Lower Respiratory Tract - PDF

23.3 Lower Respiratory Tract The structures of the lower respiratory tract include both portions of the conducting zone (the trachea, bronchi, and bronchioles, including terminal bronchioles) and the components of the respiratory zone (respiratory bronchioles, alveolar ducts, and alveoli) (see figur...

23.3 Lower Respiratory Tract The structures of the lower respiratory tract include both portions of the conducting zone (the trachea, bronchi, and bronchioles, including terminal bronchioles) and the components of the respiratory zone (respiratory bronchioles, alveolar ducts, and alveoli) (see figure 23.1). 23.3a Trachea LEARNING OBJECTIVE 10. Identify and describe the structure and function of the trachea. The trachea (trā′kē-ă; rough), which is commonly referred to as the windpipe, is a patent (open) tube that connects the larynx and the two main bronchi as it extends through the neck and into the mediastinum of the thoracic cavity, where it is partially protected by the sternum. (Its anatomic position relative to these structures is shown in figure 23.9.) The position of the trachea relative to the esophagus (the muscular tube of the gastrointestinal tract that leads from the mouth to the stomach) can be seen in figure 23.8a. Notice that the trachea is anterior to the esophagus. Here, we discuss both the gross anatomy of the trachea and the histology of the tracheal wall. Figure 23.8 Trachea. (a) The trachea connects to the larynx superiorly and to the main bronchi inferiorly and is located anterior to the esophagus. (b) A cross-sectional photomicrograph shows the histology of the trachea and its relationship to the esophagus. (c) A photo of the trachea leading into the main bronchi. Located internally at this split is the carina, which houses sensory receptors that when stimulated by irritants induce a cough. (d) The inner surface of the tracheal wall, showing the upward movement of mucus toward the pharynx. (b) ©Lester V. Bergman/Corbis; (c) ©SPL/Science Source APR Module 11: Respiratory: Dissection: Trachea: Anterior Gross Anatomy of the Trachea The trachea is a flexible, slightly rigid, tubular organ that averages approximately 13 centimeters (5.1 inches) in length and 2.5 centimeters (1 inch) in diameter. The anterior and lateral walls of the trachea are supported by 15 to 20 C-shaped rings of hyaline cartilage called tracheal cartilages. The tracheal cartilages are connected superiorly and inferiorly with one another by elastic connective tissue sheets called anular (an′ū-lăr; anulus = ring) ligaments. Each C-shaped tracheal cartilage is ensheathed in a perichondrium (see section 7.2f ) and a dense, fibrous membrane (not shown in figure 23.8). The open ends of the cartilage rings are positioned posteriorly (adjacent to the esophagus) and are connected to each other by both the trachealis muscle and an elastic ligamentous membrane ( figure 23.8b). The Cshaped cartilage portion of each of these rings reinforces and provides structural support to the tracheal wall to ensure that the trachea remains open (patent) at all times. The more flexible trachealis muscle and ligamentous membrane on the posterior aspect of the trachea allow for distension during swallowing of food through the esophagus. The trachealis contracts during coughing to reduce the diameter of the trachea, thus facilitating the more rapid expulsion of air, helping to dislodge material (foreign objects or food) from the air passageway. An internal ridge of mucosa-covered cartilage called the carina (kă-rı̄ ′nă) is located at the split of the trachea into the main bronchi ( figure 23.8c). The carina has sensory receptors that are extremely sensitive and can induce a forceful cough when stimulated by irritants. Histology of the Tracheal Wall The innermost to outermost layers that form the wall of the trachea are (a) the mucosa, which is composed of a pseudostratified ciliated columnar epithelium with goblet cells and a lamina propria ( figure 23.8b, d); (b) the submucosa, consisting of areolar connective tissue that houses blood vessels, nerve endings, serous and mucous glands, and lymphatic tissue; (c) tracheal cartilage (described earlier); and (4) the adventitia, composed of elastic connective tissue ( figure 23.8b). The movement of cilia in the mucosal epithelium propels mucus laden with dust, microbes, and other particles superiorly toward the larynx and pharynx, where it may be swallowed or expelled ( figure 23.8d). WHAT DO YOU THINK? 2 The lining of the trachea and bronchi in chronic smokers changes (through metaplasia; see section 5.6b) from pseudostratified ciliated columnar epithelium to stratified squamous epithelium. Why do you think this change occurs, and what are some consequences of this change? Page 902 WHAT DID YOU LEARN? 