Although individual terminal bronchioles have very narrow lumens, they contribute little to total airway resistance. The reason is:
- A They are arranged in parallel, producing a very large summed cross-sectional area ✓
- B Their walls lack smooth muscle and remain maximally dilated
- C Surfactant lines their epithelium and eliminates frictional losses
- D Gas flow there is turbulent, which lowers resistance at low Reynolds numbers
Correct answer: A. They are arranged in parallel, producing a very large summed cross-sectional area
Explanation
Airways beyond roughly the eighth generation divide so extensively that their combined cross-sectional area expands enormously; with thousands of bronchioles in parallel, total resistance falls despite each lumen being small. Measurements show the largest pressure drop occurs in medium-sized bronchi, which is where most resistive work is dissipated. Turbulence actually increases resistance, so option D states the opposite of laminar flow physics.
Reference: West's Respiratory Physiology: The Essentials, 11th ed.
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