Airway (non-elastic) resistance

Airway (non-elastic) resistance depends on the pattern of airflow (laminar or turbulent – see the relevant chapters), the rate of breathing and the radius and length of the airway, as described in the Hagen–Poiseuille formula. Remember that resistance is independent of flow in laminar flow but it rises linearly with flow in turbulent flow. Much higher pressures have to be achieved in turbulent flow to pass the same flow. (See also the chapter on flow for Reynolds number and the influence of viscosity and density.)

The radius decreases as the airways branch; the nasopharynx and larynx account for half of the total airway resistance, and the trachea and smaller airways constitute the other half. Airways smaller than 2 mm internal diameter contribute very little (see Figure 106). This is because their total cross-sectional area is large, and airflows in the small airways are lower, therefore laminar.

Factors affecting airway resistance are those which alter the diameter of the airway:

• Body size: the diameters of the larynx and trachea account for the large part of total resistance. Airway resistance is highest in neonates (30 cm l −1 per s) and declines with increasing body size until a functional residual capacity of 2500 is reached. Thereafter, the diameter of the airways and airways resistance does not change substantially as shown in Figure 107.

• Thickness of bronchial mucosa, e.g. swelling or secretions.

• Bronchial muscle tone: constriction due to parasympathetic stimuli, histamine release, irritant gases, dilatation produced by sympathetic stimuli or parasympathetic block.

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