The role of mucus in the lungs
Lungs allow rapid gas exchange between the atmosphere and the blood. Air is drawn into the lungs via the trachea due to low pressure in the lungs, created by the movement of the ribs and diaphragm. The trachea divided into two bronchi which carry air to and from each lung. Within each lung there is a tree-like system of tubes ending in narrow tubes, bronchioles, attached to tiny balloon-like alveoli. The alveoli are the sites of gas exchange.
Everyone normally has a thin coating of mucus in these tubes, produced continuously from goblet cells in the walls of the airways. Any dust, debris or microorganisms that enter the airways become trapped in the mucus. This is continually removed by the wave-like beating of cilia that cover the epithelial cells lining the tubes of the gas exchange system. However, people with CF, have mucus that is drier than usual resulting in a sticky mucus layer that the cilia find difficult to move.
This sticky mucus in the lungs has two major effects on health, It increases the chances of lung infection and it makes as exchange less efficient, particularly in the later stages of the disease.
Sticky mucus increases chance of lung infection
Microorganisms become trapped in the mucus in the lungs. Some of these can cause illness- they are pathogens. Mucus is normally moved by cilia into the back of the mouth cavity where it is either coughed or swallowed, reducing risk of infection. Acid in stomach kills more microorganisms that are swallowed.
With CF the mucus layer is so sticky that cilia cannot move the mucus. Mucus production still continues, as it would in a normal lung, and the airways build up layers of thickened mucus. There are low levels of oxygen in the mucus, partly because oxygen diffuses slowly through it and partly because the epithelial cells use up more oxygen in CF patients. Harmful bacteria thrive in these anaerobic conditions.
White blood cells fight the infections within the mucus, but at they die they break down, releasing DNA which makes the mucus even sticker. Repeated infections can eventually weakens the body's ability to fight the pathogens, and cause damage to the structures of the gas exchange system.