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Pleura, lungs, trachea and bronchi
The lungs are the essential organs of respiration and are responsible for the exchange of oxygen and carbon dioxide in the blood. The functional anatomy of the thorax and respiratory diaphragm facilitates this complex process. Acting together, the muscles of respiration and the diaphragm increase the intrathoracic volume, creating a negative pressure within the pleural space that causes air influx and lung expansion ( Ch. 55 ). The resultant reduction in intra-alveolar pressure prompts the flow of air down the pressure gradient, through the upper respiratory tract, trachea, bronchi and bronchioles, and finally into the alveoli, where gaseous exchange occurs. Conversely, a decrease in the intrathoracic volume increases intra-alveolar pressure, causing the expiration of air. The combined expiratory movements of the thorax and abdomen do not clear the lung of its gas with each breath; the presence of a large residual volume of gas minimizes the degree to which each new inspiration of air can affect the composition of gas in diffusion exchange with the blood ( ). Besides efficient oxygen diffusion from alveolar air to blood, the lungs must also provide an inefficient carbon dioxide transfer, thus maintaining a stable carbon dioxide blood and tissue pressure, much higher (38–42 mmHg) than that in the atmosphere. Maintenance of critical homeostasis for carbon dioxide pressure is of greater importance than precise control of oxygen because so many essential electrical and metabolic functions depend on the hydrogen ion concentration [H + ] of the fluids in which these functions are carried out.
The process of breathing exposes the lung to noxious agents, including gases, dust particles, bacteria and viruses, and to dehydration and temperature extremes. The mucous barrier (including its cellular and immunoglobulin factors), the mucociliary escalator, branching pattern of the airways and the cough reflex are all anatomical defences against these insults. Studies of trends of particle deposition in children and young adults have shown that pulmonary deposition fractions are highest in infants who are at greatest risk from exposure to airborne particulate matter ( ). Respiratory function may be compromised by either anatomical defects such as chest wall abnormalities, or by paralysis of respiratory muscles or regional pain. Similarly, ultrastructural abnormalities, such as ciliary dysfunction (as seen in Kartagener’s syndrome), lead to recurrent respiratory infections and airway damage.
Pleura
Each lung is covered by a double-layered serous membrane, the parietal and visceral pleura, arranged as a closed, invaginated sac. The visceral pleura adheres closely to the lung surface and follows the interlobar fissures, separately covering each lobe of the lung. In the region of the lung hilum, the visceral pleura lifts off the mediastinal surface of the lung, and continues along the lung root for a short distance before folding back (reflecting) on itself to continue as the mediastinal parietal pleura ( Fig. 54.1 ). The parietal pleura covers the lateral mediastinal margins (forming its lateral boundary), most of the respiratory diaphragm and the corresponding half of the thoracic wall.
The pleural cavity represents the potential space between the parietal and visceral pleura. It incorporates an intervening thin film of pleural fluid that allows close sliding contact between the two layers during all phases of respiration. The pleural fluid has a high daily turnover: fluid is produced at 0.01–0.02 ml/kg/h and is continuously absorbed to maintain a pleural fluid level of 0.1–0.2 ml/kg. Fluid movement and absorption are supported by a balance between plasmatic and pleural (hydrostatic and oncotic) pressures and thoracic lymphatic drainage. Pleural effusions occur when the mechanisms that normally resorb the fluid are destabilized.
The negative pressure developed in the pleural cavity during inspiration is the result of the opposing outward pull of the chest wall and the inward elastic recoil of the lung. Any change in the elasticity of these structures or accumulation of fluid or air within the pleural cavity will alter respiratory activity either regionally or globally. Inter-regional disparities in ventilation normally exist as a result of local differences in thoracic expansion and position-related, gravity-dependent gradients in pleural pressure, and are reflected as regional inequalities in gas exchange.
The right and left pleural sacs are separate compartments. Their parietal layers touch posterior to the superior half of the body of the sternum at the anterior junction line (see Fig. 53.4B ), and come close to each other, or may touch, posterior to the oesophagus and level with the bodies of the third to fifth thoracic vertebrae at the posterior junctional line. Both lines are visible on radiographs and/or cross-sectional imaging (see Fig. 56.20 ); changes in the appearance of either line can indicate pathology. More inferiorly, the descending aorta (on the left) and azygos vein (on the right) demarcate the medial extent of the posterior parts of the pleural sacs. Cardiac asymmetry dictates that the left pleural cavity may sometimes be smaller. Transudates accumulate earlier and more often within the larger right pleural cavity.
