Porth's Essentials of Pathophysiology, 4e

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Control of Respiratory Function

C h a p t e r 2 1

The movement of blood through the pulmonary cap- illary bed requires that the mean pulmonary arterial pressure be greater than the mean pulmonary venous pressure. Distribution of Pulmonary Blood Flow As with ventilation, the distribution of pulmonary blood flow is affected by body position and gravity. In the upright position, the distance of the upper apices of the lung above the level of the heart may exceed the per- fusion capabilities of the mean pulmonary arterial pres- sure (approximately 12 mm Hg); therefore, blood flow in the upper part of the lungs is less than that in the base. In the supine position, the lungs and the heart are at the same level, and blood flow to the apices and base of the lungs becomes more uniform. In this position, however, blood flow to the posterior or dependent portions (e.g., bottom of the lung when lying on the side) exceeds flow in the anterior or nondependent portions of the lungs. The alveolar concentration of oxygen also affects pulmonary blood flow. When the concentration of oxy- gen in air of the alveoli decreases below normal blood, the adjacent blood vessels constrict in an effort to dis- tribute blood where it will be most effectively oxygen- ated. When alveolar oxygen levels drop below 60 mm Hg, marked vasoconstriction occurs, and at very low oxygen levels, the local flow may be almost abolished. In regional hypoxia, as occurs with a localized airway obstruction (e.g., atelectasis), vasoconstriction is local- ized to a specific region of the lung. Generalized hypoxia, such as occurs at high altitudes causes vasoconstriction throughout all of the vessels of the lung. Shunt Shunt refers to blood that moves from the right to the left side of the circulation without being oxygenated. As with dead air space, there are two types of shunts: anatomic and physiologic. In an anatomic shunt , blood moves from the venous to the arterial side of the cir- culation without moving through the lungs. Anatomic shunting of blood is most commonly due to congenital heart defects (see Chapter 19). In a physiologic shunt , there is mismatching of ventilation and perfusion within the lung, resulting in insufficient ventilation to provide the oxygen needed to oxygenate the blood flowing through the alveolar capillaries. Physiologic shunting of blood usually results from destructive lung disease that impairs ventilation or from heart failure that interferes with movement of blood through sections of the lungs.

Airways

Venous blood

Arterial blood

Perfusion without ventilation

Ventilation without perfusion

Alveolus

Normal

left), there is perfusion without ventilation, resulting in a low ventilation-perfusion ratio. It occurs in conditions such as atelectasis in which there is airway obstruction (see Chapter 23). With dead air space (depicted on the right), there is ventilation without perfusion, resulting in a high ventilation–perfusion ratio. It occurs with con- ditions such as pulmonary embolism, which impairs blood flow to a part of the lung. The arterial blood leav- ing the pulmonary circulation reflects mixing of blood from normally ventilated and perfused areas of the lung as well as areas that are not perfused (dead air space) or ventilated (shunt). Many of the conditions that cause mismatching of ventilation and perfusion involve both dead air space and shunt. In chronic obstructive lung disease, for example, there may be impaired ventila- tion in one area of the lung and impaired perfusion in another area. Diffusion Diffusion takes place in the respiratory portions of the lung and refers to the movement of gases across the alveolar–capillary membrane. Diffusion of gases in the lung is affected by (1) difference in the pressure of the gas on either side of the membrane, (2) the surface area that is available for diffusion, (3) the thickness of the alveolar–capillary membrane through which the gas must pass, and (4) the diffusion characteristics of the gas. Administration of high concentrations of oxy- gen increases the difference in partial pressure between the two sides of the membrane and increases the diffu- sion of the gas. Diseases that destroy lung tissue and the surface area for diffusion and those that increase the thickness of the alveolar–capillary membrane adversely influence the diffusing capacity of the lungs. FIGURE 21-16. Matching of ventilation and perfusion. (Center) Normal matching of ventilation and perfusion; (left) perfusion without ventilation (i.e., shunt); (right) ventilation without perfusion (i.e., dead air space).

Mismatching of Ventilation and Perfusion

The gas exchange properties of the lung depend on matching ventilation and perfusion, ensuring that equal amounts of air and blood are entering the respiratory portion of the lungs. Both dead air space and shunt produce a mismatching of ventilation and perfusion, as depicted in Figure 21-16. With shunt (depicted on the