At what point does bulk gas flow not occur in the airways?

Bulk gas flow in the airways refers to the movement of large volumes of air due to pressure differences between the atmosphere and the lungs. This flow happens when there is a gradient that allows air to move from areas of higher pressure to areas of lower pressure.

When the pressures at the airway opening and the alveoli are equal, there is no pressure gradient to drive airflow, and thus, bulk gas flow does not occur. Essentially, this means that there is a cessation of movement of air into or out of the lungs because the forces that usually promote inhalation or exhalation are balanced at that moment.

In contrast, during forced inhalation and exercise, there are significant pressure gradients created due to muscle contractions, resulting in increased airflow. Similarly, at the beginning of expiration, the pressures in the lungs are often higher than in the atmosphere, which promotes the flow of air out of the lungs.