The diameter and internal volume of these bronchi vary significantly during the forced expiration maneuver.
During forced expiration the intrathoracic pressure increases so that it exceeds the pressure inside these bronchi, which causes their collapse. Two parameters characterize these bronchi:
- the parameter reflecting the outflow of air from the volume of those bronchi
During maximal inspiration the intermediate bronchi are significantly distended. During forced expiration they are collapsed. That means significant volume changes of these bronchi. VDc characterizes approximately the volume of air exhaled from these bronchi at the beginning of the forced expiration.
- the parameter connected with air outflow from the volume of lungs through those bronchi
In spite of the fact that the bronchi are practically collapsed, the air, flows through them. The rate of that flow, however, depends no more on the force of expiration and current bronchial diameter. It depends on current lung volume and on the lumen of the bronchi, i.e. on the diameter which they could have after their dissection from the chest. The inverse of the lumen (1/lumen) can be treated as reflecting the resistance of the dissected bronchi, i.e. as a parameter invariable during respiration, which characterizes the properties of the bronchi as such. That is what R(1/lumen) is.
In order to simulate a healthy respiratory system of another size, after changing the Size of the lungs, the sizes of all bronchi, including intermediate bronchi, should be appropriately changed through a change of the R(1/lumen) parameter. Then, such set of changed parameter values can be regarded as a new reference set (after simulation for that new set, the virtual key Use as ref. should be pressed).