At the approximate level of the sternal angle, the trachea bifurcates or splits, into the right and left primary (1o) bronchi. Each bronchus runs freely for a few centimeters, then enters its respective lung. Air is conducted through the primary bronchi into and out of each lung.
After entering a lung, the primary bronchi each divide into secondary (2o) bronchi. The secondary bronchi are also known as lobar bronchi because each one directly conducts air to and from one of the lung's five lobes.
Within a lobe, tertiary (3o) bronchi branch from the secondary bronchi. Each tertiary bronchus conducts air to and from a bronchopulmonary segment, which is an anatomical and functional subdivision of a lobe. There are 10 bronchopulmonary segments in the right lung and 8 in the left lung, due to the fusion of S1-2 and S7-8.
Because they conduct air in and out of the bronchopulmary segments, the tertiary bronchi are known as segmental bronchi.
