Do the exposure time and PCO2 affect the terminal PO2 in a confined atmosphere?

The effect of exposure time and the presence of CO2 on gas exchange and the terminal PO2 of rats in a confined atmosphere where PO2 decreased due to the oxygen consumption of the rats was measured. Terminal inspired PO2 was higher (46.4 Torr) in short exposure (0.47 h) than terminal PO2 (35.3 Torr) in longer exposures (0.8-13 h). Terminal PO2 was not changed when CO2 accumulated in the sealed chamber (CO2 group) as compared with the conditions where CO2 was consistently removed (no-CO2 group). Carbon dioxide caused further depression of VO2 with developing hypoxia. Both breathing frequency and heart frequency showed no response to PO2 in short exposure, maximal response in intermediate exposure time and reduced response at the longest exposure time. The rat could regulate different physiological parameters to meet its needs in developing hypoxia down to a critical PO2 below which regulation was impaired. This critical PO2 was found to change as a function of the exposure duration. The possible early adjustment to hypoxia and the common use of critical PO2 for comparison between species are discussed.