Most of the airway resistance is from:

23 generations of respiratory tree: Trachea – 0th generation Further divisions – new generations  First 7 generations  Cartilaginous Non cartilaginous Last 16 generations  Non cartilaginous Compressible Thin layer of smooth muscles +nt Some neuro endocrine cells +nt APUD KULCHITSKY CELLS First 16 generations - Conducting zone (dead space) Last 7 Non cartilaginous – Respiratory zone Note: Out of first 16 generations, trachea ( 0th generation ) offers maximum resistance. Reference: GUYTON AND HALL TEXTBOOK OF MEDICAL PHYSIOLOGY, THIRTEENTH EDITION(INTERNATIONAL EDITION)