The paired pleural cavities contain the left and right lungs. They are completely separated from each other by the mediastinum and are under negative atmospheric pressure (see respiratory mechanics,pp. 112–113).
Fig. 8.1 Pleural cavity
Pleural cavities and lungs projected onto thoracic skeleton.
Fig. 8.2 Boundaries of the pleural cavities and lungs
Fig. 8.3 Parietal pleura
The pleural cavity is bounded by two serous layers. The visceral (pulmonary) pleura covers the lungs, and the parietal pleura lines the inner surface of the thoracic cavity. The four parts of the parietal pleura (costal, diaphragmatic, mediastinal, and cervical) are continuous.
Lungs in Situ
Fig. 8.4 Lungs in situ
The left and right lungs occupy the full volume of the pleural cavity. Note that the left lung is slightly smaller than the right due to the asymmetrical position of the heart.
The oblique and horizontal fissures divide the right lung into three lobes: superior, middle, and inferior. The oblique fissure divides the left lung into two lobes: superior and inferior. The apex of each lung extends into the root of the neck. The hilum is the location at which the bronchi and neurovascular structures connect to the lung.
Fig. 8.5 Gross anatomy of the lungs
The regions of the lungs show varying degrees of lucency in chest radiographs. The perihilar region where the main bronchi and vessels enter and exit the lung is less radiolucent than the peripheral region, which contains small-caliber vascular branches and segmental bronchi. The perihilar lung region is also covered by the heart. These “shadows” appear as white or bright areas on the radiograph (radiographs are negatives: areas that are impermeable to light will appear bright).
Fig. 8.6 Radiographic appearance of the lungs
Fig. 8.7 Opacity in lung diseases
Lateral and anterior views of the right and left lungs. Opacity (decreased radiolucency) may be observed in diseased lung areas. Increased opacity may be due to 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. Note: Opacities that conform to segmental lung boundaries are almost invariably due to pulmonary inflammation.
Diseases of the lungs
Increased opacity in the lungs does not necessarly correspond to segmental boundaries. Fluid accumulation in the lungs also creates characteristic “shadows” in pulmonary radiographs.
Bronchopulmonary Segments of the Lungs
The lung lobes are subdivided into bronchopulmonary segments, each supplied by a tertiary (segmental) bronchus. Note: These subdivisions are not defined by surface boundaries but by origin.
Fig. 8.8 Segmentation of the lung
Anterior view. See pp. 110–111 for details of the trachea and bronchial tree.
Fig. 8.9 Posteroanterior bronchogram
Anterior view of right lung.
Fig. 8.10 Right lung: Bronchopulmonary segments
Fig. 8.11 Left lung: Bronchopulmonary segments
Lung cancer, emphysema, or tuberculosis may necessitate the surgical removal of damaged portions of the lung. Surgeons exploit the anatomical subdivision of the lungs into lobes and segments when excising damaged tissue.
Trachea & Bronchial Tree
At or near the level of the sternal angle, the lowest tracheal cartilage extends anteroposteriorly, forming the carina. The trachea bifurcates at the carina into the right and left main bronchi. Each bronchus gives off lobar branches to the corresponding lung.
Fig. 8.12 Trachea
See p. 574 for the structures of the thyroid.
Foreign body aspiration
Toddlers are at particularly high risk of potentially fatal aspiration of foreign bodies. In general, foreign bodies are more likely to become lodged in the right main bronchus than the left: the left bronchus diverges more sharply at the tracheal bifurcation, while the right bronchus is relatively straight.
The conducting portion of the bronchial tree extends from the tracheal bifurcation to the terminal bronchiole, inclusive. The respiratory portion consists of the respiratory bronchiole, alveolar ducts, alveolar sacs, and alveoli.
Fig. 8.13 Bronchial tree
The most common cause of respiratory compromise at the bronchial level is asthma. Compromise at the alveolar level may result from increased diffusion distance, decreased aeration (emphysema), or fluid infiltration (e.g., pneumonia).
Gaseous exchange takes place between the alveolar and capillary lumens in the alveoli (see Fig. 8.13C). At these sites, the basement membranes of capillary endothelial cells are fused with those of type I alveolar epithelial cells, lowering the exchange distance to 0.5 μm. Diseases that increase this diffusion distance (e.g., edematous fluid collection or inflammation) result in compromised respiration.
Condition of alveoli: In diseases like emphysema, which occurs in chronic obstructive pulmonary disease (COPD), alveoli are destroyed or damaged. This reduces the surface area available for gaseous exchange.
Production of surfactant: Surfactant is a protein-phospholipid film that lowers the surface tension of the alveoli, making it easier for the lung to expand. The immature lungs of a preterm infant often fail to produce sufficient surfactant, leading to respiratory problems. Surfactant is produced and absorbed by alveolar epithelial cells (pneumocytes). Type I alveolar epithelial cells absorb surfactant; type II produce and distribute it.
