B Percussion field of the lungs

Anterior view. The air-filled lungs constitute a resonant cavity that produces a sonorous lung sound on percussion of the chest. The sonorous lung field extends cranially, with attenuation, to the apices of the lungs at the thoracic inlet. It also extends to the front of the chest, again with attenuation, and closely approaches the anterior midline (costomediastinal recess with anterior lung margin on deep inspiration, see pp. 138 and 141). The fluid-filled heart dampens the lung sounds, producing an area of cardiac dullness (see p. 97). A sharp transition from lung sound to liver sound is clearly audible at the inferior border of the right lung, since the liver is a solid organ with less resonance (medium-pitched, nonsonorous percussion sound).

Note: The lung percussion field does not precisely match the anatomical extent of the lungs because only well-aerated portions of the lung are sonorous to percussion. The anatomical extent of the lungs is greater than the percussion field.

C Radiographic appearance of the normal lungs

Anterior view. Different regions of the lungs show different degrees of lucency in the chest radiograph. The perihilar region of the lung (where the main bronchi enter the lung and vessels enter and leave the lung) is less radiolucent than the peripheral region, which contains small-caliber vascular branches and segmental bronchi. Additionally, the perihilar lung region is partly covered by the heart. These “shadows” appear as white or bright areas on the radiograph. The same effect is observed in diseased lung areas, which appear more opaque as a result of fluid infiltration (inflammation) or tissue proliferation (neoplasia). These opacities are easier to detect in the peripheral part of the lung, which is inherently more radiolucent than the perihilar lung.

Anterior view of the opened thorax (depiction simplified). The heart and pericardium have been removed. The vessels surrounding the heart have been transected, and all mediastinal connective tissues have been removed. The lungs have been retracted laterally to stretch and expose the main bronchi. The abdominal cavity has been opened and eviscerated, leaving only the stomach in place. The cervical part of the trachea is still visible below the cricoid cartilage. Shortly below its entry into the chest through the thoracic inlet, the trachea is almost completely obscured by the great vessels. The thoracic part of the esophagus can be seen below the tracheal bifurcation, which lies directly behind the ascending aorta. The lungs in the pleural cavities closely approach the vertebral column posteriorly, while anteriorly they extend in front of the pericardium and narrow the anterior mediastinum. Percussion of the chest yields a “sonorous” lung sound (see B) which is dulled by the heart and pericardium. The extent of the lungs depends on the phase of respiration (see p. 159), but the apices of the lungs always extend into the thoracic inlet, which is closed by a condensation of loose connective tissue—the suprapleural membrane. The apical lung tissue is pictured here as soft and pliant, corresponding to its natural consistency. It should be noted that when the pleural cavities are opened at operation, the lungs tend to collapse toward the hilum owing to their elastic recoil; they do not completely fill the pleural cavity as shown here. (For clarity, the lungs are portrayed in an expanded state.)

A Pleura and pleural cavities: structure and topography

a Transverse section through the thorax, inferior view; b Anterior view of the right pleural cavity, which has been opened.

The pleural cavities are paired like the lungs they enclose, which is one reason they have a greater extent than the lungs:

• anteriorly, they extend past the pericardium to just behind the sternum, and in the dorsomedial direction up to the spinal column (a);

• due to the arching of the dome of the diaphragm, the inferior margin of the pleural cavities extends downward and overlaps with the abdominal cavity (b);

• due to the asymmetrical position of the heart in the mediastinum, the left pleural cavity is slightly smaller than the right pleural cavity (a);

• because the pleural cavities have a greater extent than the lungs, recesses develop in them (see also p. 141).

Completely analogous to the peritoneal and pericardial cavities, each pleural cavity is composed of two serous layers: the visceral pleura (pulmonary pleura attached to the surface of the lung) and the parietal pleura (attached to the endothoracic fascia). As a result of the attachment to the thorax, the pleura and thus the lungs (which adhere to the walls of the pleural cavities through capillary forces) automatically follow the movements of the chest wall. The line of junction between the visceral and parietal layers occurs along at the medial surface of the lungs (see p. 36). The capillary fissure-like space between the visceral and parietal pleura contains a small amount of clear serous fluid. This fluid layer allows both layers of the pleura to glide past each other and at the same time serves to hold the pleural layers together by capillary forces. For more about the topographical parts of the pleural layers see C.

For more about the function of the pleural recesses, see p. 159.

E Wall structure of the trachea and main bronchi

a Wall structure (for the ultrastructure of the bronchial tree see p. 148f and 154f):

• Tracheal mucosa with an epithelial layer and a lamina propria: The lamina propria contains seromucous glands (tracheal glands) which secrete a mucous film on the surface (for the epithelium see below).

• Fibromuscular-cartilaginous coat: This contains the C-shaped rings of hyaline cartilage and, on the posterior tracheal wall, the smooth muscle of the trachealis and abundant connective tissue.

• Adventitial sheath of connective tissue: This integrates the trachea into the adjacent connective tissue of the neck and mediastinum and allows mobility.

Note: The epithelium of the carina, unlike that of the rest of the trachea, consists of nonkeratinized squamous epithelium.

b Structure of the epithelium: The mucosa of the trachea and bronchi contains a pseudostratified columnar respiratory epithelium. All cells contact the basement membrane, but not all cells extend to the surface of the lumen. The cells in contact with the surface are ciliated. These hairlike structures propel small foreign bodies that have been inhaled toward the larynx. Smoking tobacco decreases the flow of secretions, compromising airway clearance. Mucus secreting goblet cells without cilia are interspersed among the epithelial cells.

A Gross anatomy of the left and right lungs

a, b Lateral view. c, d Medial view.

The color of the healthy lung ranges from gray to bluish-pink. Grayish-black particles are often visible beneath the pleural surface (as shown here) and are found even in nonsmokers. They do not necessarily have pathological significance, consisting of dust or carbonaceous particles that have been inhaled and deposited in the lung. A lung that has not been chemically fixed has a soft, spongy texture and collapses when taken from the chest. The shape shown above is the in vivo shape of the dynamically expanded lung (see p. 159). The right lung, with a volume of approximately 1500 cm, is slightly larger than the left lung, which has a volume of approximately 1400 cm (due to the inclination of the heart to the left side.) Each of the lungs is divided into lobes by one or more interlobar fissures:

• The left lung is divided into two lobes (superior and inferior) by one oblique fissure.

• The right lung consists of three lobes (superior, middle, and inferior) separated by one oblique fissure and one horizontal fissure. The pulmonary fissures are completely lined by visceral pleura.

Note: Owing to the steep angle of the oblique fissure in the left lung, the lingula of the upper lobe forms part of the base of the left lung. The smallest morphologically distinct and autonomous structural unit of the lung is the lobule, which is aerated by a bronchiole. The pulmonary lobules are separated from one another by (often incomplete) fibrous interlobular septa, demarcating numerous polyhedral areas that may be visible on the lung surface.

Aside from the differences noted above, both lungs have the same basic parts:

• The apex, which extends into the thoracic inlet

• The base, which rests on the diaphragm