How does venoconstriction influence preload and venous return?

• A. Venoconstriction reduces venous capacitance, raises mean circulatory pressure, increases venous return, and increases preload.
• B. Increases venous capacitance and decreases preload.
• C. Reduces mean circulatory pressure.
• D. Has no effect on venous return.

Answer: (D)

Venoconstriction tightens veins, reducing venous capacitance. With less capacity, more blood is pushed into the stressed part of the circulation, which raises the mean circulatory filling pressure. Venous return to the heart depends on the gradient between this mean pressure and the right atrial pressure; when the mean pressure goes up, more blood flows back to the heart, increasing preload (the end-diastolic volume). So venoconstriction tends to raise venous return and preload, as long as the heart can accept the extra volume. The idea that venoconstriction would increase capacitance, reduce the driving pressure, or have no effect doesn’t fit with how venous tone changes the pressure gradient driving return.