ENVIRONMENTAL-FACTORS AS DETERMINANTS IN BIMODAL BREATHING - AN INTRODUCTORY OVERVIEW

The physicochemical properties of the environment, water, air, and mor e or less humid soils, are extremely different and impose anatomical a nd physiological adaptations. Generally water breathers exchange throu gh gills and skin; in the air breathers cutaneous respiration is gener ally small or negligible, and gas exchanges take place in lungs or in tracheae. The main difference between water and air as to O-2 and CO2 is that O-2 is much less soluble in water than CO2, whereas O-2 and CO 2 capacitances in air are identical and the CO2 capacitances in water and in air are similar. This results in very different CO2 tensions in water- and in air-breathers. Since air is rich in O-2 compared to wat er, air breathers breathe much less than the water breathers, and so t heir P-CO2's are much higher. However, at the same temperature, water- and air-breathers have about the same pH, thanks to proper adjustment of the bicarbonates. In amphibious animals, those having gill-skin ex changes with water and pulmonary exchanges with air, the proportion of O-2 and CO2 exchanges are not evenly distributed among the several ex changers: the aquatic gas exchanger is the main route for the CO2, out put, whereas the gas-phase exchanger is the main route for O-2 uptake. An increase of temperature has several consequences: 1) decrease of t he O-2 and CO2 capacitances in water and, to a lesser extent, in air; 2) increase of the energy metabolism, O-2 consumption, CO2 production, etc.; 3) changes of the pH of the ambient water and of the body fluid s. These effects of the changes of temperature are seen in all living beings; in the amphibious animals, the increase of temperature augment s the O-2 uptake by the lung and the CO2 output by the branchial and/o r cutaneous routes. That is to say that the temperature variations cha nge the intensity of the gas exchanges and the distribution of O-2 upt ake and CO2 output between the gill-skin exchanger and the lung exchan ger. It is classical to oppose terrestrial life to aquatic life; the a mphibious, often bimodal animals represent intermediate forms which pr esumably play an essential role in the evolution and transition from a n aquatic to a terrestrial abode.