If blood is red why do your veins look blue?

SDStaff Hawk replies:

Blood is a bright red in its oxygenated form (i.e., leaving the lungs), when hemoglobin is bound to oxygen to form oxyhemoglobin. It’s a dark red in its deoxygenated form (i.e., returning to the lungs), when hemoglobin is bound to carbon dioxide to form carboxyhemoglobin.

Veins appear blue because light, penetrating the skin, is absorbed and reflected back to the eye. Since only the higher energy wavelengths can do this (lower energy wavelengths just don’t have the *oomph*), only higher energy wavelengths are seen. And higher energy wavelengths are what we call “blue.”

In an experiment, glass tubes were filled with blood and immersed in milk, milk having a similar ratio of fat, proteins, and water in emulsion as skin. At a certain depth, the tubes appeared blue.

As a phlebotomist (a person who draws blood) at the local university hospital, I drew hundreds of samples of venous blood. Almost all samples were dark red. Those that weren’t were usually because people were on enriched oxygen systems.

At the same time, respiratory therapists drew blood from an artery, not a vein. These samples were characteristically bright red, unless the patient was having difficulty breathing (asthma, people with one lung, pneumonia, emphysema, whatever).

So, your veins may look blue, but even a blueblood will bleed red.

SDStaff Hawk, Straight Dope Science Advisory Board

Send questions to Cecil via cecil@straightdope.com.

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