10 What is the function of the C-shaped tracheal cartilages? How do the trachealis muscle and elastic ligamentous membrane that complete each ring posteriorly function? INTEGRATE CLINICAL VIEW 23.5 Tracheotomy and Cricothyrotomy Tracheotomy (trā-kē-ot΄ō-mē; tome = incision) involves making an incision into the trachea to facilitate breathing when a patient’s airway is blocked or respiratory ventilation is compromised by disease or injury. A physician makes an incision first in the neck (1-1.5 cm superior to the suprasternal notch) and then in the trachea between the third and fourth tracheal rings. This opening (called a tracheostomy; trā-kē-os΄tō-mē) allows the insertion of a tracheotomy tube. Although a tracheotomy is a potentially lifesaving procedure, it is not without risks and should be undertaken only by trained medical personnel. Cricothyrotomy (krī′kō-thī-rot′ō-mē) is an alternative procedure often performed by EMTs and paramedics to open an individual’s airway during certain emergency situations where a patient cannot breathe on their own. A cricothyrotomy is generally performed by making a vertical incision between the cricoid cartilage and the thyroid cartilage. Once the incision is made, a tube is placed into this opening, which allows air exchange to occur. 23.3b Bronchial Tree LEARNING OBJECTIVES 11. Identify and describe the structural subdivisions of the bronchial tree. 12. Explain the processes of bronchoconstriction and bronchodilation. The bronchial (brong′kē-ăl) tree is a highly branched system of air-conducting passages that originates at the main bronchi and progressively branches into narrower tubes that diverge throughout the lungs before ending in the alveoli ( figures 23.9 and 23.10). Page 903 Figure 23.9 Bronchial Tree. The bronchial tree originates at the two main bronchi and ends at the alveoli. (a) Larynx, trachea, and bronchi are shown. (b) These major divisions of the bronchial tree are color-coded. (The bronchioles, alveolar ducts, and alveoli can be viewed in figure 23.10.) APR Module 11: Respiratory: Dissection: Lower Respiratory: Anterior: Segmental Bronchus and Branches Figure 23.10 Structure of the Bronchial Wall. Irregular cartilage plates of decreasing size support the branching bronchi. In contrast, bronchioles do not contain cartilage, but rather have a proportionately thicker layer of smooth muscle. This smooth muscle layer allows for bronchoconstriction and bronchodilation that change the size of the lumen, regulating the amount of air reaching the alveoli. Gross Anatomy of the Bronchial Tree The trachea splits at the level of the sternal angle (where the manubrium and body of the sternum articulate; see section 8.6a) into the right and left main bronchi (brong′kī; sing., bronchus) also known as primary bronchi. Each main bronchus projects inferiorly and laterally into a lung. The right main bronchus is shorter, wider, and more vertically oriented than the left main bronchus—thus, foreign particles are more likely to lodge in the right main bronchus. Both main bronchi, along with all associated pulmonary vessels, lymph vessels, and nerves, enter a lung on its medial surface (see figure 23.14b). Each main bronchus then branches into lobar bronchi (or secondary bronchi), which extend to each lobe of the lung. The right lung with three lobes has three lobar bronchi, and the left lung with two lobes has two lobar bronchi. Lobar bronchi are smaller in diameter than main bronchi. They further divide into segmental bronchi (or tertiary bronchi) that serve a division of the lung called a bronchopulmonary segment (described in section 23.4a). The right lung is supplied with 10 segmental bronchi, and the left lung is supplied by 8 to 10 segmental bronchi. The bronchial tree continues to divide into more numerous and smaller bronchi and then bronchioles. There are approximately 9 to 12 different levels, or generations, of bronchial branch divisions; the main, lobar, and segmental bronchi are the first, second, and third generations of bronchi, respectively. Page 904 INTEGRATE CLINICAL VIEW 23.6 Bronchitis Bronchitis (brong-kĪ΄tis) is inflammation of the bronchi and bronchioles caused by a viral or bacterial infection (see section 22.1), or by inhaling irritants such as vaporized chemicals, particulate matter, or cigarette smoke. Clinically, bronchitis is divided into two categories: acute and chronic. Acute bronchitis develops rapidly either during or after an infection, such as a cold. Symptoms include coughing, sneezing, pain upon inhalation, and fever. Most cases of acute bronchitis resolve completely within 10 to 14 days. Chronic bronchitis results from long-term exposure to irritants. Chronic bronchitis is defined medically as the production of large amounts of mucus, associated with a cough lasting 3 continuous months. If exposure to the irritant persists, permanent changes to the bronchi occur, including thickened bronchial walls with subsequent narrowing of their lumens and overgrowth (hyperplasia) of the mucin-secreting cells of the bronchi. These long-term changes in the bronchi increase the likelihood of future bacterial infections within the respiratory tract. INTEGRATE CLINICAL VIEW 23.7 Asthma Asthma (az΄mă) is a chronic condition