Which condition increases pulmonary blood flow?

• A. Acidosis
• B. Hypoxemia
• C. Hypercarbia
• D. Alkalosis

Answer: (D)

Pulmonary blood flow is controlled by the resistance in the pulmonary vessels; when the vessels relax and resistance drops, more blood flows through the lungs. Alkalosis lowers pulmonary vascular resistance by promoting relaxation of the pulmonary arteries, so blood flow to the lungs increases. In contrast, acidosis, hypoxemia, and hypercarbia raise pulmonary vascular resistance through vasoconstrictive pathways or CO2-related effects, which reduces pulmonary blood flow. So the state that increases flow is alkalosis because it causes pulmonary vasodilation and a decrease in resistance.