Why does the blood turns dark red as it circulates through out body?

The blood does not turn dark red as it circulates throughout the body. Oxygenated blood is bright red, while deoxygenated blood is darker red. As blood circulates through the body, it picks up oxygen from the lungs and delivers it to the tissues. As the blood delivers oxygen, it becomes deoxygenated and turns a darker red. This is why the blood in the arteries is bright red, while the blood in the veins is darker red.