Disturbances in the structural formation of the cilia can cause a number of ciliopathies, notably primary ciliary dyskinesia.

[6] In the upper part of the respiratory tract, the nasal hair in the nostrils traps large particles, and the sneeze reflex may also be triggered to expel them.

[9] 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 mucus helps maintain epithelial moisture and traps particulate material and pathogens moving through the airway, and its composition determines how well mucociliary clearance works.

[11][12] 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.

[14][15] 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.

[16] The returning cilia bend to immerse completely in the PCL which has the effect of reducing a reverse movement of mucus.