Which factor promotes the movement of secretions during postural drainage?

The factor that promotes the movement of secretions during postural drainage is the force of gravity. In postural drainage, patients are positioned in specific ways to take advantage of gravity, which helps to facilitate the drainage of mucus from the airways. By positioning the body at various angles, gravity assists in pulling secretions down into larger airways where they can be more easily expelled through coughing or suctioning.

While cilia play a role in the movement of mucus within the airways, their function is more about moving secretions along the respiratory tract rather than aiding in drainage through gravity. Involuntary muscle contractions do contribute to respiratory mechanics, but they do not influence drainage as directly as gravity does. The sweeping motion of cilia is important for clearing mucus, but again, it does not replace the gravitational assistance provided in postural drainage. Thus, the correct choice reflects the fundamental principle of using body positioning to leverage gravity for effective secretion clearance.