Organs of the Respiratory System and their Neurovasculature

4 Organs of the Respiratory System and their Neurovasculature

4.1 Lungs: Location in the Thorax


A Location of the lungs in the thorax: topographical relations

a Transverse section through the thorax, superior view. The lungs completely occupy the left and right pleural cavities flanking the mediastinum. Anteriorly, they approach each other in front of the pericardium and posteriorly they are located close to the spinal column. Due to the asymmetrical position of the heart, the left lung is slightly smaller than the right lung (see D).

b Projections of the lungs onto the thoracic skeleton, anterior view. Superiorly, both lungs extend above the superior thoracic aperture; inferiorly, the undersurface of the lungs arches over the domes of the diaphragm. The distinct notch at the inferior medial border of the left lung is due to the heart, which is partially overlapped by the medial border of the lungs.


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.


D The lungs in situ

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.)

4.2 Pleural Cavities


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.

C Portions of the parietal pleura



Adjacent layer of connective tissue

Costal portion

Inner chest wall

Endothoracic fascia

Diaphragmatic portion

Surface of the diaphragm

Phrenicopleural fascia

Mediastinal portion

Lateral to mediastinum

Unnamed, direct transition to the connective tissue of the mediastinum

Cervical portion (Pleural cupula)

Apical, above the superior thoracic aperture

Suprapleural membrane (Sibson’s fascia)


D Innervation of the pleura

The parietal pleura, as part of the trunk wall, is innervated by somatic sensory nerves: The mediastinal portion and the largest part of the diaphragmatic portion are supplied by the phrenic nerves. A small part of the diaphragmatic portion located close to the ribs is also supplied by intercostal nerves. The costal portion is innervated by intercostal nerves. The visceral pleura is the organ-related layer and as such receives a sparse innervation by visceral sensory fibers, probably from the sympathetic nervous system. The corresponding neuronal perikarya are located in spinal ganglia—their branching axons pass through the sympathetic ganglion without terminating.

4.3 Boundaries of the Lungs and Parietal Pleura


A Projections of the boundaries of the lungs and parietal pleura onto the thoracic skeleton

Anterior view (a), posterior view (b), view from the left and right sides (c and d). The diagrams show the boundaries of the parietal pleura and lungs. The table (see B) summarizes some of the projection sites of the pleura and lungs onto the anterior, posterior and lateral thoracic wall. The parietal pleura lines the inner surface of the thoracic skeleton and projects itself onto palpable or visible bony landmarks.

The connection between these landmarks forms the boundaries of the parietal pleura (important in cases of pleural inflammations with effusion – visible on radiographs).

Note: The asymmetrical position of the heart makes the pleural cavity slightly smaller on the left side than on the right side. This causes the boundaries of the parietal pleura on the left side at the level of the heart to shift more laterally than on the right side.

B Relations of the lungs and pleural boundaries to landmarks on the thoracic skeleton


4.4 Trachea


A The trachea projected onto the neck and thorax

The trachea is located in the mediastinum and lies precisely in the median plane. The initial, cervical part of the trachea begins just below the larynx, and its thoracic part ends at the tracheal bifurcation. The trachea expands during inspiration and contracts during expiration. The projection in the figure shows the appearance of the trachea at functional residual capacity (relaxed end-expiration).


B Shape of the trachea

a Anterior view; b Superior view of the tracheal bifurcation.

The trachea is a flexible air-conducting tube 10 to 12 cm long. At the approximate level of the T 3–T 4 vertebral bodies, it bifurcates into the left and right main (principal) bronchi, which form an angle of approximately 55–70°. Viewed from the anterior side, the tracheal bifurcation lies just below the junction of the manubrium and body of the sternum. Note: The right main bronchus is more vertical than the left main bronchus, and therefore it is more common for aspirated foreign bodies to enter the right main bronchus than the left. This also makes it easier to view the interior of the right main bronchus with an endoscope. Owing to the asymmetry of the heart and the associated asymmetrical position of the lungs, the left main bronchus is slightly longer than the right.


