Comparative respiratory physiology: the fundamental mechanisms and the functional designs of the gas exchangers
John N Maina
Department of Zoology, University of Johannesburg, Johannesburg, South Africa
Abstract: Acquisition of molecular oxygen (O2) from the external fluid media (water and air) and the discharge of carbon dioxide (CO2) into the same milieu is the primary role of respiration. The functional designs of gas exchangers have been considerably determined by the laws of physics which govern the properties and the flux of gases and the physicochemical properties of the respiratory fluid media (water or air and blood). Although the morphologies of gas exchangers differ greatly, certain shared structural and functional features exist. For example, in all cases, the transfer of O2 and CO2 across the water/air–blood (tissue) barriers occurs entirely by passive diffusion along concentration gradients. In the multicellular organisms, gas exchangers have developed either by evagination or invagination. The arrangement, shape, and geometries of the airways and the blood vessels determine the transport and exposure of the respiratory media and, consequently, gas exchange. The thickness of the water/air–blood (tissue) barrier, the respiratory surface area, and volume of pulmonary capillary blood are the foremost structural parameters which determine the diffusing capacity of a gas exchanger for O2. In fish, stratified design of the gills and internal subdivision of the lungs increase the respiratory surface area: the same adaptive property is realized by different means. A surface active phospholipid substance (surfactant) lines the respiratory surface. Adaptive specializations of gas exchangers have developed to meet individual survival needs.
Keywords: gas exchanger, oxygen, respiration, carbon dioxide, diffusing capacity
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