Deoxygenated blood is blue
Human blood is always red. Deoxygenated blood in veins is a dark red or maroon color, not blue. Veins appear bluish or greenish through the skin due to the way different wavelengths of light penetrate and reflect from subcutaneous tissue, not because the blood inside is blue.
What we know
Hemoglobin, the iron-containing protein in red blood cells, changes color depending on whether it is bound to oxygen. Oxygenated hemoglobin (oxyhemoglobin) in arteries is a bright, vivid red. Deoxygenated hemoglobin in veins is a darker, more purplish-red color. Neither is blue.
The visual impression that veins are blue arises from optical physics rather than the color of the blood. Light entering the skin is scattered and absorbed by different tissues at different rates depending on wavelength. Blue and violet light is absorbed and reflected near the skin surface by subcutaneous fat, while red and near-infrared light penetrates more deeply and is absorbed by hemoglobin in vessels. When light reflects from a vein area, the relatively blue component dominates in what reaches the eye, particularly against lighter skin tones. This is why veins appear blue or green even though the blood inside is dark red.
Arizona State University's Ask A Biologist, Medical News Today, and UC Santa Barbara's ScienceLine all confirm that human blood is always red. Some non-human animals do have blue blood: horseshoe crabs and octopuses use copper-based hemocyanin as their oxygen carrier instead of iron-based hemoglobin, and hemocyanin produces blue color when oxygenated. This fact may have contributed to the popular misconception about human blood.
The blue-coded diagrams of deoxygenated blood in textbook illustrations of the circulatory system are an educational convention for visual clarity, not an accurate representation of blood color.
Common claims
- Veins carry blue bloodFalse - blood is always red, dark or bright
- Blood turns red when exposed to airMisleading - it was already red inside
- Some animals have blue bloodTrue - horseshoe crabs and cephalopods use hemocyanin