OBJECTIVES

This chapter should help you to:

· Name the 3 divisions of the respiratory system and the components of each.
· Compare the right and left lungs.
· Describe the walls of the respiratory tract in terms of the arrangement, composition, and function of the component layers and describe the structure and function of the cells in each layer.
· Distinguish between different parts of the respiratory tract on the basis of regional differences in wall structure.
· Describe the structure of the interalveolar septurn.
· Describe the structure and function of the blood-air barrier and identify its components in an electron micrograph.
· Compare sympathetic and parasympathetic effects on bronchial smooth muscle.
· Describe the structure, function, and location of the pleura.
· Identify the organ, tissues, and cell types present and distinguish between the various components of the respiratory system from a slide or photomicrograph of the respiratory tract or lung tissue.

SYNOPSIS

I. GENERAL FEATURES OF THE RESPIRATORY SYSTEM

A. Components and Basic Functions of the Respiratory System: The respiratory system includes the lungs, airways tie, pharnyx, larynx, trachea, bronchi) and associated structures. Specialized for gaseous exchange between blood and air, including the uptake of oxygen and release of carbon dioxide, it is functionally divisible into 3 major parts: the conducting and respiratory portions and the ventilating mechanism.


1. Ventilating mechanism. This mechanism, which creates pressure differences that move air into (inspiration) and out of (expiration) the lungs, includes the diaphragm, rib cage, intercostal muscles, abdominal muscles, and elastic connective tissue in the lungs. In spiration (inhalation) is active, involving muscle contraction. To inhale, the intercostal muscles lift the ribs while the diaphragm and abdominal muscles lower the floor of the thoracic cavity. This enlarges the cavity, creating a vacuum that draws air through the airways. The incoming air expands the airways, inflates the lungs, and stretches the elastic connective tissue. Expiration (exhalation) is more passive: Relaxing the muscles allows the elastic fibers to retract, contracting the lungs and forcing air out.


2. Conducting portion. The walls of this system of tubes are specialized to carry air to and from the site of gas exchange without collapsing under the pressures created by the ventilat ing mechanism. This portion also conditions the air, warming, moistening, and cleaning it to enhance gas exchange. It includes the nasal cavity (II), nasopharynx (IV), larynx (V), trachea (VI), bronchi (V1I), bronchioles (VII.D), and terminal bronchioles (VII.E). 3. Respiratory portion. This portion is distinguished by alveoli (VIII), small, saccular struc tures whose thin walls enable the gas exchange between air and blood. Alveoli occur in clusters at the end of the bronchial tree. These clusters extend (like rooms from a hallway) (Fig 17-1) from the walls of respiratory bronchioles (VII.F), alveolar ducts (VII.G), and atria and alveolar sacs (VII.H).

B. Wall Structure: Like the digestive tract, the tubelike respiratory tract has layered walls whose lining epithelium derives from endoderm. The wall layers include an epithelium, a lamina propria that contains mucous glands as well as cartilage that prevent the tract from collapsing under pressure, smooth muscle that regulates the luminal diameter, and an adventitia that contains collagen and elastic fibers. Each layer undergoes gradual changes as the wall's overall thickness decreases from the nasal cavity to the alveoli (Table 17-1).

1. Respiratory epithelium a. General features. The epithelium lining most of the tract is ciliated pseudostratified columnar with goblet cells; it is generally referred to as respiratory epithelium. As the respiratory tract undergoes branching and its luminal diameter decreases, the epithelium gradually drops in height and loses first goblet cells and then cilia as it approaches the alveoli. b. Epithelial cell types (1) Ciliated columnar cells predominate in the tract. Each has about 300 motile cilia (see Chapter 3) on its apical surface; there are associated basal bodies in the apical cytoplasm. (2) Mucous goblet cells are the second most numerous type. They secrete the mucus that covers the epithelium and traps and removes bacteria and other particles from inspired air. Cilia projecting from columnar cells sweep the contaminated mucus toward the mouth for disposal. (3) Brush cells. Also columnar, these cells lack cilia; they often have abundant apical microvilli. Two types are present: One resembles an immature cell and apparently serves to replace dead ciliated or goblet cells; the other has nerve endings on its basal surface and appears to be a sensory receptor. (4) Basal cells. These small round cells lie on the basal lamina but do not reach the lumen. They appear to be stem cells that can replace the other cell types. (5) Small granule cells resemble basal cells, but they contain many small cytoplasmic granules and exhibit DNES activity (see Chapter 4). c. Metaplasia refers to the change in tissue organization or type undergone by epithelia in response to changes in the physical or chemical environment. For example, a smoker's respiratory epithelium typically develops more goblet cells in response to high pollutant levels and fewer ciliated cells in response to carbon monoxide. These changes, which are reversible, frequently cause congestion of the smaller airways.

