Question

multiple choice question
why does the body need skeletal muscle contraction, pressure changes during breathing, and vasoconstriction to move blood through the veins?
the blood pressure in the veins is too low to push blood toward the heart.
veins contain valves which partially obstruct blood flow in them.
veins are so superficial that the force of the heart cant reach them.

Explanation:

The blood pressure in veins is low. Skeletal muscle contraction, pressure changes during breathing, and vasoconstriction help overcome this low - pressure situation to move blood towards the heart. Valves in veins prevent back - flow rather than obstruct blood flow. Veins' superficial location is not the reason for the need of these辅助 mechanisms as the heart's force is not related to vein depth in this context.

Answer:

The blood pressure in the veins is too low to push blood toward the heart.