation from their lungs, their throat, and their long and extendible tongues. That we humans don’t respond to heat by extending wet tongues, panting, and drooling, isn’t due to innately better manners; it’s because we can sweat.

For humans, sweating—an obviously watery operation—is the most important means of heat loss. We sweat almost constantly, though we may not be aware of the fact. Indeed, about half of human sweat is called insensible perspiration, because it’s so slow and subtle. Under normal conditions of comfort, with relatively little activity, water loss through evaporation accounts for more than 40% of the water lost by the body.

It follows that conditions that increase—or hamper—the evaporation of water substantially affect heat disposal. A hot day is particularly uncomfortable if the air is humid and still and we are covered with a heavy layer of clothing—conditions which minimize the evaporation of water. That hot day wouldn’t feel so hot if the air were dry and breezy. What we call a cool breeze is usually, in fact, a cooling breeze. “Heat stroke” or “heat collapse” usually has its beginnings in dehydration: the body seeks to cool itself and doesn’t have enough water to do the job.

Some sense of the urgency of water replacement in terms of body heat can be seen by how fast the water we drink can be put to work by the body when we’re very dry. When water is drunk in such a state, its absorption and use is almost instantaneous. You may have experienced the phenomenon: You’re hot and dry and begin to drink. Almost at once, sweat seems to burst out on your face.