2. Lamina propria. Consisting of loose connective tissue, the larnina propria contains mucous glands in the upper tract (from the nasal cavity to the bronchi). Its elastic fiber content increases toward the alveoli. Skeletal connective tissue support begins as cartilage and bone in the nasal cavity and becomes cartilage only in the larynx. It gradually decreases, disap pearing at the level of the bronchioles. 3, Smooth muscle. This begins in the trachea, where it joins the open ends of the C-shaped tracheal cartilages (VI). In the bronchi, many layers of smooth muscle cells encircle the walls in a spiral. From this point, the thickness of the muscle layer gradually decreases until it disappears at the level of the alveolar ducts.

II. NASAL CAVITY

This cavity is divided by the nasal septum into 2 bilaterally symmetric cavities that open to the exterior through the nares (nostrils). Each cavity consists of 2 chambers--a vestibule and a nasal Fossa--which differ in position, size, and wall structure.

A. Vestibule: The smaller, wider, and more anterior chamber of each side, it lies just behind the nares. The medial septum and lateral walls are supported by cartilage, and the epithelial lining is a continuation of the epidermis (see Chapter Is) covering the nose. Just inside, the epithelium is keratinized, containing many sebaceous and sweat glands as well as thick short hairs called vibrissae, which filter large particles from inspired air. Deeper in the vestibule, the epithelium changes from keratinized to nonkeratinized stratified squamous and then to respiratory epi thelium just before entering the nasal fossa.

B. Nasal Fossa: This is the larger, narrower, and more posterior chamber on each side. Here the septum and lateral walls are lined by respiratory epitheliurn. They are supported by bone and contain mucous glands and venous sinuses in the lamina propria. Three curved bony shelves. termed conchae, or turbinate bones, project into each fossa from its lateral wall. These help warm and moisten the air by increasing the mucosal surface area and forming a system of baffles that cause turbulence and slow the air Row through the cavity. Alternating from side to side every 20-30 minutes, venous plexuses (swell bodies) in the conchal mucosa engorge with blood, causing it to swell. This action restricts air Row, directing it through the other side of the nose, and thus helps prevent overdrying of the mucosal surface. Arterial vessels in the fossa walls create a countercurrent system that warms air by directing blood flow from posterior to anterior (opposite to the flow of inspired air) in a series of small arches. Specialized olfactory epithelium (see Chapter 24) is present in the roof of each fossa.

III. PARANASAL SINUSES

These are dilated cavities in the frontal, maxillary, ethmoid, and sphenoid bones around the nose and eyes. Their thin respiratory epithclial lining has few goblet cells and is bound tightly to the periosteum of the surrounding bones by a lamina propria that contains a few small mucous glands. Mucus produced here drains into the nasal fossa through small openings protected by the conchae.

IV. NASOPHARYNX

The upper part of the pharynx (see Chapter 15), the nasopharynx is a broad single cavity overlying the soft palate. It is continuous anteriorly with the nasal fossae and inferiorly with the oral part of the pharynx (oropharnyx). The walls, lined by respiratory epithelium, are supported by bone and skeletal muscle.

V. LARYNX

A bilaterally symmetric tube, the larynx lies in the neck between the base of the oropharynx and the trachea. During swallowing, its opening is protected by the cpiglottis. Its walls, supported by several laryngeal cartilages in the lamina propria, contain skeletal muscle and house the vocal apparatus.

