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MED 101- PHYSICS I > 15. BASIC PHYSICS OF PRESSURE AND FLOW IN FLUIDS > Flashcards

15. BASIC PHYSICS OF PRESSURE AND FLOW IN FLUIDS Flashcards

1
Q
  1. What is the Law of Laplace?
A
  • it relates the Transmural Pressure
    (this is the difference between two sides of a wall)
  • TO the wall stress
  • this law gives the average stress over the wall
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2
Q
  1. In a Cylindrical Vessel, what simple relation exists?
A
  • the relation between pressure
  • and the circumferential wall stress
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3
Q
  1. What does the Law of Laplace only hold for?
A
  • it only holds for Simple Geometries
  • it applies to cylindrical and spherical geometries
  • regardless of whether the material is linear or
    nonlinear
  • regardless of whether the wall is thin or thick
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4
Q
  1. In which field is the Law of Laplace most often used?
A
  • in Hemodynamics
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5
Q
  1. What information does the Law of Laplace give us, with regards to Hemodynamics?
A
  • it gives us the relation between:
    - pressure within the lumen of a vessel (▵P)
    - the tension in the wall (T)
    - the vessel radius (R)

LUMEN= cavity or channel within a tube or tubular organ

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6
Q
  1. How do we mathematically write the Law of Laplace?
A

T = ▵P x R

  • T = tension in the wall
    (a form of stress)
  • ▵P = pressure within the lumen of a vessel
  • R = radius of the cylinder
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7
Q
  1. What is the only Limitation of Laplace’s Law?
A
  • it gives the average wall stress
  • it cannot give any information on the stress distribution across the wall
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8
Q
  1. What bodily functions does the Law of Laplace help us understand?
A
  • Cardiac Function
  • Vascular Function
  • this law is of great conceptual importance
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9
Q
  1. What is the main determinant of Wall Stress in the heart?
A
  • the ratio
    (r/h)
  • the ratio at the left ventricle apex and the base of the heart is the same
  • this means that there are similar wall stresses at both of these points
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10
Q
  1. How do the Cardiac Muscle cells increase in thickness during Hypertension?
A
  • it builds more contractile proteins in parallel
  • this leads to a concentric hypertrophy
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11
Q
  1. What do thicker Cardiac Muscle cells result in?
A
  • a thicker wall
  • this causes the systolic wall stress to return to normal levels
  • EVEN if there is a higher pressure in the systole
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12
Q
  1. What is the Equation of Continuity?
A
  • it is a statement of the conservation of mass during flow
  • mass cannot be lost without some kind of disruption
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13
Q
  1. When is the Product constant?

PRODUCT = p x A x v
p = density
A= area
v= speed

A
  • When a fluid of given density (p)
  • moves with an average speed (v)
  • in a tube with a cross-sectional area (A)
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14
Q
  1. What do the symbols A x v represent?
A
  • the volume flow per unit time
  • (m³ / s)
  • Av can also be found as Q
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15
Q
  1. What do the symbols pAv represent?
A
  • mass per unit time
  • kg/m³ x m³/s = kg/s
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16
Q
  1. What can be said about mass and volume when the substance is in a stable state?
A
  • the same mass flows into a volume
  • the same mass leaves the volume
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17
Q
  1. LOOKING AT THIS IMAGE:
    • Suppose that an incompressible fluid fills this pipe
    • this fluid flows through the pipe

What can be inferred?

A
  • the flow of the incompressible fluid at point A1 is equal to the flow of the incompressible fluid at point A2
  • the mass flow rate is the same for point A1 and point A2
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18
Q
  1. Looking at this image, how would we mathematically describe the continuity of the incompressible fluid?
A
  • Q = A1.v1
  • Q= A2.v2

THEREFORE:
A1.v1 = A2.v2

  • v1 is the average fluid speed over A1
  • v2 is the average fluid speed over A2
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19
Q
  1. What can be said abut a gas that is moving at a low speed?
A
  • the density of the gas remains the same at different positions throughout the container that the gas is found in
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20
Q
  1. What does Bernoulli’s Equation relate?
A
  • it relates blood pressure (P)
  • AND blood flow velocity (v)
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21
Q
  1. What can the Bernoulli Equation be viewed as?
A
  • an Energy Law
  • it expresses the conservation of energy in the flowing blood
22
Q
  1. What does Bernoulli’s equation state when pressure is lost due to friction?
    OR when turbulence is neglected
A
  • the sum of the fluid mechanical energy, fluid kinetic energy and fluid potential energy stay constant
  • FLUID MECHANICAL ENERGY = pressure energy
  • FLUID KINETIC ENERGY = ½.p.v²
  • FLUID POTENTIAL ENERGY = p.g.h
23
Q
  1. How can we describe Bernoulli’s Equation mathematically?
A

P + ½p.v² + p.g.h = CONSTANT

  • P = pressure energy (flow energy)
  • ½p.v² = kinetic energy
  • p.g.h = potential energy
24
Q
  1. With regards to total energy, what can be said about an organ filled with blood?
A
  • the total energy is constant
25
Q
  1. Why can the term p.g.h (Potential energy) be ignored when it comes to a blood vessel in the Supine Human?
A

