"Hydrate or Die," is the slogan of CamelBack, the company that first introduced hydration systems a few years go. Hyperbole aside, the importance of drinking enough water while exercising -- especially during the summer months -- cannot be emphasized enough.

When we exercise, we build up heat in the working muscles, causing our internal temperatures to rise. We cool ourselves by perspiring, or sweating. It is actually the evaporation of sweat on our skin that cools us, helping us maintain a cooler core temperature. That's why hot, humid conditions can be dangerous. When there is a lot of moisture in the air, evaporation doesn't occur and the sweat just runs down our arms and legs. We lose even more water and we need to drink more to keep cool and replace lost fluid.

The first drawback to loss of fluids is diminished performance. The water we lose through sweating comes from blood plasma. Plasma is the vehicle for red blood cells, which contain the oxygen that muscles need in order to perform work. As you become dehydrated, your total blood volume is reduced, and the oxygen-carrying capability of your blood decreases.

If you continue to exercise without proper fluid intake, you may experience a drop in blood pressure, dizziness and nausea, or even faint. These are the symptoms of heat exhaustion. Keep going, and your body's ability to dissipate heat is even further impaired, and you may suffer heat stroke.