Pulmonary vascular resistance is least at which lung volume?

Pulmonary vascular resistance is least at which lung volume?

• A. Total lung volume
• B. Residual volume
• C. Functional residual capacity (FRC)
• D. Expiratory reserve volume

Answer: (C)

Pulmonary vascular resistance changes with lung volume because the size and patency of the pulmonary vessels are shaped by surrounding alveoli and the tensions on the vessel walls. At very low lung volumes, such as near residual volume, alveolar walls compress the capillaries, pushing PVR higher. As the lungs inflate toward mid-volume, the capillaries become less compressed and recruit more, lowering PVR. If you keep inflating toward very high volumes, PVR rises again because the vessels are stretched and extra-alveolar vessels are affected by increased intrathoracic pressure, narrowing the bed once more. Functional residual capacity sits at that mid-volume range where the vascular bed is least compressed and most open, so PVR is minimal there. That’s why functional residual capacity is the volume at which PVR is least.

Pulmonary vascular resistance changes with lung volume because the size and patency of the pulmonary vessels are shaped by surrounding alveoli and the tensions on the vessel walls. At very low lung volumes, such as near residual volume, alveolar walls compress the capillaries, pushing PVR higher. As the lungs inflate toward mid-volume, the capillaries become less compressed and recruit more, lowering PVR. If you keep inflating toward very high volumes, PVR rises again because the vessels are stretched and extra-alveolar vessels are affected by increased intrathoracic pressure, narrowing the bed once more. Functional residual capacity sits at that mid-volume range where the vascular bed is least compressed and most open, so PVR is minimal there. That’s why functional residual capacity is the volume at which PVR is least.