All of the gastro intestinal tract is a very long moist tube to help digestion of food and then absorption of nutrients and then excretion. There is always a constant flow of enzymes which are water based. There is a mechanism by which the kidneys control the entire body water in the right proportion and when dehydration occurs, the first thing that happens is that our internal body fluid maintenance mechanism gets upset and tells on the various organs resulting in dry mouth, skin, eyes etc. until the fluid gets replaced.