Note: Readers commenting under today's Friday Happy Hour post brought up an age old question: "Is blood blue when it's inside the veins?" Matt Soniak kindly put together this response.

Why do veins look blue? One answer you're likely to hear is that veins look blue because the blood inside actually is blue, because it's deoxygenated. If you wonder why you've never seen blue blood before, someone might tell you that's because when you bleed, the blood is oxygenated upon contact with air, and immediately turns red.

First things first: Our blood is not blue. It is always red.1Even when it's deoxygenated. Even in the absence of oxygen in a vacuum. (Remember, when you get blood drawn at your doctor's office, they use a vacutainer, which is essentially a vacuum in a tube. The tube is attached to the needle in your arm, exposing the inside of the vein to the vacuum and drawing the blood out.)

How red it is varies.

After your blood is pumped to your lungs by your heart, it's bright red because hemoglobin -- the iron-containing, oxygen-transporting protein in our red blood cells -- binds to the oxygen the blood just picked up. From the lungs, the blood goes back to the heart (this is called pulmonary circulation), which pumps it out to the rest of the body via the arteries and into tiny blood vessels called capillaries, where it gives its oxygen to the body's tissues (systemic circulation). On its return trip to the heart through the veins, the oxygen-depleted blood is dark red or maroon, because the hemoglobin is no longer bound to oxygen.

Why So Blue?

Now, I'm no surgeon, but real doctors will tell you that when you poke around inside a human being and see a vein or artery in its naked glory, it isn't blue. If blood isn't blue, and veins and arteries aren't actually blue, why do our veins look blue through our skin?

When you look down at the veins in your arm, light of different wavelengths is hitting the skin, the veins and the blood. Some of that light is being absorbed, and some is getting scattered and reflected back to your eye. Different wavelengths of light have different properties and abilities. As it turns out, blue light, compared to red light 1) doesn't penetrate the skin as well, 2) is absorbed by the blood more, and 3) is more likely to be scattered and make it back to your eye.

So, if a vein is close to the surface of the skin, most of the blue light will be absorbed, and even though red light doesn't reflect as much, the red light:blue light ratio is high enough to make the vein appear red. With deeper veins, the blood doesn't absorb as much blue or red light. But the blue light's inability to penetrate as deeply as red light makes the veins appear blue.

1 Note the "our" in that statement. Humans and all other animals with backbones have red blood, but some animals, such as lobsters, crabs, crawfish, octopodes, squid, mussels and clams, do have blue blood.