The mechanics of respiration are based on a rhythmic increase and decrease in thoracic volume, with an associated expansion and contraction of the lungs. Inspiration (red): Contraction of the diaphragm leaflets lowers the diaphragm into the inspiratory position, increasing the volume of the pleural cavity along the vertical axis. Contraction of the thoracic muscles (external intercostals with the scalene, intercartilaginous, and posterior serratus muscles) elevates the ribs, expanding the pleural cavity along the sagittal and transverse axes (Fig. 8.15A,B). Surface tension in the pleural space causes the visceral and parietal pleura to adhere; thus, changes in thoracic volume alter the volume of the lungs. This is particularly evident in the pleural recesses: at functional residual capacity (resting position between inspiration and expiration), the lung does not fully occupy the pleural cavity. As the pleural cavity expands, a negative intrapleural pressure is generated. The air pressure differential results in an influx of air (inspiration). Expiration (blue): During passive expiration, the muscles of the thoracic cage relax and the diaphragm returns to its expiratory position. Contraction of the lungs increases the pulmonary pressure and expels air from the lungs. For forcible expiration, the internal intercostal muscles (with the transverse thoracic and subcostal mucosa) can actively lower the rib cage more rapidly and to a greater extent than through passive elastic recoil.
Fig. 8.14 Respiratory changes in thoracic volume
Inspiratory position (red); expiratory position (blue).
Fig. 8.15 Inspiration: Pleural cavity expansion
Fig. 8.16 Expiration: Pleural cavity contraction
Fig. 8.17 Respiratory changes in lung volume
Fig. 8.18 Inspiration: Lung expansion
Fig. 8.19 Expiration: Lung contraction
Fig. 8.20 Movements of the lung and bronchial tree
As the volume of the lung changes with the thoracic cavity, the entire bronchial tree moves within the lung. These structural movements are more pronounced in portions of the bronchial tree distant from the pulmonary hilum.
The pleural space is normally sealed from the outside environment. Injury to the parietal pleura, visceral pleura, or lung allows air to enter the pleural cavity (pneumothorax). The lung collapses due to its inherent elasticity, and the patient's ability to breathe is compromised. The uninjured lung continues to function under normal pressure variations, resulting in “mediastinal flutter”: the mediastinum shifts toward the normal side during inspiration and returns to the midline during expiration. Tension (valve) pneumothorax occurs when traumatically detached and displaced tissure covers the defect in the thoracic wall from the inside. This mobile flap allows air to enter, but not escape, the pleural cavity, causing a pressure buildup. The mediastinum shifts to the normal side, which may cause kinking of the great vessels and prevent the return of venous blood to the heart. Without treatment, tension pneumothorax is invariably fatal.
Pulmonary Arteries & Veins
The pulmonary trunk arises from the right ventricle and divides into a left and right pulmonary artery for each lung. The paired pulmonary veins open into the left atrium on each side. The pulmonary arteries accompany and follow the branching of the bronchial tree, whereas the pulmonary veins do not, being located at the margins of the pulmonary lobules.
Fig. 8.21 Pulmonary arteries and veins
Fig. 8.22 Pulmonary arteries
Fig. 8.23 Pulmonary veins
Potentially life-threatening pulmonary embolism occurs when blood clots migrate through the venous system and become lodged in one of the arteries supplying the lungs. Symptoms include dyspnea (difficulty breathing) and tachycardia (increased heart rate). Most pulmonary emboli originate from stagnant blood in the veins of the lower limb and pelvis (venous thromboemboli). Causes include immobilization, disordered blood coagulation, and trauma. Note: A thromboembolus is a thrombus (blood clot) that has migrated (embolised).
Neurovasculature of the Tracheobronchial Tree
Fig. 8.24 Pulmonary vasculature
The pulmonary system is responsible for gaseous exchange within the lung. Pulmonary arteries (shown in blue) carry deoxygenated blood and follow the bronchial tree. The pulmonary vein (red) is the only vein in the body carrying oxygenated blood, which it receives from the alveolar capillaries at the periphery of the lobule.
Fig. 8.25 Arteries of the tracheobronchial tree
The bronchial tree receives its nutrients via the bronchial arteries, found in the adventitia of the airways. Typically, there are one to three bronchial arteries arising directly from the aorta. Origin from a posterior intercostal artery may also occur.
Fig. 8.26 Veins of the tracheobronchial tree
Fig. 8.27 Autonomic innervation of the tracheobronchial tree
Sympathetic innervation (red); parasympathetic innervation (blue).
Lymphatics of the Pleural Cavity
The lungs and bronchi are drained by two lymphatic drainage systems. The peribronchial network follows the bronchial tree, draining lymph from the bronchi and most of the lungs. The subpleural network collects lymph from the peripheral lung and visceral pleura.
Fig. 8.28 Lymphatic drainage of the pleural cavity
Transverse section, inferior view.
Fig. 8.29 Lymph nodes of the pleural cavity
Anterior view of pulmonary nodes.