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You've probably heard of people having a stroke, and you're probably familiar with the notion that it has something to do with the brain, and you'd be right. In particular, it's a rapid loss of brain function because of something strange happening with the blood flow to the brain. And let me show you that in a little bit more detail. And to do that, let's think about the 2 major types of strokes. There's the ischemic strokes, and the other type of stroke is hemorrhagic, and these can kind of be sub-categorized, but I won't go into all of the details there. And if I really just define ischemia and hemorrhaging to you, I think you'll have an idea of how these strokes are different and how they interrupt the blood flow to different parts of the brain. You know from the videos on stenosis and ischemia and the videos on heart attacks that ischemia is a lack of blood flow to certain body tissues. So an ischemic stroke is actually very, very similar to what we saw in a heart attack, except it's not occurring in a coronary blood vessel, it's occurring in a blood vessel in the brain. So let me draw that right over here. Let's say that this is a blood vessel in the brain. And let's say that blood is flowing in that direction (this is an artery). And so you could imagine that maybe there is a big blood clot that forms in some part of the brain. Let me do the blood clot in magenta. This blood clot might form because -- no, that's not magenta -- the blood clot might form because maybe there's a plaque there, maybe the plaque got ruptured, either way, this clot is restricting the flow of blood. And we know that this blood clot -- we can call this a thrombus, or we could say that thrombosis has occurred over here-- either way, the blood flow is restricted, and the brain tissue that's further downstream is not going to get its oxygen, and it might die; it might experience infarction. And that's why ischemic strokes are also sometimes called cerebral infarctions. These are all very fancy words, but I think, hopefully, they're becoming a little bit more common in our vocabulary, they keep showing up over and over again. And I also want to be clear: most strokes are actually ischemic strokes. The numbers I looked up, they say, 87% of strokes are ischemic. Now, the other type of way that you could have ischemia in one of these blood vessels, and this is completely analogous to what we saw in the heart, when we had heart attacks, is: you could have thrombosis, or you could also have an embolism. Whenever someone says thrombosis, or a thrombus, or thrombi, they're talking about blood clots. Whenever someone talks about an embolus, or emboli, or embolism, they're talking about something moving through the blood that eventually blocks a blood vessel. So you can actually have a thromboembolism, you can actually have a blood clot that gets broken off -- so let me ignore this for now-- let me paint over it a little bit so that this isn't the main cause of blockage-- but you could actually have a blood clot that breaks off, becomes an embolus, and since it's an embolus due to a blood clot, you call it a thrombembolus -- I always have trouble saying all of these words-- and eventually it blocks an artery over here. So this right here is an embolism, but either way, you're blocking the blood flow further down the brain, [which] could cause infarction, that brain tissue will die, and whatever that brain tissue did for mental function, or whatever, is going to make it very hard for this person who is experiencing this stroke to do those things. Now, it's not always noticeable, that's called a silent stroke, but damage is occurring. The person experiencing the stroke-- and I'm not a doctor, so take all of this with a grain of salt-- the person experiencing the stroke could be anywhere from - well, one, they may not even notice that damage is occurring, they might have a headache, or it might be more severe, they might actually not be able to properly move a side of their body, or a side of their face, or properly be able to speak, so it really depends on what part of the brain is being damaged. But in either of these situations, an ischemic stroke is caused by some type of restriction or blockage that causes things downstream to not get proper oxygen, and then, so you can imagine, cells over here aren't going to get their oxygen, and then they might actually die. A hemorrhagic stroke - to hemorrhage means to bleed, it's literally just a fancy word for bleeding- and so in a hemorrhagic stroke you have a situation where a blood vessel can actually break, where you have a blood vessel- I'm actually trying to draw the same blood vessel- where it actually breaks. We'll talk more in the future of why a blood vessel might break - strongly related to high blood pressure and other risk factors, but I don't want to get into that right now - but you could imagine if a blood vessel breaks, you have all this blood spewing into the brain in, kind of, an uncontrolled way. So let's say this little diagram [that] I drew right here, that part of the brain, if you have a hemorrhagic stroke, you have all of this blood that's flowing into the brain, and all of that uncontrolled blood will mess up that part of the brain, that causes those neurons and brain tissue to malfunction and maybe causes some of them to die, and it would also cause the blood flow further dowstream to be impaired, so the stuff downstream aren't going to get the blood they need because all the blood is being released everywhere else. And since 87% of strokes are ischemic strokes, the remainder are hemorrhagic, so this is the remaining 13% of strokes.