23.4 Lungs.pdf

23.4 Lungs The paired lungs house both the bronchi and bronchioles of the conducting zone and the structures of the respiratory zone of the respiratory tract. Here we examine the position of the lungs in the thoracic cavity and their anatomic structure. We also discuss the blood supply and innervati...

23.4 Lungs The paired lungs house both the bronchi and bronchioles of the conducting zone and the structures of the respiratory zone of the respiratory tract. Here we examine the position of the lungs in the thoracic cavity and their anatomic structure. We also discuss the blood supply and innervation of the lungs, pleural membranes, and how lung inflation is maintained. 23.4a Gross Anatomy of the Lung LEARNING OBJECTIVES 16. Identify and describe the general structure of the lungs. 17. Compare and contrast the right versus left lung. The paired lungs are located within the thoracic cavity on either side of the mediastinum, the median region that houses the heart (see sections 1.5e and 19.2a). The lungs are enclosed and protected by the thoracic cage, which is the bony framework of the chest and is described in section 8.6 ( figure 23.13). The chest wall contains elastic tissue. This elastic tissue (and the elastic tissue that surrounds alveoli; see figure 23.11a) is essential in distending and recoiling during normal breathing (as described later). Figure 23.13 Position of the Lungs. Within the thoracic cavity, the lungs are bordered and protected by the thoracic cage. They are lateral to the mediastinum. The base of each lung rests on the diaphragm, and its apex is slightly superior and posterior to the clavicle. APR Module 11: Respiratory: Dissection: Lower Respiratory: Anterior: Right lung Each lung has a wide, concave base that rests inferiorly upon the muscular diaphragm and an apex (or cupula) that is slightly superior and posterior to the clavicle. The lung surfaces are adjacent to the ribs, mediastinum, and diaphragm and are respectively referred to as the costal surface, mediastinal surface, and diaphragmatic surface of the lungs. Each lung has a conical shape with an indented region on its mediastinal surface called the hilum, through which pass the bronchi, pulmonary vessels, lymph vessels, and autonomic nerves ( figure 23.14). Collectively, these structures that extend from the hilum are termed the root of the lung. Page 909 Figure 23.14 Lungs. The lungs are composed of lobes separated by distinct depressions called fissures. (a) Lateral views show the three lobes of the right lung and the two lobes of the left lung. (b) Medial views show the hilum of each lung, where the pulmonary vessels and bronchi extend as part of the root of the lung. APR Module 11: Respiratory: Dissection: Lower Respiratory: Anterior: Oblique fissure of right lung The right and left lungs exhibit some obvious structural differences. The right lung is larger and wider than the left lung, and is subdivided by two fissures into three lobes. The horizontal fissure separates the superior (upper) lobe from the middle lobe, whereas the oblique fissure separates the middle lobe from the inferior (lower) lobe. In contrast, the left lung—which is slightly smaller than the right lung because the heart projects into the left side of the thoracic cavity—has only two lobes. The left lung has an oblique fissure that separates the superior lobe from the inferior lobe. The lingula (= small tongue) is a tongue-shaped projection from the superior lobe of the left lung that is homologous to the middle lobe of the right lung. The left lung also has two surface indentations to accommodate the heart: the cardiac impression on its medial surface and a cardiac notch on its anterior surface. Page 910 Each lung is partitioned into bronchopulmonary (brong′kō-pul′mō-nār′ē) segments; there are 10 segments in the right lung and typically 8 to 10 in the left lung ( figure 23.15). (The discrepancy in segment number for the left lung comes from the merging of some left lung segments into combined ones that occurs during development.) Each bronchopulmonary segment is an autonomous unit, encapsulated within connective tissue and supplied by its own segmental bronchus, a branch of both the pulmonary artery and vein, and lymph vessels. Consequently, if a portion of a lung is diseased, a surgeon can remove the entire bronchopulmonary segment that is affected, and the remaining healthy segments continue to function. Figure 23.15 Bronchopulmonary Segments and Lobules of the Lungs. Both lungs are partitioned into self-contained bronchopulmonary segments (represented by different colors), each supplied by a segmental bronchus. Lobules are visible in each bronchopulmonary segment, each supplied by a terminal bronchiole. (Note: The medial bronchopulmonary segment is not visible on the left lung.) Within each segment, the lung is organized into marble-sized lobules. Each lobule is surrounded by connective tissue and supplied by a terminal bronchiole, an arteriole, a venule, and a lymph vessel. WHAT DID YOU LEARN? 