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Lecture 17

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February 10, 2017

Dr. Guevara

phgy 210

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PHGY 210 Lecture 17 Dr. Guevara

Do not redistribute. Page 2 of 7 Written by: Angelica Le

CARDIOVASCULAR

Last Lecture:

• Flow through a vessel: If you increase the pressure at both ends, the perfusion pressure is unchanged and

flow is the same

• Two things control flow: perfusion pressure and the vessel properties (viscosity, length, cross sectional area)

• Can’t directly measure resistance, so find it using equation: Resistance = Perfusion pressure/flow

This Lecture:

LAMINAR OR PARABOLIC FLOW

• Demonstrated using a collapsible umbrella holder

• Each layer is called a lamina

• The layers slide over each other, so the blood closest

to the midline moves the fastest and the fluid closest

to the walls moves the slowest.

• There’s friction between the sliding layers, and this is

what gives the viscosity of the fluid

• As fluid layers slide over each other, heat is being

generated and lost, and that energy comes from the

pressure energy

• This is also why pressure falls as we move down the vessel, it’s giving up increasing amounts of energy in

heat to the fluid

• Also called parabolic flow: if you look at the velocity as a function of the distance from the midline, you see

a parabola.

o At the walls of the vessel, the velocity is zero. As you move away from the walls, toward the midline of

the vessel, the velocity is increasing.

o Gives a shape of Y = X2, a parabola

• It has a third name: Smooth Flow

o The fluid layers slide smoothly over each other

• In many of the vessels in the body, this is the kind of flow you have

POISEUILLE’S LAW

• Poiseuille wanted to figure out the resistance to flow, so he changed the perfusion pressure and realized:

➢ Flow = Δ Perfusion Pressure P/Resistance to flow R

• In a pipe or vessel, you can describe flow using cross sectional area and length of vessel

• Demonstrates another way to find Resistance R (equation in the picture below)

➢ A = cross sectional area, L = length, r = radius

➢ Don’t memorize 8π, just know relationship between viscosity, length, cross sectional area, radius

• If you blow through a pipe vs a straw of the same length,

using the same pressure, the flow in the straw is much slower

because of the cross sectional area

• Aorta has a large cross sectional area to produce a smaller

resistance

• The equation for flow here is only for laminar flow.

➢ Turbulent flow is less smooth; equation doesn’t apply

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