Fluid Dynamics and Hemodynamics Flashcards
Fluid dynemics
Study of fluid through a flow system
what are some variables associated with fluid dynamics
Power Work Energy Potential/kinetic energy Pressure Volumetric flow Resistance Capacitance Compliance Velocity Viscosity
Power
rate at which energy is transferred. Power describes how fast work is being performed (WATTS=joules/sec)
Work
the amount of energy transferred (AVERAGE POWER X TOTAL TIME) - JOULES
Energy
quantities such as mass energy, kinetic, potential, heat, radiation
Potential energy
energy which is stored which can be converted to other forms of energy
Energy
Must be concerved
Kinetic energy
Represents energy related to movement and is proportional to the velocity squared of the movement
Pressure
force per unit area
Volumetric Flow
– volume of fluid per time which moves past a point (Litres/min…etc.)
Resistance
ratio of the pressure drop across a vessel per volumetric. Measured of the impediment that must be overcome for flow to occur.
Capacitance
the ability to hold a change in volume per change in tome (dv/dt) V is volume and t is time
Velcoity
speed with which a fluid moves in a specific direction
Viscosity
measure of the resistance of the fluid to flow due to the attraction of the molecules
Energy can occur from a
Higher to lower enrgy level
Energy is converted from what to what in ultrasound?
Electropotential energy is converted into acoustic mechanical energy and transmitted into the body – absorption is mainly the conversion of the acoustic energy into heat energy
Reflected waves are then converted back into electropotential energy
Fundamental rule #1 of energy
Energy is always conserved – energy is never lost, only converted between forms
Energy within the cardiovascular system is
Converted back and forth between kinetic and potential energy
Pressure that represents force exerted on the vessel walls
Potential energy
force of flow direction in the vessels
Kinetic energy
Increase in blood velocity=
Increased kinetic energy and therefore decreased potential energy
As flow accelerates
Decrease in potential energy and an compsentory increase in kinetic energy
Key concept Kinetic/potential energy
If we make the assumption that little or no energy is lost to heat, conservation of energy requires that a change in kinetic energy must equal a change in potential energy.
Since kinetic energy is related to velocity and since velocity can be measured by Doppler, a change in potential energy (pressure) can be determined by performing Doppler
hydrostatic pressure is a form of what
Potential energy
Hydrostatic pressure is what
is the pressure that results from the force of the fluid (gravity) which results from a column of fluid.
what is hydrostatic pressure proportional to
The hydrostatic pressure is proportional to the density of the fluid, the height of the fluid, and gravity
Any factors affecting weight will affect
hydrostatic pressure
A taller column will create higher/lower hydrostatic pressure
High hydrostatic pressure
A more dense fluid will create a higher/lower hydrostatic pressure
Higher hydrostatic pressure
clinically what affects hydrostatic pressure
Height and patient position
For normal density of blood, each inch of blood in a vertical column results in a pressure of….
2mmHG
Volumetric flow
Flow is defined as the amount or volume of a quantity which moves past a point per unit time
Doppler does not measure FLOW, it measures velocity
Velcoity
Speed
Speed
Velocity, flow and pressure are all related
Can not assume that high flow represents a high velocity
Capactiance
Capacitance is defined as a change in volume per time. Is a measure of the ability to hold a change in volume per change in time
Compliance
Is the measure of the ability to hold a change in volume per change in pressure
High compliance
implies that there is a large increase in volume for a small increase in pressure
Fluid viscoisty
Measure of the internal resistance of a fluid to flow
Fluid viscosity is caused by
Caused by molecular cohesive forces
Attraction of molecules
what does the resistance equation state
States that the resistance is directly proportional to the vessel length and the fluid viscosity and inversely proportional to the radius of the vessel to the 4th power
If the length increases
The resistance increases
More energy is required to transport the same flow in the longer pipeline
Resistance is inversely proportional to
Radius
Radius affects resistance faster then
Length
Radius is affected by
4th power
Resistance is inversely proportional to
r4
Resistance is proportional to
Viscosity
Higher viscosity results in
A higher resistance to flow
Resistance equation
R=8ln/pir4
Larger cross sectional area
Increases the volumetric flow
Higher average spatial velocity increases
Volumetric flow
Increase in velocity increases flow
Continuity (volumetric flow) equation
Q=v*area
Assumption
We know that flow only occurs from a higher to lower energy state
One for of energy is pressure exertion on a wall
If we assume no other energy it is fair to say that this higher pressure region to a lower pressure region would create flow against a resistive pathway
Pressure gradient is proportional to
Resistance
An increase in resistance results in an increase in pressure drop for fixed flow
Pressure gradient is proportional to
Volumetric flow
For fixed resistance, higher flow results in an increase in the pressure gradient
Simplified law of hemodynamics
P=Q*R
Poiseuille’s law
Is the same law as the simplified law, but written in terms of the volumetric flw (Q) and with a direct substitution for resistance
what is Poiseuille’s law
Q=Ppir4/8ln
Poiseuille’s law can only function under certain conditions
The flow conduit is rigid and cylindrical tube
The flow is in a steady state, laminar flow
The fluid is Newtonian
Bernoulli’s equation
Is derived directly from the conservation of energy theorem
Since energy must be conserved in a closed system, the sum if the energy at point 1 must be equal to the energy at point 2
By grouping the pressure terms on one side of the equation, the kinetic energy terms on the other, the expression becomes Bernouli’s
what does Bernoulli’s equation state
For a closed system, assuming no energy lose to heat (friction on walls), the energy at point 1 musy qual the energy at point 2
What are some assumptions of Bernoulli’s equation
- Rigid tube
- no friction
- steady, non pulsatile flow state
- Non-viscous fluid
- incompressible, inhomogeneous fluid
Bernoulli’s equation
Takes into account major sources of energy interacting to create flow
What are rigid tube flow asusmptions
- Flow conduit is a rigid tube
- surface of the tube is smooth with no irregulates
- fluid is Newtonian (homogeneous with constant viscosity)
- compressible fluid
- there is no energy lost to heat
- flow state is steady
flow is affected by what
Changes in a cross sectional area
Decreasing area
Causes acceleration and a blunting of parabolic laminar flow
Increasing area
flow disturbances can occur (turbulence) as a mechanism of reducing kinetic energy
Steady flow
Steady flow is constant in volumetric flow
Pulsatile flow
- Volumetric flow is dynamic with time
- dynamic pressure is generated by heart, blood flow
Laminar flow
- Well behaved manner and uniform direction
- Fluid moves in concentric rings with no crossing of ring boundaries
Plug Flow
laminar flow that occurs from an acceleration component such as early systole or ascending branch of aorta
Parabolic flow
velocity profile across vessel shaped like parabola, arterial flow in straight, unchanging arteries, venous flow
Disturbed flow
disturbed flow in any deviation from laminar flow
Turbulent flow
fluid is not uniform and is random or chaotic. Occurs distal to stenosis or narrowing
Entrance effects
change in velocity profile into a vessel of a reduced caliber. Since the caliber has decreased in area, the velocity must increase (accelerate).
