Question

why does lung compliance increase as pressure increase? or in another words, how come change in lung volume is higher as pressure increases?

Provide literature to support answer.

Answer 1

Compliance describes the distensibility of the respiratory structures. It says how much the volume of a structure changes for a given change in pressure. If compliance is high, the volume changes a lot and if it is low, the volume changes little. The lungs are both compliant and elastic. They must be compliant to fill with air during inspiration. They must be elastic to recoil and push air out during expiration. The compliance of the lungs is demonstrated by an isolated lung in a jar.

This increases lung compliance and reduces the tendency of the lungs to recoil inward. The presence of pulmonary surfactant causes the surface tension to decrease in proportion to the ratio of surfactant to alveolar surface area. Here in the given figure, the lung volume is is expressed as a function of pressure. Compliance is the slope of the relationship ΔV/ΔP . The space outside the lung is analogous to intrapleural pressurein vivo. In the experiment, the lungs and airways are open to the atmosphere and the pressure inside the lungs is equal to atmospheric. Pressure outside the lung is varied with a pump to simulate changes in intra pleural pressure. The volume of the lung is measured at different pressures. When the outside pressure is made more negative (i.e., lower than atmospheric), the lung inflates and its volume increases. When the outside pressure is made less negative, the lung deflates and its volume decreases. A sequence of inflation and deflation creates a pressure-volume loop. The slopes of the inspiration and expiration curves are compliance, and compliance is an intrinsic property of the lung. Since it’s the same lung, why would compliance be higher when we expire than when we inspire, The answer is surface tension. In the air-filled lung, there are strong inter molecular forces between liquid molecules lining alveoli. During inspiration, one begins at low lung volume where the liquid molecules are close together and strongly attracted to each other to inflate the lung, one must break these inter molecular forces.

Low compliance indicates a stiff lung and means extra work is required to bring in a normal volume of air. This occurs as the lungs in this case become fibrotic, lose their distensibility and become stiffer. In a highly compliant lung, as in emphysema, the elastic tissue is damaged by enzymes. Compliance is increased in obstructive lung disease like pulmonary emphysema, less in asthma and at a minor degree in chronic bronchitis. In emphysema, the elastic recoil is decreased and the P-V curve is shifted up and left. This is due to the loss of elastic tissue as a result of alveolar wall destruction.

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