Physiology: Dynamic instabilities in the inflating lung.
In lung diseases such as asthma, expiratory flow becomes limited, airways can collapse and the vital exchange of gases is compromised. Here we model the inflation of collapsed regions of the lung during inspiration in terms of avalanches propagating through a bifurcating network of airways, and find that the accompanying cascade of dynamic pressure instabilities -- avalanche 'shocks' -- manifests as negative elastic resistance of the lung. Our analysis of this apparent thermodynamic paradox provides a better understanding of aeration in the deep regions of the lung, which may find application in medical conditions in which gas exchange is impaired.