Erica Mattia, age 12, of Mesa, Ariz., for her question:
WHY DO PEOPLE PERSPIRE?
Perspiration, also called sweat, consists of water and certain dissolved substances produced by glands in the skin. The primary importance of perspiration and the sweat glands is to produce sweat when the body needs to lose heat.
The sweat glands are of almost no importance in ridding the body of waste materials.
Sweat glands are distributed over the entire surface of the body. But in certain areas they are larger and more concentrated. There are many large sweat glands in the armpits, for example, and also on the palms of the hands and the soles of the feet.
Sweating itself does not reduce body heat. But when the sweat evaporates, it has a cooling effect on the body. This is especially true with perspiration on the forehead.
People perspire in cool weather as well as in hot. They can also perspire at night just as they do during the day.
When the temperature is cool, the small amount of sweat produced evaporates almost as soon as it is formed. This is called "insensible" perspiration. When the weather is warm, or during strenuous exercise, the sweat glands increase their production. Then drops of water actually accumulate on the skin surface and we say a person is sweating. This is called "sensible" perspiration.
The hypothalamus is the part of the brain which has the body's heat regulating center and it helps keep body temperatures constant. This part of the brain receives impulses from warm blood and from heat receptors in the skin. It sends signals by way of the nerves to the sweat glands, which then produce sweat.
Nervous tension and excitement also activate the sweat glands, especially those in the hands and armpits.
When the water of perspiration evaporates, certain solids of urea and salts are left on the skin.
Bathing or showering often will keep the urea and salts from accumulating on the skin and clogging the pores.
Excessive sweating in the armpits can be counteracted by applying various substances sold for this purpose.
Many animals do not reduce body heat in the way that human beings do. A dog, as an example, has sweat glands, but they are not at all important in reducing the body temperature.
Many people believe that a dog perspires through its mouth. This is not true. A dog rarely perspires. Instead, it cools itself by panting.
When a liquid evaporates from the surface of something, that surface becomes much cooler because it requires heat to change a liquid into a vapor or gas. An electric fan cools us off because its current of air makes sweat evaporate quickly. The heat needed for this evaporation is taken from our own bodies. The same rule works when we sprinkle the sidewalk on a hot day. As the water evaporates, it uses up heat and cools the air above the walk.