Researchers are measuring regional variations in pulmonary ventilation (V) and blood flow (Q) in healthy individuals while they stand in the upright position. The results are graphed with the x-axis denoting the position along the lung from base to apex. Which of the following curves is most likely to be observed?

Question

Researchers are measuring regional variations in pulmonary ventilation (V) and blood flow (Q) in healthy individuals while they stand in the upright position.  The results are graphed with the x-axis denoting the position along the lung from base to apex.  Which of the following curves is most likely to be observed?

Quizplus · Accepted Answer

11ec160b_3ce9_8183_9249_d7e42b6bdffc_MD0001_00 In the upright position, regional differences in ventilation and perfusion occur vertically in the lungs due to gravity. Ventilation is lowest in the apex and highest in the base because gravity acts to stretch the lungs downward so that the alveoli at the apex are expanded more than those at the base (ie, slinky effect). Therefore, during inspiration a small amount of air goes to the apex where alveoli are distended and less compliant, while more air goes to the base where alveoli have ample potential space to fill and are more compliant. Perfusion is also lowest in the apex and highest in the base, as the increased hydrostatic pressure in the lower lung regions facilitates increased blood flow. Because blood is denser than air or lung tissue, the gravitational effect on perfusion is more pronounced than on ventilation, leading to greater variability in perfusion than ventilation from apex to base. Therefore, perfusion is very low in the apex and very high in the base, whereas ventilation is only somewhat low in the apex and somewhat high in the base. This causes the gradient of the ventilation/perfusion (V/Q) ratio to be opposite of the gradient of perfusion or ventilation taken individually. The V/Q ratio is lowest at the base and highest at the apex. (Choices A and B) These graphs depict the V/Q ratio decreasing from base to apex, which is the opposite of what occurs. (Choice C) This graph depicts a constant V/Q ratio of 1.0 throughout the lung, which does not occur. The V/Q ratio varies from approximately 0.6 at the base to 3.0 at the apex. A value of 0.8 is often considered the average V/Q ratio for normal lungs. (Choice E) This graph shows constant perfusion throughout the lung, which does not occur. Educational objective: Both perfusion and ventilation are highest in the base of the lung and lowest in the apex; however, the variability in perfusion is greater than that in ventilation. This causes the ventilation/perfusion ratio to follow the opposite gradient: it is lowest in the base and highest in the apex.

Researchers Are Measuring Regional Variations in Pulmonary Ventilation (V) and Blood