Exit effects
change in velocity profile exiting a vessel of a smaller diameter. Velocity must decrease to maintain constant flow. Inertia is dissipated by chaotic or turbulent flow
Reynolds number
Indicates the likelihood of turbulence occurring
A higher Reynold’s number implies a greater likelihood of turbulence occurring
In hemodynamics what is removed
Basic assumptions (Rigid, cylindrical tube, steady, laminar flow, Newtonian fluid) as they do not hold true for blood flow
Driving pressure in hemodynamics
Is dynamic (blood flow is pulsatile)
what is the principal parameter measured by Doppler
Velocity
Time variant velocity signal from doppler is reliant on…
Cardiac Output Pulse Pressure Mean Arterial Pressure (MAP) Peripheral Resistance Venovasmator Tone
Pressure is ______in the human body
Dynamic
Arteries are _____ and therefore not ______for flow
elastic rigid conduits for flow
why is elasticity important
- allows aorta to be capacitive
- Capacitance of aorta allows energy to be stored in walls to provide energy to propel blood during diastole
- Run off from the capacitive aorta through the resistive arterioles and capillaries reduces pulsatility, improving heart efficiency
Because of vessel compliance….
A change in pressure results in a change in cross sectional area and hence an increase in the capacity for flow volume (Vasodilation and vasoconstriction)
When pressure is applied to the vessel wall this creates….
A larger cross sectional areas of the vessel and greater volume of flow
As walls stretch in a vessel
Electricity decreaceases
Significant distention of walls does what
Decreases compliance at a point
simplified pressure volume relationship
Expresses the rate of change in pressure is proportional to the rate of change in volume
There is a _____range over which small increases in ______result in large increases in _______
Large, pressure, volume
At both low and high ends the rate of volume with increases pressure is slower/faster than in the middle
Slower
Initial filling requires an increase in ______before stretching and the rate can then become constant
pressure
Once vessel is stretched out, the ability to hold volume _______
Decreases
Non compliant vessles
At lower pressures the relationship is linear but at higher pressures, an increase in pressure=no increase in volume
Series resistance
effective (overall) resistance is the sum of the resistances of each component
Parallel resistance
overall resistance is more complicated
inverse effective resistance is calculated as the sum of individual inverse resistances
Effective resistance decreases/increases with increasing parallel vessels
Decreases
Effective resistance for a single larger dimeter vessel is much more/less than for a parallel combination
less
Amount of energy lost by transporting blood over a rough vessel is…..
greater than the energy lost transporting over smooth surfaces
kinetic energy is lost to ____
heat through the friction from both external interaction of fluid with the walls and internal interaction related to viscosity
Energy losses _____with decreasing vessel size as a result of increased frictional and viscous energy loss
increase
Varying vessel sizes is a principal mechanism in controlling the ______ throughout the arterial system
Effective resistance
Why is control of resistance important
To control pressure decrease as well as regulate volumetric flow
Resistance ________in progression from the low resistance the aorta to the high resistance of the arterioles
Decreases
effective resistance of the capillaries is high but
lower then arterioles because of the sheer number of capillaries
Velocity of the flow is controlled primarily by….
The varying total cross sectional area of the vessels
For a fixed volume as the area _______ the velocity _______
increases, decreases
Pressure in the venous system
Low
Venous system is referred to as the
Capacitive or reservoir component of the cardiovascular system
The venous pressure gradient is…..
Small, the capacitance of the veins creates a reservoir where it can stay until a gradient exists for return
blood loss situations draw from the venous reservoir
Where is most of the blood volume
Veins and venules
Calf muscle pump
The calf muscle pump helps overcome the effect of gravity to aid with venous return for a patient in the standing position. By muscle contraction, the venous volume is ratcheted back toward the right heart through a series of valves which open and close with muscle contraction.
what is transmural pressure
measure of the difference of the pressure inside the vessel (intravascular pressure) relative to the pressure outside the vessel (tissue pressure). Note that the transmural pressure is always referenced from the inside of the vessel to the outside of the vessel.
With increased intravascular pressure
tissue pressure is lower
Critical stenosis
When the disease becomes critical, the amount of energy lost to frictional and viscous effects become so severe, that volume is not maintained across the lesion. As depicted below, a point is reached at which there is a narrow stream of flow at a high velocity with most of the flow traveling at a relatively low velocity (“string flow”).
