osmotic pressure

If osmosis is water sneaking toward the saltier side, osmotic pressure is the exact amount of push you would have to apply to the salty side to stop it. It is the back-pressure that just balances the natural tendency of solvent to flow in. The more dissolved particles a solution holds, the harder water wants to enter, and the higher its osmotic pressure.

Osmotic pressure depends only on the number of dissolved particles per volume, not on what they are, so one mole of sugar and roughly two moles of dissolved ions from one mole of sodium chloride exert different pressures. For dilute solutions it follows a relationship much like the ideal gas law, rising with particle concentration and with temperature. This is why body fluids, with their fixed total solute content, sit at a well-defined osmotic pressure that formulations are matched to.

Pharmacists harness osmotic pressure deliberately. The osmotic pump tablet uses a core that draws in water through a semipermeable coat, building internal pressure that squeezes drug solution out through a tiny laser-drilled hole at an almost constant rate. Conversely, mismatched osmotic pressure is dangerous: an injection far from body osmotic pressure can rupture or shrivel red blood cells.