rate of between 10 and 20 times per second. The cilia are surrounded by a periciliary liquid layer (PCL), a sol layer that is overlain with the gel layer of mucus. These two components make up the epithelial lining fluid (ELF), also known as the airway surface liquid (ASL), the composition of which is tightly regulated. The ion channels CFTR, and ENaC work together to maintain the necessary hydration of the airway surface liquid. An important factor is the rate of mucin secretion. The mucus helps maintain epithelial moisture and traps particulate material and pathogens moving through the airway, and its composition determines how well mucociliary clearance works. Mechanism Within the thin periciliary liquid layer the cilia beat in a coordinated fashion directed to the pharynx where the transported mucus is either swallowed or coughed up. This movement towards the pharynx is either upward from the lower respiratory tract or downwards from the nasal structures clearing the mucus that is constantly produced. Each cilium is about 7 μm in length, and is fixed at its base. Its beat has two parts the power stroke, or effector stroke, and the recovery stroke. The movement of the cilia takes place in the periciliary liquid which is a little shorter in depth than the height of an extended cilium. This allows the cilia to penetrate the mucous layer during its full extension in the effector stroke, and to propel the mucus directionally, away from the cell surface. In the recovery stroke the cilium bends from one end to the other bringing it back to the starting point for the next power stroke. The returning cilia bend to immerse completely in the PCL which has the effect of reducing a reverse movement of mucus. The coordinated movement of the cilia on all the cells is carried out in a fashion that is not clear. This produces wave-like motions that in the trachea, move at a speed of between 6 and 20 mm per minute. The wave produced is a metachronal wave that moves the mucus. Many mathematical models have been developed in order to study the mechanisms of ciliary beating. These include models to understand the generation and rhythm of the metachronal wave, and the generation of the force in the effective stroke of the cilium. Clinical significance Effective mucociliary clearance depends on a number of factors including the numbers of cilia, and their