Porth's Essentials of Pathophysiology, 4e

516

Respiratory Function

U N I T 6

Z

Trachea

Trachea

0

1

Left primary bronchus

2 3 4

BR

BL

Secondary bronchi

Conducting zone

TBL

Segmental bronchi

17 18 19 20 21 22 23

RBL

Terminal bronchioles

AD

Transit and

respiratory zone

AS

A

B

FIGURE 21-3. (A) Conducting and respiratory air pathways inferior to the larynx. (From Anatomic Chart Company. Atlas of Human Anatomy. Springhouse, PA: Springhouse; 2001:175.) (B) Idealization of the human airways.The first 16 generations of branching (Z) make up the conducting airways, and the last seven constitute the respiratory zone (or transitional and respiratory zone). AD, alveolar ducts; AS, alveolar sacs; BL, bronchiole; BR, bronchus; RBL, respiratory bronchiole;TBL, terminal bronchiole. (FromWei bei ER. Morphometry of the Human Lung. Berlin: Springer-Verlag;1962:111.)

the same basic structure as the trachea. Each primary bronchus, accompanied by the pulmonary arteries, veins, and lymph vessels, enters the lung through a slit called the hilus . On entering the lungs, each primary bronchus divides into secondary or lobular bronchi that supply each of the lobes of the lung—three in the right lung and two in the left. The right middle lobe bronchus, which is particularly subject to obstruction, is relatively small in diameter and length and sometimes bends sharply near its bifurcation, making it particularly subject to obstruc- tion. The secondary bronchi, in turn, divide to form the segmental bronchi that supply the bronchopulmonary segments of the lung. These segments are identified according to their location in the lung (e.g., the apical segment of the right upper lobe) and are the smallest named units in the lung. Lung lesions such as atelectasis and pneumonia often are localized to a particular bron- chopulmonary segment. Initially, the walls of the bronchial airways have the same general structure as the trachea. At the point where the bronchi enter the the lungs to become the intrapul- monary bronchi, the structure changes (Fig. 21-6). The cartilage rings are replaced by cartilage plates of irregu- lar shape that encircle the entire circumference of the airway. A second change that is observed in the wall of

Posterior

Esophagus

Trachealis muscle

Lumen of trachea

Hyaline cartilage

Ciliated epithelium

Anterior

FIGURE 21-4. Cross-section of the trachea, illustrating its relationship to the esophagus, the position of the supporting hyaline cartilage rings in its wall, and the trachealis muscle connecting the free ends of the cartilage rings.