Airway collapse during forced expiration.

Question 1

1

Answer 1

• During normal quiet breathing, airway resistance is low, so there is little frictional loss of pressure within the airways.
• Intrapleural pressure remains less than airway pressure throughout the length of the airways, so the airways remain open.

Question 2

2

Answer 2

Even though intrapleural pressure is elevated during the active expiration accompanying routine vigorous activity, intra-alveolar pressure is also elevated and airway resistance is still low, so the friction-induced drop in airway pressure does not fall below the elevated intrapleural pressure until the level at which the airways are held open by cartilaginous rings. Therefore, airway collapse does not occur.

Question 3

3

Answer 3

During maximal forced expiration, both intra-alveolar and intrapleural pressures are markedly increased. When frictional losses cause the airway pressure to fall below the surrounding elevated intrapleural pressure, the small nonrigid airways are compressed closed, blocking further expiration of air through the airway. In healthy individuals, this dynamic compression of airways occurs only at very low lung volumes.

Question 4

4

Answer 4

In obstructive lung disease, premature airway collapse occurs for two reasons:
(1) the pressure drop along the airways is magnified as a result of increased airway resistance,.
(2) the intrapleural pressure is higher than normal because of the loss, as in emphysema, of lung tissue that is responsible for the lung’s tendency to recoil and pull away from the thoracic wall.
Excessive air trapped in the alveoli behind the compressed bronchiolar segments reduces the amount of gas exchanged between the alveoli and the atmosphere.
Therefore, less alveolar air is “freshened” with each breath when airways collapse at higher lung volumes in patients with obstructive lung disease.