Biochemical basis of physical properties of respiratory tract secretions.

The physical properties of respiratory tract secretion (RTS) play a prominent rôle in the non-specific defence mechanisms of the lung. Viscosity and elasticity, that is flow and deformation, are only two of the physical properties of RTS. Spinability, pourability, adhesiveness and tackiness are starting to be recognised as physical properties of RTS and its is likely that they may be relevant in the pathogenesis of airways obstruction. RTS is a gel, which consists of a cross-linked polymer network dispersed in a liquid solvent. The polymeric structure of the epithelial glycoprotein can be explained in terms of covalent (disulphide) linkages and/or physical entanglement between glycoproteins subunits. Other constituents of RTS such as proteins, lipids, ions and water can influence the physical properties of RTS.