characterized by episodes of bronchoconstriction coupled with wheezing, coughing, shortness of breath, and excess pulmonary mucus. Typically, the affected person develops sensitivity to an airborne agent, such as pollen, smoke, mold spores, dust mites, or particulate matter. Upon reexposure to this triggering substance, a localized inflammatory response (see section 22.3e) occurs in the bronchi and bronchioles, resulting in bronchoconstriction, swollen submucosa, and increased production of mucus. Episodes typically last an hour or two. Continual exposure to the triggering agent increases the severity and frequency of asthma attacks. The walls of the bronchi and bronchioles eventually may become permanently thickened, leading to chronic and unremitting airway narrowing and shortness of breath. If airway narrowing is extreme during a severe asthma attack, death can occur. Exercise-induced asthma is triggered by physical exertion. The physiologic requirement for additional oxygen during exercise triggers harder breathing, which typically occurs through the mouth. In a cold environment, air that is normally conditioned (warmed and humidified) by the nose and nasal cavity (see section 23.2a) enters the lungs relatively colder and drier. For those individuals susceptible to exercise-induced asthma, the colder and drier air induces smooth muscle within their bronchioles to contract to a greater degree than normal, causing excessive bronchoconstriction, which greatly reduces airflow. The reasons for exercise-induced asthma in a hot and humid environment are not completely understood, but are thought to be triggered by the: (a) moist air, which is heavier and harder to breathe, so an individual breathes deeper or more rapidly, (b) hot air, which irritates the air passageways, (c) larger amounts of mold spores that are formed in the moist environment, and (d) greater amounts of pollutants (e.g., smog). In susceptible individuals, inflammation within the air passageways is triggered and the bronchioles constrict. Because the bronchioles become more narrow, the preferred term for exercise-induced asthma is exercise-induced bronchoconstriction. The primary treatment for asthma consists of administering inhaled steroids (cortisone-related compounds) to reduce the inflammatory reaction, combined with bronchodilators to alleviate the bronchoconstriction. A treatment called bronchial thermoplasty uses heat to remove some of the outer layers of smooth muscle. This decreases the muscle contractions associated with bronchoconstriction to lessen the severity of asthma. Airway constriction occurs during an asthma attack. Page 905 Bronchi lead into smaller air passageways that do not have cartilage in their walls and that have a diameter of less than 1 millimeter called bronchioles (brong′kē-ōl). Terminal bronchioles are the last portion of the conducting pathway. They lead into respiratory bronchioles, the first segments of the respiratory zone (which is described in section 23.3c). Histology of the Bronchial Tree Bronchi have both smooth muscle and plates of hyaline cartilage that support their walls to ensure that they remain open ( figure 23.10). The extent of wall support lessens as bronchi divide and their diameter decreases. The cartilage first appears as various-sized, irregular plates in their walls, which continue to decrease in size, thickness, and number as the bronchi branch into smaller air passageways. Bronchioles are smaller in diameter than bronchi and have no cartilage within their walls. (In fact, other than being smaller, the lack of cartilage is what distinguishes bronchioles from bronchi.) Instead, bronchioles have a proportionately thicker layer of smooth muscle than do bronchi. It is their smaller diameter and relatively thicker layer of smooth muscle within their walls that normally prevent bronchioles from collapsing. The changes in the epithelium lining the bronchi and bronchioles composing the bronchial tree are summarized in figure 23.2. Regulating Airflow Within the Bronchial Tree It is within the bronchial tree where the amount of airflow between the atmosphere and alveoli is altered. It is regulated through contraction and relaxation of the smooth muscles within the walls of bronchi and bronchioles. Airflow is decreased when their smooth muscle is stimulated to contract (in response to parasympathetic division stimulation; see section 15.3a), which narrows the diameter of the lumen (opening), causing bronchoconstriction. In contrast, airflow is increased when their smooth muscle relaxes (due to sympathetic division stimulation; see section 15.4), which widens the diameter of the lumen, causing bronchodilation. Note that bronchoconstriction lessens the amount of potentially harmful substances that may be inhaled into the alveoli (e.g., smoke, toxins, allergens), helping to protect the lungs (see section 22.3a), whereas bronchodilation maximizes the amount of air moved between the atmosphere and alveoli to increase the amount of oxygen that is delivered to the alveoli and the amount of carbon dioxide that is removed. WHAT DID YOU LEARN? 