Some of the folded portions of the pleura at its sites of reflection or recess (e.g. retrosternal, interlobar fissures and the azygo-oesophageal recess), or regions of contact between the left and right pleural cavity (e.g. anterior/posterior junctional lines) are the only parts of normal pleura that can be visualized radiologically. Demonstration of significant pleural shadowing in any other region usually implies pathological pleural abnormality. At direct inspection of the pleural cavity, as during thoracoscopy, both the parietal and visceral pleura appear translucent; the underlying thoracic muscles and blood vessels are visible beneath the parietal pleura, and the lung and subpleural vascular network are visible beneath the visceral pleura, rendering the latter grey and variegated in appearance.
Parietal pleura
Although continuous, different regions of parietal pleura are customarily distinguished by name according to the region they contact. The costal (costovertebral) part lines the internal surface of the thoracic wall and the lateral aspects of the vertebral bodies and intervening intervertebral discs; the diaphragmatic part covers the superior (thoracic) surface of the respiratory diaphragm; the cervical part (pleural dome) covers the inferior surface of the suprapleural membrane (in the region of the lung apex); and the mediastinal part covers the structures between the lungs and within the mediastinum, and forms the lateral border of the mediastinum. The surface markings of the parietal pleura are described in Chapter 52 .
Costal part
The costal part of the parietal pleura lines the sternum, the lateral margins of the vertebral bodies and intervening intervertebral discs, the ribs and transversus thoracis and the intercostal muscles. External to the pleura, and located within the extrapleural space is the endothoracic fascia, a thin layer of loose connective tissue analogous to the transversalis fascia of the abdominal wall. Anteriorly, the costal pleura begins posterior to the sternum, where it is continuous with the mediastinal pleura along a line extending inferomedially, from posterior to the sternoclavicular joint to posterior to the sternal angle in the midline (see Fig. 53.4B ). From here, the right and left costal pleurae descend in contact with each other to the level of the fourth costal cartilage and then diverge. On the right side, the pleura descends almost vertically to reach the posterior aspect of the xiphisternal joint, while on the left it diverges laterally and descends at a distance of 3–5 cm from the sternal margin to the anterior end of the sixth rib, forming the cardiac notch (pericardial triangle). On each side, the costal parietal pleura passes laterally, lining the internal surfaces of the costal cartilages, ribs, transversus thoracis and intercostal muscles. Posteriorly, it passes over the sympathetic trunk and its ganglia and splanchnic branches to reach the lateral sides of the vertebral bodies and intervertebral discs, where it again is continuous with the mediastinal pleura. Superior to the first rib, the costal pleura is continuous with the cervical pleura; inferiorly, it becomes continuous with the diaphragmatic pleura along a line that differs slightly on the two sides. On the right, the line of costodiaphragmatic reflection (junction) begins posterior to the xiphoid process, passes posterior to the seventh costal cartilage to reach the eighth rib in the midclavicular line and the tenth rib in the mid-axillary line, then the twelfth rib (approximately level with the superior border of the spinous process of the twelfth thoracic vertebra) adjacent to the vertebral column. On the left, the line initially follows the sixth costal cartilage but then follows a course similar to that on the right. In neonates and young children, the domes of the respiratory diaphragm are lower and flatter, reducing the angle of this point of reflection.
Diaphragmatic part
The diaphragmatic part of the parietal pleura covers most of the superior (thoracic) surface of the respiratory diaphragm where it is tightly attached to the phrenicopleural fascia, a continuation of the endothoracic fascia. It is continuous with the costal pleura around the periphery of the diaphragm, and within the costodiaphragmatic recess, and with the mediastinal pleura medially.
