The primary limiting factor in sports that require great endurance is the time it takes for your heart to pump oxygen in your bloodstream from your lungs into your muscles.

A study from the University of Connecticut (Medicine and Science in Sports and Exercise, May, 2006) shows that with dehydration, your heart beats with far less force so it pumps far less blood with each beat, and is unable to bring as much oxygen to your muscles.

You can't depend on thirst to tell you when you lack fluids. Certain brain cells called osmoreceptors tell you when you are thirty, but only after the salt concentration of your blood has risen considerably.

When you exercise, you sweat. Sweat contains far more water than salt in comparison to blood. So you lose far more water than salt during exercise and blood levels of salt rise.