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You've probably heard of

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people having a stroke,

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and you're probably familiar with the notion

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that it has something to do

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with the brain, and you'd be right.

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In particular, it's a rapid loss of

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brain function because of

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something strange happening

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with the blood flow to the brain.

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And let me show you that

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in a little bit more detail.

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And to do that,

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let's think about

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the 2 major types of strokes.

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There's the ischemic strokes,

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and the other type of stroke

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is hemorrhagic,

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and these can kind of be sub-categorized,

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but I won't go into all of the details there.

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And if I really just define

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ischemia and hemorrhaging to you,

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I think you'll have an idea

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of how these strokes are different

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and how they interrupt the blood flow

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to different parts of the brain.

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You know from the videos on

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stenosis and ischemia

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and the videos on heart attacks

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that ischemia is

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a lack of blood flow

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to certain body tissues.

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So an ischemic stroke is actually

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very, very similar to what we saw

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in a heart attack,

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except it's not occurring

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in a coronary blood vessel,

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it's occurring in a blood vessel

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in the brain.

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So let me draw that right over here.

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Let's say that this is

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a blood vessel in the brain.

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And let's say that blood is flowing

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in that direction (this is an artery).

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And so you could imagine

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that maybe there is a big blood clot

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that forms in some part of the brain.

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Let me do the blood clot in magenta.

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This blood clot might form because --

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no, that's not magenta --

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the blood clot might form

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because maybe there's a plaque there,

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maybe the plaque got ruptured,

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either way, this clot is restricting

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the flow of blood.

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And we know that this blood clot --

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we can call this a thrombus,

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or we could say that thrombosis

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has occurred over here--

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either way, the blood flow is restricted,

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and the brain tissue that's further downstream

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is not going to get its oxygen,

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and it might die; it might experience infarction.

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And that's why ischemic strokes are also

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sometimes called cerebral infarctions.

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These are all very fancy words,

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but I think, hopefully, they're becoming

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a little bit more common in our vocabulary,

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they keep showing up over and over again.

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And I also want to be clear:

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most strokes are actually

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ischemic strokes.

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The numbers I looked up, they say,

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87% of strokes are ischemic.

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Now, the other type of way

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that you could have ischemia

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in one of these blood vessels,

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and this is completely analogous

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to what we saw in the heart,

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when we had heart attacks, is:

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you could have thrombosis,

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or you could also have an embolism.

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Whenever someone says thrombosis,

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or a thrombus, or thrombi,

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they're talking about blood clots.

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Whenever someone talks about

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an embolus, or emboli, or embolism,

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they're talking about something

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moving through the blood

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that eventually blocks a blood vessel.

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So you can actually have

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a thromboembolism,

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you can actually have a blood clot

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that gets broken off --

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so let me ignore this for now--

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let me paint over it a little bit

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so that this isn't the main cause of blockage--

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but you could actually have a blood clot

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that breaks off, becomes an embolus,

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and since it's an embolus due to a blood clot,

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you call it a thrombembolus --

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I always have trouble saying all of these words--

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and eventually it blocks an artery over here.

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So this right here is an embolism,

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but either way, you're blocking the blood flow

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further down the brain,

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[which] could cause infarction,

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that brain tissue will die,

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and whatever that brain tissue did

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for mental function, or whatever,

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is going to make it very hard

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for this person who is experiencing this stroke

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to do those things.

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Now, it's not always noticeable,

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that's called a silent stroke,

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but damage is occurring.

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The person experiencing the stroke--

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and I'm not a doctor,

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so take all of this with a grain of salt--

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the person experiencing the stroke

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could be anywhere from -

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well, one, they may not even notice

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that damage is occurring,

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they might have a headache,

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or it might be more severe,

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they might actually

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not be able to properly move

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a side of their body,

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or a side of their face,

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or properly be able to speak,

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so it really depends on what

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part of the brain is being damaged.

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But in either of these situations,

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an ischemic stroke is caused by

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some type of restriction or blockage

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that causes things downstream to not

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get proper oxygen, and then,

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so you can imagine,

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cells over here aren't going

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to get their oxygen,

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and then they might actually die.

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A hemorrhagic stroke -

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to hemorrhage means to bleed,

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it's literally just a fancy word for bleeding-

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and so in a hemorrhagic stroke

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you have a situation where a blood vessel

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can actually break, where you have a blood vessel-

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I'm actually trying to draw

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the same blood vessel- where it actually breaks.

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We'll talk more in the future

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of why a blood vessel might break -

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strongly related to high blood pressure

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and other risk factors,

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but I don't want to get into that right now -

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but you could imagine if a blood vessel breaks,

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you have all this blood spewing into the brain

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in, kind of, an uncontrolled way.

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So let's say this little diagram

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[that] I drew right here,

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that part of the brain,

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if you have a hemorrhagic stroke,

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you have all of this blood

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that's flowing into the brain,

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and all of that uncontrolled blood

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will mess up that part of the brain,

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that causes those neurons

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and brain tissue to malfunction

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and maybe causes some of them to die,

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and it would also cause

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the blood flow further dowstream

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to be impaired, so the stuff

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downstream aren't going to get

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the blood they need

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because all the blood

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is being released everywhere else.

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And since 87% of strokes are ischemic strokes,

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the remainder are hemorrhagic,

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so this is the remaining 13% of strokes.