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The upper respiratory tract (eg, nasal passages, mouth, pharynx, larynx) accounts for about half of total airway resistance.  The remainder comes from the lower respiratory tract, which begins at the trachea and proceeds through roughly 23 generations of airway bifurcation before reaching the alveoli (ie, the trachea divides into approximately 8 × 10⁶ [2²³] smaller airways before reaching the alveoli).  Because the airways at each level of the lower respiratory tract are arranged in parallel, airway resistance is determined by the total cross-sectional area of all the airways at that level; the greater the total cross-sectional area, the lower the airway resistance.

The cross-sectional area of the trachea is relatively small (~3.5 cm²), but the total area of the first few generations of bronchi is even smaller, leading to an initial small increase in resistance from the trachea to the bronchi.  However, from the medium-sized bronchi on, the total cross-sectional area at each level of the lower respiratory tract increases in step-wise fashion, creating a progressive decline in airway resistance.  By the time the alveoli are reached, the total cross-sectional area is massive (~5 × 10⁵ cm²) and airway resistance is essentially zero.

Educational objective:
The airway resistance at each level of the lower respiratory tract is inversely related to the total cross-sectional area of all the airways at that level.  Airway resistance is high in the trachea and reaches a peak in the medium-sized bronchi, where total cross-sectional area is at a minimum.  Airway resistance then progressively decreases as total cross-sectional area increases through the smaller bronchioles, terminal bronchioles, and alveoli.