Homoiosmotic Animals

aquatic animals that are capable of maintaining a more or less constant osmotic pressure of the blood and tissue fluids, which is different from the osmotic pressure of the surrounding medium. Homoi-osmotic animals are contrasted with poikilosmotic animals.

Examples of homoiosmotic animals are those freshwater invertebrates in which the osmotic pressure of the blood and tissue fluids is higher than the osmotic pressure of the surrounding medium; they maintain their osmotic pressure by discharging excess water from the body through the excretory organs. Another example of homoiosmotic animals are the teleost fish; those of them that live in seas maintain a lower osmotic pressure of the blood and tissue fluids than that of the surrounding medium by excreting relatively small amounts of urine through the kidneys and an abundant quantity of salts through the gills; on the other hand, teleost fish that live in fresh water maintain a higher osmotic pressure than their surrounding medium by excreting much urine and absorbing salts through the gills.