On the mechanics of muco-ciliary transport 1

Although the internal structure of cilia on micro-organisms and in the lung is identical the mechanical principles of fluid (mucous) transport may be vastly different. Cilia on micro-organisms are generally quite long, less densely packed on the epithelium, have a pronounced aperiodic beating pattern and a highly coordinated metachronal wave compared to the short densely packed, multicellular coordination of cilia found in the lung. A simple hydrodynamic theory using two layers of Newtonian fluid of differing viscosity is developed to model muco-ciliary transport. Features include estimates for the mean force distribution, the likelihood that a cilium is located at a given height above the epithelium for an idealised beat pattern, and allowance for the retardation of the serous layer by the cilia. Initial calculations suggest that if the cilia just penetrate the upper, much more viscous, layer that mucous transport rates are substantially enhanced. Other features of the results are the significant retardation of flow in the serous layer and the ‘plug’ flow in the mucous layer.