Porth's Essentials of Pathophysiology, 4e

528

Respiratory Function

U N I T 6

inversely related to its radius; therefore, the small air- ways close first, trapping some air in the alveoli. This trapping of air may be increased in older persons and persons with chronic lung disease owing to a loss in the elastic recoil properties of the lungs. In these persons, airway closure occurs at the end of normal instead of low lung volumes, trapping larger amounts of air that cannot participate in gas exchange. Dead Air Space Dead space refers to the air that must be moved with each breath but does not participate in gas exchange. The movement of air through dead space contributes to the work of breathing but not to gas exchange. Some of the air that enters the respiratory tract during breath- ing fails to reach the alveoli. This volume (about 150 to 200 mL), which remains in the conducting airways of the nose, pharynx, trachea, bronchi, and bronchioles and does not participate in gas exchange, is referred to as anatomic dead space . A second type of dead space, physiological dead space, consists of the total amount of air that does not participate in gas exchange. It includes the anatomic dead space plus the dead space in alveoli that are perfused, but not ventilated. Physiologic dead space tends to be the same as anatomic dead space in persons with normal respiratory function, but can be considerably larger in the presence of lung disease. Perfusion The term perfusion is used to describe the flow of blood through the gas exchange portion of the lung. Deoxygenated blood enters the lung through the pul- monary artery, which has its origin in the right side of the heart and enters the lung at the hilus, along with the primary bronchus. The pulmonary arteries branch in a manner similar to that of the airways. The small pulmo- nary arteries accompany the bronchi as they move down the lobules and branch to supply the capillary network that surrounds the alveoli (see Fig. 21-7). The oxygen- ated capillary blood is collected in the small pulmonary veins of the lobules, and then it moves to the larger veins to be collected in the four large pulmonary veins that empty into the left atrium. The pulmonary blood vessels are thinner, more com- pliant, and offer less resistance to flow than those in the systemic circulation, and the pressures in the pulmo- nary system are much lower (e.g., 22/8 mm Hg versus 120/70 mm Hg). The low pressure and low resistance of the pulmonary circulation accommodate the delivery of varying amounts of blood from the systemic circulation without producing signs and symptoms of congestion. The volume in the pulmonary circulation is approxi- mately 500 mL, with approximately 100 mL of this volume located in the pulmonary capillary bed. When the input of blood from the right heart and output of blood to the left heart are equal, pulmonary blood flow remains constant. Small differences between input and output can result in large changes in pulmonary vol- ume if the differences continue for many heartbeats.

Exchange of Gases Within the Lungs

The primary functions of the lungs are oxygenation of the blood and removal of carbon dioxide. Pulmonary gas exchange is conventionally divided into three pro- cesses: (1) ventilation or the flow of gases into and out of the alveoli of the lungs, (2) perfusion or flow of blood in the adjacent pulmonary capillaries, and (3) diffusion or transfer of gases between the alveoli and the pulmo- nary capillaries. Alveolar Ventilation The ultimate importance of the alveolar ventilation is to continually renew the air in the gas exchange areas of the lungs where the air is in close proximity to the blood. These areas include the alveoli, alveolar sacs, alveolar ducts, and respiratory bronchioles. It is affected by body position and lung volume as well as by disease conditions that affect the heart and respiratory system. Distribution of Alveolar Ventilation The distribution of ventilation between the base (bot- tom) and apex (top) of the lung varies with body posi- tion and reflects the effects of gravity on intrapleural pressure and lung compliance. Compliance reflects the change in volume that occurs with a change in intra- pleural pressure. It is lower in fully expanded alveoli, which have difficulty accommodating more air, and greater in alveoli that are less inflated and can more eas- ily expand to accommodate more air. In the seated or standing position, gravity exerts a downward pull on the lung, causing intrapleural pressure at the apex of the lung to become more negative. As a result, the alveoli at the apex of the lung are more fully expanded and less compliant than those at the base of the lung. The same holds true for lung expansion in the dependent portions of the lung in the supine or lateral position. In the supine position, ventilation in the lowermost (posterior) parts of the lung exceeds that in the uppermost (anterior) parts. In the lateral position (i.e., lying on the side), the alveoli in the dependent lung is better ventilated. The distribution of ventilation also is affected by lung volumes. During full inspiration (high lung volumes) in the seated or standing position, the airways are pulled open and air moves into the more compliant portions of the lower lung. At low lung volumes, the opposite occurs. At functional residual capacity, the intrapleural pressure at the base of the lung exceeds airway pressure, compressing the airways so that ventilation is greatly reduced. In contrast, the airways in the apex of the lung remain open, and the alveoli in this area of the lung are well ventilated. Even at low lung volumes, some air remains in the alveoli of the lower portion of the lungs, preventing their collapse. According to the law of Laplace (dis- cussed previously), the pressure needed to overcome the tension in the wall of a sphere or an elastic tube is