The colloid osmotic pressures of invertebrate body fluids.

Colloid osmotic pressures of the body fluids of twenty invertebrate species were measured directly. The results, which are generally lower than predicted values for the same species, pertain to several physiological questions: (1) they do not quantitatively explain the frequently observed hyperosmoticity of body fluids in species believed to be osmoconformers, indicating that the condition cannot be merely a consequence of a Gibbs-Donnan equilibrium; (2) the excess of hydrostatic over colloid osmotic pressure is very small. This result supports the hypothesis that the oxygen transport function of bloods with extracellular haemocyanins and haem proteins is limited by their colligative properties; (3) the pressure relationships and the absence of colloid osmotic activity in urine indicates that filtration contributes to urine formation in several species.