During which phase do veins refill passively?

Veins refill passively during the relaxation phase. In this phase, the pressure within the veins decreases due to the reduction in blood volume being pumped from the heart. This creates a negative pressure gradient that allows blood to flow back into the veins without the need for muscular contractions.

The relaxation phase specifically refers to the points in the cardiac cycle where the heart muscle relaxes, leading to a decrease in intravascular pressure. This allows venous return to occur more easily as the surrounding tissues create a pull on the blood within the veins.

In contrast, the contraction phase involves active muscular engagement that pushes blood through the veins, while the inflow phase pertains to when blood is actively being delivered into a particular region, and the pressure phase refers to the level of pressure inside the vessels. None of these accurately describe the passive nature of venous filling that characterizes the relaxation phase.