Trachea and Bronchi

• Begins at the inferior border of the cricoid cartilage (C6) as a continuation of the larynx and ends by bifurcating into the right and left main stem bronchi at the level of the sternal angle (junction of T4 and T5).
• Is about 9 to 15 cm in length and has 16 to 20 incomplete hyaline cartilaginous rings that open posteriorly toward the esophagus and prevent the trachea from collapsing.
• Has the carina , a downward and backward projection of the last tracheal cartilage, which lies at the level of the sternal angle and forms a keel-like ridge separating the openings of the right and left main bronchi.

• Is shorter, wider , and more vertical than the left main bronchus; therefore, more foreign bodies that enter through the trachea are lodged in this bronchus or inferior lobar bronchus.
• Runs under the arch of the azygos vein and divides into three lobar or secondary (superior, middle and inferior ) bronchi and finally into 10 segmental bronchi. The right superior lobar (secondary) bronchus is known as the eparterial (above the artery) bronchus because it passes above the level of the pulmonary artery. All others are the hyparterial bronchi.

• Runs inferolaterally inferior to the arch of the aorta , crosses anterior to the esophagus and thoracic aorta, and divides into two lobar or secondary bronchi , the upper and lower, and finally into 8 to 10 segmental bronchi.
• Is also crossed superiorly by the arch of the aorta over its proximal part and by the left pulmonary artery over its distal part.
• Dilates its lumen by sympathetic nerves and constricts by parasympathetic stimulation.