Clinical Correlates of pH Levels Problem Set

Clinical Correlates of pH Levels
Problem Set

Problem 7: Chronic compensation for collapsed lung

Tutorial to help answer this question

Suppose a patient has a collapsed lung and cannot expel CO₂ at a normal rate. Chronic compensation would involve:

A. increased activity of carbonic anhydrase.

B. kidney retention of H⁺.

C. kidney retention of HCO₃^-.

D. kidney expulsion of HCO₃^-.

E. switching to anaerobic metabolism.

Tutorial

Compensation for respiratory acidosis

A patient with a collapsed lung who cannot expel CO₂ at a normal rate will suffer from respiratory acidosis. This happens because the resulting rise in blood CO₂ triggers an increase in the H₂CO₃ intermediate, which, in turn, dissociates to bicarbonate and H₊, thus lowering pH.

CO₂

H₂O

[H₂CO₃]

H⁺

HCO₃^-

Because the main problem in respiratory acidosis affects the lung, the only other organ that can really compensate is the kidney. As might be predicted by the above equation, the kidney retains HCO₃^- (see green arrow in table below) in order to drive the reaction back to the left and reduce H⁺. (The kidney also has a limited ability to excrete H⁺ directly.)

	respiratory				metabolic
	acidosis		alkalosis		acidosis		alkalosis
	U	C	U	C	U	C	U	C
pH
HCO₃^-:CO₂ ratio
[HCO₃^-]
pCO₂
total CO₂