Britannica Sources used in this answer

How do bronchi branch into bronchioles inside the lungs?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Illustration showing different animals and their respiratory organs: a spider with book lungs, a chicken with lungs, a sponge with labeled inflow and outflow pores, a fish with gills, and a caterpillar with spiracles. — Respiratory system Different methods of respiration in animals.

Inside the lungs, the bronchi undergo a process of repeated subdivision to form smaller and narrower airways known as bronchioles. This branching pattern generally follows the rules of irregular dichotomy, meaning that each airway divides into two daughter branches that may differ significantly in length and diameter.12

As the bronchi extend deeper into the lungs, their structural composition changes:
* Cartilage: The cartilage rings found in the stem bronchi are replaced by irregular cartilage plates in the bronchi, and eventually disappear entirely in the bronchioles.13
* Epithelium: The lining changes from a typical respiratory epithelium to a simple cuboidal epithelium. While ciliated cells remain present, mucus-secreting goblet cells are replaced by Clara cells in the bronchioles.23
* Support: Because bronchioles lack cartilage, they gain structural stability through their integration into the surrounding gas-exchanging tissues.2

On average, the airway tree consists of about 24 to 25 generations of branches, with the transition from conducting airways to the respiratory (gas-exchanging) zone typically occurring around the 16th generation.2