Practice Questions: Answers -- Alveolar Gas Exchange

Alveolar Gas Exchange Answers

1. A. Pulmonary edema increases diffusion distance between the alveoli and the pulmonary capillaries; therefore both DO2 and DCO2 would be reduced. Note, however, that the diffusing capacity decrease has a greater effect on blood oxygen than on blood carbon dioxide because DCO2 is normally so much higher than DO2.

2. D. Systemic venous blood contains both the CO2 that was in systemic arterial blood plus the CO2 that is added to the blood by tissue metabolism as the blood passes through the systemic capillaries. Question: What would be the answer is the question read "At which of the following sites is PCO2 lowest?"

3. C. At the end of inspiration, the dead space contains ambient or inspired air (saturated), which has a PO2 of 150 mmHg.

4. D. Breathing though a tube increases the dead space and the wasted ventilation. If the tidal volume remains constant, then the increased dead space means the alveolar ventilation is reduced, so alveolar PA-CO2 increases. If PA-CO2 increases then systemic arterial Pa-CO2 must increase also.

5. E. PA-O2 would be lower than normal due the reduced PI-O2.   PA-CO2 would probably be lower due to hyperventilation caused by the hypoxia. However, PA-H2O would be normal, as water vapor pressure in the alveoli depends only on the temperature. Note: I would also accept choice A as the correct answer.

6. E. Pa-O2 decreases when DO2 decreases to less than one-third its normal value. But DCO2 is so high normally that even a decrease to one-fourth will still permit carbon dioxide to equilibrate in the time that blood passes through pulmonary capillaries.

7.  C.  Since the Dead Space is about 30% of the tidal volume at rest, then the Wasted Ventilation will be about 30% of the Total Ventilation at rest, also.