1. Weibel’s Airway Generations | Pulmonary Physiology | USMLE Step 1

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1. Weibel’s Airway Generations | Pulmonary Physiology | USMLE Step 1

Weibel’s model of airway generations provides a detailed anatomical and functional map of the human respiratory tract, illustrating how the airway branches into 23 generations from the trachea to the alveolar sacs. This model is critical for understanding gas transport and ventilation dynamics in USMLE Step 1 physiology. The conducting zone comprises Generations 0 to 16, which include the trachea (Gen 0), bronchi (Gens 1–3), and bronchioles up to terminal bronchioles (Gens 4–16). These airways are lined with ciliated epithelium, contain no alveoli, and are responsible for conducting air — not gas exchange — thereby contributing to the anatomical dead space (~150 mL). Generation 17 onward marks the respiratory zone, where alveoli appear and gas exchange begins. This includes respiratory bronchioles (Gens 17–19), alveolar ducts (Gens 20–22), and alveolar sacs (Gen 23). With each branching, airway diameter decreases but total cross-sectional area increases, resulting in reduced airflow velocity and optimized gas diffusion. This structure-function transition explains the shift from bulk flow in proximal airways to passive diffusion in distal airways. Understanding this model helps clarify key physiological concepts like airway resistance, dead space, and V/Q matching, and is frequently tested through both conceptual and case-based MCQs on Step 1.

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