What function is performed by sweat? Why does the organism give it off in such large quantities? It is the mechanism by which a human organism protects itself against overheating. Evaporation uses large amounts of heat, 600 calories per litre of sweat. If all this heat were given oil by a human body, its temperature would fall by approximately ten degrees. Unfortunately, our body expends only a small proportion of its heat on evaporation and, therefore, sweating cannot ensure cooling of the body, but can only protect it against overheating. Normal body temperature, about 37°C (in the armpit), is only maintained due to the evaporation of water from the lungs and skin, even when the temperature of the ambient air is as high as 40 or 50°C.
Sweating is not always beneficial. When humidity is high, sweat evaporates very slowly. It collects in large drops and flows over the body surface bringing no relief as there is no cooling without evaporation. For this reason heat is easier to tolerate in dry deserts than in damp tropical forests.
Is it harmful to sweat a lot? Loss of three to five litres of water, whatever the cause, brings about intolerable thirst, but this is not fatal if it is compensated sufficiently quickly. There was a well-known case in France in 1821 when a man doomed himself to death by stubbornly refusing to drink anything. The struggle between life and death continued for 17 days. The man could have been saved had he been given sufficient water to drink, even as late as the fifteenth day of his amazing fast.
Where does the water contained in sweat come from? Where does man store the liquid he has drunk? Sweat glands draw water from the blood. As long as sweating is not excessive, the blood does not become thicker and no decrease occurs in its volume. But as soon as the water content of the blood begins to drop, the same amount of water flows into the blood vessels from the stores. (The chief sites of water storage are the subcutaneous tissue, muscles and other organs.) Conversely, the water consumed by man is absorbed by the blood from the intestines; the corresponding amount of water is immediately transported to the stores.
The amount of water which can be stored in the body is limited, especially in birds and flying insects. It is scarcely enough to ensure the vital functions of the organism for a day or two, even in cool weather. But there must always be a reserve of water in the organism. The most original method for storing water has been invented by bees. A bee family, consisting of one thousand adult insects and a great number of larvae, cannot exist without water. What happens to the young when non-flying weather occurs? The bees have found a way out. If you open a hive you can see large worker bees hanging motionlessly on the combs. They are living reservoirs of water. The bee water-carriers pour the excess water into the crops of the worker bees till they become too heavy to fly or crawl. After a day or two of non-flying weather their abdomens return to size and the reservoirs become empty.