Final answer:
Muscle contraction and valves play a crucial role in venous and lymphatic flow in the human body.
Explanation:
In the human body, venous and lymphatic flow are primarily accomplished by muscle contraction, which helps push fluid upwards, and valves, which prevent the flow of fluid backward. Additionally, subatmospheric pressure plays a role in helping maintain the pressure gradient needed for venous blood flow back to the heart.
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