Cervical part
The cervical part of the parietal pleura is the continuation of the costal pleura over the apex of the lung and inferior to the suprapleural membrane, conforming tightly to both. It ascends medially from the internal border of the first rib to the apex of the lung, and then descends lateral to the trachea to become the mediastinal pleura. As a result of the obliquity of the first rib, the cervical pleura extends 3–4 cm superior to the first costal cartilage, up to or above the level of the neck of the first rib, reaching as high as the body of the seventh cervical vertebra (see Figs 54.1 , 52.5 ). The cervical pleura is strengthened by the suprapleural membrane (Sibson’s fascia), a thickening of the endothoracic fascia that is attached anteriorly to the internal border of the first rib and posteriorly to the anterior border of the transverse process of the seventh cervical vertebra. Scalenus minimus, when present, extends from the anterior border of the transverse process of the fifth, sixth and/or seventh cervical vertebra to the inner border of the first rib behind the subclavian groove, the cervical pleura and the suprapleural membrane (which it tenses). The cervical pleura and suprapleural membrane are related to the supraclavicular part of the brachial plexus, the stellate (cervicothoracic) ganglion and the vertebral vessels; the subclavian artery passes laterally in a groove located anterior to the most superior level of the membrane ( Fig. 54.2 ).
Mediastinal part
The mediastinal part of the parietal pleura represents the lateral boundary of the mediastinum and forms a continuous covering above the level of the hilum of the lung, from the sternum anteriorly to the vertebral column posteriorly. On the right, it covers the right brachiocephalic vein, upper part of the superior vena cava, terminal part of the azygos vein, right phrenic and vagus nerves, trachea and oesophagus (see Figure 55.10, Figure 56.7 ). On the left it covers the aortic arch, left phrenic and vagus nerves, left brachiocephalic and left superior intercostal veins, left common carotid and subclavian arteries, thoracic duct and oesophagus. At the root of the lung, it turns laterally to form a tube (sleeve) that encloses the structures passing to the lung hilum (see Fig. 54.1 ) and is continuous with the visceral pleura.
Visceral pleura
The visceral (pulmonary) part of the parietal pleura is adherent to the lung over all external surfaces, including those in the fissures, except at the hilum of the lung and along a line descending from this, that marks the formation and attachment of the pulmonary ligament (see Fig. 54.1 ; Fig. 54.3 ).
Pulmonary ligament
The pulmonary ligament, a thin almost vertically aligned double-layered fold of pleura, extends inferiorly from the region of the lung hilum and root. It passes between the mediastinal surface of the lung, where it joins the visceral pleura, and the lateral aspect of the mediastinum in the region of the oesophagus, where it joins the mediastinal part of the parietal pleura (see Figs 54.1 , 54.3 ). It is continuous superiorly with the pleura around the lung hilum and root. Inferiorly, the two layers of pleura join, ending in a free sickle-shaped contour.
Parietal pleural recesses
The parietal pleura extends considerably beyond the inferior border of the lung but not as far as the attachment of the respiratory diaphragm, which means that the diaphragm is in contact with the costal cartilages and intercostal muscles below the line of pleural reflection from the thoracic wall to the diaphragm. In quiet inspiration, the inferior margin of the lung does not reach the point of parietal pleural reflection and the costal and diaphragmatic parts of the pleura are separated by a narrow slit, the costodiaphragmatic recess; the lower limit of the lung is normally about 5 cm superior to the lowest point of the recess. A similar costomediastinal recess exists between the mediastinum and the posterior surfaces of the sternum and costal cartilages, where the thin anterior margin of the lung projects toward the line of pleural reflection. The extent of this recess, the anterior costomediastinal line of pleural reflection, and the position of the anterior margin of the lung, exhibit individual variation.
The parietal pleural recesses allow the proportionate inspiratory expansion of the lungs with minimal friction. The same thin layer of pleural fluid within the recesses separates the costal and diaphragmatic parts of the parietal pleura during expiration, or the parietal pleura from the intervening visceral pleura during inspiration. Pleural adhesions and air or fluid accumulation limit this mechanism to a variable degree.
The inferior border of the costodiaphragmatic recess is an important consideration in surgical approaches to the kidney. The superior pole of the kidney crosses the twelfth rib and sometimes also the eleventh rib, which means that part of each kidney is associated with the parietal pleural reflection within the costodiaphragmatic recess. The parietal pleura may be damaged by flank incisions passing below the twelfth rib, but is more likely when rib resection is required during nephrectomy.
The aortico-oesophageal and azygo-oesophageal recesses project medially between the aorta and oesophagus on the left side and the azygos vein and oesophagus on the right side (see Fig. 56.7A ).
