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I wanna figure out how blood gets from my heart, which I'm going to draw here,

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all the way to my toe, and I'm going to draw my foot over here and show you which toe I'm talking about.

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Let's say this toe right here.

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Now, to start the journey, it's gonna have to out of the left ventricle and into the largest artery of the body.

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This is going to be the aorta, and the aorta is very, very wide across, and that's why I say it's a large artery.

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And, from the aorta, I'm actually not drawing all the branches of the aorta,

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but from the aorta, it's going to go down into my belly, and it's going to branch towards my left leg and my right leg.

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So, let's say we follow just the left leg,

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so this artery over here on the top,

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it's gonna get a little bit smaller, and maybe I would call this a medium-sized artery by this point.

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Ah, this is actually now getting down towards my ankle.

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Let's say we've gone quite a distance down to my ankle,

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and then, you know there are, of course, little branches, and let's just follow the branch that goes towards my foot,

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which is this top one.

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Let's say this one goes towards my foot,

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and this is going to be now an even smaller artery.

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Let's call it small artery.

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Small artery.

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From there,

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we're actually going to get into what we call arterioles.

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So it's going to get even tinier, right? It's going to branch.

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Now, these are very, very tiny branches coming off of my small artery, and let's follow this one right here,

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and this one is my arteriole.

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Arteriole.

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So, these are all the different branches I have to go through,

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and finally, I'm going to get into tiny little branches,

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I'm going to have to draw them very, very skinny just to convince you that we're getting smaller and smaller.

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Let me draw three of them.

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Nah, let's draw four, just for fun. Ah, four of them,

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and this is actually gonna now get towards my little toe cells, so let me draw some toe cells in here to convince you that I actually have gotten there.

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Let's say one, two over here, and maybe one over here, and these are my toe cells.

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Toe cells.

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And after the toe cells have kind of taken out whatever they need.

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Maybe they need glucose, or maybe they need some oxygen.

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Whatever they have taken out,

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they are also gonna put in their wastes,

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so they have, of course, some carbon dioxide waste that we need to drag back.

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This is now going to dump into what we call a venule.

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A venule,

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and this venule is gonna basically then feed into many, many other venules.

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Maybe there's a venule down here coming in, and maybe a venule up here coming in.

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Maybe from the second toe,

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and it's gonna basically all kind of gather together,

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and again, to a giant, giant set of veins.

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You know, maybe veins are dumping in here now, maybe another vein dumping in here,

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and these veins are all going to dump into an enormous vein that we call the inferior vena cava.

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I'll write that right here.

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Inferior vena cava,

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and this is the large vein that brings back all the blood from the bottom half of the body.

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There's also another one over here called the superior vena cava,

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and this is bringing back blood from the arms and head.

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So these two veins, the superior vena cava and the inferior vena cava, are dragging the blood back to the heart,

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and, generally speaking, these are all considered, of course, veins.

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Let's back up now and start with the large and medium arteries.

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These guys together are sometimes referred to as elastic arteries.

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Elastic arteries,

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and the reason they're called elastic arteries,

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one of the, one of the good reasons why they're called that is that they have a protein in the walls of the blood vessel called elastin.

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They have a lot of this elastin protein,

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and, if you think about the word elastin or elastic,

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you know, obviously similar words,

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you might think of something like a rubber band or a balloon,

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and that's probably the easiest way to think about it.

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If you have a blood vessel, one of these large arteries for example,

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and let's say blood is under a lot of pressure, right, because the heart is squeezing out a lot of high pressure blood.

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This artery is literally going to balloon out, and if you actually looked at it from the outside,

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it would look like a little sausage.

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Something like this, where it's puffed out.

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So what's happened there between the first and second picture is that the pressure energy,

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so the heart is squeezing out a lot of pressurized blood, and of course, there's energy in that blood, right?

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That pressure energy has been converted over into elastic energy.

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It's actually converting energy. We don't really always think about it that way, but that's exactly what's happening,

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and when you convert from pressure energy to elastic energy,

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what you are really then doing is you're balancing out those high pressures.

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So you're balancing out high pressures.

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High pressures,

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and this is actually very important because the blood that's coming into our arteries is under, let's not forget, high pressure.

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So the arteriole system, we know, is a high pressure system.

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So this makes perfect sense that the first few arteries, those large arteries and even those medium-sized arteries

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are going to be able to deal with the pressure really well.

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Now let me draw a little line here just to keep it straight.

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The small artery and the arteriole, these two, these are actually sometimes called the muscular arteries.

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Muscular arteries,

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and the reason, again, if you just want to look at the wall of the artery, you'll get the answer.

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The wall of the artery is actually very muscular.

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In fact, specifically, it's smooth muscle.

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So not that kind of muscle you have in your heart or in your bicep, but this is smooth muscle

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that's in the wall of the artery, and there's lots of it.

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So again, if you have a little blood vessel like this, if you imagine tons and tons of smooth muscle on the outside,

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so let's draw it like this.

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Little bands of smooth muscle.

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If those bands decide that they want to contract down, if they want to squeeze down,

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you're going to get something that looks like a little straw because those muscles are now tight.

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They are tightly wound, right?

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So you're going to create like a little straw, and this process is called vasoconstriction.

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Vaso- just means blood vessel, and constriction is, kinda, tightening down.

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So vasoconstriction, tightening down of the blood vessel, and what that does is it increases resistance, right?

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Just like if you're trying to blow through a tiny, tiny little straw, there's a lot of resistance.

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Well, it's the same idea here, and actually a lot of that resistance and change in the constriction, or vasoconstriction, is happening at the arteriole level.

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So that's why they're very special, and I want you to remember them.

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From there, blood is going to go through the capillaries.

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I didn't actually label them the first time, but let me just write that here.

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Sometimes, they call them capillary beds.

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I'll write that out,

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and then it's going to go and get collected in the venules and eventually into the veins,

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and the important thing about the veins,

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I'm going to stop right here and just talk about it very briefly, is that they have these little valves,

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and these valves make sure that the blood continues to flow in one direction.

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So one important thing here is the valves,

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and remember the other important thing is that they are able to deal with large volumes.

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So unlike the arteriole side, where it's all about large pressure,

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down here with the vein side, we have to think about large volumes.

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Remember, about two-thirds of your blood at any point in time is sitting in some vein or venule somewhere.