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.
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.
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.
While the lungs are similar, they are not identical, an aspect reflected in the number of bronchopulmonary segments. The right lungs has three bronchopulmonary segments in its superior lobe, while its middle lobe consists of two segments. Last but not least, there are five segments in its inferior lobe, making a total of ten bronchopulmonary in the entire right lung. Compared to the right lung, the left one only has two lobes, hence a lower number of bronchopulmonary segments. Its upper lobe consists of four segments, while the structure of the lower lobe is made up of four, totalling up to eight segments in the left lung.
A layer of connective tissue separates each segment from the next. During development, there are initially ten segments per lung, but since the left lung only has two lobes, two pairs of bronchopulmonary segments fuse to give eight total segments, with four for each lobe. The right lung retains all 10 segments.
Bronchopulmonary segments are 'independent' entities, hence each bronchopulmonary segment requires its own source of nutrients and air to help with gas exchange. The air is brought to each bronchopulmonary segment by a segmental (tertiary) bronchus, while the blood is brought by two separate arteries which travel together, side-by-side through the centre of their respective bronchopulmonary segment: the pulmonary and bronchial arteries. The bronchial artery supplies the respective segment with nutrition and oxygenated blood, while the pulmonary artery carries deoxygenated blood from the heart in order to be oxygenated in the lungs. 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. They 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.