Lung Lining Liquid – The Hidden Depths

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Abstract
The volume of liquid which lines the lumen of the lung is small and thus exists as an extremely thin layer. Nevertheless, it appears to have important functions which include contributing to the mechanical and chemical defences of the pulmonary epithelium and providing the correct ionic environment for surfactant function. Energy is expended by the epithelium to prevent excess liquid accumulating, a process involving active transport of sodium ions out of the lumen. Precisely how lung lining liquid (LLL) is formed and how its volume is controlled remain uncertain. Control of volume is important to prevent excess liquid interfering with gas exchange: yet too little liquid would inhibit efficient functioning of the airway ciliary escalator and thus removal of mucus, debris and pathogens. The chemical content of LLL is also controlled by the epithelium. LLL pH is acidic which favours calcium ionisation, important for surfactant function. Glucose is removed from LLL so decreasing the likelihood of bacterial growth and reducing extra-cellular glycosylation of proteins such as immunoglobulins and surfactant apoproteins which would destroy their function. LLL is rich in antioxidant species, protecting the lung from atmospheric oxygen and free radicals formed by inflammation. The physico-chemical properties of water itself may be important when the lining layer is very thin. The structure of water can be semi-crystalline under some circumstances which could impart another role to LLL – that of contributing to the structural integrity of the surfactant/LLL/epithelial complex.