What characterizes physiological dead space?

Physiological dead space is characterized by the presence of alveoli that are not participating effectively in gas exchange, despite being ventilated. This means that while these alveoli may be intact and capable of holding air, they are not involved in the oxygen and carbon dioxide exchange that is critical for respiration. Typically, this can occur due to issues such as poor perfusion of blood to these areas, or other factors affecting the alveolar-capillary interface.

This concept is essential in understanding how certain conditions can impair respiratory efficiency, even when ventilation seems adequate. The other choices refer to conditions or structures that do not accurately define physiological dead space. For instance, healthy alveoli actively contribute to gas exchange, non-functioning airways do not refer to the ventilated state of alveoli, and obstructed airways indicate a restrictive problem that affects airflow but does not directly illustrate the concept of dead space within functioning alveoli.