Chapter 22: The pleurae and lungs
The two lungs and their pleural sacs are situated in the thoracic cavity (figs. 22-1 and 22-2). The pleura is a thin, glistening, slippery serous membrane, inflammation of which is called pleurisy. The pleura lines the thoracic wall and diaphragm, where it is known as the parietal pleura. It is reflected onto the lung, where it is called the visceral pleura. The visceral pleura covers the lung and dips into its fissures. The facing surfaces of the parietal and visceral pleurae slide smoothly against each other during respiration. The contact between the parietal and visceral pleurae depends on the atmospheric pressure (1) on the outside of the chest wall and (2) inside the alveoli (which are connected to the exterior by the bronchial tree). On the other hand, the two pleural layers tend to be separated by the elasticity of (1) the thoracic wall (directed outward) and (2) the lungs (stretched by inspiration). The pleural cavity, which is the potential space between the two layers, contains only a thin film of fluid. Air in the pleural cavity (pneumothorax) results in collapse of the lung. The pleura is supplied by adjacent arteries and nerves and has numerous lymphatics. Irritation of the parietal pleura causes pain referred to the thoraco-abdominal wall (intercostal nerves) or to the shoulder (phrenic nerve).
The parietal pleura has costal, mediastinal, and diaphragmatic parts and a cupola (fig. 22-3). The costal pleura is separated from the sternum, costal cartilages, ribs, and muscles by a loose connective tissue termed endothoracic fascia, which provides a natural cleavage plane for surgical separation of the pleura from the thoracic wall. Anteriorly, the costal pleura turns sharply onto the mediastinum, and the underlying portion of the pleural cavity is called the costomediastinal recess. Inferiorly, the costal pleura is continuous with the diaphragmatic pleura, and the underlying space is termed the costodiaphragmatic recess. In the adult, the anterior borders of the right and left pleurae probably meet at or near the median plane during a part of their course. The left anterior border sometimes diverges to leave a part of the pericardium uncovered (bare area). Posteriorly, the pleura crosses the twelfth rib. At the root of the lung, the mediastinal pleura turns laterally, enclosing the structures at the root and becoming continuous with the visceral pleura. This reflection projects downward as a tapering double fold called the pulmonary ligament. The mediastinal pleura is adherent to the pericardium except where the phrenic nerve descends between them. Above the arch of the aorta, the right and left pleurae approach each other behind the esophagus. The diaphragmatic pleura covers most of the diaphragm except the central tendon. The cupola (or cervical pleura) is the continuation of the costal and mediastinal parts of the pleura over the apex of the lung. The cupola is strengthened by a thickening of the endothoracic fascia termed the suprapleural membrane, which is attached to the inner margin of the first rib and the transverse process of the seventh cervical vertebra (C7). Some muscular fibers (scalenus minimus) may be inserted into the membrane. Because of the slope of the first rib, the cupola of the pleura and the apex of the lung project upward into the neck, posterior to the sternomastoid, and hence may be injured in wounds of the neck. Their highest point is 2 to 3 cm or more above the level of the medial third of the clavicle. The sympathetic trunks and first thoracic nerves are found posterior to the cupola.
The anterior border of the pleura extends downward from the cupola, passing posterior to the sternoclavicular joint, then to the middle of the sternal angle, and next to approximately the level of the xiphisternal joint. The inferior border of the pleura extends laterally from the xiphisternal joint, crosses rib 8 in the midclavicular line and rib 10 in the midaxillary line, and then proceeds toward the spine of the 12th thoracic vertebra (see fig. 22-2). Considerable individual variation occurs, but generally the pleura extends two fingerbreadths inferior to the lung.
The lungs are the essential organs of respiration. The Latin word pulmo, lung, gives rise to the adjective pulmonary. The corresponding Greek word provides pneumonia, inflammation of the lungs. Each lung is attached by its root and pulmonary ligament to the heart and trachea but is otherwise free in the thoracic cavity. The lungs are light, soft, spongy, and elastic, and, because they contain air, they float in water. If an infant has not drawn a breath, however, the lungs will not float. (From two thirds of the way through prenatal life, a fetus is viable, i.e., has sufficient pulmonary development to live ex utero.) The air in the lungs renders them translucent to x-rays. The surface of an adult lung is usually mottled, and it presents dark gray or bluish patches caused by inhalation of atmospheric dust.
The main bronchus enters the hilus and subdivides within the substance of the lung to form the "bronchial tree." The tubes of the tree carry air to the alveoli, where respiratory exchanges with the blood occur. The bronchial tree elongates on deep inspiration.
The right lung, which is heavier than the left, is also shorter (the right dome of the diaphragm being higher) and wider (the heart bulging more to the left).
Each lung has an apex, three surfaces (costal, medial, and diaphragmatic), and three borders (anterior, inferior, and posterior). The right lung is divided into upper, middle, and lower lobes by oblique and horizontal fissures, whereas the left lung has usually only upper and lower lobes, separated by an oblique fissure.
The bronchi and pulmonary vessels, which extend from the trachea and heart, respectively, collectively form the root of the lung. The part of the medial surface where these structures enter the lung is known as the hilus (figs. 22-4 and 22-5).