Vascular supply and lymphatic drainage
The costal part of the parietal pleura is supplied by branches from the intercostal and internal thoracic arteries; the mediastinal part by branches from the bronchial, superior phrenic, internal thoracic and mediastinal arteries; the cervical part by branches from the subclavian artery; and the diaphragmatic part by the superficial part of the diaphragmatic microcirculation. Veins draining the parietal pleura join the thoracic wall veins, eventually draining into the superior vena cava mainly via the azygos system (see Fig. 56.2 ). Lymph from the costal part of the parietal pleura drains into the parasternal nodes anteriorly and intercostal nodes posteriorly, while that from the diaphragmatic pleura drains into the mediastinal, parasternal, para-aortic (phrenic) and coeliac nodes (see Figure 56.4, Figure 56.5 ).
The visceral pleura forms an integral part of the lung and, accordingly, its arterial supply and venous drainage are provided by the bronchial vessels. The bronchial arteries at the hilum form a circle, surrounding the main bronchus, and pleural branches from this anulus supply the visceral pleura covering the mediastinum, interlobar and apical surfaces of the lung, and part of the diaphragmatic (base) surface. The visceral pleura is drained by pulmonary veins, apart from an area around the hilum that drains into bronchial veins. The lymphatic drainage of the visceral pleura is initially via the subpleural lymphatic plexus. Lymph can pass via segmental or intersegmental lymphatic pathways, and sometimes via peribronchial lymphatic pathways; the exact pattern of drainage varies depending on lobe, segment, side and individual ( ).
Innervation
The costal and peripheral regions of the diaphragmatic parietal pleurae are innervated by intercostal nerves. The mediastinal and central regions of the diaphragmatic parietal pleurae are innervated by the phrenic nerves (C3–C5). Irritation of the intercostal nerves causes pain that is referred to the similarly innervated part of the thoracic or abdominal wall, whereas irritation of the central diaphragmatic pleura causes pain that is referred to the lower neck and shoulder tip, i.e. to the C3 and C4 dermatomes of the affected side.
Pneumothorax
A breach of the parietal or visceral pleura (e.g. in chest wall injury) may lead to the accumulation of air within the pleural cavity (pneumothorax). Fluid (hydrothorax), blood (haemothorax) and, rarely, lymph (chylothorax) can also accumulate in this space. Pneumothoraces may occur spontaneously (rupture of emphysematous bullae, or subpleural ‘blebs’ typically in tall, asthenic males) or following rib fracture or penetrating trauma.
Occasionally, a valve-like effect occurs, so that air enters the pleural cavity during inspiration (due to the presence of a negative pressure) but cannot escape in expiration. This may result in a ‘tension pneumothorax’, which can be life-threatening and should be suspected whenever there are unilateral decreased breath sounds, hypotension, jugular venous distension and contralateral tracheal deviation (due to mediastinal shift). A tension pneumothorax requires immediate decompression. This may be carried out using a wide-bore needle; or a decompression cannula apparatus, inserted into the second intercostal space in the midclavicular line; or via a thoracostomy with an incision made through the fourth or fifth intercostal space between the anterior and mid-axillary lines followed by digital insertion to breach the parietal pleura and sweep away pleural adhesions ( ), followed by tube insertion for longer-term drainage.
Fluid collection in the pleural cavity may be due to congestive cardiac failure, hypoalbuminaemia or inflammatory, infective or neoplastic conditions. A developing pleural effusion causes obliteration of the costodiaphragmatic recess, where a lateral fluid meniscus is often visible from a non-loculated effusion on a frontal chest radiograph. Drainage of the fluid and subsequent analysis are required for diagnostic purposes. Where there is a collection of purulence (empyema) or blood, pleural cavity drainage is essential for therapeutic purposes. Ultrasonography is useful in assessing the size and characteristics of an effusion, such as the presence of loculation and debris and can demonstrate underlying consolidated lung. Computed tomography (CT) is used to assess the underlying lung parenchyma, endobronchium and mediastinal lymph nodes.
Microstructure
The smooth pleural surface consists of a single layer of flat mesothelial (serosal) cells separated from an underlying lamina propria of loose connective tissue by a basal lamina. Ultrastructurally, pleural and peritoneal mesothelial cells are similar in having a highly folded basal plasma membrane, in that their adjacent cell surfaces interdigitate and are joined by desmosomes, and in that their luminal surfaces bear numerous microvilli and some cilia. Cytoplasmic pinocytotic vesicles are common. The connective tissue covers the entire pulmonary surface; it extends from the hilum along the bronchial tree and accompanying blood vessels within the substance of the lung and divides the lung into numerous small polyhedral lobules, each of which receives a terminal bronchiole and arteriole, venules, lymphatics and nerves. Lobules vary in size; the superficial ones are the largest, visible as polygonal areas 5–15 mm across.
