Main content
Health and medicine
Course: Health and medicine > Unit 2
Lesson 5: Blood vessels- Arteries vs. veins - what's the difference?
- Arteries, arterioles, venules, and veins
- Layers of a blood vessel
- Three types of capillaries
- Pre-capillary sphincters
- Compliance and elastance
- Bernoulli's equation of total energy
- Stored elastic energy in large and middle sized arteries
- Compliance - decreased blood pressure
- Compliance - increased blood flow
© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice
Compliance - increased blood flow
Learn how compliant arteries allows for a "Constant Pressure System" like a modern water gun! Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.
Want to join the conversation?
- does rigid vessels influence the differential between the systole and diastole pressure?(4 votes)
- I found a graph in "The Textbook of Medical Physiology" by Guyton and Hall, that compared normal pressure, to pressure in arteriosclerosis (a condition that would make the vessels more rigid). It showed Systolic pressure increasing greatly, but diastolic pressure remaining about the same. So, the difference between systolic and diastolic pressures would increase.(1 vote)
- At, Rishi says that you wouldn't even notice the difference between systole and diastole, but wouldn't you feel the pressure go up in systole and stillness in diastole (with pressure against your hand)? 4:15(1 vote)
- At, Rishi is right. You cannot feel the difference in pressure with your hand because the pressure is not transmitted to the surface of the body from the vessels. What you can feel is the reverberations of the valves closing and the blood slamming against the heart as a result. 4:15(4 votes)
- Then where does the blood go in diastole? Rishi says that there's less blood pushing out on the walls, but where does all the extra blood go, the "more" blood minus the "less" blood that is pushing on the wall? Will the blood be more dense in the middle? Pushing out on your hand?(1 vote)
- There is a large bolus of blood initially that cannot immediately end up on the small arteries, capillaries, and veins. After the initial burst in systole, the blood is able to make its way to these smaller parts of the vascular system.(2 votes)
- Would the blood back flow during diastole?
The only reason that the arteries doesn't have back flow is because of the fast moving blood, it doesn't have a chance to turn around.
But because our hand is there, the blood has a chance to turn around. so wouldn't the blood back flow on the aortic valve and since so much pressure is building up, would the valve break?(1 vote)- There is some retrograde aortic flow in the descending aorta. The pressure wave rebounds off the bifurcation at the iliacs and does put pressure on the aortic valve. Luckily, the aortic valve is strong and typically does not leak. There is a disease (aortic regurgitation) that occurs when the aortic valve leaks and it can lead to lots of problems.(1 vote)
Video transcript
So let's suppose that I take my
little drawing of a heart-- I'm going to make it nice and
small, so it illustrates the big point-- and I have
my aorta there stretched out. And I put my hand right there. Let's say somehow I could
get my hand in there, and I could feel the blood
pulsing up against my hand. So let's imagine
that for a second. Here's the question. What would it feel like? Would it feel like,
let's say, this, with each arrow
representing systole? Maybe this is when the
heart is squeezing here, and the heart is
squeezing here and again. Maybe this would be five
little spurts of blood? Is that what it would feel like? Because again, in diastole,
the heart is resting. So presumably any kind
of flow I'm getting would be coming from
the heart, right? So is that what it feels
like or something else? And we're going to answer
that question right now. So let's draw it a
little bit larger. I'm going to draw it right here. And you're going to see
something very, very cool. So let's say this is the
aorta now stretched out. I'm going to draw it like that. And I'm going to keep the
center of the aorta here, so you can see
what it would look like if it wasn't stretched out. But we know, of course, now
that I have compliant arteries and that it will do this. As the blood goes
in, this is all going to fill up with blood. And of course, blood is moving
through the middle as well. But this is the blood
that's pushing out on the walls of the aorta. On both sides, I've got lots of
blood pushing out on the walls. And let me draw with
arrows how it got there. And of course, I
have blood that's going straight through as well. And so here's my blood that's
going straight through. And let me draw
in my hand, so you can see where that's oriented. This is my hand as before. And I feel the blood
going straight through. And that's happening
during systole. And then in diastole, you
have, we know, recoil. So in diastole, the heart
is, again, right here, and the vessels are right here. And they are not as
wide or plump as they were before--
something like that. This is the inner tube, if
you will, all the way through. And the blood is
still in the walls. There's still blood
in the walls here. That's the only reason they're
pushed out even a little bit. There has to be something
there holding it out. And again, on the other
side-- but you can see there's a lot less
blood than there was. And so you have to wonder, where
did all that extra blood go that was in the walls? Or I shouldn't
say in the walls-- that was pushing
out on the walls? And the extra blood that you
don't see is gushing out. So it's actually moving out, and
that's pushing out on my hand. So in diastole, I actually have
stuff pushing out on my hand as well. And I would feel that. So I would actually feel blood
pushing on my hand in diastole. So initially, I thought
maybe I would just feel blood in systole, but I actually
get flow in diastole as well. And these arrows are a little
bit longer because of the fact that diastole, we know, is a
little bit longer than systole. Diastole we think is about 2/3
of the time of a heartbeat. And systole we think is about
1/3 of the time in a heartbeat. So what you get, basically,
is a continuous flow, a steady stream of flow. And you wouldn't even notice
the difference necessarily between systole and diastole. It would be a steady stream. And actually, thinking
about steady streams, I'm just going to move this
up a little bit because I want to show you something
pretty cool. So let's make a
little bit of space. And that is that we've
actually taken this idea-- and when I say we, I
mean toy manufacturers have taken this idea--
and actually made water guns using
a similar system. So they actually
said, OK, well, we know that people like to spray
their friends with water. This is, let's say,
a little water gun. And they like to do it
after sneaking up on them. If you're like me, you like
sneaking up on your cousins. And this is, let's say,
the little water chamber. What happens in these
water guns that they make is that they have
you pump them up. You have to pump up
this little air pump. And it actually sends
water from this chamber right here over into here. So water goes into this little
balloon as you pump it up. So you're pumping,
pumping, pumping. Let's say you're
waiting for your cousin to come around a corner. You're just sitting
there quietly pumping. And this balloon gets
bigger and bigger. And it gets very big. And now, you're ready. Now you're ready for your cousin
to come around the corner. And let's say at that
moment, as you're ready, you see your cousin. You pull the trigger. I just drew a little
trigger for you to pull. You pull the trigger, and
all this water gushes out. And it comes out in
a nice, even flow. It's not like you have
to pull the trigger and you get a squirt, and
then you pull it again and you get another squirt. You can actually
hold the trigger, and you get a nice, even
flow just like we have here. So the idea is that
you get these even flows by having stored up energy
in this elastic balloon, which is right here. So this balloon is
actually storing up energy. And in fact, they actually
call this a constant pressure system. So think about that. The water gun manufacturers
are calling this a constant pressure system. And the idea comes
from something that's very similar
to what happens in your body-- very, very cool. So when you think about
maintaining flow of blood, remember, again, the
compliant arteries are really, really
helpful for that.