Final answer:
When the muscles of the legs contract and relax, it increases pressure on nearby veins, helping counteract the force of gravity and move blood back to the heart. Failure to flex the legs while standing for prolonged periods can lead to blood pooling in the lower limbs and insufficient oxygenated blood reaching the brain.
Step-by-step explanation:
When the muscles of the legs contract and relax, it has an effect on blood flow. The contraction of leg muscles during activities like walking or running puts pressure on nearby veins, causing blood to flow upward. This increases pressure, helps counteract the force of gravity, and moves blood back to the heart.
The one-way valves in the veins open above the contracting muscles, allowing blood to flow through, while the valves below the contracting muscles close to prevent blood from seeping back down. Failure to flex the legs while standing for prolonged periods can lead to blood pooling in the lower limbs and insufficient oxygenated blood reaching the brain.