What happens in chronic hypercapnia? 1. The central chemoreceptive response to CO2 is decreased. 2. The cerebrospinal fluid pH is restored to normal. 3. Responsiveness to increased CO2 is decreased
a. 1 only
b. 1 and 2 only
c. 2 and 3 only
d. 1, 2, and 3
ANS: D
If PaCO2 rises gradually over time, as might occur in severe COPD because of steadily deterio-rating lung mechanics, the kidneys compensate by increasing the plasma bicarbonate concentra-tion, keeping the arterial pH within normal limits. As plasma bicarbonate levels increase, these ions slowly diffuse across the blood-brain barrier, keeping cerebrospinal fluid pH in its normal range. The central chemoreceptors respond to hydrogen ion concentration, not the CO2 molecule; thus, they sense a normal pH environment, even though the PaCO2 is abnormally high.
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