Humans typically produce a large amount of saliva per day (1.5+ liters in adults). I've searched around but can't find a direct answer to the question of where, since saliva is 99% water, all the water comes from. My guess (and forgive me if this seems obvious) is that it's delivered via the blood, and the relevant cells in salivary glands (unlike cells in most other tissues) are constantly pumping large amounts of water out of the blood in order to make saliva.
A related question is, does the body in some way have mechanisms for delivering more water to these glands? Or is the blood leaving those tissues just lower in water content?