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Airway generation

FIGURE 7 Resistance to air flow changes with airway generation, where 0 is the trachea. Resistance is greatest in the segmental bronchi and decreases rapidly in smaller but more numerous peripheral airways. (After Pedley et al, Respir. Physiol. 1970;9:387.)

The physical factors determining whether flow is laminar or turbulent include tube radius (r), gas velocity (v), density (d), and viscosity (r), which are used to define the Reynolds number (Re):

Airway generation

FIGURE 7 Resistance to air flow changes with airway generation, where 0 is the trachea. Resistance is greatest in the segmental bronchi and decreases rapidly in smaller but more numerous peripheral airways. (After Pedley et al, Respir. Physiol. 1970;9:387.)

The Reynolds number quantifies the ratio of inertial to viscous forces, and turbulence occurs when this number exceeds 2000. In the airways, turbulence occurs when velocities are high relative to airway radius, or mainly in the upper airways before extensive branching increases total cross-sectional area and decreases linear gas velocity. Air flow is laminar in the small airways, where velocity is low; Re approaches 1 in the terminal bronchioles.

Flow in most of the airways is transitional, or somewhere between being laminar and turbulent. In general, the pressure gradient for ventilation is described by the flow rate and the flow rate squared:

AP = KjV + K2V2, where K1 and K2 are constants. This is Rohrer's equation, and under most conditions the second term is relatively small, so the equation simplifies to Ohm's law (K1 = R). Factors increasing the importance of turbulence and increasing the pressures necessary for ventilation include high gas velocities, such as may occur during exercise, or breathing high-density gases, for example, during deep-sea diving.

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