The apex is rounded and fills the cupola of the pleura. The costal surface, which is related to the sternum, costal cartilages, and ribs, joins the medial surface at the anterior and posterior borders and the diaphragmatic surface at the inferior border. The medial surface is related posteriorly to the sides of the bodies of the vertebrae. Anteriorly, the medial surface is related to the superior, middle, and posterior parts of the mediastinum and includes the hilus. The diaphragmatic surface, or base, rests on the dome of the diaphragm, which separates the lung from the liver (on the right side) or the stomach, spleen, and sometimes liver and left colic flexure (on the left side).
Rarely, the azygos vein, instead of arching over the hilus of the right lung, arches over the upper lobe so that it isolates a medial part of the lung, called the lobule of the azygos vein.
The anterior border of the lung corresponds to that of the pleura, although it is uncertain whether the costomediastinal recess of the pleura is completely filled by the lung during quiet breathing, as it is in deep inspiration. The anterior border of the left lung probably deviates more to the left (cardiac notch) than does that of the pleura. The portion of the upper lobe of the left lung that lies between the cardiac notch and the oblique fissure is known as the lingula, and it corresponds to the middle lobe of the right lung. The inferior border of the lung occupies the costodiaphragmatic recess of the pleura, although it is too thin to be demonstrated by percussion during quiet breathing. The liver, stomach, spleen, colon, kidney, and peritoneal cavity extend to a higher level than the periphery of the diaphragm and the inferior border of the lung. Hence any perforation of the lower intercostal spaces should be considered an abdominal as well as a thoracic wound.
The inferior limit of the lung that can be outlined by percussion extends laterally from the xiphisternal joint and about two intercostal spaces higher than the pleura. It crosses rib 6 in the midclavicular line and rib 8 in the midaxillary line and then proceeds toward the 10th thorac vertebra. Considerable individual variation occurs, however.
Lobes and fissures (see fig. 22-2)
The right lung is divided into upper, middle, and lower lobes by an oblique and a horizontal fissure. The left lung is divided into upper and lower lobes by an oblique fissure. The oblique fissure follows approximately the line of rib 6 as far as the inferior border of the lung. When the arm is abducted and the hand placed on the back of the head, the medial border of the scapula indicates approximately the oblique fissure. The horizontal fissure begins at the oblique fissure near the midaxillary line (of the right side), at about the level of rib 6. It extends forward to the anterior border at the level of costal cartilage 4. It may be incomplete or even absent.
The root of the lung consists of the structures entering and emerging at the hilus. It connects the medial surface of each lung to the heart and trachea. It is surrounded by pleura, which is prolonged below as the pulmonary ligament. The roots of the lungs descend on deep inspiration. The chief structures in the root are the bronchi and pulmonary vessels. Also included are nerves, bronchial vessels, and lymphatics and nodes. The heart and great vessels are anterior to the trachea and main bronchi, and this relationship is maintained in the root of the lung, where the anterior-posterior order, is veins, artery, and bronchus, with the artery superior to the veins.
Bronchopulmonary segments (fig. 22-6)
The main bronchus divides into lobar (second-order) bronchi, each of which then divides into segmental (third-order) bronchi. The portion of lung supplied by a third-order bronchus is known as a bronchopulmonary segment. A given segment may be located by radiography or bronchoscopy. Pulmonary disorders may be localized in a bronchopulmonary segment, and surgical removal of a segment is feasible. The segments are separated from each other by connective tissue septa. Although variations are not uncommon, the bronchopulmonary segments have been named and numbered (fig. 22-6 and table 22-1). There are slight differences between the right and left lungs: briefly, in the left lung, segments 1 and 2 are generally combined, and commonly segments 7 and 8 are also. The branches of the pulmonary artery accompany the bronchi but are more variable. Pulmonary veins do not accompany the bronchi, but run between the segments; hence they are guides to intersegmental planes.
Blood to be oxygenated is carried by the pulmonary arteries, whereas the tissue of the bronchial tree and alveoli is nourished by the bronchial arteries. The branches of the pulmonary arteries within the lungs accompany the bronchi and end in capillary networks in the alveoli. The arteries at the hilus are visible radiographically and form a pattern that extends into the lung. The pulmonary veins collect oxygenated blood from the lung and deoxygenated blood from the bronchi and visceral pleura. Pulmonary veins are intersegmental in location. Usually four pulmonary veins enter the left atrium.
Bronchial arteries, usually one on the right and two on the left, arise commonly from the aorta, but variations are frequent. They supply oxygenated blood to the non-respiratory tissues of the lungs, including the visceral pleura. Bronchial veins carry deoxygenated blood from the first few bronchial divisions to the azygos system.
Carbon particles in the superficial lymphatics give the lung a grayish and mottled appearance. Superficial and deep lymphatic vessels drain toward the hilus and end in pulmonary and bronchopulmonary nodes. These in turn drain into the tracheobronchial nodes.
