When engaged in physical activity, body temperature rises as much as three degrees.  Your body's natural cooling system, sweating, kicks in to lower body temperature under extreme exercise and heart stress, a body can lose half a gallon of water per hour.  If the lost water is not replaced, dehydration occurs and serious consequence may follow.   

To understand what happens, here presented the analogy of your car. When your car's cooling system is running smoothly, excess heat from the engine is transferred to the water in the tubes, which goes to the radiator to be cooled by the air.  The cooled water goes back to heated once again by heat drawn from the engine.  If there is not enough water in the system to allow for proper heat dissipation, your engine over heats, your car stops running and you are left cursing at the side of the road.  Now picture your body as the car- your muscles are the engine, your skin is the radiator and your blood vessels are the water tubes that connect engine with the radiator.   

When your cooling system is running smoothly, excess heat from your muscles is drawn in to your blood vessels where is circulates to your skin.  Evaporating sweat draws heat away from the blood vessels.  The cooler blood then recirculates throughout the body, lowering body temperature.  When too much water is lost through sweating, your blood volume decreases.  This decreases blood pressure which in future, reduces blood flow between the muscles and skin.  To over come this, your heart rate increases.  Because less blood reaches the skin, heat loss is reduced and the body over heats. 

If you fail to replace the water you lose, you run the risk of becoming dehydrated.  When a person is dehydrated by more that 4% or 5% of body weight, their exercise performance declines by 20% to 30%.  Not only that, the impact of dehydration on the cardiovascular system can produce heart problems in people with coronary heart disease and diabetes.

Sweating is not the only way you lose water.  Another is through respiration - you lose water every time you exhale.  This water loss increases as your physical activity increases because you breathe more.  So if you are just replacing how much you are sweating out, it is not enough.  If you just drink when you are thirsty, you are not getting enough water because thirst alone is not the best measures of a body's fluid needs.  Generally speaking, you should drink water before, during and after exercise.  Drinking about two cups (160z) of water one hour before and one cup half an hour before is a good start.  Then you should drink half a cup to one cup or more, every 15 - 20 min.  The amount depends on the air temperature, your body weight and how hard you are exercising.