Human blood is red in color because it contains a large number of red blood cells. Red blood cells contain hemoglobin, which contains a red 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 other parts of the body. The hemoglobin that binds to oxygen is called oxyhemoglobin and absorbs blue-green light, causing red-orange light to reflect back to our eyes in a red color, which is why blood turns a bright red cherry color when oxygen binds to hemoglobin, and a dark red color when no oxygen is bound.