Human blood is red inside the body because of the numerous red blood cells, which contain hemoglobin. However, the blood color ranges from bright red to dark red.
When blood leaves the heart to be transported to other organs, it carries oxygen in its hemoglobin. Blood looks bright red when oxygen is bound to the iron within it. Once oxygen is released to the tissues and carbon dioxide binds the deoxygenated blood, it looks darker in color. All vertebrates have red blood, and there are no circumstances in which it has been otherwise. This refutes the myth that deoxygenated blood is blue.