Is blood ever blue inside the human body?

No. Human blood is never blue, inside the body or outside it. It ranges from bright cherry red to a dark brick or mahogany red, depending on how much oxygen it carries. The idea that blood in veins is blue until it meets air is a myth. The blue appearance of veins is an optical effect, not the true color of the blood. [1] [2] [3]

Red blood cells contain hemoglobin, a protein that carries oxygen. Iron is a key part of hemoglobin, and iron bonding with oxygen gives blood its red color. When more oxygen is attached to hemoglobin, the blood is a brighter red. When less is attached, it is a darker red. Blood in the arteries, freshly oxygenated in the lungs, is therefore brighter than blood in the veins. [3] [1]

Hematologist David Steensma notes that even when venous blood is very low in oxygen and almost black, it still keeps a mahogany tint. [4]

Veins near the skin surface, such as those on the back of the hand, can look blue through lighter skin. Hematologist Alan Lichtin of the Cleveland Clinic describes this as an optical illusion created by how light passes through the skin and the vein wall. Red light, with its longer wavelength, travels farther into tissue and is partly absorbed by the blood, while blue light penetrates only a short distance and is mostly reflected back to the eye. [1] [3] [5]

Contrast also matters. We judge color against its surroundings, so a vein looks darker and bluer against lighter skin. The Naked Scientists add that vessel size, the blood's oxygen content and the thickness of the skin above all affect how much light is absorbed or scattered. Textbook diagrams that draw veins in blue may have reinforced the misconception. [3] [6] [4]

Some animals do have blue blood. Octopuses, horseshoe crabs, some other crabs, spiders and other invertebrates carry oxygen with hemocyanin, a protein built around copper rather than iron. Oxygenated hemocyanin is blue. Humans and other mammals always have red blood. [3] [1] [4]

In people, a rare condition called sulfhemoglobinemia can give blood a dark green tint. It happens when a sulfur atom alters the chemistry within hemoglobin and is typically caused by medications containing sulfur compounds. The color is temporary and fades as new red blood cells form. [1]

The phrase "blue blood" for nobility comes from the Spanish sangre azul. According to Steensma, Iberian families used it to show pale, untanned skin and so distance themselves from darker-skinned Moors. It was about skin pigment, not blood. [4]