Porth's Essentials of Pathophysiology, 4e

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Respiratory Function

U N I T 6

of gestation; consequently, many premature infants have difficulty producing sufficient amounts of surfactant. This can lead to alveolar collapse and severe respiratory distress. This condition, called infant respiratory distress syndrome , is the single most common cause of respira- tory disease in premature infants. Recombinant forms of surfactant are now available for use in treatment of the disorder (see Chapter 22). The synthesis of surfactant can also be impaired in the adult. This usually occurs as the result of severe injury or infection and can contrib- ute to the development of a condition called the acute respiratory distress syndrome (see Chapter 24). Airway Resistance During breathing, the flow or volume of air that moves into and out of the lungs is directly related to the pres- sure difference between the lungs and the atmosphere and inversely related to the resistance that the air encounters as it moves through the conducting airways. Airway radius. The primary determinant of airway resistance to airflow is the radius of the conducting airway. Normally, the radius is large enough so that a gradient of less than 1 cm/H 2 O pressure is needed for sufficient airflow during quiet breathing. The site of most of the resistance occurs in the larger bronchioles and bronchi near the trachea, with the smallest air- ways contributing very little to the total airway resis- tance. The low resistance of the smaller bronchioles can be explained in terms of their large number and their parallel arrangements. Many airway diseases, such as emphysema and chronic bronchitis, begin in the small airways. Early detection of these diseases is often dif- ficult because a considerable amount of damage must be present before the usual diagnostic measurements of airway resistance can detect them. Lung Volume. Airway resistance is also affected by lung volume, being less during inspiration than during expiration. This is because elastic-type fibers connect the outside of the airways to the surrounding lung tis- sues. As a result, these airways are pulled open as the lungs expand during inspiration, and they become nar- rower as the lungs deflate during expiration (Fig. 21-13). This is one of the reasons persons with conditions that increase airway resistance, such as bronchial asthma, usually have less difficulty during inspiration than dur- ing expiration. Neural and Local Control of Airway Diameter. Constriction of bronchial smooth muscle, which controls airway diameter, also contributes to airway resistance. The smooth muscles in the airways are under autonomic nervous system control. Stimulation of the parasympa- thetic nervous system produces bronchial constriction as well as increased mucus secretion, whereas sympathetic stimulation has the opposite effect. Parasympathetic nerves can be stimulated by reflexes that originate in the lungs, most of which begin with irritation of the epi- thelial cells by cigarette smoke, dust, noxious gases, or

Low lung volume

bronchial infections. Inflammatory mediators such as the leukotrienes, histamine, and prostaglandins released by resident mast cells and airway epithelial cells can also cause bronchial constriction. Many of the drugs used in the treatment of bron- chial asthma and other respiratory conditions that produce bronchial constriction act at the level of the autonomic nervous system or inflammatory mediators to relieve airway obstruction. For example, β -adrenergic stimulants are often used to achieve short-term relief of asthma symptoms. Anti-inflammatory agents, such as the corticosteroids or leukotriene antagonists, are used to achieve long-term relief (see Chapter 3, Fig. 3-4). Laminar and Turbulent Airflow. Depending on the velocity and pattern of flow, airflow can be laminar or turbulent. Laminar , or streamlined , airflow occurs at low flow rates in which the air stream is parallel to the sides of the airway. With laminar flow, the air at the periphery must overcome the resistance to flow; and as a result, the air in the center of the airway moves faster. In the bronchial tree with its many branches, laminar airflow probably occurs only in the very small airways, where the velocity of flow is low. Because the small air- ways contribute little resistance to airflow, they consti- tute a silent zone in terms of respiratory sounds. Turbulent airflow is disorganized flow in which the molecules of the gas move laterally, collide with one High lung volume FIGURE 21-13. Interaction of tissue forces on airways during low and high lung volumes. At low lung volumes, the tissue forces promote folding or collapsing and place less tension on the airways, which become smaller; during high lung volumes, the tissue forces stretch and pull the airways open.