The trachea divide into two bronchi, each leading to one lung. The structure of the bronchi resembles that of the trachea.
The branching sub-divisions the bronchi. The larger bronchioles are supported by cartilage, but this gradually disappears as bronchioles smaller. Their walls are made of smooth muscle lined with cuboidal epithelial cells.
Their walls are made of smooth muscle-lined with cuboidal cells. Minute air-sacs with a diameter between 100 and 300 Hm. They contain some collagen elastic fibres, and are lined with squamous epithelium