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If Blood Is Red, Why Do Veins Look Blue?

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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.

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iStock // Ekaterina Minaeva
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Man Buys Two Metric Tons of LEGO Bricks; Sorts Them Via Machine Learning
May 21, 2017
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iStock // Ekaterina Minaeva

Jacques Mattheij made a small, but awesome, mistake. He went on eBay one evening and bid on a bunch of bulk LEGO brick auctions, then went to sleep. Upon waking, he discovered that he was the high bidder on many, and was now the proud owner of two tons of LEGO bricks. (This is about 4400 pounds.) He wrote, "[L]esson 1: if you win almost all bids you are bidding too high."

Mattheij had noticed that bulk, unsorted bricks sell for something like €10/kilogram, whereas sets are roughly €40/kg and rare parts go for up to €100/kg. Much of the value of the bricks is in their sorting. If he could reduce the entropy of these bins of unsorted bricks, he could make a tidy profit. While many people do this work by hand, the problem is enormous—just the kind of challenge for a computer. Mattheij writes:

There are 38000+ shapes and there are 100+ possible shades of color (you can roughly tell how old someone is by asking them what lego colors they remember from their youth).

In the following months, Mattheij built a proof-of-concept sorting system using, of course, LEGO. He broke the problem down into a series of sub-problems (including "feeding LEGO reliably from a hopper is surprisingly hard," one of those facts of nature that will stymie even the best system design). After tinkering with the prototype at length, he expanded the system to a surprisingly complex system of conveyer belts (powered by a home treadmill), various pieces of cabinetry, and "copious quantities of crazy glue."

Here's a video showing the current system running at low speed:

The key part of the system was running the bricks past a camera paired with a computer running a neural net-based image classifier. That allows the computer (when sufficiently trained on brick images) to recognize bricks and thus categorize them by color, shape, or other parameters. Remember that as bricks pass by, they can be in any orientation, can be dirty, can even be stuck to other pieces. So having a flexible software system is key to recognizing—in a fraction of a second—what a given brick is, in order to sort it out. When a match is found, a jet of compressed air pops the piece off the conveyer belt and into a waiting bin.

After much experimentation, Mattheij rewrote the software (several times in fact) to accomplish a variety of basic tasks. At its core, the system takes images from a webcam and feeds them to a neural network to do the classification. Of course, the neural net needs to be "trained" by showing it lots of images, and telling it what those images represent. Mattheij's breakthrough was allowing the machine to effectively train itself, with guidance: Running pieces through allows the system to take its own photos, make a guess, and build on that guess. As long as Mattheij corrects the incorrect guesses, he ends up with a decent (and self-reinforcing) corpus of training data. As the machine continues running, it can rack up more training, allowing it to recognize a broad variety of pieces on the fly.

Here's another video, focusing on how the pieces move on conveyer belts (running at slow speed so puny humans can follow). You can also see the air jets in action:

In an email interview, Mattheij told Mental Floss that the system currently sorts LEGO bricks into more than 50 categories. It can also be run in a color-sorting mode to bin the parts across 12 color groups. (Thus at present you'd likely do a two-pass sort on the bricks: once for shape, then a separate pass for color.) He continues to refine the system, with a focus on making its recognition abilities faster. At some point down the line, he plans to make the software portion open source. You're on your own as far as building conveyer belts, bins, and so forth.

Check out Mattheij's writeup in two parts for more information. It starts with an overview of the story, followed up with a deep dive on the software. He's also tweeting about the project (among other things). And if you look around a bit, you'll find bulk LEGO brick auctions online—it's definitely a thing!

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May 23, 2017
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