Flow in the subclavian vein may increase when the patient is supine due to what reason?

• A. Decrease in hydrostatic pressure
• B. Increase in hydrostatic pressure
• C. No change in venous pressure
• D. Increased venous resistance

Answer: (A)

Position and hydrostatic pressure drive venous return. When a person lies down, the vertical distance between the subclavian veins and the heart is greatly reduced, so the hydrostatic pressure in those veins falls. With less hydrostatic load opposing the return of blood to the heart, the pressure gradient toward the right atrium becomes more favorable, and flow in the subclavian vein increases.

In contrast, if hydrostatic pressure were increased (as in standing), more pressure would be present in the arm veins to overcome and pooling can occur, which tends to reduce forward flow. Changes in venous resistance or the idea of no change in venous pressure don’t fit the positional effect observed.