In interstitial lung disease, inflammation and/or scarring (fibrosis) occurs in the interstitium of the lung. The interstitium of the lung refers to the microscopic area within the walls of the alveoli (air sacs) between the membrane of the air sac and the membrane of the surrounding blood vessels. Like the leaves on a tree, the alveoli arise from the tiniest bronchioles (airways).
There are hundreds of millions of alveoli in a lung: more than a couple would fit on the period at the end of this sentence. Each alveolus (individual air sac) is surrounded by a network of tiny blood vessels (capillaries) — like mesh encircling the alveolus. The air sac and blood vessels together are called a respiratory unit. This is where the exchange of oxygen and carbon dioxide takes place.
Normally, oxygen passes easily from inside the air sac, through its membrane, across the interstitium and through the membrane of the blood vessel in which red blood cells are lined up and ready to be loaded with oxygen like empty train cars waiting to be filled with cargo. In interstitial lung disease, inflammation, scarring or fibrosis thickens the interstitium, making the lung thick and restricted from filling to their normal capacity and preventing oxygen from passing freely into the bloodstream.