Chapter 3. Lungs

Big Picture

The lungs exchange oxygen and carbon dioxide and are the functional organs of the respiratory system. To serve that vital function, the lungs are located adjacent to the heart within the pleural sacs. The pleurae are serous membranes that line the internal surface of the thoracic cage and the outside surface of the lungs. The plurae secrete fluid that decreases resistance against lung movment during breathing.

Description of Pleural Sacs

Each lung (right and left) is contained within a serous membrane called the pleural sac (Figure 3-1A). The right and left pleural sacs occupy most of the thoracic cavity and flank both sides of the heart. Each pleural sac is composed of two layers of serous (secretory) membrane, the parietal pleura and the visceral pleura (Figure 3-1B).

• Parietal pleura. The external serous membrane lining the internal surface (wall) of the thoracic cavity.
• Visceral pleura. The internal serous membrane intimately attached to the surface of each lung.
• Pleural fluid. A layer of fluid located between the parietal and visceral pleurae in what is called the pleural space.

Regions of the Parietal Pleura

As previously described, the parietal pleura lines the internal surface (wall) of the thoracic cavity, including the lateral sides of the mediastinum (Figure 3-1C and D). The parietal pleura is separated from the thoracic wall by the endothoracic fascia, a thin layer of connective tissue located between the parietal pleura and the innermost intercostal muscles and membrane. The parietal pleura is assigned specific names, depending on the structures that it lines.

• Mediastinal parietal pleura. Lines the lateral surface of the mediastinum (the space between the lungs where the heart is located).
• Costal parietal pleura. Lines the internal surface of the ribs.
• Diaphragmatic parietal pleura. Lines the superior surface of the diaphragm.
• Cervical parietal pleura (cupula). Extends above rib 1 to the root of the neck.

Innervation and Vascular Supply of the Parietal Pleura

Intercostal nerves supply the costal parietal pleura and the peripheral portion of the diaphragmatic parietal pleura. Phrenic nerves supply the central portion of the diaphragmatic parietal pleura and the mediastinal parietal pleura. The parietal pleura is innervated by general sensory neurons, and therefore, it is sensitive to pain.

The parietal pleura receives its vascular supply via branches of the internal thoracic, superior phrenic, posterior intercostal, and superior intercostal arteries.

Visceral Pleura

The visceral pleura is intimately attached to each lung and follows the contour of the lobes of the lung (Figure 3-1C and D). The visceral pleura is contiguous with the parietal pleura at the site where bronchi, vessels, nerves, and lymphatics pass from the mediastinum into the lung (the root, or hilum, of the lungs). In contrast to the parietal pleura, the visceral pleura is insensitive to pain because its visceral sensory neurons originate from the autonomic vagus nerve (CN X). The visceral pleura receives its blood supply via the bronchial arteries, whereas its venous drainage is via the pulmonary veins.

Pleural Space

Each pleural cavity has a pleural space. The pleural space is located between the parietal and visceral pleurae and is a closed space, which means that there is no communication between the right and left pleural spaces. The pleural space contains a thin film of pleural fluid that lubricates the surface of the pleurae and facilitates the movement of the lungs across the thoracic wall and diaphragm during inspiration and expiration.

During expiration, air flows out of the lungs, causing the internal pressure of the lungs to decrease and the lung tissue to potentially collapse. However, the thin layer of pleural fluid, working in conjunction with surfactant in the alveoli, keeps the lungs from collapsing by keeping the visceral pleura coupled to the parietal pleura and the alveoli open, respectively. As a result, the lungs remain inflated, even at the end of a deep expiration.

Significance of the Pleural Reflections and Recesses

In quite respiration, the lung, with its covering of visceral pleura, does not fill the entire pleural sac. The locations where the lung does not completely fill the pleural sac are called pleural recesses. There are two clinically important recesses for the right and left lungs that are termed the same as the reflection next to it (Figure 3-1C and D):

• Costodiaphragmatic recess. The recess where the costal parietal pleura meets the diaphragmatic parietal pleura. This recess is located at the inferior limit of the pleural sac.
• Costomediastinal recess. The recess where the costal parietal pleura meets the mediastinal parietal pleura anteriorly, near the midline.

The pleural recesses are sites where pleural fluid accumulates during quiet breathing. When a deep breath is taken, the greatly expanded lungs push into the recesses, allowing lung volume to increase and, consequently, the pleural fluid becomes displaced around each lung.

Big Picture

The lungs are the organs of gas exchange. To serve that function efficiently, the lungs are attached to the trachea and the heart. The pulmonary circulation supplies the respiratory tissues with deoxygenated blood, whereas the bronchial vessels nourish the nonrespiratory tissues with oxygenated blood. Parasympathetic innervation to the lung causes bronchoconstriction, whereas sympathetic innervation causes with oxygenated blood.

