A cell is isotonic to its environment if the osmotic pressure of the intra- and extra- cellular fluid compartments are equal. If the extracellular salt concentration is lower than the intracellular fluid, the cell is hypertonic relative to its environment and water tends to flow into the cell. When the cellular fluid is less concentrated, the cell is hypotonic to its environment and water tends to leave the cell and the cell shrinks. Moving three sodium particles out for every two potassium particles that enter, the sodium-potassium pump is critical to increasing the osmotic pressure of the environment, which reduces the tendency of water to rush in due to the osmotic pressure of cellular contents.

In addition to cellular processes, physiological regulation of the osmotic pressure of the extracellular fluid compartment takes place through the actions of the urinary system.