The main purpose of sweating is to cool the body. When your body temperature rises because of exercise, fever or a hot day, your heart responds by pumping faster to move blood around to all the organs of your body more quickly and absorb their heat. If it is an extra hot day, or you are exercising rigorously, the blood gets too warm to cool off hot organs and at that point, the sweating reflex kicks in.
Hard exercise produces enough heat to evaporate two quarts of water in one hour. The human body produces enough heat to evaporate the accumulated perspiration in your body in one hour. Water evaporating from the skin has a fantastic cooling effect not just on the skin but on the blood flowing just under the skin.
To get rid of the extra heat produced by exercise, capillaries under the skin open up and allow heat to get closer to the surface. That is why people get flushed red cheeks after exercise. Most of the water lost by sweating is drawn from the blood, dropping blood volume up to 27 percent.
A person playing tennis who doesn’t stop to drink some water will soon miss a lot of easy shots. What happens is that blood has gotten less watery. In this state it cannot move as easily and cannot pick up oxygen readily when it goes to the lungs, and thus cannot give muscles the amount of oxygen it needs. Muscles deprived of oxygen cannot function very well. Clumsiness, dizziness and disorientation are the results of dehydration.
If you continue to exercise in spite of being dehydrated, the muscles start to really need oxygen, which they cannot get from the blood in this state. To assure that the blood gets to your muscles in this dehydrated state, the flow of blood to the skin decreases in order to decrease sweat, which, in turn, conserves water. The time that body heat builds up dangerously and the sweating mechanism is most needed, sweating is prevented. This causes body heat to be raised too much, causing what is called a heat stroke. Victims of heat stroke feel extremely hot to touch, but are not perspiring.
In order to gauge how much water you lose in the process of exercising, weigh yourself after exercising. If you lose 4 percent of your weight, you have lost about 20 percent of body water. For every pint lost during exercise, replace it with a pint of water, which is equivalent to two cups.
To avoid getting a heat stroke, drink a cup of mineral water half an hour before exercising. Drink three to six ounces every fifteen minutes during exercise. Do not wait until you get thirsty to drink water. If you wait until you notice your thirst, you are already in the process of becoming dehydrated. Drink cool fluids, with temperatures of about 40 to 50 degrees Fahrenheit. At this temperature, fluids pass from the stomach to the bloodstream faster than at any other temperature. After exercise, sports drinks and fruit juices are advisable as well.
To avoid heat exhaustion, wear clothes that allow your body to breathe. Do not exercise when you have a fever. On very hot days, exercise either early in the morning or late in the evening. Also on very hot days, swimming is probably the best exercise.