In considering the mechanisms of aerosol deposition within the lung and the factors that may influence them, it is of some importance to consider first the anatomy and air velocities within the respiratory tract. The temporal aspects of the passage of air through the various anatomic regions and the point during the breathing cycle are also relevant factors.
The angles of branching, the diameter and lengths of different elements of the airways, and the pulmonary air spaces must be visualized in an arbitrary and oversimplified form to make practical use of anatomic data. Some of the details ofthe divisions of the respiratory system that have been assumed by Landahl  and the anatomic flow rate and transitional features in these areas are shown in Table 5.
Table 5 Schematic Representation of the Respiratory Tract
Diameter Length Velocity Passage time (cm) (cm) (cm sec"1) (sec)
Mouth Pharynx Trachea Bronchi
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If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.