There are several different reasons dogs lose water from their body. A few ways this can happen is through panting, urinating, and evaporation through the feet and other body surfaces. Obviously, dogs replenish their fluids by drinking water or other liquids, and also by eating moist foods.
I once had a dog named Lightning that was so afraid of thunder he would dart behind our couch for cover. Ironic, I know. Dogs are often afraid of the booming noise and will sometimes be so nervous that they shake.