Dark-colored blood, called venous blood, is dark because it has become deoxygenated after having traveled through the body before returning to the heart drained of oxygen. Blood leaving the heart, called arterial blood, is pumped from the lungs back to the heart by the pulmonary artery, and it contains a large amount of oxygen for delivery around the body. Arterial blood, therefore, appears a brighter red than deoxygenated venous blood.
Blood gets its color from red blood cells called hemoglobin, to which oxygen binds. Ninety-seven percent of the oxygen transported by the blood from the lungs is carried by hemoglobin. Besides carrying oxygen around the body, hemoglobin helps remove carbon dioxide from the body. The difference in color between venous blood and arterial blood is known as the arteriovenous oxygen difference. Venous blood is also colder than arterial blood due to its lower oxygen content and pH.
A popular and persistent misconception indicates that deoxygenated blood is blue because veins may appear to be blue due to the way light is diffused by the epidermis. Human skin may also take on a bluish color, known as cyanosis, sometimes as a result of certain medical conditions; however, deoxygenated human blood is never blue.