Chapter 23 Respiratory System
Pleural Membranes and 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 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 pleural layer is continuous with the visceral pleural layer 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 23.17Pleural 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).^^
Paranasal Sinuses The paranasal (par-ă-nā′săl; para = alongside) sinuses, first described in section 8.2d, are associated with the nasal cavity (figure 23.4). These sinuses, which are spaces within the skull bones, are named for the specific skull bones in which they are located. Thus, from a superior to inferior direction, they are the paired frontal, ethmoidal, and maxillary sinuses; the sphenoidal sinuses Page 905are located posterior to the ethmoidal sinuses. Ducts connect all paranasal sinuses to the nasal cavity. Both the paranasal sinuses and their ducts are lined by a pseudostratified ciliated columnar epithelium that is continuous with the mucosa of the nasal cavity. The mucus, with its trapped particulate matter, is swept by cilia from each paranasal sinus into the nasal cavity and then into the pharynx, where it is swallowed.
***Figure 23.4 Paranasal Sinuses. The paranasal sinuses are air-filled cavities named for the bones in which they occur: frontal, ethmoidal, sphenoidal, and maxillary.^^
Respiratory Zone: Respiratory Bronchioles, Alveolar Ducts, and Alveoli - 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 smallest 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-lī; alveus = hollow sac) is a small (about 0.25 to 0.5 millimeter in diameter), saccular outpocketing (similar to a hollow grape).
Figure 23.11Bronchioles 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 wrap around the alveoli for gas exchange. Elastic tissue is also wrapped around 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.
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 within 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, occur between some adjacent alveoli; these openings provide for collateral ventilation of alveoli (i.e., for air to circulate between alveoli).^^ Pulmonary capillaries surround each alveolus to facilitate gas exchange between the alveolus and blood within the pulmonary capillaries. The interalveolar septum contains elastic fibers that contribute to the ability of the lungs to stretch during inspiration and recoil during expiration.
Figure 23.12Alveoli and the Respiratory Membrane. (a) Microscopic alveoli form the terminal end of the air passageway. (b) 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 covering layer is not shown here.)
Gross Anatomy of the 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 described in section 8.6 (figure 23.13). 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.^^
Figure 23.13Position 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.
Volume Changes in the Thoracic Cavity - Only small movements of the diaphragm are required for breathing, and usually the changes in vertical dimension measure only a few millimeters during quiet breathing. Greater changes in the superior movement of the diaphragm occur during forced expiration because of contraction of the abdominal muscles. Lateral dimension changes occur either as the rib cage is elevated and the thoracic cavity widens or as the rib cage depresses and thoracic cavity narrows. This action can be mimicked by placing your hands at the sides of your ribs and then abducting and adducting your hands relative to your ribs. Anterior-posterior dimension changes occur as the inferior portion of the sternum moves anteriorly and then posteriorly. This action can be visualized by placing one hand on the front of your lower chest and lifting it outwardly away from the chest and then back. In general, lateral and anterior-posterior dimensional changes both occur as a result of the contraction and relaxation of all the muscles of breathing shown in figure 23.19, except for the diaphragm.
Figure 23.20Thoracic Cavity Dimensional Changes Associated with Breathing. The boxlike thoracic cavity changes size during inspiration and expiration. The box increases in vertical, lateral, and anterior-posterior dimensions during inspiration due to movement of the diaphragm, ribs, and sternum. These dimensions decrease upon expiration.
Integration of Concepts: Quiet Breathing - Quiet breathing was described as the normal breathing that occurs when you are relaxed (see section 23.5a). Refer to figure 23.22 as you read through this description of the stepwise sequence of events that alter volume and pressure during quiet breathing:
Figure 23.22Volume and Pressure Changes Associated with the Mechanics of Quiet Breathing. Circled numbers correspond to the steps described in the text. (a) Schematic of changes in volume, pressure, and airflow that are associated with breathing. (b) Approximately 500 mL of air is inspired and then expired during quiet breathing; these volumes are associated with (c) relatively small changes in intrapulmonary and intrapleural pressures.
Efficiency of Gas Exchange at the Respiratory Membrane - The efficiency of both O2 and CO2 diffusion during alveolar gas exchange is dependent upon anatomic features of the respiratory membrane: its large surface and its minimal thickness. The aggregate surface area of the respiratory membrane in a healthy lung measures approximately 70 square meters—a little less than half the size of a tennis court. The minimal thickness of this barrier measures approximately 0.5 micrometer. The distance of the respiratory membrane is effectively increased in pneumonia (see Clinical View 23.7: "Pneumonia"). Physiologic adjustments also contribute to maximizing gas exchange at the alveoli. Some alveoli, at any given time, are well ventilated and some are not; similarly, some regions of the lung have ample blood moving through pulmonary capillaries, and some do not. The smooth muscle of both the bronchioles that lead into the alveoli and the Page 938arterioles that carry blood to pulmonary capillaries can contract and relax to maximize gas exchange. This inherent ability of bronchioles to regulate airflow and arterioles to regulate blood flow simultaneously is called ventilation-perfusion coupling (figure 23.26).
