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.
Surgically, there is an advantage when dealing with bronchopulmonary segments since they are separated from the rest of the lung by connective tissue septa, which means that a segment can be removed without affecting other nearby segments. Knowledge of bronchopulmonary segment anatomy is important for understanding and interpreting radiographs and other medical images of the lungs, especially when needed as reference for surgical resection of diseased lung segments. Removal of a bronchopulmonary segment is known as segmentectomy.
Segmental atelectasis can occur if there is blockage of a segmental bronchus (e.g. due to aspiration of an object) that prevents air from reaching the bronchopulmonary segment it supplies. Air that is blocked in the segment will be gradually absorbed by the blood, which leads to the bronchopulmonary segment collapsing. Segmental collapse does not require compensatory space in the pleural cavity since adjacent segments will then expand to compensate for the reduced volume of air reaching the collapsed segment.