Veins

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to create the [|Voki] \ 1.Veins have three layers,an outer layer of tissue muscle in the middle and a smooth inner layer of epithelial cells. 2. The vein valves are necessary to keep blood flowing toward the heart, but they are also necessary to allow blood to flow against the force of gravity. 3.When you faint, your head comes down to the same level as your heart, making it easy for the blood to quickly reach the brain. 4.The vein valves, however, provide footholds for the blood as it climbs its way up.Blood that flows up to the brain faces the same problem. If the blood is having a hard time climbing up, you will feel light-headed and possibly even faint.

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