Which mechanism is primarily responsible for inertial impaction in the upper airways?

The mechanism primarily responsible for inertial impaction in the upper airways is the change in momentum of particles. Inertial impaction occurs when airborne particles, such as dust, pollen, or pathogens, are inhaled into the respiratory tract. As these particles travel through the air, they gain momentum. When they encounter the bends or turns in the airway, such as the branching points of the tracheobronchial tree, their trajectory changes. This change in direction requires some force, and due to their inertia, heavier particles are unable to follow the air stream's curve and instead collide with the airway walls, leading to their deposition in the upper airways. This process is critical in filtering out larger particles from the air before it reaches the lower respiratory tract, helping to protect the lungs from potential irritants and pathogens.

In contrast, other mechanisms such as diffusion, evaporation, and settling due to gravity play different roles in respiratory function and are not the primary drivers of inertial impaction. While gravity can have an effect on the settling of larger particles over time, it is the change in momentum during inhalation that directly influences how particles interact with airway surfaces.