The lungs, both right and left, are covered by a thin membrane called the visceral pleura and surrounded by another membrane known as the parietal pleura. The meeting point of these two pleurae allows for smooth movement of the lung as it expands within the chest, creating a potential space. Air can enter this space between the visceral and parietal pleurae due to trauma, surgery, or rupture of a group of alveoli, resulting in a condition called pneumothorax. Fluid can also enter this space, leading to the formation of a pleural effusion or, in severe cases of infection, an empyema.

The lung's topography is illustrated in Figure 20.2, depicting the lobes, segments, and fissures. The fissures divide the lobes in each lung, with specific bronchopulmonary segments assigned numbers as shown in Figure 20.3. The trachea divides into two main stem bronchi, which further branch into lobar bronchi corresponding to each lobe. These lobar bronchi then divide into segmental bronchi and continue to divide into smaller branches called bronchioles, ultimately terminating in the alveolus.

Bronchi and bronchioles differ not only in size but also in the presence of cartilage, type of epithelium, and blood supply. Beyond the segmental bronchi, the airways divide in a dichotomous or asymmetrical branching pattern. Bronchi, characterized by their size and cartilage, eventually transform into terminal bronchioles, which are the smallest airways without alveoli. Each branching of the airway system increases the number of smaller-diameter airways, consequently expanding the total surface area for the next generation of branches. Terminal bronchioles connect to a group of alveoli through an opening, forming respiratory bronchioles.

A bronchopulmonary segment is the region of the lung supplied by a segmental bronchus and represents the functional anatomical unit. Surgically removing irreversibly diseased bronchopulmonary segments is feasible due to their specific structure. The gas-exchanging unit of the lung, known as the respiratory unit, comprises the respiratory bronchioles, alveolar ducts, and alveoli. Bronchi, containing cartilage, and terminal bronchioles, lacking alveoli and cartilage, facilitate the movement of gases from the airways to the alveoli, forming the conducting airways. The conducting airways, approximately 150 mL in volume, do not participate in gas exchange and constitute the anatomical dead space. The respiratory bronchioles with alveoli and the subsequent areas are responsible for gas exchange.
Fig. 20.2 ). Both the right and left lungs are covered by a thin membrane called the visceral pleura and are encased by another membrane called the parietal pleura. The interface of these two pleurae allows for smooth gliding of the lung as it expands in the chest and produces a potential space. Air can enter between the visceral and parietal pleurae by trauma, surgery, or rupture of a group of alveoli; the resulting condition is a pneumothorax. Fluid can also enter this space and create a pleural effusion or, in the case of severe infection, an empyema.
•Fig. 20.2 Illustrations of the Topography of the Lung With Anterior and Posterior Views, Demon-strating the Lobes, Segments, and Fissures. Thefissures(orchasms)demarcatethelobesineachlung.Numbersrefertospecificbronchopulmonarysegments,asdepictedin
Fig.20.3 .SVC,superiorvenacava.Lateral 4 Anterior 3 Anterior 3 1 1 Apical 1 Apical 1 Medial 5 Inferior 5 Superior 4 Superior 6 Posterior basal 10 POSTERIOR VIEW Left Right ANTERIOR VIEW LeftRight Posterior 2 2 Lateral basal 9 Lateral basal 9 Posterior basal 10 Superior 6 8 8 8 Heart Anterior basal Lower lobe Anterior basal Oblique fissure SVC Trachea Upper lobe Middle lobe Lower lobe Anterior basal Oblique fissure Aortic arch Apical Upper lobe Lower lobe Upper lobe Lower lobe Upper lobe Posterior Apical Horizontalfissure
The trachea branches into two main stem bronchi (
Fig. 20.3 ). These main stem bronchi then divide (like the branches of a tree) into lobar bronchi (one for each lobe), which in turn divide into segmental bronchi (Fig.
20.4
Fig. 20.3 ) and then into smaller and smaller branches (bronchioles) until ending in the alveolus (
Fig.
20.5 ). Bronchi and bronchioles differ not only in size but also by the presence of cartilage, the type of epithelium, and their blood supply (
Table 20.1). Beyond the segmental bronchi, the airways divide in a dichotomous or asymmetrical branching pattern. Bronchi, distinguished by their size and the presence of cartilage, eventually become terminal bronchioles, which are the smallest airways without alveoli. Each branching of an airway results in an increase in the number of airways with smaller diameters; as a result, the total surface area for the next generation of branches increases. Terminal bronchioles terminate in an opening (duct) to a group of alveoli and are called respiratory bronchioles.
The region of the lung supplied by a segmental bronchus is called a bronchopulmonary segment and is the functional anatomical unit of the lung. Because of their structure, bronchopulmonary segments that have become irreversibly diseased can easily be removed surgically. The basic physiological unit of the lung is the gas-exchanging unit (respiratory unit), which consists of the respiratory bronchioles, the alveolar ducts, and the alveoli (see
Figs.
20.4 20.5). The bronchi, which contain cartilage, and the terminal bronchioles (i.e., lacking alveoli), in which cartilage is absent, serve to move gas from the airways to the alveoli and are referred to as the conducting airways. This area of the lung (≈150 mL in volume) does not participate in gas exchange and forms the anatomical dead space. The respiratory bronchioles with alveoli and the area beginning •Fig. 20.3 Illustration of Bronchopulmonary Segments, Anterior View. Thenumberscorrespondtothosein