Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger,
Peer Review Status: Externally Peer Reviewed
Plate 11.230 Alveolus
The respiratory system is composed of conducting and respiratory passages, which serve several functions. The primary function of this system is the exchange of oxygen and carbon dioxide; secondary functions are phonation and olfaction.
The conducting portion of the respiratory system includes the nasal cavity, paranasal sinuses, nasopharynx, larynx, trachea, and bronchi. The conducting portion, from the nostrils to the lungs, warms, humidifies, and filters the air. Mucus secreted by goblet cells (approximately 1 liter/day) entraps particulate matter, and the cilia of the pseudostratified columnar epithelium lining carry a continuous carpet of secreted mucus toward the pharynx for elimination from the respiratory system. The familiar "smoker's cough" results from paralysis of ciliary action by nicotine and other products of tobacco smoking with resultant stasis and accumulation of mucus in the lower respiratory system; this, in turn, triggers the cough reflex in an effort to clear the distal airway. Bronchitis is a frequent sequel to such ciliary impairment.
The trachea is a relatively rigid tube about 11 cm in length and 2.5 cm in diameter in adults. At the inferior border of the superior mediastinum of the thorax, the trachea bifurcates into two primary bronchi, which are structurally similar to the trachea. The mucosa of these tubes is supported by incomplete cartilaginous rings. Three layers of the wall are distinguishable. (1) The mucosa is composed of a ciliated pseudostratified columnar epithelium and numerous goblet cells, both of which rest on a prominent, thick basement membrane (thickest in the body). A thin lamina propria composed of collagenous, elastic, and reticular fibers may harbor accumulations of lymphocytes, which play an important role in safeguarding the body from inhaled pathogenic organisms. There is no muscularis mucosae. (2) The submucosa contains seromucous glands, located primarily at the interspaces between the cartilaginous rings, and some fat cells. (3) The adventitia contains the cartilaginous rings interconnected by connective tissue. Each ring is composed of hyaline cartilage, appears in the form of the letter C or Y, and is open posteriorly. The open ends are connected by fibroelastic tissue and a band of smooth muscle (trachealis muscle). This soft tissue band on the posterior surface of the trachea, which faces the esophagus, is capable of yielding to esophageal dilation resulting from the passage of food or liquid. The cartilaginous rings mechanically hold the airway open but also give it flexibility. By preventing the collapse of the conducting pathway, respiration is not impeded (see Plates 224 and 225).
The lungs are contained within the paired pleural cavities in the thorax. The lungs lie free within the pleural sac but are firmly attached at the hilus to the mediastinum, where a primary bronchus and the pulmonary vessels are located.
The designated subdivisions of the respiratory tree distal to the bronchus are the bronchiole, respiratory bronchiole, alveolar duct, alveolar sac, and alveolus.
The epithelium of the bronchus is pseudostratified columnar ciliated, with numerous goblet cells. The subsequent branching and reduction in size of the bronchi result in a change in the epithelium to simple columnar ciliated, with abundant goblet cells. The lamina propria is encircled by a smooth muscle layer, which, when contracted, gives the tube a folded appearance. The connective tissue outside the muscle contains seromucous glands. Solitary lymph nodules may be present in the mucosa and in the connective tissue around the cartilage. As the bronchi undergo progressive reduction in size through dichotomous branching, the cartilage support is eventually lost, at which time the passageway is termed a bronchiole.
As a smaller subdivision of the conducting tube, the bronchiole varies in size from about 0.5 to 1 mm in diameter. The ciliated epithelium is columnar and diminishes to a cuboidal form as the branching to successively smaller bronchioles continues. The muscle layer becomes the dominant structure and is composed of smooth muscle and elastic fibers. Within the adventitia, there is a reduction and then elimination of glands and lymph nodules. The mucosa of the smaller bronchioles may be highly folded owing to loss of firm supporting structures (see Plates 30, 225, and 226). Distal to the smallest bronchiole (terminal bronchiole) is the respiratory bronchiole. These crucially important segments of the lung mark the transition from the conducting to the respiratory passages, where oxygen and carbon dioxide are exchanged. The respiratory passages include (1) respiratory bronchioles, (2) alveolar ducts, (3) alveolar sacs, and (4) alveoli.
The epithelium of the respiratory bronchiole is primarily cuboidal and may be ciliated. Goblet cells are absent. The epithelium is supported by a thin collagenous layer in which smooth muscle and some elastic fibers are found. Alveoli appear as small pockets that interrupt the main wall. Alveoli become more numerous distally. The respiratory bronchiole branches to form alveolar ducts. These thin- walled, fibroelastic tubes are lined with a squamous epithelium and possess alveoli that appear as outpockets of the main wall. The main wall of the duct between alveoli contains smooth muscle. The terminal portion of the respiratory duct (atrium) gives rise to the alveolar sacs, composed of a variable number of alveoli that appear as small compartments opening into the alveolar sac. The alveoli are the smallest and most numerous subdivisions of the respiratory system. The interalveolar septum often contains 10 to 15 µm openings between neighboring alveoli that function to equalize air pressure in adjoining alveoli.
The alveolar wall is lined by a very thin (as thin as 25 nm) squamous epithelium (so-called Type I cells) covered with a thin film of fluid rich in hydrophilic phospholipid. This coating (pulmonary surfactant) is produced by Type II cells (also known as great alveolar cells or septal cells). Surfactant aids in keeping the alveoli open by reducing the surface tension of the moist interface between opposing alveolar surfaces, and thus reducing the inspiratory work required in breathing. Hyaline membrane disease in newborns has been correlated with insufficient pulmonary surfactant; neonates with this disorder have great difficulty in opening and expanding alveoli so that oxygen and carbon dioxide exchange can take place. The respiratory epithelium is composed largely of Type I cells (97 per cent), with the remainder being Type II cells. These cell types both rest on a basal lamina, which is, in turn, intimately associated with capillaries of the pulmonary vascular system. The extremely thin wall and rich capillary bed favor the transfer of oxygen to the red blood corpuscles and the release and transfer of carbon dioxide to the alveolar airway. The airway contains macrophages, which are monocytes derived from bone marrow. These so-called dust cells phagocytize debris that is found on alveolar surfaces and are important in the defense by the body against pathogens. Dust cells, laden with phagocytized material, are carried on a moving carpet of mucus to the pharynx, where they are swallowed and destroyed.
Several important arrangements are essential to respiratory function. The lungs are enclosed in the pleural cavities, which enlarge as the thorax expands during inspiration. This results in a negative pressure within the respiratory tree and air enters the system. During the filling of the lungs, the rich elastic network throughout the system becomes stretched, expanding and elongating the alveolar ducts and drawing oxygen-rich air into, and mixing it with, the carbon dioxide- rich air. During expiration, air is expelled from the system by elastic recoil as the thoracic cavity decreases in size. During muscular exercise, skeletal muscle assists in this process.
If the pleural cavities are exposed to air from the outside through an artificial opening in the thoracic wall (pneumothorax), the lungs will collapse and will not fill during inspiration owing to the equalization of the air pressure inside and outside the lungs.
Loss of elasticity and breakdown of interalveolar septa gives rise to emphysema, also often a sequel to long-term smoking. Smoking may cause a wide variety of pulmonary disorders quite apart from the well-documented correlation with the incidence of lung cancer.
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