Comparative physiology of body fluid regulation in vertebrates with special reference to thirst regulation

The origin of life took place in the ancient sea where the ionic concentrat ion is thought to have been somewhat lower than that of the present day sea s, This may partly explain why most vertebrate species have plasma ionic co ncentrations roughly one-third of seawater. Exceptions are primitive marine cyclostomes whose plasma is almost identical to seawater, and marine carti laginous fishes that accumulate urea in plasma to increase osmolarity to a seawater level. The mechanisms for regulation of water and electrolyte bala nce should have evolved from these animals into those of more advanced ones in which plasma ions are regulated to one-third of seawater irrespective o f the habitat. Although most extant terrestrial and aquatic animals maintai n similar plasma osmolarity and ionic concentrations, the mechanisms of reg ulation differ greatly among different groups of animals according to their habitat, An outstanding difference is that while plasma Na+ concentration is a primary factor of regulation in terrestrial mammals and birds, blood v olume is most strictly regulated in aquatic teleost fishes. Consistently, w hile an increase in plasma osmolarity (cellular dehydration) is a major dip sogenic stimulus for habitat. aquatic birds and mammals, hypovolemia (extra cellular dehydration) is a much stronger stimulus for elicitation of drinki ng in teleost fishes. Furthermore, fish cells in culture are tolerant to ch anges in environmental osmolarity compared with mammalian cells, further su ggesting a secondary role of plasma osmolarity as a target of regulation in fishes. A secondary role of blood volume for body fluid regulation in bird s is further assessed by the fact that volume receptors for thirst, salt gl and secretion, and vasotocin secretion are localized in the extravascular, interstitial space in some species of birds. All terrestrial animals includ ing mammals have derived from the fishes in phylogeny, during which the mec hanisms for body fluid regulation underwent adaptive evolution in the cours e of transition from aquatic to terrestrial life, Therefore, much can be le arned from comparative studies of body fluid regulation that reveals the di versity and uniformity of the mechanisms. In this review, important compara tive studies that may contribute to an understanding of body fluid regulati on throughout vertebrate species will be summarized.