The diagram below depicts four alveoli. Alveoli A and C are normal. Alveolus B is experiencing increased airway resistance because of airway narrowing. Alveolus D has decreased compliance. How would a positive-pressure tidal volume be distributed among these alveoli?
A. All alveoli would likely be equally distended.
B. Alveolus B would likely be overdistended.
C. Alveolus D would likely be overdistended.
D. Alveoli A and C would likely be overdistended.
ANS: D
A. Incorrect response: See explanation D.
B. Incorrect response: See explanation D.
C. Incorrect response: See explanation D.
D. Correct response: Regarding alveolus B, narrowed airways produce a high resistance to flow that can result in high peak inspiratory pressures. Much of this pressure is dissipated in providing gas flow through obstructed airways and does not overdistend distal alveoli. Heterogeneous distribution of airflow obstruction seen in most airway diseases, however, means that less obstructed alveoli (i.e., alveolus A) may transiently be exposed to these high peak inspiratory pressures and be at risk for overdistention injury.
In lungs with compliance inhomogeneities (alveoli C and D), a positive-pressure tidal volume distributes to the alveoli (i.e., alveolus C) with the highest compliance. The resulting regional tidal volume may be sufficiently large to produce regional overdistention.?
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