Why is human blood red? Why arterial blood is bright red, and venous blood is dark red.

1) the red color of human blood is given by the hemoglobin protein contained in erythrocytes, it consists of a protein part – globin and a compound containing iron – heme;
2) arterial blood has a bright red color due to oxyhemoglobin – the combination of hemoglobin with oxygen;
3) venous blood has a dark red color due to carbhemoglobin – the combination of hemoglobin with carbon dioxide

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