WHEN VELOCITY IS VERY HIGH:
- the pressure is low
- this is because we have to incorporate the effects of
friction and the change in the size of the vessels
- this will make the flow of the blood turbulent

26
Q
  1. What does Bernoulli’s equation predict with regards to pressure in blood vessels?
A

ACCORDING TO BERNOULLI:
- after the blood vessel narrows:
- the pressure is recovered completely

IN REALITY:
- pressure distal due to stenosis does not recover
completely
- this is due to energy losses by the fluid friction and by
the turbulence

STENOSIS= narrowing
DISTAL = a part of the body that is farther away from the
centre of the body than another part

27
Q
  1. What does Bernoulli’s Law tell us with regards to fluid particles?
A

WHEN A FLUID PARTICLE DECELERATES:
- the pressure increases

WHEN A FLUID PARTICLE ACCELERATES:
(such as with severe stenosis)
- the pressure drops
- the flow of blood is more turbulent

28
Q
  1. With regards to Bernoulli’s Equation, what can be said about the energy per unit volume for any two points?
A
29
Q
  1. In the Supine Human, how can we work out pressure differences using Bernoulli’s Equation?
A
30
Q
  1. How is Bernoulli’s Equation Clinically applied?
A
  • it is used for estimating the severity of aortic valve stenosis
  • it is used for estimating the severity of mitral valve stenosis
  • it is used for estimating the severity of arterial stenosis and aneurysms
31
Q
  1. What is the result of stenosis by plaque deposits?
A
  • the blood velocity must be increased
  • this will result in a decrease in pressure
  • this will lead to further narrowing of the artery
  • the artery may then close entirely
32
Q
  1. What happens to the blood flow when the artery is narrowed?
A
  • the blood flow will become more turbulent
  • the blood flow could possibly damage the arterial wall
  • this will damage the elasticity of the arterial wall
33
Q
  1. What is the result of the arterial wall becoming less elastic?
A
  • the wall’s vibrational characteristics will change
  • this can lead to resonant vibrations
  • these will dislodge the plaque deposits
34
Q
  1. What is an aneurysm?
A
  • it is a localised bulge in the artery
  • it is ballon like
35
Q
  1. What happens when the radius of the Aneurysm increases?
A
  • the velocity of the blood flow increases
  • the pressure decreases
  • the wall of the artery is weakened further
  • this increases the chance of the aneurysm rupturing
36
Q
  1. What does Poiseuille’s Law describe?
A
  • the relation between:
    - the pressure drop (▵P)
    - the fluid flow (Q)
  • this is all measured under steady conditions
37
Q
  1. What are the characteristics of the laminar flow of fluids through a tube/vessel?

LAMINAR= a flow that takes place along constant streamlines, without turbulence

A
  • each fluid layer will stay at the same constant distance from the centre
  • the velocity profile (v.r) is parabolic
38
Q
  1. What are three factors that flow is strongly dependent on?
A
  1. the radius of the tube
    (this is known as the fourth power)
  2. the pressure drop over the tube length
    (▵P / l)
  3. the viscosity of the fluid
    (η)
39
Q
  1. What is the formula that describes the velocity (v) as a function of the radius (r)?
A

v = velocity

▵P =change in pressure

ri = initial radius

r = centre line
= usually equal to zero
= this is the maximum velocity

η = viscosity of the fluid
= fluid coefficient

l = length of the tube

40
Q
  1. How is Blood Flow (Q) worked out?
A

Q = blood flow

▵P = change in pressure

η = viscosity of fluid

l = length of tube

π = pi

ri = initial radius

NB: this shows how Poiseuille’s Law relates ▵P and
Q to each other
: it does this through a uniform (constant) radius
: and through a stiff blood vessel

41
Q
  1. How would we work out the Volume Flow Rate?
A
  • velocity x surface area
  • v x (P . r² / 4)
42
Q
  1. What are the three major assumptions for Poiseuille’s Law to hold?
A
  1. the tube is stiff
    . it is straight
    . it is uniform
  2. the fluid is Newtonian
    . meaning that the viscosity is constant
  3. the flow is laminar
    . it is steady
    . it it not pulsatile (strong regular rhythm)
    . the velocity at the wall is zero
43
Q
  1. What is a more general form of Poiseuille’s Law?
A
  • Q = ▵P / R
  • R = resistance
44
Q
  1. How is R (Resistance) calculated?
A

R = Resistance

η = viscosity of fluid

l = length of the tube/vessel

π = pi

ri = initial radius

45
Q
  1. What is this law used in analogy with?
A
  • it is used in analogy to Ohm’s Law of electricity
  • this is where the resistance is equal to:
    - voltage drop / current
  • the voltage difference is compared to the pressure drop
  • the current is compared to the volume flow
  • the resistance can be calculated from pressure and flow measurements
46
Q
  1. Read through Summary 1.
    Does everything make sense?
A
  • yes
47
Q
  1. Read through Summary 2.
    Does everything make sense?
A
  • yes
48
Q
  1. Read through Summary 3.
    Does everything make sense?
A
  • yes
49
Q
  1. Read through Summary 4.
    Does everything make sense?
A
  • yes
50
Q
  1. Read through Summary 5.
    Does everything make sense?
A
  • yes

MED 101- PHYSICS I (28 decks)

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