Critical appraisal of some factors pertinent to the functional designs of the gas exchangers.

Respiration acquires O2 from the external fluid milieu and eliminates CO2 back into the same. Gas exchangers evolved under certain immutable physicochemical laws upon which their elemental functional design is hardwired. Adaptive changes have occurred within the constraints set by such laws to satisfy metabolic needs for O2 , environmental conditions, respiratory medium utilized, lifestyle pursued and phylogenetic level of development: correlation between structure and function exists. After the inaugural simple cell membrane, as body size and structural complexity increased, respiratory organs formed by evagination or invagination: the gills developed by the former process and the lungs by the latter. Conservation of water on land was the main driver for invagination of the lungs. In gills, respiratory surface area increases by stratified arrangement of the structural components while in lungs it occurs by internal subdivision. The minuscule terminal respiratory units of lungs are stabilized by surfactant. In gas exchangers, respiratory fluid media are transported by convection over long distances, a process that requires energy. However, movement of respiratory gases across tissue barriers occurs by simple passive diffusion. Short distances and large surface areas are needed for diffusion to occur efficiently. Certain properties, e.g., diffusion of gases through the tissue barrier, stabilization of the respiratory units by surfactant and a thin tripartite tissue barrier, have been conserved during the evolution of the gas exchangers. In biology, such rare features are called Bauplans, blueprints or frozen cores. That several of them (Bauplans) exist in gas exchangers almost certainly indicates the importance of respiration to life.