Flow, pressure and resistance in blood vessels and the lung

Flow, pressure and resistance in blood vessels and the lung


– [Narrator] This is Sarah
Wilkinson from Humber College and the University of Guelph Humber. In this video I’m gonna discuss how air and blood move in the body with regards to pressure
differences and resistance. I would encourage you to
watch the previous video that I created in this series about pressure-volume relationships. Flow as in either blood flow
or air flow or water flow is going to be equal to
the difference in pressures between one end of the
system and the other and inversely proportional
to the resistance to flow. A classic example of this
is your bathroom plumbing. We know that water flows faster if there’s a large difference in pressure between one end of the
system and the other and it’s inversely
related to the resistance. As an example, if you have small pipes or if you have a blockage in your pipe like a tree root invading it,
you’re gonna have lower flow. We’re gonna look at this
concept in the human body. One other way of looking
at pressure differences is gravity or waterfalls. I’ve got two waterfalls here. One on the left has a
much larger difference. It’s going to have a faster flow. Whereas this one on the
right has a lower difference so it’s going to have a lower flow. So we can kinda think
about pressure differences in terms of waterfalls. So let’s look at pressure
difference in the lungs. If two people are
breathing in at sea level where atmospheric pressure is
760 millimeters of Mercury, but one person has a lower
intrapulmonary pressure compared to the other so
the person on the left has an intrapulmonary pressure
of 756 millimeters Mercury while the other one on the right has 758. All other things being equal,
I resistance everything else. Air flow will move much quicker
in the person on the left so they’re gonna bring air in faster because of the larger
pressure differential. If we look at the effective resistance in bringing air into the lung, if we have the same
two people who now have the same intrapulmonary pressure so there’s no difference
in pressure difference, but they have more resistance, IE, a asthma attack or something
clogged in their lungs, there’s gonna be a much lower flow because of the higher resistance. So flow is inversely related to resistance as long as the difference
in pressure is the same. We can look at similar
examples in blood vessels. So here we’ve got two
different blood vessels. The one lower has a lower resistance because it’s got a bigger lumen. So if I had the same pressure differential between one side of the other, because of the lower resistance,
blood flow would be higher. And this is exactly what
happens to deliver more blood to the skeletal muscle during exercise. Capillaries and arterial, small arteries, these have dilate, get bigger. When that happens with the
same pressure difference, more flow goes to the skeletal muscle. Conversely, in tissues that don’t need lots of blood flow during
exercise such as the kidneys, you’ll have vasoconstriction,
smaller blood vessels, therefore, less blood flow. If we take another example and we have two blood vessels of the same size, but now one has more resistance, if they both meet at the same
blood flow going through them so if they meet at the same flow, but one set have higher resistance, this one would have to have
a bigger pressure difference. So the heart’s gonna have to work harder to get the same flow going through it to create a bigger pressure difference. There’s lots of other
examples of this relationship throughout the whole body and you’re gonna see
it time and time again in that air or blood flow is going to be directly proportional to the difference in pressure at one end of the system compared to the other and inversely proportional
to the resistance of flow. So if there’s a higher resistance, there’s gonna be lower flow. You’ll see this when we
talk about heart disease, you’ll see this in blood
flow changes during exercise, you’ll see this in how we
breathe or in going to altitude. There’s countless examples
of this exact relationship within the human body.

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