Why must PO2 be decreased substantially before ventilation increases, but even small increases in PCO2 stimulate ventilation?
What will be an ideal response?
Answer: Oxygen delivery to the tissues depends on the amount of oxygen delivered by hemoglobin. The hemoglobin oxygen dissociation curve is a plateau between 80 and 100 mm Hg PO2 over this range, hemoglobin is virtually 100% saturated, so a drop in blood PO2 from 100 to 80 has very little change on oxygen delivery to the tissues. Even a drop in blood PO2 from 100 to 60 mm Hg only decreases oxygen delivery to the tissues by about 10%. This explains why ventilation does not need to increase substantially until air PO2 drops below 60 mm Hg. In contrast, a rise in PCO2 produces linear increases in dissolved CO2, carbonic acid and H+ production, producing a near-linear increase in ventilation in order to homeostatically control PCO2 and pH.
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