Trachea and Bronchi

The trachea, or windpipe, courses from the larynx into the thorax, where the trachea bifurcates into the right and left primary (principal or mainstem) bronchi at the T4–T5 vertebral level (Figure 3-2B). The right primary bronchus divides into superior, middle, and inferior secondary (lobar) bronchi, corresponding to superior, middle, and inferior lobes of the right lung, respectively. The left primary bronchus divides into superior and inferior secondary bronchi, corresponding to superior and inferior lobes of the left lung, respectively. Each secondary bronchus further divides into tertiary (segmental) bronchi, which further divide. The smallest bronchi give rise to bronchioles, which terminate in alveolar sacs where the exchange of gases occurs.

Regions of the Lung

Each lung has the following regions corresponding to its respective regions of the chest:

• Costal lung surface. The large convex area related to the inner surface of the ribs.
• Mediastinal lung surface. The medial concave surface containing the root (hilum) of the lungs related to the heart. This surface contains a mainstem bronchus and a pulmonary artery and pulmonary veins, nerves, and lymphatics.
• Diaphragmatic lung surface. The base of the lungs, which are convex because they rest on the domed diaphragm.
• Apex of the lung. This area projects into the root of the neck and is crossed anteriorly by the subclavian artery and vein.

Lobes of the Lung

Each lung is divided into lobes with its own covering of visceral pleura.

• The right lung has three lobes (superior, middle, and inferior), which are divided by a horizontal and an oblique fissure (Figure 3-2A). The right lung is shorter and wider than the left because of the higher right dome of the diaphragm and because the heart bulges more into the left side of the thorax.
• The left lung has only two lobes (superior and inferior), which are divided by an oblique fissure along the sixth rib (Figure 3-2C). Instead of having a middle lobe, the left lung has a space occupied by the heart. Therefore, the left lung has a cardiac notch as well as the lingula, an extension of the left superior lobe into the left costomediastinal recess.

Big Picture

For gas exchange to occur, the lung must be connected to the heart so that oxygenated blood and deoxygenated blood flow between both organs. The location where blood vessels and other structures enter and leave the lungs is called the hilum of the lung.

Vasculature of the Lung

The pulmonary and bronchial arteries and veins provide dual vascular supply. These two supplies can be confusing. In a nutshell, here is the information you need to know about the vascular supply: Pulmonary arteries and veins deal with gas exchange and the circulation of blood between the heart and lungs, whereas the bronchial arteries and veins are the vascular supply to the structural elements of the lungs, such as the bronchial tree.

• Pulmonary arteries. Branch from the pulmonary trunk, which receives blood from the right ventricle of the heart (Figure 3-3A). Pulmonary arteries deliver deoxygenated blood from the systemic circulation to exchange carbon dioxide with oxygen in the lungs. At the bifurcation of the pulmonary trunk into the pulmonary arteries, there is a connection to the aortic arch via the ligamentum arteriosum, the fibrous remnant of the fetal ductus arteriosus.
• Pulmonary veins. Transport oxygenated blood from the pulmonary capillaries to the left atrium of the heart. They do not accompany the bronchi or the segmental arteries within the lung parenchyma. Two pulmonary veins exit the left lung and three pulmonary veins exit the right lung (one for each lobe), but the right upper and middle veins usually join so that usually only four pulmonary veins enter the left atrium.
• Bronchial arteries. Branch from the thoracic (descending) aorta and supply the bronchial tree. There is usually one bronchial artery for the right lung and two for the left lung.
• Bronchial veins. Drain the bronchi. The bronchial veins receive blood from the larger subdivisions of the bronchi and empty into the azygos vein on the right and the accessory hemiazygos vein on the left. Some bronchial veins become tributaries to the pulmonary veins.

Lymphatics of the Lung

Lymph from lobes of the lungs drains into pulmonary and bronchopulmonary (hilar) nodes and then into the tracheobronchial (carinal) nodes and into the paratracheal nodes en route to drain into either the right lymphatic duct (for the right lung) or the thoracic duct (for the left lung). It should be noted that no lymphatics are present in the walls of the alveolar sacs.

Innervation of the Lung

The pulmonary plexus follows the trachea and bronchial tree, providing parasympathetic and sympathetic innervation to the smooth muscle and glands of the lungs (Figure 3-3B). The pulmonary plexus is divided and named according to its position to the root of the lung, where the anterior pulmonary plexus lies anterior and the posterior pulmonary plexus lies posterior to the corresponding bronchus. Branches of the pulmonary plexus accompany the blood vessels and bronchi into the lung as well.