16 Match the component of the lung with its air passageway. main bronchus bronchopulmonary segment lobar bronchus lobe of lung segmental bronchus lobule of lung terminal bronchiole lung (right and left) 23.4b Circulation to and Innervation of the Lungs LEARNING OBJECTIVES 18. Compare and contrast the two types of blood circulation through the lungs. 19. Describe the innervation of the lungs by the two divisions of the autonomic nervous system. The circulation of blood to and from the lungs, lymph circulation from the lungs, and innervation of the lungs by the autonomic nervous system are described here. Blood Supply Two types of blood circulation are associated with the lungs: the pulmonary circulation and the bronchial circulation. Recall from our examination of the heart and blood vessels that the pulmonary circulation ( figure 23.16) transports blood to and from the lungs to pick up oxygen and get rid of excess carbon dioxide (see section 20.8a). Pulmonary arteries carry deoxygenated blood to pulmonary capillaries within the lungs. The deoxygenated blood that enters these capillaries is reoxygenated here before it returns through a series of pulmonary venules and veins to the left atrium (see figure 19.3a). Figure 23.16 Pulmonary Circulation of the Lungs. Pulmonary circulation delivers blood to the lungs for reoxygenation and removal of carbon dioxide. Blood flow through the pulmonary circulation is indicated by numbers 1 through 5. In comparison, the bronchial circulation is a component of the systemic circulation and transports oxygenated blood to the tissues of the lungs (see figure 20.23). It consists of both small bronchial arteries and veins that supply the walls of bronchi and bronchioles. The cells of the smallest respiratory structures (such as alveoli and alveolar ducts) exchange their respiratory gases directly with the inhaled air. Approximately three or four bronchial arteries branch from the anterior wall of the descending thoracic aorta (or its branches) (see section 20.10c). Thereafter, bronchial arteries divide to form capillary beds supplying structures in the bronchial tree. Bronchial veins collect venous blood from these capillary beds. Some of this deoxygenated blood drains into pulmonary veins, where it mixes with the freshly oxygenated blood. Consequently, blood exiting the lungs within the pulmonary veins, which will be returned to the left side of the heart and circulated throughout the body, is slightly less oxygenated than the blood immediately leaving the pulmonary capillaries following gas exchange. Lymph Drainage Lymph vessels and lymph nodes are located within the connective tissue of the lung, around the bronchi, and in the pleura. Lymph vessels are important in removing excess fluid from the lungs. Lymph, absorbed by lymph vessels, is filtered through lymph nodes, which collect carbon, dust particles, and pollutants that were not “swept out” by cilia lining the respiratory tract. Lymph nodes may become dark in color as they accumulate the microscopic matter we inhale over a lifetime. Page 911 INTEGRATE CLINICAL VIEW 23.9 Smoking and Lung Cancer Watch Video: Smoking risks Smoking results in the inhalation of over 200 chemicals into the respiratory passageways of the lungs. These chemicals blacken the respiratory passageways and cause respiratory changes that increase the risk of (a) respiratory infections, including the common cold, influenza, pneumonia, and tuberculosis, (b) cellular and genetic damage to the lungs that may lead to emphysema or lung cancer, and (c) a more severe and fatal outcome from a COVID-19 infection. Lung cancer is a highly aggressive and frequently fatal malignancy that originates in the epithelium of the respiratory system. It claims over 150,000 lives annually in the United States. Smoking causes about 85% of all lung cancers. Symptoms include chronic cough, coughing up blood, excess pulmonary mucus, and increased likelihood of pulmonary infections. The deleterious effects of smoking are not limited to the respiratory system. Nicotine causes vasoconstriction in the cardiovascular system, carbon monoxide interferes with oxygen binding to hemoglobin, and the risk and severity of atherosclerosis are increased. These changes decrease blood flow, thus resulting in decreased delivery of nutrients and oxygen