Osmoregulation: keeping water and salts in balance

A goldfish in a bowl never takes a single sip. A sailor stranded at sea can die of thirst while surrounded by water. Both are stuck with the same rule, just from opposite sides of it.

Water always moves from a more dilute solution toward a more concentrated one, across a membrane, until both sides match. The goldfish's body fluid is already saltier than the bowl, so water keeps pushing in on its own. Seawater is saltier than the sailor's blood, so drinking it only pulls more water out of his cells.

Every cell, in every organism, faces this same tug of war between itself and its surroundings. The mechanism that keeps water and salts in balance despite it is called osmoregulation.