• Parasympathetic innervation. The pulmonary plexus receives preganglionic parasympathetic and visceral sensory innervation via the vagus nerves. The vagus nerves are the tenth pair of cranial nerves and have the widest field of distribution to the body. They innervate structures in the head, neck, thorax, and abdomen. In the thorax, the vagus nerves provide all the parasympathetic innervation of the viscera. Parasympathetic innervation causes bronchoconstriction of the smooth muscle of the bronchial tree, vasodilation of the pulmonary vessels, and secretion from bronchial glands.
• Sympathetic innervation. Postganglionic sympathetic fibers from the T1 to T4 levels of the sympathetic trunk and cervical sympathetic ganglia contribute to the pulmonary plexus. Sympathetic innervation causes bronchodilation, vasoconstriction of the pulmonary vessels, and inhibition of secretion from the bronchial glands. Visceral sensory fibers from the visceral pleura and bronchi may also accompany sympathetic fibers.

Bronchopulmonary Segments

A bronchopulmonary segment consists of a lobar (segmental or tertiary) bronchus, a corresponding branch of the pulmonary artery, and the supplied segment of the lung tissue, all surrounded by a connective tissue septum. A bronchopulmonary segment refers to the portion of the lung supplied by each segmental bronchus and segmental artery. The pulmonary veins lie between bronchopulmonary segments.

Big Picture

Respiration is the vital exchange of oxygen and carbon dioxide as blood circulates through the lungs. The thoracic skeleton, thoracic wall muscles, bronchial tree, and pulmonary circulation all aid in this process. Breathing is a mechanical process resulting from volume changes in the thoracic cavity with inverse changes in pleural pressure. Pressure changes lead to gas flow.

Inspiration

The process of inspiration (inhalation) is easily understood if you visualize the thoracic cavity as a closed box with a single opening at the top called the trachea. The trachea allows air to move in and out of the thoracic cavity (“the box”). The volume of the thoracic cavity is changeable and can be increased by enlarging all its diameters (superior to inferior, anterior to posterior, and medial to lateral), thereby decreasing the pressure in the pleural space (pleural pressure) (Figure 3-4A and B). This, in turn, causes air to rush from the atmosphere (positive pressure relative to the lungs) into the lungs (negative pressure relative to the atmosphere) because gas flows down its pressure gradient. The muscles that primarily expand the thoracic cavity during inspiration are the diaphragm and the intercostal muscles.

• Diaphragm. In the relaxed state, the diaphragm is dome shaped. When the diaphragm contracts, it flattens, increasing the vertical dimensions and thus the volume of the thoracic cavity.
• Intercostal muscles. Contraction of the external intercostal muscles lifts the rib cage and pulls the sternum anteriorly. Because the ribs curve downward as well as forward around the chest wall, the broadest lateral and anteroposterior dimensions of the rib cage are normally directed downward. However, when the ribs are raised and drawn together, they also swing outward, expanding the diameter of the thorax both laterally and in the anteroposterior plane. This is similar to the action that occurs when a curved bucket handle is raised away from the bucket (Figure 3-4C).

Although these actions expand the thoracic dimensions by only a few millimeters along each plane, this expansion is sufficient to increase the volume of the thoracic cavity by approximately 0.5 L, the approximate volume of air that enters the lungs during normal inhalation. The diaphragm is by far the most important structure that brings about the pressure, gas flow, and volume changes that lead to normal inhalation.

As the thoracic dimensions increase during inspiration, pleural pressure becomes more negative and “pulls” on the lungs as thoracic volume increases. The consequence is that the lungs expand (fill with gas) and intrapulmonary volume increases. Inspiration ends when thoracic volume ceases to increase, resulting in no further reduction in pleural pressure. Gas flow ceases and thus lung volume does not change.

During the deep or forced inspirations that occur during vigorous exercise, the volume of the thoracic cavity is further increased by activation of the accessory muscles. Accessory respiratory muscles, including the scalenes, sternocleidomastoid, and pectoralis minor, elevate the ribs more than occurs during quiet inspiration.

Expiration

Quiet expiration (exhalation) is largely a passive process that depends more on the natural elasticity of the thoracic wall and lungs than on muscle contraction. In contrast, forced expiration is an active process (Figure 3-4A–C).

• Quiet expiration. As the inspiratory muscles relax, the diaphragm ascends, the rib cage descends, and the stretched elastic tissue of the lungs recoils. Thus, both thoracic and lung volumes decrease. Decreased lung volume compresses the alveoli, resulting in increases above atmospheric pressure, thereby forcing gas flow out of the lungs. For example, when the diaphragm relaxes, it passively moves superiorly. Consequently, the vertical dimension of the thorax is decreased and thus the volume of the thoracic cavity decreases.
• Forced expiration. When the expiratory muscles (e.g., the external and internal oblique and transverse and rectus abdominis) contract, they increase intra-abdominal pressure. This forces the abdominal organs superiorly against the diaphragm, raising it. The same muscles depress the rib cage. Both actions forcibly reduce the volume in the thoracic cavity, increasing pleural volume and thence pressure in the lungs, forcing air to move from the lungs and out of the trachea.