Retention, clearance and absorption in the respiratory tract

The respiratory tract of man consists of many structures but they can be divided into three main parts  

The air passages of the larynx, trachea and large bronchii, by which air is conducted to and from the lung. 

The gaseous exchange area of the lung consisting of the bronchi and alveoli. 

The nasopharynx and trachea provides a good filtration system against particles of diameter greater than 20 im since particles of this size tend to be impacted in the nasal passages. The area is lined with ciliated epithilium which transfers deposited material upward, and according to Druett is cleared with a half-time of minutes. 

Particles of unit density and a diameter of 0.5 pm are deposited relatively inefficiently in the lung but particles below this size become trapped by diffusion processes. 

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