Sweat, Perspiration and Glow

Many years later, as he was facing the firing squad, Colonel Aureliano Buendia was to remember that distant afternoon when his father took him to discover ice.
                                       Gabriel Garcia Marquez,  One Hundred Years of Solitude

Ice. Water in its solid phase, the molecules of H-two-0 so low in heat energy that they have  lost  their liquid dance and settled into the rigid structure of a crystalline grid. In the summer of 1881, as President James Garfield lay dying in a sweltering Washington summer, a crude air conditioner was made in which melted ice water dripped onto flaglike pieces of cloth over which blew fanned air. This device lowered the room temperature by 20F, and in the course of 2 months consumed more than a half million pounds of ice.

Ice rarely plays such a key role in human physiology during hot weather conditions. It is the evaporation of water in the form of sweat which cools us.

Sweat. Sweat is a clear, colorless liquid secreted by the eccrine sweat glands which lie in the dermal layer of the skin. Humans have 2 million to 5 million sweat glands, with an average density of 150 to 340 per square centimeter of skin, a space about one-third the area of a dime. By age 3, a person will have acquired his lifelong number of sweat glands, although the sweat glands of children appear to be smaller and less effective than those of adults. The eccrine glands of the hairy skin surfaces respond to thermal stimulation and provide cooling, while those of the palms and soles (which appeared earlier in evolution), respond to psychological stimuli and serve mainly to prevent friction.  Eccrine sweat glands are most numerous on the soles and palms and then, in decreasing order, on the head, trunk and extremities.

Sweat  is 99% water. It contains sodium at a concentration of 20 to 100 mEq/liter, equal to 15%-74% of the sodium concentration found in serum. The sweat concentration of sodium is genetically determined, and can be considerably decreased with physical training. As an athlete becomes heat acclimatized,   the sodium concentration of sweat decreases, sweating begins earlier in exercise, and sweating is more copious. Therefore, a "heat-hardened" athlete will have an even greater need for water intake during exercise than one who is not similarly well-conditioned.

Sweat losses during vigorous exercise in hot weather can be as high as 3L/hour (=3.5 quarts, 6.6 lb). Such losses pose serious problems for the athlete, since gut absorption of water may be only 0.8-1.25 L/hour. In other words, the athlete may lose as much as 6.6 pounds per hour via sweating while being able to replace only 2.2 pounds per hour by absorbing fluids. Athletic events associated with high sweat rates for several hours guarantee player dehydration. If dehydration reaches a critical level, blood pressure will fall, sweating will decrease, core temperature will rise, and (if the athlete continues vigorous effort in the face of multi-system compromise) severe illness or death will inevitably follow. Sweat literally becomes the water of life.

For sweat to have its beneficial cooling effect, it must evaporate. For each liter of sweat which evaporates, the athlete loses 580 Kcal of heat. The sweat which dramatically drips to the ground at the feet of the athlete is water wasted. Wiping sweat from the face and hands provides comfort and a better grip, but little cooling. The athlete's sweat-drenched shirt offers mute testimony to the ineffectiveness of his sweating. In fact, the saturated shirt becomes a barrier to effective evaporation, and will lead to even more heat storage and temperature-control difficulty.

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