Thus far, we have assumed that if ventilation and cardiac output are normal, gas exchange will also be normal. Unfortunately, this is not the case. Even though total ventilation and total blood flow (i.e., cardiac output) may be normal, there are regions in the lung where ventilation and blood flow are not matched, so that a certain fraction of the cardiac output is not fully oxygenated.
The matching of airflow and blood flow is best examined by considering the ventilation-perfusion ratio, which compares alveolar ventilation to blood flow in lung regions. Since resting healthy individuals have an alveolar ventilation (Va) of 4 L/min and a cardiac output (pulmonary blood flow or perfusion) of 5 L/min, the ideal alveolar ventilationperfusion ratio (Va/Qratio) should be 0.8 (there are no units, as this is a ratio). We have already seen that gravity can cause regional differences in blood flow and alveolar ventilation (see Chapter 19). In an upright person, the base of the lungs is better ventilated and better perfused than the apex.
Regional alveolar ventilation and blood flow are illustrated in Figure 20.11. Three points are apparent from this figure:
• Ventilation and blood flow are both gravity-dependent;
airflow and blood flow increase down the lung.
• Blood flow shows about a 5-fold difference between the top and bottom of the lung, while ventilation shows about a 2-fold difference. This causes gravity-dependent regional variations in the Va/Qratio that range from 0.7 at the base to 3 or higher at the apex. Blood flow is proportionately greater than ventilation at the base, and ventilation is proportionately greater than blood flow at the apex.
The functional importance of lung ventilation-perfusion ratios is that the crucial factor in gas exchange is the matching of regional ventilation and blood flow, as opposed to total alveolar ventilation and total pulmonary blood flow. The distribution of Va/Q in a healthy adult is shown in Figure 20.12. Even in healthy lungs, most of the ventilation and perfusion go to lung units with a Va/Q ratio of about 1 instead of the ideal ratio of 0.8. At the apical region, where the Va/Q ratio is high, there is overventilation relative to blood flow. At the base, where the ratio is low, the opposite occurs (i.e., overperfusion relative to ventilation). In the latter case, a fraction of the blood passes through the pulmonary capillaries at the base of the lungs without becoming fully oxygenated.
The effect of regional Va/Q ratio on blood gases is shown in Figure 20.13. Because overventilation relative to blood flow (high Va/Q) occurs in the apex, the Pao2 is high and the Paco2 is low at the apex of the lungs. Oxygen tension (Po2) in the blood leaving pulmonary capillaries at the base of the lungs is low because the blood is not fully oxygenated as a result of underventilation relative to blood
Regional alveolar ventilation and blood flow. Gravity causes a mismatch of blood flow and alveolar ventilation in the base and apex of the lungs. Both ventilation and perfusion are gravity-dependent. At the base of the lungs, blood flow exceeds alveolar ventilation, resulting in a low ventilation-perfusion ratio. At the apex, the opposite occurs,-alveolar ventilation is greater than blood flow, resulting in a high ventilation-perfusion ratio.
flow. Regional differences in Va/Q ratios tend to localize some diseases to the top or bottom parts of the lungs. For example, tuberculosis tends to be localized in the apex because of a more favorable environment (i.e., higher oxygen levels for Mycobacterium tuberculosis).
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