Environmental Factors as Determinants in Bimodal Breathing: An Introductory Overview

Abstract

SYNOPSIS. The physicochemical properties of the environment, water, air, and more or less humid soils, are extremely different and impose anatomical and physiological adaptations. Generally water breathers exchange through gills and skin; in the air breathers cutaneous respiration is generally small or negligible, and gas exchanges take place in lungs or in tracheae. The main difference between water and air as to O2 and CO2 is that O2 is much less soluble in water than CO2, whereas O2 and CO2 capacitances in air are identical and the CO2 capacitances in water and in air are similar. This results in very different CO2 tensions in waterand in air-breathers. Since air is rich in O2 compared to water, air breathers breathe much less than the water breathers, and so their Pco2'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 exchanges with water and pulmonary exchanges with air, the proportion of O2 and CO2 exchanges are not evenly distributed among the several exchangers: the aquatic gas exchanger is the main route for the CO2 output, whereas the gas-phase exchanger is the main route for O2 uptake. An increase of temperature has several consequences: 1) decrease of the O2 and CO2 capacitances in water and, to a lesser extent, in air; 2) increase of the energy metabolism, O2 consumption, CO2 production, etc.; 3) changes of the pH of the ambient water and of the body fluids. These effects of the changes of temperature are seen in all living beings; in the amphibious animals, the increase of temperature augments the O2 uptake by the lung and the CO2 output by the branchial and/or cutaneous routes. That is to say that the temperature variations change the intensity of the gas exchanges and the distribution of O2 uptake and CO2output between the gill-skin exchanger and the lung exchanger. It is classical to oppose terrestrial life to aquatic life; the amphibious, often bimodal animals represent intermediate forms which presumably play an essential role in the evolution and transition from an aquatic to a terrestrial abode.