Notes

Lung Compliance Changes in Disease

Compliance refers to the distensibility of the lungs, chest wall, or both, and is a measure of how much volume changes as a result of changes in pressure.
– Compliance is inversely related to elastance.

Review compliance in health

First we show the compliance curve for healthy systems.
– Indicate the volume at which functional residual capacity is achieved; as we saw in our earlier diagram, this is the volume of air in the lungs after tidal expiration.
– At this point, the pulmonary system is in equilibrium, because, at this volume, the collapsing force of the lungs is equal to the expanding force of the chest wall.
– When a person expires and volume is lower than Functional Residual Capacity, the system "wants" to expand (the expanding force of the chest wall is greater than the collapsing force of the lungs at this point).
– When a person inspires and volume is above Functional Residual cCapacity, the system "wants" to collapse (the collapsing force of the lungs is greater than the expanding force of the chest wall at this point).
Diseases that alter lung compliance alter the slopes of the lines for the lungs and combined system (not for the chest wall, alone).
– So, we show that, in emphysema, which is an obstructive disorder, the slope of the line representing compliance is increased.
Thus, at any volume, the collapsing force of the lungs is reduced, so more volume must be added to establish equilibrium; a new, higher functional residual capacity is established.
– In fibrosis, which is a restrictive disorder, the slope of the line is reduced, and a new, lower functional residual capacity is established.