Vessel Direction, Not Oxygen Content, Defines Artery and Vein
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An artery is defined by carrying blood away from the heart, a vein by carrying blood toward it, per Bio 3203 and 3205 — oxygen content varies by circuit.
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The pulmonary vein carries oxygenated blood toward the heart, matching its vein classification.
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"Arteries carry oxygenated blood," per Bio 3209, breaks down exactly at the pulmonary loop this definition explains.