Alveolar ventilation and its distribution

Approximately two-thirds of total ventilation reach the alveoli; one-third remains in the bronchi and is not available for gas exchange – dead space gas. Fresh gas that enters the alveoli per minute is known as alveolar ventilation; the normal value is about 5 l min−1.

Alveolar ventilation is not distributed evenly in the lung: dependent regions are better ventilated during spontaneous respiration than the upper zones. This is because of the effects of gravity: pleural pressure at the base is less negative, and the alveoli at the base are less expanded than apical alveoli. In Figure 109, apical alveolus, represented by point A, is already nearly 80% expanded, while the basal alveolus at point B is only about 30% expanded. At maximum inspiration the change of size is higher for the basal alveoli than the apical ones, which had less scope for expansion. The above only applies for the normal lung at normal lung volumes. If basal alveoli are collapsed they receive no ventilation if the tidal breath, and the pleural pressure change is small. This is illustrated by point C (closed alveolus) in Figure 109. Such a situation occurs in the normal lung at residual volume, in old age when closing volume exceeds functional residual capacity or in chronic obstructive airways disease when functional residual capacity is high and the tidal breathing range is near the closing volume.