1. Is the Blood Inside Bluish Wrist Veins Really Blue?
Some people can see bluish lines on the inside of their wrists. These are veins located just under the skin. However, the blood itself is not blue. A computer simulation study investigated why veins appear bluish by looking at how light passes through skin, rather than the color of the blood itself [6].
It is often thought that venous blood has used up all its oxygen. However, data shows that about 95% of hemoglobin in arterial blood is bonded to oxygen, while about 75% remains bonded in mixed venous blood before it enters the lungs [4]. Subtracting these figures shows that about 20% of the total hemoglobin amount releases oxygen as it circulates through the body. This means venous blood returns to the heart still holding most of its oxygen. Venous blood keeps most of its oxygen, and its color is a darker red.
2. How Iron-Containing Heme in Red Blood Cells Makes Blood Red
Red blood cells are disc-shaped cells with a diameter of 7 to 8 micrometers and a dimpled center. They do not have a nucleus. Most of their components are water and hemoglobin [5]. Since 1 micrometer is 1/1000 of a millimeter, about 130 of these cells would fit side-by-side in a 1 mm length.
Hemoglobin is a combination of "heme," which contains iron, and a protein called "globin." The Japanese Ministry of Health, Labour and Welfare explains that blood is red because heme acts as a red pigment [1]. Heme contains divalent iron and is found not only in hemoglobin but also in pigment parts of other substances like myoglobin [2]. Another encyclopedia states that the redness of blood is due to pigment proteins containing iron atoms within red blood cells [3]. The true identity of the red color is the color of this iron-containing pigment.
3. Why Blood Is Bright Red with Oxygen and Dark Red Without It
One hemoglobin molecule contains 4 iron atoms. Each iron atom can bond with one oxygen molecule [3]. This bonding is reversible: it occurs where oxygen is plentiful and separates where oxygen is scarce.
Hemoglobin becomes a vivid red (bright red) when bonded to oxygen and a dark red when not bonded. Arterial blood that has passed through the lungs is bright red, while venous blood is dark red, because of this difference in oxygen bonding [4]. An encyclopedia also notes that arterial blood appears bright red because most of its hemoglobin is bonded to oxygen [3]. Even though it is the same red pigment, the color becomes brighter depending on how much oxygen it holds.
4. Why Red Blood Looks Blue Under the Skin: Scientists' Explanations Differ
One explanation suggests that blue light scatters in the tissue closer to the surface, while the remaining red and green light is absorbed by pigments deeper down, making the area look blue [8]. A simulation study also suggests that collagen fibers in the shallow layers of skin may play a role in scattering light [7]. This is based on the property that blue light is more easily scattered than other colors.
On the other hand, some researchers state that veins are not blue, and the blue appearance is an optical illusion caused by color contrast [8]. A paper reported in 1996 claimed that the reason veins look blue is not simply because more blue light returns than red light [8]. The simple explanation that "blue light bounces back" may need to be reconsidered. There is no single agreed-upon answer for why veins look blue. Separately, it is a fact that the blood itself is red.
5. Hemocyanin: The Copper Pigment That Turns Blue in Some Animals' Blood
The blood of squid, octopus, and crayfish contains a pigment protein called hemocyanin. Like hemoglobin, it carries oxygen, but it contains copper instead of iron. Hemocyanin is colorless on its own, but it turns blue when it bonds with oxygen [9].
While the "blue veins" in humans are a matter of appearance, the blue color of hemocyanin is a real change in the pigment itself when oxygen bonds to it.
6. Try It Yourself: Compare Your Vein Color in Bright and Dim Light
If you can see bluish veins on your wrist, try observing how they look. If they are hard to see, do not force it. Compare the color of the same vein in a bright area near a window and in a slightly darker room. Note in words how the color of the vein differs from the color of the surrounding skin.
This is an observation of appearance. Results may vary from person to person, and this alone will not determine which scientific theory is correct. For further reading, the literature review by Akiyoshi Kitaoka lists the arguments for the scattering theory and the illusion theory side by side, showing that the explanation is not yet settled [8].