What forces establish the sub-atmospheric pressure found in the pleural space?

a. Contraction of accessory muscles of in-spiration
b. Contraction of expiratory muscles
c. Equal opposing tendency of the chest wall to expand and lung to collapse
d. Effect of gravity, particularly at the base of the lungs

ANS: C
This tendency of the lung to collapse is counteracted by the thoracic wall's tendency to spring outward and to hold the lung inflated. The "tension" developed by these two opposing tenden-cies results in development of sub-atmospheric intrapleural pressure.