If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:7:43

Video transcript

I want to figure out how blood gets from my heart which I'm going to draw here 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 let's say this toe right here now to start the journey it's going to have to go out of the left ventricle and into the largest artery of the body 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 and from the aorta I'm actually not drawing all the branches of the aorta 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 so let's say we follow just the left leg so this artery over here on the top it's going to get a little bit smaller and maybe I would call this a medium-sized artery by this point this is actually now getting down towards my ankle we've let's say we've gone quite a distance down to my ankle and then you know they're of course little branches and let's just follow the branch that goes towards my foot which is this top one let's say this one goes towards my foot and this is going to be now an even smaller artery let's call it small artery small artery from there we're actually going to get into what we call arterioles so it's going to get even tinier right it's going to branch now these are very very tiny branches coming off of my small artery and let's follow this one right here and this one is my arterial arteriole so these are all the different branches I have to go through and finally I'm going to get into tiny little branches I'm going to have to draw them very very skinny just to convince you that we're getting smaller and smaller let me draw three of them now let's draw four just for fun four of them and this is actually going to now get towards my little toe cells let me draw some toe cells in here to convince you that I actually have gotten there let's say one two over here and maybe one over here these are my toe cells toe cells and after the toe cells have kind of taken out whatever they need maybe they need glucose or maybe they need some oxygen whatever they've taken out they're also going to put in their waste so they have of course some carbon dioxide waste that we need to drag back this is now going to dump into what we call a venule a venule and this venule is going to basically then feed into many many other venules maybe there's a venule down here coming in and maybe a venule up here coming in maybe from the second toe and it's going to basically all kind of gather together and again into a giant giant set of veins you know maybe veins are dumping in here now maybe another vein dumping in here and these veins are all going to dump into an enormous vein that we call the inferior vena cava I'll write that right here inferior vena cava and this is the large vein that brings back all the blood from the bottom half of the body there's also another one over here called the superior vena cava and this is bringing back blood from the arms and head so these two veins the superior vena cava and the inferior vena cava are dragging the blood back to the heart and generally speaking these are all considered of course veins let's back up now and start with the large and medium arteries these guys together are sometimes referred to as elastic arteries elastic arteries and the reason they're called elastic arteries one of the one of the good reasons while they're called that is that they have a protein in the walls of the blood vessel called elastin they have a lot of this elastin protein and if you think about the word elastin or elastic you know obviously very similar words you might think of something like a rubber band or a balloon and that's probably the easiest way to think about it if you have a blood vessel one of these large arteries for example and let's say blood is under a lot of pressure right because the heart is squeezing out a lot of high pressure blood this artery is literally going to balloon out and if you actually looked at it from the outside it would look like a little sausage something like this where it's puffed out so what's happened there between the first and second picture is that the pressure energy so the heart is squeezing out a lot of pressurized blood and of course there there's energy in that blood right that pressure energy has been converted over into elastic energy it's actually converting energy we don't really always think about it that way but that's exactly what's happening and when you convert from pressure energy to elastic energy what you're really then doing is you're balancing out those high pressures so you're balancing out high pressures high pressures and this is actually very important because the blood that's coming into our arteries is under let's not forget high pressure so the arterial system we know is a high pressure system so this makes perfect sense that the first few arteries those large arteries and even those medium-sized arteries are going to be able to deal with the pressure really well now let me draw a little line here just to keep it straight the small artery and the arterial these two these are actually sometimes called the muscular arteries muscular arteries and the reason again if you just want to look at the wall of the artery you'll get the answer the wall of the artery is actually very muscular in fact specifically it's smooth muscle so not that kind of muscle you have in your heart or in your bicep but this is smooth muscle that's in the wall of the artery there's lots of it so again if you have a little blood vessel like this if you imagine tons and tons of smooth muscle on the outside so let's draw it like this little bands of smooth muscle if those bands decide that they want to contract down if they want to squeeze down you're going to get something that looks like a little straw because those muscles are now tight they're tightly wound right so you're going to create like a little straw and this process is called vasoconstriction vaso just means blood vessel and constriction is kind of tightening down so vasoconstriction tightening down to the blood vessel and what that does is it increases resistance right just like if you're trying to blow through a tiny tiny little straw there's a lot of resistance well the same idea here and actually a lot of that resistance and change in the constriction or vasoconstriction is happening at the arteriole level so that's why they're very special and I want you to remember them from their blood is going to go through the capillaries I didn't actually label them the first time let me just write that here sometimes they call them capillary beds I'll write that out and then it's going to go and get collected in the venules and eventually into the veins and the important thing about the veins I'm going to stop right here and just talk about very briefly is that they have these little valves and these valves make sure that the blood continues to flow in one direction so one important thing here is is the valves and remember the other important thing is that they are able to deal with large volumes so unlike the arterial side where is all about large pressure down here with the vein side we have to think about large volumes remember about two-thirds of your blood at any point in time is sitting in some vein or venule somewhere