Biorheological matching: mucociliary interaction and epithelial clearance.

In mucociliary clearance mucus has been found to be essential as coupler between ciliary beat and transport. The rheological properties of mucus are carefully matched to this task. It has to be an incipient gel and deviations from this state, to a sol or to a stiff gel, both render it defective in function. The known rheological properties of normal mucus are shown to be consistent with our understanding of how a periodically interacting set of cilia produce mucus layer flow. A dual requirement has to be met: Adequate tip penetration and adequate force transfer laterally. As a result any biorheologically properly matched system can replace mucus. Mucociliary clearance is a clear cut, rather well understood case of a general principle of biorheological matching.