The color of blood depends on its oxygen level. When there is plenty of oxygen, the color is a brighter red. The reason is because of a protein called hemoglobin, which carries the oxygen in your blood. When hemoglobin has a full oxygen load, it has a bright red color, and when it's deoxygenated it's a darker red. For example, arterial and capillary blood is bright, and venous blood is dark.
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