L14 Circulation- Hemodynamics

Question 1

Explain Poiseuille’s law.

Answer 1

Applies to laminar flow of Newtonian fluids through uniform cylindrical tubes (blood is non-Newtonian and circulatory system isn’t uniform)

• Flow is direction proportional to pressure gradient and inversely proportional to resistance
• Flow is also directly proportional to radius^4= diameter double give flow increase by 16

Question 2

How does Poiseuille’s law relate to blood flow?

Answer 2

It puts flow in relationship to resistance, pressure gradient and radius of vessel

Question 3

How does Poiseuille’s law related to BP?

Answer 3

Flow is directly proportional to pressure gradient

Question 4

How does Poiseuille’s law related to resistance?

Answer 4

Flow is indirectly proportional to resistance

Question 5

Define viscosity.

Answer 5

Internal frictional resistance between adjacent layers of fluid
-viscosity= shear stress/ shear rate (pressure/velocity)

Question 6

How does viscosity change with vessel diameter?

Answer 6

Increasing vessel diameter increases viscosity b/c of increase in hematocrit

Question 7

How does hematocrit change viscosity?

Answer 7

Increase in hematocrit= increase in viscosity

Question 8

How does hematocrit change with vessel diameter?

Answer 8

Hematocrit is lesser in smaller vessels than in larger b/c of plasma skimming and axial streaming

Question 9

Define laminar blood flow.

Answer 9

Fluid moves in parallel concentric layers within a tube

-Laminar flow is silent

Question 10

Define turbulent blood flow.

Answer 10

Fluid moves in a disorderly pattern

• Murmurs, bruit, thrombi
• Korotkoff sounds- based on changes in V of blood flow

Question 11

Define Reynold’s number.

Answer 11

Dimensionless number indicating propensity of turbulent blood flow

• Higher Reynold’s number= greater chance for turbulent blood flow
• N(R)= (density x Diameter x velocity)/ viscosity
• Larger diameter = higher chance for turbulence (aorta, smokers)

Question 12

Define Bernoulli’s principle.

Answer 12

In a constant flow system, total energy (potential + kinetic) remains constant

• Energy conserved by velocity of flow (kinetic) or lateral pressure (potential)
• Total energy (E)= potential energy (P) + velocity of blood flow (pv^2 / 2)
• When velocity of blood flow increases, lateral pressure decreases

Question 13

How does Bernoulli’s apply to the circulation system?

Answer 13

• Blood flow moves from higher to lower total energy, not from higher to lower pressure
• Abrupt decreases in vessel size (stenosis) converts potential energy into kinetic= lateral pressure decreases and velocity of blood flow increases

Question 14

Define the Laplace relationship.

Answer 14

Wall Tension= Pressure x Radius / Wall Thickness

Question 15

How does wall tension affect fxn of dilated hearts?

Answer 15

Large radius= high wall tension, more systolic work/higher oxygen consumption to overcome tension
-High wall tension opposes shortening

Question 16

How does wall tension affect arteriolar vasoconstriction?

Answer 16

Large wall thickness/lumen diameter ratio= low wall tension, provides greater control of vessel diameter and blood flow

Question 17

What is the relationship between velocity of blood flow and vessel cross sectional area?

Answer 17

As total cross-sectional area increases, velocity of blood flow decreases
-Velocity is slowest in capillaries to allow for nutrient reabsorption

Question 18

Explain series resistances in circulation.

Answer 18

1

Question 19

Explain parallel resistances in circulation.

Answer 19

1

Question 20

Define how velocity of blood flow and pressures change throughout circulation in relation to changes in total cross-sectional area.

Answer 20

1

Question 21

Define shear stress

Answer 21

Resistance to movement between laminae (pressure)

Question 22

Define shear rate

Answer 22

Relative velocities between laminae (velocity of blood flow)

Question 23

Explain the shearing laminae in blood vessels.

Answer 23

The shearing laminae of blood are concentric cylinders that move with different velocities.

• Inner most moves with highest velocity, outermost moves slowest
• Velocity profile becomes parabolic with maximum velocity at central axis
• Lower viscosity = sharper parabolic profile

Question 24

Define Newtonian vs. non-Newtonian fluids.

Answer 24

• Newtonian fluid has constant viscosity over a range of shear rates and stress, homogenous fluid (water)
• non-Newtonian fluid has viscosity changes over ranges of shear rate and stress, non-homogenous fluid (blood)

Question 25

How does velocity change blood viscosity?

Answer 25

As velocity of blood flow increases, viscosity decreases due to loss of interactions

Question 26

Why do higher altitudes lead to higher hematocrit?

Answer 26

Increased erythopoietin (EPO) occurs to increase RBCs in blood to carry more oxygen= increase in hematocrit= increased viscosity

Question 27

Define plasma skimming.

Answer 27

Tendency of smaller vessels to have more plasma and less RBCs due to axial streaming -As blood flow slows down from larger to smaller vessels, viscosity should increase, but it doesn't b/c of axial streaming

Question 28

Define axial streaming.

Answer 28

Tendency of RBCs to accumulate in axial laminae at high shear rates - Important b/c it changes your hematocrit - Highest velocity in a vessel is in the center

Question 29

How does wall tension affect fxn of capillaries?

Answer 29

Small radius = low wall tension, can withstand very large amounts of transmural pressures

Question 30

How does wall tension affect fxn of aneurysms?

Answer 30

Large radius= high wall tension, can't withstand transmural pressures and eventually rupture