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. When you exercise your body demands higher volumes of oxygen. If you are dehydrated your plasma volume decreases and therefore cannot deliver the oxygen required to your muscles and organs as efficiently. You will not be able to train to your optimum level, no matter how much you try. With a reduced plasma volume your heart will have to work harder to pump less blood around the body. You’re already pushing it while you train, why make it work any harder for less output?
Another symptom is reduced blood pressure against the vascular walls. You may feel faint, dizzy or nauseous. These are the symptoms of heat exhaustion. If you keep going, your body’s ability to dissipate heat is further impaired and the consequences become progressively more severe.
Your brain is 85% water. Why do you think you get headaches when you’re dehydrated? The water carries the electrical signals for you to function. In basic terms a drop in the volume of water will make those signals slower. If you want to stay alert and on top of your game, you need to ensure you’re hydrated, before, during and after your session.
The sensation of thirst is triggered by the hypothalamus in the brain. It measures the concentration of salts in your blood. As blood volume decreases due to sweating, your concentration of salts goes up, and the hypothalamus sends the signal to start drinking. By the time you feel thirsty, you are already dehydrated. It is thought that this concentration of salts and electrolytes may also be one of the causes of muscle cramps.