Ask a Scientist: Why does blood look red, blue or brown?

Dr. James Faber is professor of cell biology and physiology at the UNC School of Medicine. Here he explains how principles of chemistry and physiology combine to create the color scheme of blood. Questions and answers have been edited.

Q. If blood is always red, why do the vessels that carry it appear blue?

A. Blood flowing throughout the body’s vessels has slightly different shades of red versus blue. These shades depend on the amount of oxygen that is carried by the hemoglobin protein present in the blood’s “red blood cells.” Hemoglobin gives these cells their red color (blood also has “white blood cells,” which are immune cells that lack hemoglobin). In a healthy person, the arteries that carry blood away from the heart are fully loaded with oxygen. This oxygen abundance causes the blood to be a bright red color that penetrates the walls of the vessels. When this blood flows through the capillaries in the tissues, the hemoglobin releases the oxygen to the surrounding tissue cells, where it is converted to energy molecules.

The blood flowing from the capillaries into the veins on its way back to the heart and lungs has less oxygen bound to hemoglobin, so blood in veins appears bluish-red. When looking at the skin, we can’t see the red arteries because they are located deep to keep them out of harm’s way. Instead, we see only veins. And they appear blue rather than red, especially in individuals with little melanin pigment in their skin.

In some people – those with compromised lung function, or mountaineers climbing at high altitude without bottled oxygen – the blood flowing from the heart to the tissues might be more bluish in tint than normal. We can readily detect this difference by looking at the color of certain tissues with capillaries that are dense and close to the surface, such as the lips and nail-beds of the fingers. As a result, these individuals will appear to have blue lips and fingernails – not a good sign, because not enough oxygen is getting to their tissues.

Q. Where does the term “blue blood” come from?

A. Chemistry and physiology don’t seem to have much to do with the term “blue bloods,” an English idiom that denotes noble birth or descent. But historians believe the term originates from ancient and medieval societies of Europe, where it was presumably used to distinguish an upper-class person, whose skin veins appeared especially blue due to their untanned skin, from a working-class person who spent most of their time working in the fields and thus had tanned skin.

Q. Why does blood sometimes appear not only red and blue, but brown?

A. Blood can also have a rust or brown color after it has been outside of the body for a time. Here again, it is oxygen at work. The breakdown of the red blood cells and their hemoglobin releases iron atoms that are normally protected from the highly chemically reactive oxygen molecule. And just as with a rusty nail, oxygen in the air oxidizes the iron to a rust-brown color.