What occurs at a lung volume equivalent to the functional residual capacity (FRC)? 1. The forces of the chest wall and lungs are in balance. 2. Chest wall expansion is offset by lung contraction. 3. Opposing chest wall-lung forces generate negative Ppl
a. 1, 2, and 3
b. 1 and 2 only
c. 1 and 3 only
d. 2 and 3 only
ANS: A
The lung-chest wall system may be compared with two springs pulling against each other. The chest wall spring tends to expand, whereas the lung spring tends to contract. At the resting level, the forces of the chest wall and lungs balance. The tendency of the chest wall to expand is offset by the contractile force of the lungs. This balance of forces determines the resting lung volume, or FRC. The opposing forces between the chest wall and lungs are partially responsible for the sub-atmospheric pressure in the intrapleural space.
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