11 What are the significant structural differences between bronchi and bronchioles? 23.3c Respiratory Zone: Respiratory Bronchioles, Alveolar Ducts, and Alveoli LEARNING OBJECTIVES 13. Identify and describe the structure and function of the components of the respiratory zone. 14. List three types of cells found in alveoli, and describe the function of each. INTEGRATE CLINICAL VIEW 23.8 Pneumonia Pneumonia (nū-mō΄nē-ă) is an infection of the lung, which results in the alveoli filling with fluid, exudate, or pus. (Exudate is described in section 22.3e and pus in Clinical View 22.1: “Pus and Abscesses.”) Pneumonia has numerous causes, but it is most likely the result of a bacterial or viral infection (see section 22.1). This contagious disease is usually spread by respiratory droplets. Symptoms include cough, fever, difficulty breathing, weakness, chills, increased heart rate, and chest pain while inhaling. Additionally, the bronchi produce and expel sputum, which may be colored (rust- or green-tinged). The infection may involve an entire lung or just one lobe. Diagnosis of pneumonia depends on symptoms and characteristic changes seen on a chest x-ray (see photo). A sputum culture is often helpful in identifying the specific organism. Pneumonia that results in milder symptoms is often called walking pneumonia. Pneumonia results in tissue swelling, accumulated fluid and leukocytes within the lung, which increases the thickness of the respiratory membrane. The capacity for gas exchange is impaired, decreasing the diffusion of oxygen (O2) and carbon dioxide (CO2) between the alveoli and blood within the pulmonary capillaries. A healthy, young adult who is diagnosed relatively early will typically recover from pneumonia in about 2 to 3 weeks. However, the duration of the illness is lengthened in older individuals and those who are immunocompromised (having an impaired immune system), or if the pneumonia was caused by a bacterium (those caused by a virus are typically less severe). In addition, the later pneumonia is diagnosed and treated, the more severe will be the symptoms and the longer its duration. Chest x-ray of a patient with pneumonia in the left lung. A normal lung appears as a black space on an x-ray because its spongy structure is not dense. In contrast, a lung infected with pneumonia appears white or opaque on an xray due to accumulation of fluid and cells. Collection CNRI/Phototake Normal alveoli (left) and alveoli affected by pneumonia (right). The respiratory zone was described in section 23.1b as being composed of respiratory bronchioles, alveolar ducts, and alveoli. These are all microscopic structures. The respiratory bronchioles subdivide into thin airways called alveolar ducts that lead into alveolar sacs, which are composed of a cluster of alveoli ( figure 23.11). An alveolus (al-vē′ō-ŭs; pl., alveoli, al-vē′ō-lı̄ ; alveus = hollow sac) is a small (about 0.25 to 0.5 millimeter in diameter), saccular outpocketing (similar to a hollow grape). Page 906 Figure 23.11 Bronchioles and Alveoli. Bronchioles and alveoli form the terminal ends of the respiratory passageway. (a) Terminal bronchioles branch into respiratory bronchioles in the respiratory zone, which then branch into alveolar ducts and alveoli. Pulmonary vessels are positioned alongside the bronchioles, and the pulmonary capillaries form a vascular network around the alveoli, allowing for gas exchange. Elastic tissue also surrounds the alveoli. (b) A photomicrograph shows the relationship of respiratory bronchiole, alveolar ducts, and alveoli. (c) An SEM of a terminal bronchiole, a respiratory bronchiole, alveolar duct, and alveoli reveals the honeycomb appearance of alveoli. (b) ©McGraw-Hill Education/Alvin Telser; (c) ©David Phillips/Science Source APR Module 11 Respiratory: Histology: Alveolus: SEM Low magnification: Alveolus Respiratory bronchioles typically are composed of a simple cuboidal epithelium, whereas both the alveolar ducts and alveoli are composed of a simple squamous epithelium (see figure 23.2b). The epithelium lining the passageways of the respiratory zone is much thinner than in the conducting portion, thus facilitating gas diffusion between the respiratory zone and pulmonary capillaries. Each lung contains approximately 300 to 400 million alveoli by the time a person is about 8 years old. The packing of these millions of air-filled alveoli gives the lung its spongy nature. Alveoli abut one another, so their sides become slightly flattened. Thus, an alveolus in cross section actually looks more hexagonal or polygonal in shape than circular. Small openings in the walls, called alveolar pores, are between some adjacent alveoli; these openings allow air to circulate between alveoli. Pulmonary capillaries form a vascular network around each alveolus. It is as if each alveolus has its own “hair net” of delicate, microscopic blood vessels that covers it. The interalveolar septum (the wall