Why does blood change color after a period of death?

When people breathe, the oxygen tension in the lungs is high, and hemoglobin in red blood cells combines with oxygen in capillaries to form oxygenated hemoglobin. When hemoglobin combines with oxygen, blood turns bright red and becomes arterial blood. When it combines with carbon dioxide, blood turns dark red and becomes venous blood.

After death, the oxygen in the body will be consumed, so the color of blood will be different before and after death. In addition, some components in blood will fail under the condition of lack of oxygen and water, which will also cause the change of blood color.