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What is Most of the airway resistance is from: and explain in detail?
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)