Up to 60% of the human body is water, and only about 10% of this water is in the bloodstream. A lot of the water in the bloodstream can be lost through sweating. In fact, during intense exercise in hot and humid conditions, we can sweat up to 3 liters, which is almost all of the water in the bloodstream. To replace the water that is lost from the bloodstream, the body takes water from its tissues or uses the fluids that you drink during and after exercise.