Distribution of Lung Airway Resistance
About 40% to 50% of the total airway resistance is located between the nose and larynx (see top figure). The remaining resistance resides between the larynx and alveolar ducts (intrathoracic airways). Total airway resistance is slightly higher with nasal than with open-mouth breathing. However, it is within the tracheobronchial tree that several lung disorders are most likely to alter airway resistance. Within the tracheobronchial tree, most resistance to airflow occurs in the medium-sized bronchi between the fourth-to-eighth order of branching (see figure bottom left). These midsized bronchi offer more air flow resistance than larger or smaller airways because of the complex relationship between air flow velocity, total cross-sectional area, airway length and diameter, and branching frequency. Even though airways become narrower after each branching, which would be expected to increase flow resistance, the flow is divided into two parallel paths, so flow velocity in individual airways decreases (see figure bottom right). A lower flow velocity diminishes the likelihood of turbulent flow. Also, resistances arranged in parallel are added as reciprocals of their individual resistances. As a result, airways distal to the medium-sized bronchi account for progressively less of the total airway resistance. Increased airway resistance offered by the bronchioles and smaller airways is often difficult to detect because they represent such a small fraction of total airway resistance. As a result, these distal airways are sometimes referred to as the "silent airways" because they can have a marked increase in resistance without appreciably altering total airway resistance. However, some pulmonary function tests are capable of detecting increases resistance in the distal airways.