Why does ventilation–-perfusion mismatching affect the PaO2 more than the PaCO2? I. CO2 is highly diffusible across the alveolar-capillary membrane. II. The PaCO2 is essentially linear to the CO2 content of the blood. III. The O2 is less than the CO2. IV. The relationship between the PaO2 and the O2 content in the blood is nonlinear

A. III only
B. I, II only
C. I, II, IV only
D. II, III, IV only

?ANS: C
A. Incorrect response: Neither the O2 nor the CO2 has an influence on ventilation–perfusion mismatching.
B. Incorrect response: See explanation C.
C. Correct response: Because of the high diffusibility of CO2 across the alveolar-capillary membrane, and because of the linear relationship between the PaCO2 and the CO2 content in the blood, CO2 diffusion across the alveolar-capillary membrane is only minimally affected by ventilation–perfusion mismatching. The diffusion of O2 across the alveolar-capillary membrane is much more affected because the relationship between the PaO2 and the O2 content in the blood is not linear.
D. Incorrect response: See explanation C.