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Human lips have a reddish color because of the great concentration of tiny capillaries just below the skin. The blood in these capillaries is normally highly oxygenated and therefore quite red. This explains why the lips appear pale when a person is anemic or has lost a great deal of blood. It also explains why the lips turn blue in very cold weather. Cold causes the capillaries to constrict, and the blood loses oxygen and changes to a darker color.
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