Vein Structure
This video presents the same text shown beside it, spoken and on screen. It adds nothing the text does not say.
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A vein carries blood toward the heart through a thinner wall than an artery, relying on one-way valves and surrounding muscle to keep low-pressure flow moving.
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Valves inside leg veins stop blood pooling backward under gravity while a person stands.
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"Vein" names a direction of flow, toward the heart — it does not specify what the blood contains.