The anterior and posterior pulmonary plexuses around the root of the lung are formed by branches of the vagi and sympathetic trunks. Parasympathetic fibers (of vagal origin) supply the smooth muscle and glands of the bronchial tree. Spasm of the bronchial musculature occurs in asthma and can be relieved by epinephrine. Sympathetic fibers supply blood vessels and probably relax bronchial smooth muscle. Afferent fibers (vagal) from the visceral pleura and bronchi are concerned with the reflex control of respiration. Irritation of endings in the bronchial mucosa provokes coughing.
The classic methods of physical examination are inspection, palpation, percussion, and auscultation. Corresponding areas on the two sides of the chest should be compared systematically. Inspection includes examination of the scapulae, clavicles, ribs, sternal angle, subcostal angle, expansion of the chest, and movement of the abdominal wall. The rate of respiration is normally 11 to 14 per minute, i.e., about one fourth of the pulse rate. It is higher in children, and the neonatal rate is two to three times as high as the adult rate. Palpation involves feeling the trachea and also the vibrations ("vocal fremitus") of the chest wall. Percussion produces "resonance" over air-containing organs such as the lungs and "dullness" over solid organs. Auscultation ofthe breath sounds is undertaken with a stethoscope.
The chief x-ray methods used in the examination of the chest are plain films and CT scans. Bronchoscopy is used to examine the air passages and to biopsy masses and pulmonary function tests evaluate the movement of air.
The most frequently employed view in radiography of the chest is the "PA" view, i.e., postero-anterior projection, in which the x-ray tube is behind the erect patient and the film is vertically in front of the chest. Other views are indicated in figure 23-15. The bronchi and pulmonary tissue are radiolucent, but the branches of the pulmonary arteries form a visible pattern. The dense hilar shadows are produced by a combination of vascular, bronchial, lymphatic, and connective tissue components. The bronchial tree can be outlined by a contrast medium injected by way of an intratracheal catheter introduced through the mouth or nose. This procedure is termed bronchography (fig. 22-8). This has largerly been replaced by fiber-optic bronchoscopy which directly visualizes the passages.
Boyden, E. A., Segmental Anatomy of the Lungs, Blakiston (McGraw-Hill), New York, 1955. A classic on the patterns of the segmental bronchi and related pulmonary vessels.
Hayek, H., von, Die menschliche Lunge, 2nd ed., Springer, Berlin, 1970. The first edition (1960) is available in English (The Human Lung).
Nagaishi, C., et al., Functional Anatomy and Histology of the Lung, University Park, Baltimore, 1972. A mine of pulmonary information.
22-1 What are the serous membranes?
22-2 What is pneumothorax?
22-3 What is the cupola?
22-4 How does the right lung differ from the left?
22-5 What is the inferior limit of the lung and pleura?
22-6 Why is a perforating wound of the lower intercostal spaces considered to be an abdominal as well as a thoracic wound?
22-7 What is a bronchopulmonary segment?
22-8 What is the main constituent of the hilar shadows seen radiographically?
22-9 What are the classic methods of physical examination?
22-10 Which is the most frequently employed view in radiography of the chest?
Figure 22-1 Thorax of an adult. Note the clavicles, ribs, diaphragm, cardiovascular shadow (including the right atrium, aortic knuckle, and left ventricle), trachea, and lungs (including vascular markings). Large descending branches of the pulmonary arteries are visible on each side of the heart.
Figure 22-2 The lobes and fissures of the lungs. The pleurae are shown in blue (based on Brock.)
Figure 22-3 Diagrams of pleural reflections. A, Coronal section of the right lung and pleura. Lines B to E indicate the respective planes and levels of sections shown in diagrams B to E. B, Upper horizontal section. Note the costomediastinal and retro-esophageal recesses. C, Middle horizontal section. Note that the anterior border of the pleura forms the edge of the costomediastinal recess and that the oblique fissure reaches almost to the hilus. D, Lower horizontal section, showing also relationships to the pericardium. Note that the pulmonary ligament is formed by the double reflection of the pleura below the hilus of the lung. The mediastinal pleura is adherent to the fibrous pericardium, except where the phrenic nerve descends between them (not shown). E, Sagittal section.
Figure 22-4 Mirror-image views of the right lung and mediastinal structures. The line of reflection of the parietal to the visceral pleura is shown as a white line around the hilus, prolonged below as the two-layered pulmonary ligament. Impressions produced by mediastinal structures have been indicated on the medial surface of the lung as they might appear in a hardened lung. (Based on Mainland and on Mainland and Gordon.) The inset shows the relationships at the hilus, including the eparterial (upper lobe) bronchus. Note that in this and in similar drawings, because the pulmonary veins contain oxygenated blood, they are shown in red; correspondingly, the pulmonary arteries are shown in blue.
Figure 22-7 The primary lung volumes. The outermost line represents the greatest size to which the lung can expand. (Based on Comroe et al.)
Figure 22-8 Postero-anterior bronchograms. A, Right lung. B, Left lung. (From Medical Radiography and Photography, courtesy of J. Stauffer Lehman, M.D., and J. Antrim Crellin, M.D., Philadelphia, Pennsylvania.) For terminology see table 22-1.