C Structure of the trachea and bronchial tree

a Anterior view; b Posterior view with opened posterior wall.

The trachea consists of 16–20 horseshoe-shaped rings composed of hyaline cartilage (the tracheal cartilages) and a membranous posterior wall composed of connective tissue and tracheal muscle (not shown here). The tracheal cartilages are interconnected longitudinally by collagenous connective tissue (anular ligaments). The two parts of the trachea are clearly distinguishable:

Cervical part: extends from the first tracheal cartilage below the cricoid cartilage of the larynx at the level of the C 6/C 7 vertebrae to the thoracic inlet (see A);

Thoracic part: extends from the thoracic inlet to the tracheal bifurcation, where the trachea divides into the right and left main bronchi at the level of the T4 vertebra. A cartilaginous spur (carina, see Bb) at the tracheal bifurcation (the carina, see Bb) projects upward into the tracheal lumen.

The left and right main bronchi divide into two or three lobar bronchi, respectively, which subsequently branch into segmental bronchi (see D).

D Divisions of the trachea and bronchial tree

Right main bronchus

Left main bronchus

Right superior lobar bronchus

Apical segmental bronchus (I)

Posterior segmental bronchus (II)

Anterior segmental bronchus (III)

Left superior lobar bronchus

Apicoposterior segmental bronchus (I, II)

Anterior segmental bronchus (III)

Right middle lobar bronchus

Lateral segmental bronchus (IV)

Medial segmental bronchus (V)

Superior lingular bronchus (IV)

Inferior lingular bronchus (V)

Right inferior lobar bronchus

Left inferior lobar bronchus

Superior segmental bronchus (VI)

Superior segmental bronchus (VI)

Medial basal segmental bronchus (VII)

Medial basal segmental bronchus (VII)

Anterior basal segmental bronchus (VIII)

Anterior basal segmental bronchus (VIII)

Lateral basal segmental bronchus (IX)

Lateral basal segmental bronchus (IX)

Posterior basal segmental bronchus (X)

Posterior basal segmental bronchus (X)


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.

4.5 Lungs: Shape and Structure


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


Surfaces of the lung:

Costal surface: relates laterally and posteriorly to the ribs. The vertebral part of the costal surface faces the vertebral column (see c, d);

Mediastinal surface: relates medially to the mediastinum.

Diaphragmatic surface (see c, d): relates inferiorly to the diaphragm.

Interlobar (fissural) surfaces. In the chemically fixed specimen, impressions from the ribs are visible on the costal surface, a cardiac impression on the mediastinal surface, and an impression from the diaphragm leaflet on the diaphragmatic surface. The left lung additionally has a distinct cardiac notch in its anterior border.

Borders of the lung:

Anterior border: sharp, thin border located at the junction of the costal and mediastinal surfaces (inserts into the costomediastinal recess).

Inferior border: located at the junction of the diaphragmatic and costal or mediastinal surfaces, sharp at the costal surface (inserts into the costodiaphragmatic recess) and blunt at the mediastinal surface.

Hilum: area where bronchi and neurovascular structures enter and leave the mediastinal surface. The root of the lung comprises all of the blood vessels, lymphatics, bronchi, and nerves that enter and emerge at the hilum. Elements of the bronchial tree are generally located in the posterior part of the hilum. Pulmonary venous branches are anterior and inferior, and pulmonary arterial branches are found mainly in the upper part of the hilum. A simple mnemonic to help remember the relative positions is this: bronchi are posterior, veins are ventral, arteries are apical. The main difference between the left and right hilum is that the most superior structure on the right is a bronchus (eparterial bronchus) whereas on the left it is the artery (hyparterial bronchus).

Both lungs are invested by a serous membrane, the visceral pleura (pulmonary pleura), which is reflected at the mediastinal surface to continue as the parietal pleura. This pleural fold is ruptured when the lung is removed, appearing as the pulmonary ligament.

Aug 4, 2021 | Posted by in GENERAL SURGERY | Comments Off on Organs of the Respiratory System and their Neurovasculature
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