Bronchi are passageways leading into the lungs. Bronchi will branch into smaller tubes that become bronchioles. Main bronchi (left and right) will divide into secondary or lobar bronchi. Secondary bronchi further subdivide into the tertiary (segmental) bronchi, each of which supplies a bronchopulmonary segment.
A bronchopulmonary segment is a division of the lung; in fact, these segments are the largest subdivisions of a lung lobe. In other words, lung lobes are subdivided into bronchopulmonary segments that are each supplied by a tertiary (segmental) bronchus. Segments are pyramidal-shaped, with their apices facing the lung root and their bases at the pleural surface. An important side note is that these subdivisions are not defined by surface boundaries, but instead by origin, so they are named according to the segmental bronchi supplying them.
There are 10 bronchopulmonary segments in the right lung (3 in the superior lobe, 2 in the middle lobe, 5 in the inferior lobe), and 8 segments in the left lung (4 in the upper lobe, 4 in the lower lobe). A layer of connective tissue separates each segment from the next. During development, there are initially 10 segments per lung, but since the left lung only has 2 lobes, 2 pairs of bronchopulmonary segments fuse to give 8 total segments, with 4 for each lobe. The right lung retains all 10 segments.
Each bronchopulmonary segment has its own artery, and is therefore independently supplied by a segmental bronchus, and two arteries—a pulmonary artery, and a bronchial artery, which run together through the center of the segment. Usually, the tertiary segmental artery will be located on the anterior aspect of the corresponding bronchus. Bronchopulmonary segments are drained by intersegmental parts of the pulmonary veins that lie in the connective tissue between and drain adjacent segments, and are also drained by the lymphatics.