The right answers are a. Veins have one-way valves... AND b. The contraction of skeletal muscles helps push the blood...
The venous system is equipped with an anti-reflux device: valves "check valves":
these are multiple "check valves", called valves, inserted at different levels within all the veins. The valves are open and help the venous return by directing the blood in the right direction (upwards) during muscle contraction. Valves close, preventing blood from flowing back down during muscle relaxation.
The muscles have a facilitating role in the venous return:
The muscular pumps namely the muscles of the sole of the foot, the muscles of the calf and the thigh, relayed by a passive effect of abdomino-thoracic aspiration linked to breathing, allow the venous return. By contracting, the muscles compress and massage the veins that surround them, chasing the blood they contain.
Indeed, in the absence of physical activity, the muscular pumps are stopped, the valves are open. Thus in the standing position, the difference in pressure between the ankles and the heart prevents an effective venous return.
When the body starts moving, the muscle pumps come into action and after a few steps, the pressure difference between the ankles and the heart decreases allowing a good venous return.
In the sitting position, the pressure difference between the ankles and the heart is still too great. Prolonged immobility, even when seated, compromises drainage. Such conditions promote typical postural venous insufficiency.
