In a static CO model, what is the effect of increasing right atrial pressure on the venous return equilibrium?

• A. Higher RAP increases venous return.
• B. Higher RAP reduces the pressure gradient driving venous return, lowering venous return and CO.
• C. RAP has no effect on venous return.
• D. Higher RAP increases venous return but lowers CO.

Answer: (B)

The main idea being tested is how right atrial pressure (RAP) shapes the driving force for venous return in a static model. Venous return depends on the pressure difference between the mean systemic filling pressure (MSFP) and RAP. RAP acts as a back pressure on the venous side: as RAP rises, the gradient (MSFP − RAP) shrinks, so less blood can flow back to the heart. In a static model, MSFP is treated as a constant, so increasing RAP directly reduces the venous return. Since cardiac output must balance venous return at steady state, CO also falls. If RAP rose high enough toward MSFP, venous return could approach zero.

That’s why the best answer says that higher RAP reduces the pressure gradient driving venous return, lowering venous return and CO.