A physiological phase separation in airway mucus

The pathology of the disease cystic fibrosis is known to be associated very generally with ionic imbalance in the mucosal secretion which lines the respiratory tract, so-called airway surface liquid (ASL). The imbalance is caused by mutation of a transmembrane protein (CFTR) implicated in the control of ion traffic across the airway epithelium. It is feasible that CFTR malfunction undermines a putative phase-separated texture of healthy ASL which is apparent in electron microscopy images. A molecular statistical description of ASL is presented here with the aim of illustrating this hypothesis at the phenomenological level. The model predicts that a volume criterion and a salt criterion must be met in order to achieve the phase-separated texture. These predictions help to rationalise the findings of clinical trials. In conjunction with further experimental investigation, molecular statistical approaches in this spirit have the potential to play a useful role in the drive towards treatment strategies.