to cells of systemic tissues. Women who smoke during pregnancy typically have babies with lower birth weight. This condition occurs in part because the umbilical arteries vasoconstrict, decreasing blood flow to the placenta. Smoking increases the risk of both stomach ulcers caused by Helicobacter pylori infection and cancer of the esophagus, stomach, and pancreas. In the reproductive system, smoking increases the risks associated with human papillomavirus (HPV) infection. (HPV is linked with increased risk of cervical cancer.) Smoking also increases the risk of Alzheimer disease. Studies indicate an association between secondhand smoke exposure and increased risk of bronchitis, asthma, and ear infections in children. New evidence also shows that thirdhand smoke, the toxins that are present in clothes, furniture, carpet, and other materials even after the cigarette is extinguished, poses a health risk—especially to infants and young children. (a) ©Stefan Zaklin/EPA/Newscom; (a [inset]) ©McGrawHill Education/Al Telser; (b) ©Jonathan Hordle/REX/Shutterstock; (b [inset]) ©Mike Peres RBP SPAS/CustomMedical Page 912 Innervation of the Respiratory System The smooth muscle and glands of the larynx, trachea, bronchial tree, and lungs are innervated by the lower motor neurons of the autonomic nervous system (see controlled by autonomic nuclei within the brainstem (see section 15.1a) and section 15.1c). The bronchioles are dually innervated (see section 15.7b) and, thus, are regulated by both the sympathetic and parasympathetic divisions. Sympathetic innervation to structures within the lungs originates generally from the T1–T5 segments of the spinal cord (see section 15.4a). Stimulation of the bronchioles by the sympathetic division relaxes smooth muscle within the bronchial walls, causing bronchodilation, which increases air flow (see section 23.3b). Parasympathetic innervation to structures within the lungs is the vagus nerves (CN X; see section 15.3a). The vagus nerves stimulate smooth muscle contraction within the bronchial walls, causing bronchoconstriction, which decreases air flow (see section 23.3b). The vagus nerves are also the primary source of innervation to the larynx. Thus, damage to one of the vagus nerves going to the larynx can cause a person to develop a monotone or a permanently hoarse voice. WHAT DID YOU LEARN? 17 Which arteries deliver oxygenated blood to tissues of the lungs, and which veins drain deoxygenated blood from the lungs? What vessels receive some of this deoxygenated blood? 23.4c Pleural Membranes and Pleural Cavity LEARNING OBJECTIVE 20. Identify and describe the pleural membranes and the function of serous fluid within the pleural cavity. The outer lung surfaces and the adjacent internal thoracic wall are lined by a serous membrane called pleura (plūr′ă) (see sections 1.5e and 5.5b). It is composed of a simple squamous epithelium and a thin layer of areolar connective tissue. The visceral pleura tightly adheres to the lung surface, whereas the parietal pleura lines the internal thoracic walls, the lateral surfaces of the mediastinum, and the superior surface of the diaphragm ( figure 23.17). The parietal pleura meets the visceral pleura at the hilum of each lung (see figure 1.9c). Each lung is enclosed in a separate visceral pleural membrane, and the heart in a visceral pericardial membrane (see section 19.2b); thus, these organs are compartmentalized, which helps limit spread of infections. Figure 23.17 Pleural Membranes and Pressures Associated with the Lungs. The two pleural membranes include both the visceral pleura, which covers the outer surface of the lungs, and the parietal pleura, which lines the inner surface of the thoracic wall. Between the two pleural membranes is a potential space called the pleural cavity. Two pressures associated with the lungs are the intrapulmonary pressure (the pressure within the lungs) and the intrapleural pressure (pressure within the pleural cavity). The pleural cavity is located between the visceral and parietal serous membrane layers. When the lungs are fully inflated, the pleural cavity is considered a potential space because the visceral and parietal pleurae are almost in contact with each other. An oily serous fluid is produced by the serous membranes and released into the pleural cavity. Serous fluid acts as a lubricant, ensuring that the pleural surfaces slide by each other with minimal friction during breathing. Each pleural cavity normally contains less than 15 mL of serous fluid and is drained continuously by lymph vessels within the visceral pleura. A balance normally exists between formation and removal of serous fluid in the pleural cavity. WHAT DID YOU LEARN? 