Blood is generally bright red in the arteries and dark red in the veins. What accounts for the color difference?

• A. The amount of hemoglobin
• B. The size of arteries
• C. The exchange of carbon dioxide for oxygen
• D. Oxygen content of the blood

Answer: (C)

The color difference comes from how much oxygen is bound to hemoglobin in the blood. In the lungs, hemoglobin binds oxygen and becomes oxyhemoglobin, which is bright red. After releasing oxygen to tissues, the hemoglobin is in a deoxygenated form, which is a darker red. Arterial blood, rich in oxygen, appears bright red, while venous blood, with less oxygen, appears darker. The exchange of carbon dioxide for oxygen happens at the tissues and in the lungs, but it is the oxygen content of the blood that directly determines the arterial versus venous color. The amount of hemoglobin or the size of the arteries does not explain the color change.