A. Epiglottis: This flap of tissue extends toward the oropharynx from the anterior border of the larynx. It is covered on its superior surface by nonkeratinized stratified squamous epithelium and on its inferior surface by respiratory epithelium. The lamina propria contains a few mucous glands and a small plate of elastic cartilage. During swallowing, the backward motion of the tongue forces the epiglottis over the laryngeal opening, directing food away from the airway and into the esophagus. After swallowing, the elastic cartilage helps to reopen and maintain the airway.

B. Laryngeal Cartilages: Several cartilages frame the laryngeal lumen and serve as attachments for the skeletal muscles that control the vocal apparatus. The larger thyroid, cricoid, and most of the paired arytenoid cartilages are hyaline, while the smaller ones--the paired cuneiform and comiculate, the epiglottic, and the tips of the arytenoids--are elastic.

C. Vocal Apparatus: The broad part of the larynx, below the epiglottis and surrounded by the thyroid cartilage, contains 2 bilaterally symmetric pairs of mucosal folds.


1. False vocal cords (vestibular folds). These are the upper pair of folds in the larynx. They are covered by respiratory epithelium and contain serous glands whose ducts open mainly into the cleft that separates them from the lower pair of folds.

2. True vocal cords. This lower pair of folds is covered by stratified squamous epithelium. Each contains 2 major structures: a large bundle of elastic fibers that run front to back, called the vocal ligament; and a bundle of skeletal muscle that runs parallel to the ligament, called the vocalis muscle. Air forced through the larynx by the ventilating mechanism (I.A.I) causes the true cords to vibrate. The vocalis muscle regulates the tension of the cords, while other muscles control the shape and position of the laryngeal lumen. In this way, the laryngeal muscles control the pitch (frequency) and other aspects of the sounds produced by the vibrating cords. The cords also assist the epiglottis in preventing foreign objects from reaching the lungs; they close to build up pressure when coughing is required to dislodge materials blocking the airway.

VI. TRACHEA

This 10-cm tube extends from the larynx to the primary bronchi. It is lined by respiratory epithelium, and its lamina propria contains mixed seromucous glands that open onto its lumen. Its most characteristic feature is the presence of 16-20 C-shaped cartilage rings whose open ends are directed postcriorly. The opening is bridged by a fibroelastic ligament that prevents overdistension as well as by smooth muscle bundles (tracbealis muscle) that constrict the lumen and increase the force of air flow during coughing and forced expiration.

VII. BRONCHIAL TREE

This begins where the trachea branches to form 2 primary bronchi, one of which penetrates the hilum of each lung. The hilum is also the site at which arteries and nerves enter and veins and lymphatic vessels exit the organ. These structures, together with the dense connective tissue that binds them, form the pulmonary root. The bronchial tree undergoes extensive branching within the lungs. The changes in wall structure that accompany the progress of the bronchial tree toward the alveoli (Table 17-1) occur gradually and not at sharp boundaries.

A. Primary Bronchi: There are 2 primary bronchi, one entering each lung. Their histologic appearance is quite similar to that of the trachea, but their cartilage rings and spiral bands of smooth muscle completely encircle their respective lumens. The path of the right primary bronchus is more vertical than that of the left. As a result, foreign objects that reach the bronchi are more likely to lodge in the right side of the bronchial tree.

B. Secondary Bronchi: These lobar broncbi are branches that arise directly from the primary bronchi; each supplies one pulmonary lobe. Since the right lung has 3 lobes and the left only 2, the right primary bronchus gives rise to 3 secondary bronchi and the left primary bronchus gives rise to 2. Their histologic structure is similar to that of the primary bronchi except that their supporting cartilages (and those of the smaller bronchi) are arranged as irregular plates, or islands, of cartilage, rather than as rings.

C. Tertiary Bronchi: Arising directly from the secondary bronchi, which they resemble histo logically, each of these segmental bronchi supplies one bronchopulmonary segment (pulmo nary lobule). Although each lung has 10 such segments, the different number of secondary bronchi causes the tertiary branching pattern to differ between the right and left lungs. Except for a decrease in overall diameter, the histologic appearance of tertiary bronchi is identical to that of secondary bronchi. Tertiary bronchi may branch several times to form successively smaller branches, which are considered bronchi as long as their walls contain cartilage and glands.

