Sweating takes place, you lose water, it evaporates, cooling the blood, and in turn circulates back again and the process is repeated. It's similar to the cooling system of a car. When training or competing, you can lose a lot of water by sweating . which must be replaced to perform your best. Losing as little as two to three percent of your body weight by sweat can cause a decrease in concentration,coordination, strength, and stamina. During heavy physical activity you will need to drink more.
Some athletes lose three to five liters of water a day.
You should drink water before, during,and after practice or competition. Weigh in before and after a workout. Each one pound lost during a workout should be replaced with 2 cups of fluid. If the water you lose through sweat is not replaced, blood volume decreases. In the milder forms of heat illness, dizziness or fainting may occur.