In med school they have you learn FEV1 / FVC ratios for obstructive and restrictive lung disease.
To calculate FEV1 and FVC, they tell you to breath in as much as possible, then breath out as fast as you can until your lungs are empty.
- FEV1 = the volume of air you can blow out in 1 second, “forced expiratory volume, 1 second”
- FVC = total volume of air you can blow out, “forced vital capacity”
The reason these are important is they diagnose which category of lung disease you have:
In obstructive lung disease, the bronchi are narrowed, but the alveoli are normal (narrow bronchi in the picture). It is hard to get air out through the bronchi, so the air pushed out in 1 second (FEV1) is reduced. I drew a small air cloud in the picture to show this. Since FEV1 is low and FVC is normal, FEV1/FVC is reduced.
In restrictive lung disease, the alveoli are inelastic (shown by the thick wall in the picture), but the bronchi are normal. There is no trouble getting air out of the alveolus, but the total lung capacity, and the FVC, is smaller due to the inelastic alveoli not being able to fill up with air. I drew a big cloud in the picture showing a normal FEV1. Since FEV1 is normal and FVC is reduced, FEV1/FVC is high, and can in fact be greater than 1.