D. Bronchioles: These are branches of the smallest bronchi. The largest bronchioles differ from the smallest bronchi only by the absence of cartilage and glands in their walls. Large bronchioles are lined by typical respiratory epithelium; as they branch further, the epithelial height and complexity decrease to simple ciliated columnar or cuboidal. Each bronchiole gives rise to 5-7 terminal bronchioles.

E. Terminal Bronchioles: The smallest components of the conducting portion of the respiratory system, these are lined by ciliated cuboidal or columnar epithelium and have few or no goblet cells. (The elimination of goblet cells before the cilia in the lower reaches of the bronchial tree is important in preventing individuals from drowning in their own mucus). The lining here also includes dome-shaped cilia-free Clara cells, whose cytoplasm contains glycogen granules, lateral and apical Golgi complexes, elongated mitochondria, and a few secretory granules. The function of these cells is unclear. Each terminal bronchiole branches to form 2 or more respira tory bronchioles.

F. Respiratory Bronchioles: These are the tirst part of the respiratory portion, with a cuboidal epithelial lining which resembles that of the terminal bronchioles but which is interrupted by thin-walled saccular evaginations called alveoli. The number of alveoli increases as the respira tory bronchioles proceed distally. As the alveoli increase in number, the cilia decrease until they disappear. Goblet cells are absent.

G. Alveolar Ducts: These are simply the distal extensions of the respiratory bronchioles where the alveoli are so dense that the wall consists almost entirely of these sacs, and the lining has been reduced to small knobs of smooth muscle covered by cilia-free simple cuboidal cells. The knobs appear to project into the elongated lumen of the duct, each resting atop a thin septum that separates adjacent alveoli. The alveolar duct can thus be likened to a long hallway with so many doorways leading to small rooms (alveoli) that the hallway (the alveolar duct) appears almost to lack walls.

H. Atria and Alveolar Sacs: Atria are the distal terminations of alveolar ducts. The arrangement is comparable to a long hallway (alveolar duct) leading to a rounded foyer (atrium). The foyer has small doorways leading to some small rooms (alveoli), but also has 2 or more larger doorways leading into short, dead-end hallways (alveolar sacs). The short hallways are also lined by small rooms (alveoli). Put simply, the difference between atria and alveolar sacs is that the atria open into alveolar ducts, alveoli, and alveolar sacs, while the alveolar sacs open only into alveoli and atria. Although these distinctions can be made fairly easily in sections cut longitudinally through the entire system of passageways beginning with the alveolar duct, such perfect cuts are relatively rare in standard slides of lung tissue. More often, the various compo nents are cut in oblique or cross section and only the openings to the alveoli are seen, making it hard to distinguish between the sacs and the atria. In such cases, the only useful clue is the size of the knobs that project into the passageways. Those projecting into alveolar sacs lack smooth muscle and are thus smaller than those projecting into either the atria or the alveolar ducts.

VIII. ALVEOLI

Occurring only in the respiratory portion (which their presence distinguishes from the conducting portion), these small (about 200 um in diameter) sacs open into a respiratory bronchiolc, an alveolar duct, an atrium, or an alveolar sac. They are separated by thin walls termed interalveolar (or alveolar) septa (Fig 17-2).

A. Interalveolar Septa: The structural features of these septa, which are specialized for gas exchange, are critical to respiratory function. The septa consist of 2 simple squamous epithelial layers with the interstitium sandwiched between them. The interstitium consists of continuous (nonfenestrated) capillaries embedded in an elastic connective tissue that includes elastic and collagen fibers, ground substance, fibroblasts, mast cells, macrophages, leukocytes, and contractile interstitial cells that contract in response to epinephrine and histamine. This elastic tissue is an important component of the ventilating mechanism. Gas exchange occurs between the air in the alveolar lumen and the blood in the interstitial capillaries.


1. Blood-air barrier. This term refers to the structures that oxygen and CO, must cross to be exchanged. Varying from 0. 1-1.5 um in thickness, it includes the following layers: a. The film of pulmonary surfactant on the alveolar surface. b. The cytoplasm of the squamous cpithelial (type I alveolar) cells. c. The fused basal laminae sandwiched between the type I alveolar and capillary endothelial cells. d. The cytoplasm of the squamous endothelial cells lining the intcrstitial capillaries. 2. Alveolar pores. Each septum may be interrupted by one or more pores from 10 to 15 um in diameter. These connect adjacent alveoli and may help to equalize pressure and allow collateral air circulation, thus maximizing the use of available alveoli when some small airways are blocked.

