Extracellular Acid-Base Status and the Nature of the Respiratory Medium

Abstract

Life originated in an aquatic medium; indeed the early evolution of all metazoan phyla occurred in water. A major event of biological evolution, the transition from water to air breathing, however, occurred independently in several animal groups, the most successful of which are the arthropods and the tetrapod vertebrates. This fundamental change required many structural and physiological transformations. Among others, the nature of the respiratory medium profoundly influences the extracellular acid-base state, principally via one of its independent variables, the carbon dioxide partial pressure. This results from unavoidable physical constraints with which the acid-base regulatory system must comply.