Diagrams of the human circulatory system often depict arteries in red and veins in blue, leading to a common question: Is blood in arteries truly red, and is blood in veins blue? The perceived color difference has a scientific explanation rooted in the actual color of blood and how light interacts with human tissue.
The True Colors of Blood
Human blood is consistently red, regardless of its oxygen content. The shade of red, however, varies significantly depending on how much oxygen it carries. This color is primarily due to hemoglobin, a complex protein in red blood cells. Hemoglobin contains iron, which binds with oxygen and is responsible for blood’s characteristic red hue.
When hemoglobin binds with oxygen, it forms oxyhemoglobin, giving blood a vibrant, bright red color. This oxygen-rich blood is often described as cherry red. Conversely, when blood has released most of its oxygen to tissues, deoxygenated hemoglobin results in a darker, maroon color. This distinction in shade is a direct consequence of oxygenation level, not a change to a blue pigment.
Why Arteries Appear Bright Red
Arteries transport oxygenated blood away from the heart and lungs to tissues and organs throughout the body. Blood within these arteries has recently been replenished with oxygen in the lungs. Consequently, this blood is rich in oxyhemoglobin, imparting a bright, vivid red color.
Arteries typically lie deeper within the body, making them less visible through the skin. If an artery is cut, the blood that emerges is a striking bright red due to its high oxygen content.
The Optical Illusion of Blue Veins
Blood inside veins is a dark, maroon-red because it is deoxygenated. The appearance of veins as blue through the skin is an optical illusion resulting from factors involving light and human tissue. Veins are often located closer to the skin’s surface than arteries, making them more visible.
When white light, containing all colors, strikes the skin, different wavelengths interact differently. Red light has a longer wavelength and is absorbed more readily by the skin and hemoglobin. Blue light, with a shorter wavelength, does not penetrate as deeply and is scattered more efficiently by tissues and blood, reflecting back to our eyes.
Our perception of this scattered blue light from the depth at which veins lie contributes to their bluish appearance, similar to how deep water can appear blue.