When breathing changes or when metabolism changes, it alters the PO2 and the PCO2 in the blood. We should therefore expect the blood levels of one or both of these gases to provide feedback information to the breathing rhythm generator in the medulla. Experiments in which subjects breathe air with different PO2 and PCO2 concentrations lead us to conclude that humans (and other mammals) are remarkably insensitive to falling blood levels of O2, but very sensitive to increases in the PCO2 of the blood (Figure 48.16).
We may ask whether it is a rise in the PCO2 of the blood that stimulates increased breathing when we exercise. To answer this question, researchers ran dogs on treadmills at different speeds. As the speed
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