02/28/2026
Human blood appears in two different shades depending on its oxygen content.
Arterial blood, the blood that has just passed through the lungs, carries a high amount of oxygen bound to hemoglobin. When oxygen attaches to hemoglobin, it slightly changes the molecule’s structure and how it reflects light. This makes oxygen-rich blood appear bright red.
Venous blood, on the other hand, has already delivered much of its oxygen to body tissues. With less oxygen attached, hemoglobin reflects light differently, giving this blood a darker, deeper red or maroon tone.
Despite a common myth, blood inside the human body is never blue. Veins only appear bluish through the skin because of how light passes through tissue and how our eyes interpret color. The blood itself remains red at all times, just varying in brightness depending on oxygen levels.
This difference is a straightforward result of oxygen chemistry and light interaction with hemoglobin.
Source: American Heart Association; National Library of Medicine