Main bronchus [right and left]

The right and left main bronchi are the first two major air-conducting tubes that branch directly from the trachea, serving as the entry points for air into their respective lungs.

The main bronchi are the primary divisions of the trachea, emerging at the tracheal carina. The right main bronchus extends towards the right lung, and the left main bronchus towards the left lung, each entering the pulmonary parenchyma at the hilum. Their wall structure is similar to that of the trachea, composed of cartilaginous rings or plates (which maintain the lumen open), smooth muscle (allowing for diameter regulation), and a respiratory mucous membrane lined with pseudostratified ciliated epithelium containing goblet cells. They then branch into lobar bronchi, and subsequently into segmental bronchi, forming the intrapulmonary bronchial tree.

Significant variations exist, primarily concerning the right main bronchus:

• Ruminants (Cattle, Sheep): In these species, the right main bronchus is often shorter. A distinctive feature is the presence of a tracheal bronchus (or right cranial lobar bronchus) that branches directly from the trachea before the main bifurcation, to ventilate the right cranial lobe. The right main bronchus, after the carina, then ventilates the middle, caudal, and accessory lobes of the right lung. The left main bronchus originates from the carina and ventilates the cranial and caudal lobes of the left lung.

• Cat, Dog and Horse: In these species, there is no tracheal bronchus. The trachea divides into right and left main bronchi at the carina. The right main bronchus gives rise to the lobar bronchus for the right cranial lobe, followed by the other lobar bronchi of the right lung. The left main bronchus branches similarly for the left lung.

In humans, the anatomy of the main bronchi is similar to that of cats and horses, meaning there is no tracheal bronchus. The trachea divides into the right main bronchus and the left main bronchus at the carina. The right main bronchus is generally shorter, wider, and more vertical than the left, making it more susceptible to foreign body aspiration. It gives rise to the superior, middle, and inferior lobar bronchi. The left main bronchus is longer, narrower, and more horizontal, giving rise to the superior and inferior lobar bronchi. Their roles and histological structures are comparable to those observed in animals.