The resistance to airflow depends on bronchi current diameter. The diameter of the peripheral bronchioles depends on their lumen (their size, e.g. internal diameter after dissection) and on the current lung volume. Since the resistance depends on the diameter, it also depends on the bronchi lumen and lung volume.
At least two parameters are needed to characterize the relationship between resistance and lung volume:
- one of them — R(1/lumen) — determines the rate of resistance rise, as lung volume is decreasing,
- the second parameter — CCC — determines for which lung volume the resistance increases so much that further lung emptying is practically impossible.
R(1/lumen) — The inverse of that parameter can be associated — although only in rough approximation — with the size (lumen) of the bronchioles and their tonus, that is the rate of reduction of their diameter with decreasing lung volume during expiration.
CCC is the equivalent of closing capacity (closing volume + residual volume).