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MLA Full: "Blood!!!" YouTube, uploaded by vlogbrothers, 13 November 2010,
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In which Hank gives a liter of his life away because someone needs it more than him, and introduces the biology of the human circulatory system.

Thanks to Karen Kavett for the graphics:


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Good morning, John. I'm on my way to the Western Montana Blood Bank. I am O Negative. Universal donor. I'm like a total blood stud. They can't get enough of this. I figured this would be an excellent time to talk about the human circulatory system!

So at first I didn't know how to start this video, because there is no beginning or end to the human circulatory system, except for birth and death, and if you've died, then this really isn't an interesting concept anymore, because you're dead. Or I guess you could be a zombie. If you're a zombie, I don't think it'd be interesting to you either. So I decided to do it from a perspective where there is a beginning an an end.

The life of a red blood cell. Now, we're gonna be talking about one red blood cell, and this one red blood cell's name is going to be Arby. We're calling him Arby not because he was created from the meat of a roast beef sandwich, but because back in my Bio-chem days we called red blood cells RBC's, and I'm just shortening that to RB. And Arby is one of the trillions of red blood cells that you have swimming around in your veins right now, delivering oxygen to all the places that need oxygen. This, aside from attracting the lust of vampires, is all that red blood cells do. And if they stop doing it, you would die instantly. So be grateful!

I don't know why it creeps me out that the blood bank is in the old Creamery building, but it kinda does.

So the process of creating a blood cell is called erythropoiesis. Blood cells being erythrocytes and poiesis from the Latin 'to make' which, somewhat beautifully, is also the root of the word poetry. The weirdest thing about Arby is that near the end of his creation, his nucleus--NOT NUCULUS--and all of his organelles are kicked out of the cell. So by the time Arby lost his nucleus, he had 270 million molecules of hemoglobin. I know this because I counted.

Hemoglobin is a marvelous protein complex that has four molecules of heme attached to it. Heme is an organic compound with an iron center. The iron, of course, is there to bond to oxygen, and it is very, very good at bonding to oxygen. If bonding to oxygen was punk rock, hemoglobin would be The Clash. If it was soccer, hemoglobin would be Cristiano Ronaldo. If it was being delicious, hemoglobin would be corn dogs.

Hemoglobin also only binds to oxygen. Actually, and unfortunately, and somewhat accidentally, hemoglobin actually prefers to bond to carbon monoxide, which is why when you breathe in too much carbon monoxide, you die. But Arby's hemoglobin has never been exposed to oxygen, much less carbon monoxide, so he has to go on a little adventure. Assuming that Arby was created in say your femur, he now moves up the inferior vena cava, which happens to be the largest vein in your body, and into your right atrium. The right ventricle expands, sucking the blood into it, and then contracts, pumping the blood cell through the pulmonary arteries. Pulmonary is the word for anything having to do with the lungs. Now, it is important here to note: arteries carry blood away from the heart, and veins carry blood to the heart. Usually veins have de-oxygenated blood in them and arteries have oxygenated blood in them. But, in the pulmonary artery, blood is being pumped away from the heart and toward the lungs, but there's actually de-oxygenated blood in them, because they haven't gone to the lungs yet. It seems a little weird, at first, but it's not. It's not weird at all.

Anyway, Arby heads to your lungs, where it goes into smaller and smaller blood vessels, until the blood vessels are in fact smaller than the diameter of the blood cell itself. These are called capillaries, and in this case, they're on the inside of little sacs called alveolar sacs that contain the air that you just breathed in. So as Arby is squeezing through a capillary, a few things happen. First, we realize why he kicked out his nucleus and all of his organelles. Because if he still had all that stuff, he'd be way too chubby to fit through a capillary. Without the nucleus, Arby can engorge with 230 million molecules of hemoglobin and still fold into all sorts of crazy shapes to fit through even the smallest blood vessels. But more importantly, because the hemoglobin doesn't have any oxygen attached to it, and because there's tons of oxygen in the alveolar sacs, oxygen from the air moves down it's concentration gradient, and into the capillary and then into Arby, where his hemoglobin sucks it all up. And because the hemoglobin keeps sucking up all the oxygen, the O2 concentration remains low in the blood, and it keeps flowing in from the air. And this continues until the hemoglobin has sucked up all of the oxygen it can. The now oxygenated Arby heads back into the heart, through the pulmonary vein, it's a vein, remember, because it's headed to the heart, and he enters the left atrium, gets sucked into the left ventricle, and then FWOOM! Out! Into the body where he know...somewhere in the body.

Amazing thing about hemoglobin: it knows exactly when to dump all of its oxygen. And how it does this is fascinating. Okay so Arby is full of an enzyme called carbonic anhydrase. And carbonic anhydrase converts carbon dioxide and water into carbonic acid, or H2CO3. H2CO3 is an acid, meaning it dissociates into H+, and HCO3-. And if you've done your chemistry homework, you know what's gone on here. It means the contents of Arby are getting more and more acidic. Hemoglobin actually reacts with these protons, the H+, the things that are making these things acidic, and as it does that, it starts to really suck at holding on to oxygen. Now this makes sense, because the product of any work being done by your body is carbon dioxide, and wherever CO2 is created, there is a need for O2. So wherever CO2 concentrations are high, your body needs the O2 and Arby's hemoglobin is dumping it like crazy. Am I a huge dork for thinking that's beautiful and elegant? Maybe. But it is!

So now Arby is squeezing his way through the capillaries and dumping all of his O2 into tissues that need it, and we're pretty much back where we started. And all that's left is for Arby to do it again. Every 20 seconds. For the next hundred or so days, before he's filtered out by your liver, and yes, helps to make your poop brown.

And that is really the only way that a red blood cell will ever leave your body. Unless you're bleeding. Which I hope you aren't. Or giving blood. Which I hope you are.

John, I'll see you on Monday.

Okay, I just gave blood, and then I said 'Arby' like 25 times, so now I really, really, really want a roast beef sandwich. But I'm not going to go get one, because Arby's is--

sooo good

So everyone, down here in the pants area, tell me when the last time you gave blood was. And what did you think? I hadn't done it in a long time and I thought that it was really, really easy.

Give blood! It saves lives!