This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Which of the following is TRUE about paial pressures of alveolar gases? 
Select and explain. 
A. Alveolar PCO2 is more than mixed venous pCO2, B. Alveolar pO2 is less than pO2 of expired air, C. Alveolar pCO2 is twice that of the room air, D. Mixed venous pCO2 is more than alveolar pCO2
D: Mixed venous pCO2 is more than alveolar pCO2
Answer: D: Mixed venous pCO2 is more than alveolar pCO2 
Explanation: In a normal person, the mixed venous paial pressure of carbon dioxide is 45 mm Hg and alveolar paial pressure of carbon dioxide is 40 mm Hg. An equilibrium is reached in about 0.25 of a second, or about the same time as that for oxygen. The diffusivity of carbon dioxide is about 20 times that of oxygen, but the paial pressure gradient is normally only about 5 mm Hg for carbon dioxide, whereas it is about 60 mm Hg for oxygen. Carbon dioxide transfer is therefore normally perfusion-limited, although it may be diffusion-limited in a person with an abnormal alveolar-capillary barrier. Ref: Levitzky M.G. (2007). Chapter 6. Diffusion of Gases. In M.G. Levitzky (Ed), Pulmonary Physiology, 7e.