Lungs
The lungs are the organs of respiration. They are situated on either side of the heart and other mediastinal structures and occupy most of the thoracic cavity. Each lung is free to move in its pleural cavity, except at the hilum and pulmonary ligament, where it is attached to the mediastinum. When removed from the thorax, a fresh normal lung is spongy, can float in water, and crepitates when handled because of the air within its alveoli. It is also highly elastic and so retracts on removal from the thorax. Its surface is smooth and shiny, and is divided by fine, dark lines into numerous small polyhedral lobules, each crossed by numerous finer lines, indicating the areas of contact between its most peripheral lobules and the visceral pleural surface.
In proportion to body stature, the lungs are heavier in males than in females ( ). At post mortem examination, the adult right lung usually weighs 340 g (142–835 g) in females and 445 g (185–967 g) in males; the adult left lung weighs 299 g (108–736 g) in females and 395 g (186–885 g) in males (Molina and DiMaio 2012, 2015). A fetal lung pair at 40 weeks post fertilization weighs 51 ± 16 g ( ). Lung volume, air volume and pulmonary tissue volume all increase linearly with body length ( ).
Pulmonary surface features
Each lung roughly resembles a halved cone and has an apex, base, distinct anterior and inferior borders and three outer facing surfaces (costal, diaphragmatic (base), mediastinal) (see Fig. 54.3 ). A posterior border is sometimes defined, although it is often rounded and relatively indistinct. During post mortem examination of preserved tissue each lung bears the impression of neighbouring structures that the lung was pressed against through the pleural coverings in life.
Apex
The apex, the rounded upper limit of the lung, extends above the superior thoracic aperture where it contacts the cervical pleura, and is covered in turn by the suprapleural membrane. As a consequence of the obliquity of the aperture, the apex rises 3–4 cm above the level of the first costal cartilage. The highest point of the apex is up to 2 cm above the medial third of the clavicle, at or above the neck of the first rib, and level with the body of the seventh cervical or first thoracic vertebra, or the intervening intervertebral disc; it sits in the root of the neck (see Fig. 35.18 ). Some consider the apex to be an intrathoracic structure when it does not rise above the neck of the first rib. The subclavian artery arches superolaterally over the suprapleural membrane and grooves the anterior surface of the membrane and lung apex just below its highest point, separating it from scalenus anterior (see Fig. 54.2 ). The cervicothoracic (stellate) sympathetic ganglion, ventral ramus of the first thoracic spinal nerve and supreme intercostal artery all lie posterior to the lung apex. Scalenus medius is lateral and the brachiocephalic trunk, right brachiocephalic vein and trachea are adjacent to the medial surface of the apex of the right lung. The left subclavian artery and left brachiocephalic vein are adjacent to the medial surface of the left lung apex.
Diaphragmatic surface (base)
The diaphragmatic surface (base) of the lung, when static, is semilunar and concave, moulded on the superior surface of the respiratory diaphragm, which separates each lung from the corresponding lobe of the liver, and the left lung from the gastric fundus and spleen. Since the respiratory diaphragm extends higher on the right than on the left, the concavity is deeper on the base of the right lung. Posterolaterally, the diaphragmatic surface has a more tapered shape that projects a little into the costodiaphragmatic recess. The change in diaphragmatic surface shape and relations is best viewed with inspiration/expiration radiographs, or with a ventilation CT series.
Costal surface
The costal surface of the lung is smooth and convex, and its shape is adapted to that of the thoracic wall, which is vertically deeper posteriorly. It is in contact with the costal pleura; in lung specimens that have been preserved in situ , this surface exhibits grooves that correspond with the overlying ribs, a situation similar to the hyperinflated lungs of patients with asthma. The vertebral part of the costal surface faces, and is impressed by, the anterolateral margins of the thoracic vertebrae and intervening intervertebral discs, posterior intercostal vessels and splanchnic nerves. Some consider the vertebral surface of the lung separate to the costal part.
