ANATOMY OF THE LUNG AND TRACHEOBRONCHIAL TREE
The chest has two lungs (a right lung and a left lung) (Fig. 66-1). Each lung is divided into independent lobes, with separate segments. Each segment (and therefore each lobe) maintains its own individual vascular and lymphatic network such that removal of a segment or a lobe does not disturb the vascular or lymphatic patterns of neighboring lung segments. Furthermore, tumors that arise in one segment usually follow a separate and individual drainage pattern which allows for the curative removal of subunits of each lung without jeopardizing the viability of the whole lung. Thus, knowledge of pulmonary architecture is crucial to the management of lung cancer. The two most common nomenclatures for classification of the tracheobronchial tree are the Jackson-Huber classification (Fig. 66-2A),1 which uses an anatomic space orientation, and the Boyden classification,2 which uses a numerical classification (Fig. 66-2B).
A. Segments of the left and right lungs. B. Tracheobronchial tree.
The right lung is marginally larger because the left lung accommodates the heart by having only 8 segments compared with the right lung, which has 10 segments. Each lung has at least one fissure that divides the lung into smaller lobes. The left lung is divided in two by a single horizontal fissure that creates an upper and lower lobe. The right lung has two fissures, one horizontal and one oblique. These fissures delineate three lobes: upper, middle, and lower. A normal anatomic variant includes the presence of an azygos lobe (see Fig. 66-1, inset), which is usually found at the apex of the right lung. This small variant lobe is separated from the upper lobe by a deep fissure-like groove that cradles the azygos vein.
The lobes of the left and right lung, in turn, are divided into segments representing areas of lung served by different bronchioles, as shown in Fig. 66-2. This figure also shows the intimate relationship between the lungs and tracheobronchial tree. The trachea lies anterior to the esophagus (not shown). At the bifurcation of the trachea, or carina, the left and right mainstem bronchi branch off, and each branch enters the hilus of its respective lung. These, in turn, divide into progressively smaller airways, called bronchioles, that form a rootlike network that extends through the spongelike tissues of the lung. The exterior layer of the bronchi is composed of cartilage with rings of smooth muscle that permit the bronchi to expand and retract on inspiration and expiration. The cartilaginous segments become more irregular at the distal end of this network, and there are none on the bronchioles.