B. Alveolar Cell Types:


1. Type I cells. Also called type I alveolar cells, type I pneumocytes, and squamous alveolar cells, these are squamous epithelial cells that make up 97% of the alveolar surfaces. They are specialized to serve as very thin (often only 25 nm in width) gas-permeable components of the blood-air barrier. Their organelles leg, Golgi complex, endoplasmic reticulum, mitochondria) cluster around the nucleus. Much of the cytoplasm is thus unobstructed by organelles, except for the abundant small pinocytotic vesicles that are involved in the turn over of pulmonary surfactant and the removal of small particles from the alveolar surfaces. They attach to neighboring epithelial cells by desmosomes and occluding junctions. The latter reduce pleural effusion--leakage of tissue fluid into the alveolar lumen. Type I cells can be distinguished from the nearby capillary endothelial cells by their position bordering the alveolar lumen and by their slightly more rounded nuclei.
2. Type II cells. These cells, which are also called type II alveolar cells, type II pneumocytes, great alveolar cells, and alveolar septal cells, cover the remaining 3% of the alveolar surface. They are interspersed among the type I cells, to which they attach by desmosomes and occluding junctions. Type II cells are roughly cuboidal with round nuclei; they occur most often in small groups at the angles where alveolar septal walls converge. At the electron microscope level, they contain many mitochondria and a well-developed Golgi complex, but they are mainly characterized by the presence of large (0.2-um), membrane-limited lamellar (mutlilamellar) bodies. These structures, which exhibit many closely apposed concentric or parallel membranes (lamellae), contain phospholipids, glycosaminoglycans, and proteins. Type II cells are secretory cells. Their secretory product, pulmonary surfactant, is assembled and stored in the lamellar bodies, which also carry it to the apical cytoplasm. There, the bodies fuse with the apical plasma membrane and release surfactant onto the alveolar surface.
3. Alveolar marcrophages. Known also as dust cells, these large monocytc-derived repre sentatives of the mononuclear phagocyte system are found both on the surface of alveolar septa and in the interstitium. Macrophages are important in removing any debris that escapes the mucus and cilia in the conducting portion of the system. They also phagocytose blood cells that enter the alveoli as a result of heart failure. These alveolar macrophagcs, which stain positively for iron pigment (hemosiderin), are thus designated heart failure cells.




C. Pulmonary Surfactant: Continuously synthesized and secreted by type II alveolar cells onto the alveolar surfaces, pulmonary surfactant is removed from these surfaces by alveolar mac rophages and by type I and II alveolar cells. Its composition and continuous turnover allow it to serve 2 major functions. Not only does it reduce surface tension in the alveoli, but also it is thought to have some bactericidal effects, cleaning the alveolar surface and preventing bacterial invasion of the many capillaries in the septa. The surfactant forms a thin 2-layer film over the entire alveolar surface. The film consists of an aqueous basal layer (bypopbase) composed mainly of protein, which is covered by a monomolecular film of phospholipid (mainly di palmitoyl lecithin) whose fatty acid tails extend into the lumen. By reducing surface tension, the surfactant helps prevent collapse of the alveoli during expiration. It thus eases breathing by decreasing the force required to reopen the alveoli during the next inspiration. Because surfac tant secretion begins in the last weeks of fetal development, premature infants often suffer a condition called hyaline membrane disease, evidenced by respiratory distress (labored Ibreathing) caused by the lack of surfactant. Surfactant secretion can be induced by administering glucocorticoids, significantly improving the infant's condition and chances for survival.

D. Alveolar Lining Regeneration: Daily turnover of about 1G/o of the type II cells, whose mitotic progeny form both type I and type 11 cells, allows for normal alveolar lining renewal. When these lining cells are destroyed by inhalation of toxic gases, replacements for both types of cells are similarly derived from the surviving type II cells.

