Molecules passively diffuse from regions of high to low concentration.
Because of this, aquatic animals must develop physiological mechanisms to prevent excess flow of water into their bodies. They must also develop mechanisms to prevent the loss of solutes as excess water is excreted. The process by which organisms actively maintain their internal solute concentration is called osmoregulation. Animals must actively transport solutes from their surroundings into their blood against the concentration gradient. The work required for osmoregulation depends on properties of the organism and its environment.