What happens to alveolar ventilation if dead space ventilation increases significantly?

• A. It increases
• B. It decreases
• C. It remains unchanged
• D. It doubles

Answer: (B)

When dead space ventilation increases significantly, alveolar ventilation decreases. Dead space refers to the portions of the respiratory system where gas exchange does not occur, such as the trachea and bronchi. When more ventilation is allocated to these areas, it leads to a less efficient overall process of gas exchange, as not all inhaled air reaches the alveoli where oxygen and carbon dioxide exchange takes place.

Alveolar ventilation is the amount of fresh air that reaches the alveoli and is available for gas exchange. If a larger proportion of the total ventilation goes into dead space rather than into the functional respiratory areas (the alveoli), then the volume of air that participates in gas exchange diminishes. Consequently, since alveolar ventilation is calculated by subtracting dead space ventilation from total ventilation, any significant increase in dead space results in a reduction in the effective ventilation for gas exchange, thereby decreasing overall alveolar ventilation.