Osmoregulation is the process by which animals control solute concentrations and balance water gain and loss. This is essential for fluid environments in cells, tissues, and organs. Water enters and leaves cells through osmosis, with osmolarity referring to osmotic pressure. Osmoregulators must expend energy to maintain osmotic gradients through active transport, accounting for 5% or more of resting metabolic rate in many fish. Osmoregulation relies on transport epithelia that move specific solutes in specific directions.