Which of the following is NOT a resistance variable to venous return?

The correct answer centers on the distinction between resistive variables that specifically affect venous return and those that do not. Height differences between the patient and the venous reservoir primarily contribute to the hydrostatic pressure gradient rather than resistance itself. In the context of venous return, hydrostatic pressure significantly influences flow, with gravity playing a fundamental role, but this factor does not directly alter the resistance to flow.

On the other hand, factors such as cannula diameter and length, venous line diameter and length, and blood viscosity directly influence the resistance to flow by affecting the ease with which blood travels through the vascular system. Larger diameters reduce resistance, while longer lengths increase it. Blood viscosity plays a significant role as well; thicker blood naturally encounters more resistance to flow than thinner blood. Therefore, the right choice recognizes that while hydrostatic pressure impacts venous return, it doesn't constitute a resistance variable in the same way that the other options do.