The airway epithelium is covered with a thin, liquid protective layer -Airways Surface Liquid (ASL), which is a hydrogel, 3-50μm thick. ASL is created and maintained by water (AQPs) and ion transport, especially sodium (ENaC), chloride (CFTR, ClC), potassium, bicarbonate and calcium. Epithelial cells secrete mucins, which role is to transport ASL towards the upper airways. ASL is the first protection mechanism against solid particles and microorganisms inhaled with air. The proper ASL hydration and functioning are influenced by internal factors, such as the activity of ion channels (especially ENaC and CFTR) and water (AQPs), ciliary function, nerve endings irritation, influx of immunocompetent cells and current hormonal status. External factors affecting hydration are colonization by microorganisms and inhalation of irritants. Developing inflammation, cough, and drugs affect the functions of ion channels, thereby changing the ASL volume and composition, hindering or facilitating self-purifying and flow through airways. The aim of this study was to collect reliable scientific data concerning physiological and pathological changes in ASL volume and composition.
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