In the circulatory system, veins (from the Latin vena) are blood vessels that carry blood toward the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood. They differ from arteries in structure and function; for example, arteries are more muscular than veins and they carry blood away from the heart.

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