Exactly... "hydration" is a vague term, and where that fluid is matters a great deal. It's even more subtle than 'in the stomach vs out of the stomach'. Is it i

Exactly... "hydration" is a vague term, and where that fluid is matters a great deal. It's even more subtle than 'in the stomach vs out of the stomach'. Is it in your circulatory system? Inside your cells? In the extracellular space? Your body is very good at leveraging osmosis to move water around. It's main goal is to maintain adequate volume in your circulatory system (low volume means low pressure, just like any pumping system, low pressure means not enough blood gets to your brain). It will shift fluid around from various places to keep enough in the blood. This effect is actually leveraged in traumatic injuries with significant blood loss. "Extra" salty water will be given via IV, and that will 'pull' water into the circulatory system, via osmotic pressure. If you're peeing out the water you drank within an hour or two, it's because your body didn't need it (or it needed to use it to carrying some stuff out with it). Urinating is a _good_ thing.