The trachea divides into two branches forming the main (primary) bronchi. Anatomically, these divisions are more frequently described as simply the right and left main bronchi, terms that are more useful because of the physical difference between the two. The right bronchus is wider and significantly shorter than the left. On entering the hilum of the lung, each main bronchus divides into the lobar bronchi (secondary bronchi). The left lung is divided into two lobes; the right lung is divided into three lobes. Thus, the right bronchus divides into three lobar bronchial branches, and the left into two lobar bronchial branches, with each branch supplying one lobe. The left lung is further divided into 8 bronchopulmonary segments and the right lung into 10 such segments. Thus, in the right lung the lobar bronchi give rise to 10 segmental bronchi (tertiary bronchi); the lobar bronchi of the left lung give rise to 8 segmental bronchi.
A segmental bronchus and the lung parenchyma that it supplies constitute a bronchopulmonary segment. The significance of the bronchopulmonary segment in the human lung becomes apparent when considering the need for  surgical resection, which may be required in certain disease states. The segments, each with their own blood supply and connective tissue septa, are convenient subunits that facilitate surgical resection.
Initially, the bronchi have the same general histologic structure as the trachea. At the point where the bronchi enter the lungs to become intrapulmonary bronchi, the structure of the bronchial wall changes. The cartilage rings are replaced by cartilage plates of irregular shape. The plates are distributed in a linear array around the entire circumference of the wall, giving the bronchi a circular or cylindrical shape in contrast to the ovoid shape with a flattened posterior wall of the trachea. As the bronchi decrease in size because of branching, the cartilage plates become smaller and less numerous. The plates ultimately disappear at the point where the airway reaches a diameter of about 1 mm, whereupon the branch is designated a bronchiole.
Bronchi can be identified by their cartilage plates and a circular layer of smooth muscle.
The second change observed in the wall of the intrapulmonary bronchus is the addition of smooth muscle to form a complete circumferential layer. The smooth muscle becomes an increasingly conspicuous layer as the amount of cartilage diminishes. Initially, the smooth muscle is arranged in interlacing bundles forming a continuous layer. In the smaller bronchi, the smooth muscle may appear discontinuous.
Because the smooth muscle forms a separate layer, namely, a muscularis, the wall of the bronchus can be regarded as having five layers:  Mucosa, composed of a pseudostratified epithelium with the same cellular composition as the trachea. The height of the cells decreases as the bronchi decrease in diameter. In H&E specimens the “basement membrane” is conspicuous in the primary bronchi but quickly diminishes in thickness and disappears as a discrete structure in the secondary bronchi. The lamina propria is similar to that of the trachea but is reduced in amount in proportion to the diameter of the bronchi.
 Muscularis, a continuous layer of smooth muscle in the larger bronchi. It is more attenuated and loosely organized in smaller bronchi, where it may appear discontinuous because of its spiral course. Contraction of the muscle regulates the appropriate diameter of the airway.
 Submucosa remains as a relatively loose connective tissue. Glands are present as well as adipose tissue in the larger bronchi.
 Cartilage layer consists of discontinuous cartilage plates that become smaller as the bronchial diameter diminishes.
 Adventitia is moderately dense connective tissue that is continuous with that of adjacent structures, such as pulmonary artery and lung parenchyma.