This extensive branching system of the airway passages transmits air from the atmosphere into the alveoli (the gas exchange units in the lungs). The bronchus are made of cartilage until they branch into the smaller bronchioles which are not made of cartilage. The trachea is where the divide into the bronchi begins, both on the left and right side at the anatomical point called the carina.

The right main bronchus is shorter, wider, and more  vertical than the left main bronchus. The right main bronchus divides into three lobar bronchi, while the left bronchus divides into two. The lobar bronchi divide into tertiary bronchi which are known as segmental bronchi, each of the segmental bronchi supply the bronchopulmonary segment.

A bronchopulmonary segment is a division of a lung separated from the rest of the lung by a connective tissue septum. This property allows a bronchopulmonary segment to be surgically removed without affecting other segments. There are ten segments per lung. Although sometimes the left lung fuses these segments into eight instead of ten segments.

The segmental bronchi divide into primary bronchioles which divide into terminal bronchioles, each of which then gives rise to several respiratory bronchioles, which then continue dividing into two to eleven alveolar ducts. There are five or six alveolar sacs associated with each alveolar duct. The alveolus is the basic anatomical unit of gas exchange in the lung.

The hyaline cartilage forms an incomplete D-shaped ring in the larger bronchi. Smooth muscle is present continuously around the bronchi. The cartilage and mucous membrane of the primary bronchi is similar  to those of the trachea.

Divisions of the Bronchus:

• Starting at the trachea the bronchi divide both left and right at the carina.

• Right main bronchi is shorter, wider, and more vertical than the left.

• Right main bronchi divide into 3 lobar bronchi.

• Left main bronchi divide into 2 lobar bronchi

• Lobar bronchi divide into tertiary bronchi known as segmental bronchi

• Each segmental bronchi supply the bronchopulmonary segments.

• Bronchopulmonary segments are a division of the lung by a connective tissue septum.

• There are 10 bronchopulmonary segments per lung on the right side

• There are sometimes only 8 bronchopulmonary segments on the left side where they become fused.

• Segmental bronchi divide into primary bronchioles.

• Primary bronchioles divide into respiratory bronchioles.

• Respiratory bronchioles divide into 2 to 11 alveolar ducts.

• Alveolar ducts terminate with 5 to 6 of the alveolar sacs associated to each alveolar duct.

• The alveolus or alveolar sacs are the unit within the lungs where gas exchange takes place.