Hank Green: Given our seemingly permanent obsession with all things vampire, chances are you've been exposed to some kind of lovesick brooding vampire heartthrobs on a screen somewhere, and whether those are pretty vamps slurping blood from nubile necks, sipping it from antique goblets, or splattering it on walls, the blood itself, whether crimson, burgundy, or bright red is kinda hard to miss.  The shade of that blood may vary, but the blood out in the open air is always red.  And yet, when you look down at your wrist, you see that blood in your veins is blue-ish.  What?!  So, what color is the blood in your body really?  

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Before we answer that question, though, I wanna talk a little bit about more what your blood is made of.  Blood is the fluid that our hearts pump through our circulatory systems to deliver oxygen and other materials to all of the cells in our bodies.  The average adult human has about five liters of blood sloshing around inside it, and that fluid is made up of clearish liquid plasma in the red and white blood cells and platelets that float within it.  You probably know that white blood cells fight off infection, platelets help blood clot and the red blood cells carry oxygen.  Red blood cells are 1/3 hemoglobin, an iron-rich protein that binds to Oxygen and gives our blood its rusty red hue.  And hemoglobin is a kind of hemochrome, a protein that largely influences blood's color, different organisms use different proteins, and not all animals use hemoglobin.  So not all animals' blood is red.  Which means that your blood color depends on what you are and how your blood moves oxygen around your body.  

For example, most mollusks, including octopi, squids, snails, slugs, and some arthropods, bleed blue blood.  This is because their blood contains higher levels of hemocyanin, a copper containing protein that turns from basically colorless to dark blue when exposed to oxygen.  Certain marine worms use hemerythrin to transport oxygen in their blood, which turns a pinkish violet color when oxygenated.  There's even a genus of skinks in New Guinea that have green blood because of a buildup of a greenish pigment known as biliverdin, a byproduct of the breakdown of hemoglobin.  You've actually seen a little burst of biliverdin in your own body, it's the stuff that makes a three-day old bruise look green, an excess of it can lead to jaundice and yellowing of the skin pigments.  But back to your blood.

There's been a longstanding misconception that human blood is blue until it hits the open air and gets oxygenated to its crimson hue.  That is not true, unless you've been hitting up Willy Wonka's magic blueberry gum, your blood is not blue.  Your blood is never blue, whether exposed to air by accident at a cutting board, or safely contained within your body, your blood is always red.  It only appears blue in your veins from the outside because of the way different light wavelengths penetrate your skin.  It is true that the blood in your arteries and capillaries are a brighter red than the deep red blood in your veins, which has to do with how recently that blood picked up new oxygen.  I actually once had a phlebotomist take blood out of my veins and then she said, 'watch this', and then she shook it up and I could see the color change, and it was really cool, but it did not go from blue to red!

So here's how it goes.  Your heart sends blood through your lungs to grab an oxygen snack, then pumps it through your arteries and all your capillaries where it delivers that oxygen to cell tissues everywhere.  As your blood runs through your veins on its way back to your heart, it's just like, darker, because it has delivered that oxygen.  You may have noticed the difference the last time you got a paper cut.  Maybe you took notice that the blood coming out of your finger seemed lighter and brighter than the darker, syrupy looking blood you saw coming out of your forearm at a blood drive.  The blood from your paper cut was capillary blood, it was full of oxygen, while the stuff that the phlebotomist tapped at the blood drive was venous blood that had just delivered its oxygen payload.  They're slightly different shades of red because they're carrying different phases of the oxygenation process, but they are both still decidedly not blue.  

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