Its quite simple. If you don't have the salts in the IV bag, the water would automatically diffuse by osmosis out of the blood vasculature into the interstitial tissue space.
((IV fluids that have salts such as saline or hartmann's, tend to still have the water diffuse out of the blood, but much slower than if it was plain water - there is a lot of argument about which fluids should be used for what clinical situation))