Figure 23.26Ventilation-Perfusion Coupling. (a) Bronchioles dilate or constrict in response to changes in CO2 in air within the bronchioles. (b) Pulmonary arterioles dilate or constrict in response to changes in either blood Po2 or blood Pco2. Ventilation is altered by changes in bronchodilation and bronchoconstriction. Bronchioles dilate in response to an increase in Pco2, whereas they constrict in response to a decrease in Pco2.
The upper respiratory tract, as described previously, includes the nose, nasal cavity, and pharynx
Figure 23.3Upper Respiratory Tract. (a) Anatomic regions that compose the upper respiratory tract, (b) supporting structures of the nose, (c) midsagittal section of the nasal cavity, and (d) coronal view of the nasal cavity in a cadaver
The intrinsic ligaments are located within the larynx and include both vocal ligaments and vestibular ligaments (figure 23.7). The vocal ligaments are composed primarily of elastic connective tissue and extend anterior to posterior between the thyroid cartilage and the arytenoid cartilages. These ligaments are covered with a mucosa to form the vocal folds. Vocal folds also are called the true vocal cords because they produce sound when air passes between them. They are distinctive from the surrounding tissue because they are avascular and white. The opening between these folds is called the rima glottidis (rī′mă glo-tī′dis; rima = slit). Together the vocal folds and the rima glottidis form the glottis.
Figure 23.7Vocal Folds. The vocal folds (true vocal cords) are elastic ligaments covered with a mucosa that extend between the thyroid and arytenoid cartilages. These folds surround the rima glottidis and are involved in sound production. Adducted (closed) and abducted (open) vocal folds are shown in (a) a superior view of the cartilages and ligaments only and (b) a diagrammatic laryngoscopic view of the coverings around these cartilages and ligaments. (c) A photo of a superolateral laryngoscopic view, showing the vocal folds and the rima glottidis, and the vestibular fold and rima vestibuli.
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 23.8Trachea. (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
Bronchial Tree - ***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 (figure 23.9) and (figure 23.10).^^
Figure 23.9Bronchial 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.)
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 20.8). 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 19.4a).
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
Volume and Capacity - ***The volume of air that enters and leaves the lungs can be measured with an instrument called a spirometer. Respiratory volumes vary throughout a 24-hour period and during different stages of your life. They also vary from individual to individual. The variation is significant enough to be used as a diagnostic tool for determining the health of an individual's respiratory system. Values for an individual are compared to standard values of a reference population. Respiratory measurements are often used to diagnose respiratory disease, monitor changes in respiratory impairment over time, and assess effectiveness of treatment.^^ Four major respiratory volumes are typically measured (figure 23.24 and table 23.4). ***Tidal volume (TV) is the amount of air inhaled or exhaled per breath during quiet breathing.^^ Inspiratory reserve volume (IRV) is the amount of air that can be forcibly inhaled beyond the tidal volume (after a normal inspiration). IRV is a measure of lung compliance. Expiratory reserve volume (ERV) is the amount that can be forcibly exhaled beyond the tidal volume (after a normal expiration). ERV is a measure of lung and chest wall elasticity. Finally, residual volume (RV) is the amount of air left in the lungs even after the most forceful expiration.
Respiratory Volumes and Capacities. Pulmonary volumes include tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume. Capacities are the sum of two or more volumes. Inspiratory capacity includes tidal volume and inspiratory reserve volume. Functional residual capacity includes expiratory reserve volume and residual volume. Vital capacity includes tidal volume, inspiratory reserve volume, and expiratory reserve volume. Total lung capacity is the sum of all four volumes.
Partial Pressure and Dalton's Law - ***Partial pressure is the pressure exerted by each gas within a mixture of gases and is measured in mm Hg; it is written with a P followed by the symbol for the gas. For example, the partial pressure for oxygen is written as Po2.^^ We can use atmospheric pressure and the mixture of gases in air to more fully explain partial pressure. ***Atmospheric pressure is the total pressure all gases collectively exert in the environment. These molecules include nitrogen (N2), oxygen (O2), carbon dioxide (CO2), water vapor (H2O), and a number of other minor gases. We have seen that at sea level atmospheric pressure has a value of 760 mm Hg.^^ The amount each gas in the atmosphere contributes to the total pressure—that is, its partial pressure—is determined by multiplying the total pressure exerted by the gas mixture by the percentage of the specific gas of interest: Thus, the partial pressure exerted for each gas in the atmosphere can be calculated from the total pressure (760 mm Hg at sea level) and the percentage of each of the most common gases—nitrogen (78.6%), oxygen (20.9%), carbon dioxide (0.04%), and water vapor (0.46%)—as follows: When these partial pressures are added together, their sum must equal the total atmospheric pressure. As just shown, Pn2 + Po2 + Pco2 + Ph2O = 760 mm Hg The relationship of partial pressure to total pressure is summarized by Dalton's law, which states that the total pressure in a mixture of gases is equal to the sum of all of the individual partial pressures.
The other 0.2 mm Hg is contributed by the minor gases within the atmosphere.
APR Module 11: Respiratory: Dissection: Lower Respiratory: Anterior: Right lung - 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. ***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 of the left lung is a projection from the superior lobe that is homologous to the middle lobe of the right lung. The left lung also has two surface Page 919indentations to accommodate the heart: the cardiac impression on its medial surface and a cardiac notch on its anterior surface.^^ 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.
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.
