Human blood is red because it contains a large number of red blood cells. The red blood cells contain hemoglobin, which contains a red colored compound called hemoglobin, which is essential for the transport of oxygen in the blood. Hemoglobin contains iron atoms that bind to oxygen and transport it from the lungs to the rest of the body. Hemoglobin bound to oxygen is called oxyhemoglobin and absorbs blue-green light, causing red-orange light to reflect back to our eyes as red, which is why blood turns a bright red-cherry color when oxygen is bound to hemoglobin, and a darker red color when there is no oxygen to bind to.