An area of the lung has no ventilation but is normally perfused by the pulmonary circulation. Which of the following statement(s) is/are correct?

1. Blood exiting the pulmonary capillary will have a PO2 = 40 and a PCO2 = 46.
2. The area represents an alveolar shunt.
3. The is 0.
a. 2 and 3 only
b. 1 and 3 only
c. 2 only
d. 1, 2, and 3

ANS: D
With no ventilation to remove carbon dioxide and restore fresh O2, the makeup of gases in these areas is like that of mixed venous blood (PO2 = 40 mm Hg; PCO2 = 46 mm Hg). The / is 0. Ve-nous blood entering areas with / ratios of 0 cannot pick up O2 or unload carbon dioxide and leave the lungs unchanged. For such areas to be distinguished from true anatomical shunts, exchange units with / values of 0 are called alveolar shunts. Although small anatomical shunts are normal, alveolar shunts are not.