Basic and Clinical Sciences (BCSE) Practice Exam

How does effective alveolar ventilation relate to physiological dead space?

1. It is inversely related; as dead space increases, effective ventilation decreases.

2. It is directly proportional regardless of dead space.

3. It remains constant in healthy individuals.

4. It only applies in cases of respiratory illness.

The correct answer is: It is inversely related; as dead space increases, effective ventilation decreases.

Effective alveolar ventilation refers to the portion of the respiratory rate that actually participates in gas exchange, as opposed to ventilation that occurs in areas of the lungs that do not engage in this process, known as anatomical or physiological dead space. When physiological dead space increases—meaning that a larger portion of the air you breathe doesn’t take part in gas exchange—effective ventilation decreases as a result. This relationship is important because it illustrates that not all inhaled air contributes to oxygenating the blood. When dead space is high, even if tidal volume (the amount of air moved in and out with each breath) remains the same, the volume of air that actually reaches the alveoli, where gas exchange occurs, declines. Therefore, with more dead space, the effectiveness of the ventilation in terms of gas exchange diminishes, underpinning the inverse relationship between effective alveolar ventilation and physiological dead space. In contrast, the other options do not accurately represent this relationship. The direct proportionality suggested in one choice overlooks the critical role dead space plays in effective ventilation. The idea that effective ventilation remains constant in healthy individuals ignores the variability that can occur with changes in physical activity, posture, or respiratory function. Lastly, implying that this concept only applies in cases of