Perspiration is a sort of evaporation of water from living bodies. It occurs both in plants and animals. In plants, it occurs to regulate the water level while in animals it occurs to regulate the body temperature from higher to a normal level, that is 37°C.
But sometimes people perspire excessively, why?
Water evaporates when the higher energy molecules at the surface escape the attractive forces of the other lower energy molecules below it. They become gaseous water molecules. The molecules that remained in water are low energy molecules resulting in the cooling of the water. That's why we call evaporation a cooling process.
The body transpires whenever it's hot in the surroundings to maintain the internal temperature normal because of its cooling impact. But this mechanism pushes its limit in some conditions. For example when the humidity of the surrounding air increases the rate of evaporation and therefore the cooling rate decreases. When the humidity is very high, evaporation is severely retarded and the perspiration may be so plentiful that it drips off the body before it has a chance to the evaporated and cool body.
From a physiological standpoint, it would be better if our bodies released just enough perspiration to keep the skin moist. Since nature is normally very conservative, we must wonder what advantage exists in producing excessive perspiration. To answer this question, we need to examine what would happen if our bodies did produce just enough perspiration to keep the skin moist. Perspiration contains solutes such as sodium chloride and potassium chloride. These solutes remain on the skin as the water evaporates. As the body produces more perspiration, the solutes deposited on the skin accumulate.
We just learned that the vapor pressure of a solution is directly proportional to the solute concentration. As the solute concentration increases, the vapor pressure and the evaporation rate of the perspiration decrease. Calculations have shown that increasing the concentration of solute in perspiration can decrease its vapor pressure by 20 percent.
So now we see that there is an advantage to excessive perspiration. As the body perspires excessively, the perspiration rinses off the excess solutes and prevents them from becoming too concentrated. This in turn increases the vapor pressure of the perspiration and cooling is more efficient.
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