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Fibrosing lung disease or lung fibrosis encompasses a large number of different clinical syndromes. These syndromes are all accompanied by a change in the delicate structure of the lung and disorders of gas exchange. They are diseases of the connective tissue of the lung (interstitium or parenchyma), from which the terms “interstitial lung disease” (ILD) or “diffuse parenchymal lung disease” are derived.

Some of these lung diseases start with a persistent inflammatory reaction which takes place in the air sacs (alveoli). In other forms, it is not so much the inflammatory reaction that is the prominent feature but, as we now believe, more the damage to the lining cells (epithelium) of the alveoli. Both processes subsequently result in increased formation of connective tissue and thus in fibrosis. This scar tissue forms both in the alveoli and between them, and in some forms around the airways (bronchi). Eventually there is also extensive loss of normally formed alveoli.

These processes make the lung stiffer with the result that more force needs to be used to stretch the lung and thus breathing becomes more strenuous(decrease in the elasticity or compliance of the lung). With the increase in the amount of connective tissue, the so-called diffusion distance increases. This is the distance that oxygen must cover when passing from the air side of the alveolus to the blood vessels (capillaries). As a result of this increase in the diffusion distance – as well as the loss of normally formed alveoli and thus the surface area for gas exchange overall – the uptake of oxygen into the bloodstream is made more difficult, giving rise to a decrease in exercise capacity.