Osmosis
The reason we care about blood osmolarity is that osmolarity controls the flow of water through cell membranes.
Water can pass through most cell membranes. This means that when a molecule of water hits the membrane, it can go through.
The number of water molecules that goes through depends on the number that hit the membrane.
Look at the membrane below. Which side of the membrane will be hit by more water molecules?
(Blue = water: pink = solutes)
