Abstract
NaCl is a major component of the extracellular fluid. The kidney plays critical roles in the maintenance of the homeostasis of the body fluid. The regulation of NaC1 balance is accomplished by glomerular filtration followed by tubular reabsorption of NaCl. The evolution of vertebrates from fishes to mammals were accomplished by adjustment of animals to changes in external environments from sea to terrestrial. The development of diluting nephron segments made it possible for fishes to adapt from seawater to fresh water. Such segments were succeeded to reptiles and amphibians. The development of renal medulla in the avian kidney made it possible for birds to adapt to the terrestrial environment by producing concentrated urine, which is established by the countercurrent multiplication system utilizing active NaC1 transport in Henle's loop. In mammals, urine concentrating ability was much advanced due to additional contribution of urea in the countercurrent system of the renal medulla. The evolutional changes of the renal medulla from birds to mammals can be traced through the ontogeny of the mammalian kidney, where avian type nephron was observed during the fetal period.
