osmotic pressure
/ oz-MOT-ik PRESH-er /
Imagine a U-shaped tube split in the middle by a membrane that lets water through but not salt. Put salty water on one side and pure water on the other. Water flows by osmosis into the salty side, and its level there climbs higher and higher. The rising column of water pushes back down with its weight, and eventually the push exactly balances the flow. The amount of pressure needed to stop the inflow is the osmotic pressure of that salty solution.
More precisely, osmotic pressure measures how strongly a solution pulls water in across a membrane — the more dissolved particles it contains, the harder it tugs, and the higher its osmotic pressure. It depends on the number of particles, not their kind: dissolve a spoon of salt that splits into two ions and it pulls twice as hard as a spoon of sugar that stays as one piece. It is the hidden force behind every osmosis story, given a number.
Osmotic pressure is surprisingly strong and runs your body around the clock. Your blood proteins create an osmotic pull that draws fluid back into your capillaries from the tissues; when that pull is too weak (as in severe malnutrition), fluid leaks out and the body swells. Plants stand upright partly because water pulled into their cells by osmotic pressure presses out against the cell walls, keeping stems stiff — lose that pressure and the plant wilts.
Children with severe protein starvation often have swollen bellies: with too little protein in their blood, the osmotic pull that should keep fluid in the vessels is too weak, so water leaks into the belly.
Blood proteins generate an osmotic pull that holds fluid in the vessels.
Osmotic pressure counts particles, not weight: a solution can pull water hard simply because its solute splits into many small pieces, even if the total amount of stuff is small.