Most terrestrial animals live in environments in which the abundance of gases is relatively constant with oxygen at 21% and CO2 is <0.04%. Yet, many environments are characterized by different gas compositions, including high CO2 and low O2 concentrations in some aquatic environments and low barometric pressures at high altitudes (with gas composition remaining equal to that at sea level). At high altitudes, animals respond by increasing the affinity of hemoglobin and the hematocrit to facilitate the acquisition and transport of oxygen, whereas in stagnant aquatic environments similar changes will occur along with an increased capacity to buffer high levels of environmental CO2. Furthermore, the evolution of a terrestrial from an aquatic habit was associated with a shift from an oxygen-poor environment, due to the limited solubility of oxygen in water, to an oxygen-rich environment. This shift is usually associated with reduced levels of gas exchange, which further enhances life in a terrestrial environment through the reduction in evaporative water loss.
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