In the lower airways there are three basic mechanisms due to which particles may be deposited on the airway wall: a. impaction b. sedimentation c. diffusion
Impaction (see particle size) occurs especially at places where a relatively large particle changes course, such as at airway bifurcations. Let us consider this in some more detail.
Lower intrathoracic airways. Small airway diameter is 2 mm or less.
Starting at the trachea airways divide into daughter branches, each of which is narrower than its mother branch. The system is about dichotomous, one branch giving on average two daughter branches. Whilst each daughter has a smaller cross-sectional area, the sum of the cross-sectional areas of the two daughters is larger than that of its mother. If the trachea is the airway with order number 0, then the left and right main stem bronchus are order one, each giving rise to two airways of order 2. The further we move away from the trachea, the larger the number of airways in parallel. If there is regular dichotomy and there are 20 orders of airways, then at the level of order 20 we have 2 to the power 20 or 1,048,576 airways! Since the total cross-sectional area increases with each order, the system resembles a trumpet: the further away from the mouthpiece, the wider the tube, although a thumbtack is a more correct analogue. We can depict the relationship between airway order and total cross-sectional area on a log-linear scale.
The airway system resembles a river which issues into a delta. The further we get into the estuary, the larger the cross-section and the lower the flow. In terms of particles suspended in air the largest particles have the greatest chance of impacting in the low order airways. The number of such airways and hence their total surface area is small. The volume of a 10 µm particle is 10³ = 1000 times as large as that of a 1 µm particle. Hence the concentration per unit area of any substance delivered to the airway wall is likely to be larger in the central than in the more peripheral airways. The further we go down the airways, the smaller the linear velocity of airflow and the particle. Now sedimentation, i.e. the process in which the particle falls down due to the effect of gravity, becomes an increasingly important determinant of deposition onto the airway wall. The speed at which a particle drops is proportional to its density and to the square of its diameter. In the most distal airways and alveoli, motion due to kinetic energy of the particle (diffusion), becomes the primary mechanism contributing to particle deposition for the following reasons:
The diffusion distance covered by a particle is inversely proportional to its diameter, and proportional to time0.5.