13 What is the function of serous fluid within the pleural cavity? INTEGRATE CLINICAL VIEW 23.10 Pleurisy and Pleural Effusion Pleurisy (plūr΄i-sē) is inflammation of the pleura. Usually, only one lung is affected because the lungs are located within distinct compartments. The inflamed pleura increases the friction between the visceral and parietal layers, causing the pleural layers to rub against one another. Patients with this condition report severe chest pain associated with breathing. Pleural effusion is excess fluid in the pleural cavity, which occurs when the capacity of the lymph vessels to remove fluid from the pleural cavity is exceeded by its formation. This accumulation may be caused by (a) systemic factors, such as failure of the left side of the heart, pulmonary embolism, or cirrhosis of the liver; (b) viral or bacterial infections of the lung; or (c) lung cancer that triggers the inflammatory response of the immune system within the lungs. Page 913 23.4d How Lungs Remain Inflated LEARNING OBJECTIVE 21. Explain the properties that keep lungs inflated. Lung inflation occurs due to the expanding properties of the chest wall, the recoiling elastic properties of the lungs, and the anatomic arrangement of the pleural cavity between the chest wall and lungs. The chest wall is anatomically configured to expand outwardly. This is readily observed when the thoracic cage is opened surgically, because cutting through the chest wall causes it to “spring” open. The lungs cling to the internal surface of the chest wall as it expands outward because of the surface tension caused by the serous fluid within the pleural cavity. However, the lungs are composed of vast amounts of elastic connective tissue that is “stretched” as the lungs expand. The natural tendency of elastic connective tissue is to recoil, which causes the lungs to exhibit a noticeable inward pull. The contrasting outward pull of the chest wall and the opposing inward pull of the lungs results in a vacuum, or “suction,” within the pleural cavity. Consequently, the pressure generated in the pleural cavity, called the intrapleural pressure, is lower than the pressure inside the lungs, called the intrapulmonary (or intra-alveolar) pressure ( figure 23.17). This difference in pressure keeps the lungs inflated. The lungs remain inflated similar to the way in which a balloon remains inflated—namely, the pressure inside is greater than the pressure outside. Note that when intrapleural pressure becomes equal to intrapulmonary pressure, the lungs deflate, like the deflation of a balloon when the stem is released. WHAT DID YOU LEARN? 19 Why is the intrapleural pressure normally lower than intrapulmonary pressure? What is the function of this difference in pressure? INTEGRATE CLINICAL VIEW 23.11 Pneumothorax and Atelectasis Pneumothorax (nū΄mō-thōr΄aks; pneuma = air) occurs when air gets into the pleural cavity. Pneumothorax may develop in one of two ways. Air may be introduced externally from a penetrating injury to the chest, such as a knife wound or gunshot, or it may originate internally when either a broken rib lacerates the surface of the lung or an alveolus ruptures. Lung expansion is dependent upon intrapleural pressure being lower than the intrapulmonary pressure. However, pneumothorax sometimes causes intrapleural and intrapulmonary pressures to become equal, so the lungs are released from the “outward pull” of the chest wall and collapse, or deflate. A collapsed lung is termed atelectasis (at-ĕ-lek΄tă-sis; ateles = incomplete, ektasis = extension). The collapsed portion of the lung remains down until the air has been removed from the pleural space. If the amount of air is small, it exits naturally within a few days. However, a large introduction of air is a medical emergency and requires insertion of a tube into the pleural space to suction out the air. INTEGRATE CONCEPT CONNECTION Cellular respiration is the metabolic pathway whereby glucose and other fuel molecules, such as fatty acids, are oxidized and their chemical energy is transferred to form adenosine triphosphate (ATP), as described in section 3.4. The carbon atoms of the oxidized fuel molecules are released as carbon dioxide, a waste product during the intermediate stage and citric acid cycle (see sections 3.4c and 3.4d). Oxygen functions as the final electron acceptor in the electron transport chain, then joins with hydrogen ions to form water as the final step in the electron transport chain (see section 3.4e). The respiratory system and cardiovascular system support the process of cellular respiration by delivering the oxygen and removing the carbon dioxide waste.