Which of the following statements accurately describe alveolar and extra-alveolar vessels? I. Pulmonary vascular resistance increases through alveolar vessels during inspiration. II. Pulmonary vascular resistance decreases throughout the extra-alveolar vessels during expiration. III. Extra-alveolar vessels behave as if they were surrounded by atmospheric pressure. IV. These two types of vessels

respond in the same manner to changes in lung volume.

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

ANS: A
A. Correct response: Alveolar vessels are exposed primarily to alveolar pressure. Therefore, when the lungs inflate the alveolar vessels experience an increased pulmonary vascular resistance. In other words, as alveolar volume increases, alveolar vessels are compressed; therefore pulmonary vascular resistance increases through these vessels during inspiration. On the other hand, the extra-alveolar vessels are influenced essentially by pleural pressure. Consequently, when the lungs deflate, and pleural pressure increases, the extra-alveolar vessels experience an increased pulmonary vascular resistance.
B. Incorrect response: See explanation A.
C. Incorrect response: See explanation A.
D. Incorrect response: See explanation A.