A bronchopulmonary segment is that portion of the lung supplied by the primary branch of a lobar bronchus. Each segment is surrounded by connective tissue that is continuous with visceral pleura and forms a separate, functionally independent respiratory unit. The artery supplying a segment follows the segmental bronchus but the segmental veins are at the periphery of the segment and thus can be helpful in delineating it.
The right main bronchus gives rise to three lobar bronchi: upper, middle, and lower. An two of these may occasionally have a common stem.
Right Upper Lobe
The apical segment (S1) of the right upper lobe forms the apex of the right lung. It extends into the root of the neck as high as the vertebral end of the ﬁrst rib. Toward the lateral aspect of the lung, the apical segment dips downward slightly between the posterior and anterior segments. This boundary line is roughly at the level of the ﬁrst rib anteriorly and almost down to the second rib posteriorly.
The posterior segment (S2) extends from the apical segment down to the lateral portion of the horizontal ﬁssure and the upper part of the oblique ﬁssure.
The anterior segment (S3) extends from the apical segment above down to the horizontal ﬁssure at about the level of the fourth rib.
Right Middle Lobe
The middle lobe bronchus branches into two segmental bronchi, the complete branchings of which become the lateral segment (S4) and medial segment (S5) of the lobe. These segments are separated by a vertical plane extending from the hilum out to the costal surface of the lung and reaching its inferior border just anterior to the lower end of the oblique ﬁssure. The segments are related to the anterior parts of the fourth and ﬁfth ribs and their costal cartilages.
Right Lower Lobe
The lower lobe bronchus gives off a posteriorly directed superior segmental bronchus just below the level of the oriﬁce of the middle lobe bronchus. The superior segment (S6) of the lower lobe occupies the entire superior part of the lower lobe and extends from the upper part of the oblique ﬁssure at about the level of the vertebral end of the third rib to the level of the vertebral end of the ﬁfth or sixth rib.
Inferior to the level at which the superior segmental bronchus arises, the lower lobe divides into four basal segmental bronchi: medial (S7), anterior (S8), lateral (S9), and posterior (S10). The basal segments of the lower lobe form the base of the lung and rest on the diaphragm. The medial basal segment is sometimes partially separated from other basal segments by an extra ﬁssure; in this event, it has sometimes been called the cardiac lobe of the lung.
The left main bronchus is longer than the right and not in such direct a line with the trachea. Foreign bodies, therefore, are somewhat more likely to enter the right than the left bronchus.
Left Upper Lobe
The upper lobe bronchus subdivides into a superior division bronchus and an inferior or lingular division bronchus. The superior division can be thought of as corresponding to the right upper lobe, with the lingular division corresponding to the right middle lobe; there is usually no ﬁssure separating the two, and their segmental subdivisions are not the same.
Unlike the situation on the right, the superior division of the left upper lobe has only two segments: the apicoposterior segment (S1 and S2), which corresponds to a combination of the right apical and posterior segments, and the anterior segment (S3). The inferior or lingular division also has two segments, the superior (S4) and inferior (S5) segments.
Left Lower Lobe
The segments here are similar to those of the right lower lobe except that the portion corresponding to the right anterior basal and medial basal segments is supplied on the left by two bronchi that have a common stem and thus forms a single anteromedial basal (S8) segment. Other left lower lobe segments are rior (S6), lateral basal (S9), and posterior basal (S10).