An acute increase in PaCO2 results in respiratory acidosis (pH < 7.35), whereas an acute decrease in PaCO2 results in respiratory alkalosis (pH > 7.45). Hypercapnia is defined as an elevation in PaCO2, and it occurs when CO2 production exceeds alveolar ventilation (hypoventilation). Conversely, hyperventilation occurs when alveolar ventilation exceeds CO2 production, and it decreases PaCO2 (hypocapnia).
• Fig. 23.1 The Alveolar Partial Pressure of Carbon Dioxide (PCO2; y-axis) as a Function of Alveolar Ventilation per Minute (VA; x-axis) • Fig. 23.2 Regional Distribution of Lung Volume, Including in the Lung. Each line corresponds to a given metabolic rate asso-
Alveolar Size (Circles) and Location on the Pressure-Volume ciated with a constant production of CO2 CO2 isometabolic line).
Curve of the Lung at Different Lung Volumes. Because the lungs are suspended in the upright position, the pleural pressure (Ppl) and translung pressure (PL) of lung units at the apex are greater than those at the base. These lung units are larger at any lung volume than are those at the base. The effect is greatest at residual volume (RV), less so at functional residual capacity (FRC), and absent at total lung capacity (TLC). Note also that because of their “location” on the pressure-volume curve, inspired air is differentially distributed to these lung units; those at the apex are less compliant and receive a smaller Normally, alveolar ventilation is controlled to maintain an alveolar PCO2 of approximately 40 mm Hg. Thus at rest, when VCO2 is approximately 250 mL/minute, alveolar ventilation of 5 L/minute results in an alveolar
PCO2 of 40 mm Hg. A 50% decrease in ventilation at rest (i.e., from 5 to 2.5 L/minute) results in doubling of alveolar PCO2. During exercise,
CO2 production is increased (V̇ CO2 = 750 mL/min), and to maintain normal alveolar PCO2, ventilation must increase (in this case, to 15 L/ minute). Again, however, a 50% reduction in ventilation (from 15 to 7.5 L/minute) results in doubling of the alveolar PCO2.
proportion of the inspired air than do the lung units at the base, which are more compliant (i.e., are represented at a steeper part of the
Distribution of Ventilation
Ventilation is not uniformly distributed in the lung, largely because of the effects of gravity. In the upright position, at most lung volumes, alveoli near the apex of the lung are more expanded than are alveoli at the base. Gravity pulls the lung downward and away from the chest wall. As a result, pleural pressure is lower (i.e., more negative) at the apex than at the base of the lung, and static translung pressure (PL = PA − Ppl) is increased; this results in an increase in alveolar volume at the apex. Because of the difference in alveolar volume at the apex and at the base of the lung (Fig.
23.2 ), alveoli at the lung base are represented along the steep portion of the pressure-volume curve, and they receive more of the ventilation (i.e., they have greater compliance). In contrast, the alveoli at the apex are represented closer to the top or flat portion of the pressure-volume curve. They have lower compliance and thus receive proportionately less of the tidal volume. The effect of gravity is less pronounced when a person is supine rather than upright, and it is less pressure-volume curve).
Alveolar units with long time constants fill and empty slowly. Thus an alveolar unit with increased airway resistance or increased compliance takes longer to fill and longer to empty. In adults, the normal respiratory rate is approximately 12 breaths per minute, the inspiratory time is approximately 2 seconds, and the expiratory time is approximately 3 seconds. In normal lungs, this time is sufficient to approach volume equilibrium (