Mediastinal surface
The mediastinal surface of the lung is deeply concave because it is adapted to the heart at the cardiac impression: the impression is much larger and deeper on the left lung where the heart projects more to the left of the median sagittal plane. The hilum of the lung, posterosuperior to this concavity, is the area where various structures of the lung root enter or leave the lung. The lung root and hilum are surrounded by a sleeve of pleura that extends inferiorly, posterior to the cardiac impression, as the pulmonary ligament. The mediastinal surface and vertebral part of the costal surface are sometimes collectively termed the medial surface (border) of the lung.
Other impressions on the lung surface
Cadaveric lungs that have been preserved in situ can show a number of other impressions that indicate their relations with surrounding structures, especially on the mediastinal surface (see Fig. 54.3 ). All these impressions reflect the intimate relationship between the lungs and adjacent mediastinal structures and offer a good perspective on surgical approaches or considerations, either from the anterior or posterior aspect of the corresponding pleural cavity.
On the mediastinal surface of the right lung (see Fig. 54.3A ), the cardiac impression is related to the anterior surface of the right auricle, the anterolateral surface of the right atrium and partially to the anterior surface of the right ventricle. A wide groove ascends anterior to the hilum for the superior vena cava, the terminal portion of the right brachiocephalic vein and the right subclavian vein. Posteriorly, this groove is joined by a deep sulcus that arches forwards immediately superior to the hilum and is occupied by the azygos vein. The right side of the oesophagus makes a shallow vertical groove posterior to the hilum and the pulmonary ligament, and posterior to this is a vertical groove for the azygos vein. Towards the respiratory diaphragm it inclines left and leaves the right lung, and therefore does not reach the lower limit of this surface. Posteroinferiorly, the cardiac impression is confluent with a short, wide groove adapted to the inferior vena cava. Between the lung apex and the groove for the arch of the azygos vein, the trachea and right vagus nerve are close to the lung but do not mark it.
On the mediastinal surface of the left lung (see Fig. 54.3B ), the cardiac impression is related to the anterolateral surface of the left ventricle and left auricle. The anterior surface of the conus arteriosus and adjoining part of the right ventricle are related to the lung as it ascends anterior to the hilum to accommodate the pulmonary trunk. A large groove arches over the hilum, and descends behind both it and the pulmonary ligament, corresponding to the aortic arch and descending thoracic aorta. From its summit, a narrower groove ascends to the apex for the left subclavian artery. Posterior to this, and superior to the aortic groove, the lung is impressed by the oesophagus (with potential contribution from the thoracic duct as it passes along the posterolateral surface of the oesophagus). Anterior to the groove for the subclavian artery there is a faint linear depression for the left brachiocephalic vein. Inferiorly, the oesophagus may create a groove posterior to the lower end of the pulmonary ligament.
Pulmonary borders
The inferior border of the lung is thin and sharp where the diaphragmatic and costal surfaces extend into the costodiaphragmatic recess and meet, and more rounded medially where the diaphragmatic and mediastinal surfaces meet (see Fig. 54.3 ). The surface anatomy of the inferior border of the lung is described in Chapter 52 (see Figure 52.5, Figure 52.6 ). The respiratory diaphragm rises more superiorly on the right to accommodate the liver, which means that the right lung is vertically shorter (by about 2–3 cm) than the left. However, cardiac asymmetry means that the right lung is broader, and has a greater capacity and weight than the left. Although not obvious, the posterior border separates the costal surface from the mediastinal surface and corresponds to the region of the lung adjacent to the heads of the ribs. It has no recognizable markings and is represented by a rounded region where the costal and vertebral surfaces of the lung meet. The thin, sharp, anterior border of the lung overlaps the pericardium (see Fig. 54.7 ). On the right it corresponds closely to the costomediastinal line of pleural reflection and is almost vertical. On the left it approaches the same line posterior to the sternum and superior to the fourth costal cartilage; inferior to this point it shows a variably shaped cardiac notch, the edge of which passes laterally for about 3.5 cm before curving inferomedially to the level of the sixth costal cartilage, about 4 cm from the midline. This region of the left lung does not reach the line of parietal pleural reflection (see Fig. 52.5 ) and so the fibrous pericardium is covered only by a double layer of parietal pleura (over the area of superficial cardiac dullness). Observations during surgery suggest that the line of pleural reflection, the anterior pulmonary margin and the costomediastinal pleural recess are all variable.
