The airways (bronchial tubes) in the lung are the conduit by which oxygen from outside air is moved to the tiny air sacs, called alveoli, at the ends of the bronchial tubes where gas exchange occurs, allowing oxygen to enter the blood stream to be used by the cells of the body and eliminating waste products like carbon dioxide in exhaled breath.

The airways are not just static tubes, however. They are lined with specialized tissue called ciliated epithelium that allows them to perform the critical function of keeping debris and pathogens from inhaled air from reaching the alveoli. They do this via mucociliary clearance, a coordinated effort between mucus produced in the airways to trap unwanted particles and the robust activity of cilia moving in one direction, that propels trapped particles towards the throat where they can be expelled by cough or swallowed.

In order for mucociliary clearance to be effective, the epithelial tissue lining the airways must be able to produce a mucus layer of the right depth, hydration, and consistency and must produce ciliated cells that have the right number of motile cilia (hundreds per cell) that are the right length, are able to move in a coordinated fashion, and are structurally sound. When these components are in place and are balanced, mucociliary clearance is remarkably effective at keeping the airways clean and functional.

In cystic fibrosis, genetic defects in a gene called CFTR impair the ability of certain cells to control the flow of sodium and chloride across the cell membrane, resulting in mucus that is dehydrated and extremely thick. This is a systemic problem affecting not just the lungs, but also organs of digestion, like the pancreas, further complicating underlying lung disease with nutritional deficiency issues. The airways in CF are characterized by extremely thick, difficult to remove mucus that also impairs ciliary activity.

In PCD, genetic defects on many genes affect the structure, function, or number of cilia on cells, resulting in cilia that are not able to perform their function of mucociliary clearance.

When mucociliary clearance is impaired, mucus laden with debris is trapped in the airways and becomes very thick and infected (called suppurative or ‘pus-producing’ lung disease). This triggers a ‘vicious cycle’ of inflammation, production of even more mucus, and increased infection.

Everyone experiences temporary mucociliary clearance impairment when they have a respiratory virus or cold. However, when the impairment is permanent, as in CF and PCD, over time the airways themselves become damaged and ineffective, a condition known as bronchiectasis. For some, the damage becomes so severe, respiratory failure may occur, necessitating lung transplantation.

Because mucociliary clearance is so critical to lung health, people with impaired mucociliary clearance from PCD, CF and bronchiectasis often do daily airway clearance therapies to help clear their airways. These therapies are an important part of the treatment plan for people with suppurative lung diseases.