between adjacent alveoli) often contains elastic fibers that contribute to the ability of the lungs to stretch during inspiration and recoil during expiration. Two cell types form the alveolar wall: alveolar type I cells and alveolar type II cells ( figure 23.12a). Alveolar type I cells (or squamous alveolar cells) are the most common of the two types of cells (making up approximately 95% of the alveolar surface). These simple squamous cells are the primary cells that form each alveolus. Alveolar type I cells collectively form the alveolar epithelium of the respiratory membrane, which is described in section 23.3d. Alveolar type II cells (or septal cells), which are much less numerous, are cuboidal epithelial cells that secrete an oily fluid called pulmonary surfactant (ser-fak′tănt). The function of surfactant is to prevent the collapse of alveoli. Figure 23.12 Alveoli and the Respiratory Membrane. (a) Microscopic alveoli form the terminal end of the air passageway. (b) An alveolus without surfactant, which tends to collapse because of the high surface tension, and an alveolus with surfactant, which decreases surface tension helping to prevent collapse. (c) Gas exchange between the alveoli and the blood within the pulmonary capillaries occurs across a thin respiratory membrane. The respiratory membrane consists of an alveolar epithelium (composed of an alveolar type I cell), a capillary endothelium (composed of an endothelial cell), and their fused basement membranes. Oxygen diffuses from an alveolus into the blood within the capillary, and carbon dioxide diffuses in the opposite direction. (Note that the pulmonary surfactant layer is not shown in figure a and c.) Page 907 A significant feature of alveoli is that the internal surface is moist, causing a high surface tension because of the attractive forces of the water molecules (see section 2.4b). This makes alveoli prone to collapse and to remain collapsed ( figure 23.12b). Surfactant is an oily fluid (which contains a mixture of lipid and protein molecules) that when released from alveolar type II cells coats the inner alveolar surface. If an alveolus begins to collapse, which occurs with each expiration, the surfactant molecules within each alveolus become more tightly packed together and tend to collectively oppose the collapse of the thin walls of the alveolus. Note that alveolar type II cells begin to produce surfactant beginning about 2 months prior to birth. This is discussed in greater detail in section 23.5d. A third type of cell that is part of the alveolus is the alveolar macrophage, also called a dust cell ( figure 23.12a). This cell is a leukocyte that may be either fixed or free. Fixed alveolar macrophages remain within the connective tissue of the alveolar walls, whereas free alveolar macrophages are migratory cells that continually move across the alveolar surface within the alveoli. Both types of alveolar macrophages engage in phagocytosis to engulf microorganisms and particulate material that reaches the alveoli (see section 22.3c). The alveolar macrophages are able to leave the lungs either by entering the lymph vessels or by being coughed up in sputum and then expectorated from the mouth. WHAT DID YOU LEARN? 12 Which of the following respiratory structures are supported by cartilage: nose, larynx, trachea, bronchi, bronchioles, and alveolar sacs? 13 The respiratory tract can be damaged from desiccation (drying out), cold air, microbes, or exposure to chemicals or particulate matter. Which of the following help(s) protect the respiratory tract: nasal hairs, mucus, tonsils, cilia, macrophages, sneezing, or coughing? Explain how for each. 14 Diagram and label the conducting and respiratory structures (in order) that air passes through from the atmosphere to the alveoli. Page 908 23.3d Respiratory Membrane LEARNING OBJECTIVE 15. Describe the structure and function of the respiratory membrane. The respiratory membrane is composed of an alveolar epithelium (specifically alveolar type I cells), a capillary endothelium, and their fused basement membranes ( figure 23.12c). These simple squamous cells and their fused basement membranes form an extremely thin barrier (only 0.5 micrometers), which allows for efficient diffusion of the respiratory gases between the air in the alveoli and the blood in the pulmonary capillaries. When you breathe, this is the anatomical structure at the end of the respiratory passageway that the respiratory gases must move across. Oxygen diffuses from the alveolus through the respiratory membrane into the pulmonary capillary, whereas carbon dioxide diffuses from the blood within the pulmonary capillary through the respiratory membrane to enter an alveolus. Diffusion of respiratory gases across the respiratory membrane is impaired in certain respiratory diseases (e.g., pneumonia; see Clinical View 23.8: “Pneumonia”). WHAT DID YOU LEARN? 15 Diagram and label the structures of the respiratory membrane and indicate the directions in which oxygen and carbon dioxide move.