Pulmonary fissures and lobes
Left lung
The left lung is commonly divided into superior and inferior lobes by an oblique fissure that extends from the costal to the mediastinal surfaces, superior and inferior to the hilum (see Fig. 54.3B ). When observing the surface of the lung, this fissure begins on the medial surface at the posterosuperior part of the hilum, near the pulmonary artery; ascends obliquely and posteriorly to cross the posterior border of the lung about 6 cm below the apex; next descends anteriorly across the costal surface, to reach the inferior border almost at its anterior limit; and finally ascends posteriorly on the mediastinal surface to the inferior part of the hilum. The surface and positional anatomy of the lung fissures is described in Chapter 52 . The extent of completeness and/or orientation of the left oblique fissure is variable ( ); for example, the left oblique fissure is atypical (incomplete or absent) in 18% ( ) to 73% ( ) of cases. The additional presence of a left horizontal fissure is a normal, occasional variant. Incomplete separation of the lobes and identification of the completeness of the fissures are important prior to lobectomy because individuals with incomplete fissures are more prone to develop postoperative air leaks and may require further procedures, such as stapling and pericardial sleeves.
The superior lobe lies anterosuperior to the oblique fissure and includes the apex, anterior border and much of the costal and most of the mediastinal surfaces of the lung. A small process, the lingula, is usually present at the inferior end of the cardiac notch. The larger inferior lobe lies posterior and inferior to the fissure and contributes almost the whole of the diaphragmatic surface, much of the costal surface and most of the posterior border and vertebral surface of the left lung.
Right lung
The right lung is divided commonly into superior, middle and inferior lobes by its oblique and horizontal fissures (see Fig. 54.3A ). The oblique fissure separates the inferior from the superior and middle lobes, and although less vertical is similar in position on the lung surface to the left oblique fissure (described above). The surface and positional anatomy of the lung fissures is described in Chapter 52 . On the surface of the lung, the right oblique fissure follows almost the same course as the left oblique fissure. A range of variation in the degree of completeness and/or orientation of the fissures is reported ( ). For example, the right oblique fissure is atypical (incomplete to absent) in 30% ( ) to 71% ( ) of cases, whereas the right horizontal fissure is atypical in 62% ( ) to 88% ( ) of cases. The horizontal fissure is usually visible on a frontal chest radiograph and the oblique fissure is usually visible on a lateral radiograph and on a high-resolution CT scan as a curvilinear band passing from the lateral aspect of the costal surface to the hilum ( Fig. 54.4 ). The smaller middle lobe is wedged between the superior and inferior lobes, and forms the inferior part of the anterior border, the anterior part of the inferior border and the associated part of the anteriorly located costal surface of the lung (see Fig. 54.7 ). The azygos vein may sometimes course in a more lateral position, within a four-layered pleural septum within the superior lobe, creating an ‘azygos lobe’. This is not a true lobe, but a separated portion of the superior lobe that lacks its own bronchi, arteries or veins. The condition represents an incidental finding on chest radiographs or CT scans, with no associated morbidity; it may be confused with a pulmonary nodule, and loculated effusions may appear in the fissure. Accessory fissures may separate either the medial basal segment (Twining’s line) or the superior segment from the remainder of the inferior lobe.
Trachea and Bronchi
Trachea
The adult trachea is a 10–11 cm long tube with cervical and thoracic parts. It is formed of cartilage and fibromuscular membrane and lined internally by mucosa. It descends from the cricoid cartilage of the larynx at the level of the body of the sixth (female) or seventh (male) cervical vertebra and divides into right and left main bronchi typically inferior to the sternal plane by 1–3 cm, and level with the body of the fifth or sixth thoracic vertebra or the T5–T6 intervertebral disc (see Fig. 52.7 ). It is located approximately in the midline sagittal plane but its point of bifurcation is usually a little to the right. In children less than 5 years of age a chest radiograph may demonstrate the trachea to appear to sharply deviate laterally to the right (buckle) at or just superior to the superior thoracic aperture and is considered a normal variant ( ). The variable position of the trachea in children makes it unreliable as an indicator of the presence of a right-sided aortic arch on a chest radiograph ( ).