IX. PULMONARY CIRCULATION

A. Blood Supply: The lungs have a dual blood supply: the functional (pulmonary) circulation and the systemic (nutrient) circulation. The 2 systems communicate through extensive anas tomoses near the capillary beds.

1. Functional circulation. This is provided by the pulmonary arteries and veins. a. Pulmonary arteries. Arising from the heart's right ventricle as large-diameter elastic arteries, the pulmonary arteries branch and enter the lung at the pulmonary root (VII). of They follow the branching pattern of the bronchial tree to carry oxygen-poor blood to the lungs' capillary beds for oxygenation. Smaller branches (less than 1 mm in diameter) are of the muscular type, with a definitive internal elastic lamina. Pulmonary arteries have a thin intima and thinner media than do other arteries of equal size. b. Pulmonary veins. These collect oxygenated blood from the capillaries of the lungs and return it to the left atrium of the heart for distribution through the aorta and its branches. The larger branches of these veins accompany the bronchi, but the smaller branches travel unaccompanied in the connective tissue septa that separate the bronchopulmonary segments (VII.C). The thin intima of these vessels differs from other veins in that it lacks valves and contains a rich elastic fiber network in its subendothelial layer. While the media is absent in vessels smaller than 100 um, in larger vessels it contains both smooth muscle and elastic fibers. The adventitia is thicker than that of pulmonary arteries. branching pattern of the bronchial tree to the level of the respiratory bronchioles. Here they anastomose with branches of the pulmonary artery. Branches of the bronchial ar teries carry oxygen-rich blood to capillaries in the bronchi, bronchioles, interstitium, and pleura. The blood collects in submucosal venous plexuses in various parts of the bronchial tree before entering the bronchial veins. c. Bronchial veins. Histologically, these are typical small veins that carry blood from the submucosal bronchial venous plexuses and always accompany the bronchial tree. Bronchial veins following the larger bronchi empty into the azygous, hcmiazygous, or posterior intercostal veins. Those associated with the smaller portions of the bronchial tree empty directly into branches of the pulmonary veins.

B. Lymphatic Drainage: The lungs' lymphatic vessels are divided into superficial and deep networks, both draining toward the lymph nodes near the hilum. Vessels of the deep network have few valves; they accompany either the bronchial tree or the pulmonary veins in the intersegmental connective tissue. Vessels of the superficial network, which have many valves, are found in the visceral pleura. Lymph in the superficial network travels to the hilar nodes through vessels that either traverse the pleural surface or penetrate the lung surface and empty into intersegmental vessels. Lymphatic vessels are notably absent from interalveolar septa; at this level, the rich capillary network is responsible for draining excess interstitial fluid.

X. INNERVATION

Autonomic motor and general sensory nerves penetrate the pulmonary root, accompanying the blood vessels and the bronchial tree. Sensory nerves, which carry poorly localized pain sensations, monitor irritants in the airway and are involved in the cough reflex. Parasympathetic motor fibers (branches of the vagus nerve) stimulate bronchial constriction, while sympathetic fibers cause bronchial dilation. 1Sympathomimetic drugs such as isoproterenol are used to stimulate bronchodilation during asthma attacks.

XI. PLEURA

This serous membrane has 2 layers, one covering the lungs (visceral pleural and the other covering the internal wall of the thoracic cavity (parietal pleural. Like the peritoneum and the pericardium, the pleura consists of a thin squamous mesothelium attached to the organ or wall by a thin layer of connective tissue that contains collagen and elastic fibers. Bordered by the mesothelial cells, the narrow pleural cavity lies between the parietal and visceral pleurae. The cavity normally contains only a thin film of lubricating fluid which (together with the smooth mesothelial surfaces) reduces the friction between the lung surfaces and thoracic walls that would otherwise accompany the respiratory movements. Certain diseases and wounds allow excess air or fluid to enter the pleural cavity, increasing its size and restricting respiratory movement. While small amounts of air and fluids can be absorbed, larger amounts may precipitate lung collapse and require medical intervention.

Systemic circulation. This is provided by the bronchial arteries and veins. a. Broncbial arteries. Typical muscular arteries (Chapter I I) arising from the aorta or from intercostal arteries, these are always smaller than the accompanying branches of the pulmonary arteries. The bronchial arteries enter at the pulmonary root and follow the

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