When a patient inhales a drug, different mechanisms determine where the drug particles will deposit in the respiratory tract:
1. Inertial Impaction:
Predominantly affects larger particles (>5 µm).
As air flows through the airways, these particles are unable to change direction rapidly due to their inertia and impact the airway walls, especially at bifurcations.
2. Sedimentation (Gravitational Settling):
Affects particles in the range of 1-5 µm.
These particles tend to settle out of the inhaled air and deposit in the bronchioles and alveoli.
Sedimentation increases with longer breath-holds.
3. Brownian Diffusion:
Relevant for very small particles (<1 µm).
These particles move randomly due to their interaction with gas molecules. This random motion causes them to collide with the airway walls.
This mechanism is predominant in the alveoli and is particularly relevant for ultrafine particles or gases.
4. Electrostatic Precipitation:
Inhaled particles can sometimes hold an electrostatic charge, which may influence their deposition if the respiratory tract walls have an opposite charge.