The anterior and lateral surfaces of the trachea consist of 16–20 superimposed incomplete horseshoe-shaped ‘rings’ of hyaline cartilage and intervening anular ligaments formed from fibroelastic tissue that allows tracheal elongation during inspiration or neck movement (extension). During deep inspiration the tracheal bifurcation reaches the level of the body of the sixth thoracic vertebra. In children, the tracheal cartilages retain a high degree of elasticity that is often lost in adults: during full neck extension the length of the trachea increases by 0.95± 0.43 cm in a child ( ).
The posterior membranous wall is a flat, fibromuscular structure containing smooth muscle (trachealis). During the first postnatal year of life, tracheal diameter does not exceed 4 mm. Mean transverse diameter is greater than anteroposterior diameter up to the age of 6 years, after which the two diameters are nearly equal ( ). During later childhood, tracheal diameter in millimetres is approximately equal to age in years. The external transverse diameter is typically 2 cm in adult males and 1.5 cm in adult females. The lumen is about 12 mm in diameter in live adults but increases as a result of relaxation of the smooth muscle post mortem. The transverse shape of the lumen is variable, especially in the later decades of life, and it may be round, lunate or flattened. The trachea is funnel-shaped in neonates, when the upper end is wider than the lower end and gradually becomes cylindrical with increasing age. At bronchoscopy, the posterior membranous wall of the trachea bulges into the lumen and this is exaggerated during expiration and coughing. The distal end of the trachea is visible as a concave spur. A tracheal bronchus may occasionally arise, mainly from the right aspect of the trachea around 2 cm superior to the carina, as either a supernumerary lobar or apical bronchus, or a displaced apical or superior lobar (porcine) bronchus ( ). This occurs more commonly in males and may be associated with axial skeletal anomalies. A tracheal bronchus was noted in 2.4% of children undergoing bronchoscopy for respiratory symptoms ( ).
Relations of the cervical part of the trachea
Anterior relations
The cervical part of the trachea is covered anteriorly by skin and the intervening superficial and deep cervical fasciae, and the suprasternal space. It is crossed by the jugular venous arch and overlapped by sternohyoid and sternothyroid. The second to fourth tracheal cartilages are crossed by the isthmus of the thyroid gland, above which an anastomosis connects the two superior thyroid arteries; the pretracheal fascia, inferior thyroid veins, thymic remnants (in adults) and the thyroid ima artery (when present) are all anteroinferior. In children, the thymus (especially the left lobe) may extend anterior to the trachea to the level of the thyroid gland and the brachiocephalic trunk crosses obliquely in front of the trachea at, or a little above, the upper border of the manubrium; the left brachiocephalic vein may also rise a little above this level. An enlarged thyroid gland may cover the cervical part of the trachea and reach the superior mediastinum, usually anterior to the left brachiocephalic vein.
Obstruction of the upper airways, craniofacial trauma and neck cancers (especially laryngeal), in either the acute or the chronic setting, require a tracheostomy. This is the creation of a hole (stoma) in the anterior aspect of the trachea that can serve either independently or as a site for an indwelling tube allowing the spontaneous or mechanical ventilation of the patient. The procedure usually involves incising the second and third tracheal cartilages and may be performed surgically or percutaneously ( , ). Surgical approaches are via a horizontal or vertical incision, about 2–3 cm in length, made midway between the jugular (suprasternal) notch and the thyroid cartilage; platysma, sternohyoid and then sternothyroid are separated along the midline to expose the trachea. Alternatively, in some cases, particularly in an emergency, the lumen of the larynx can be accessed directly via incision of the cricothyroid membrane (cricothyroidotomy).
Posterior relations
The cervical part of the oesophagus is the main posterior relation of the cervical trachea, separating it from the vertebral column and the prevertebral fascia.
Lateral relations
The paired lobes of the thyroid gland, that commonly descend to the level of the fifth or sixth tracheal cartilage, and the common carotid and inferior thyroid arteries, are all lateral relations. The thyroid gland is covered in a fascial capsule, an extension of the pretracheal fascia. This capsule condenses posteriorly to form the suspensory ligament of the thyroid gland (Berry) which is connected firmly to the trachea at the posterior margin of the adjacent cartilage rings, and blends with the posterior membranous wall. The recurrent laryngeal nerves ascend on each side, in or near the tracheo-oesophageal groove. The posterior suspensory ligament and the nearby terminal branches of the inferior thyroid artery are important landmarks for intraoperative identification of the recurrent laryngeal nerve.
Relations of the thoracic part of the trachea
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