hemodynamics Flashcards
Ohm’s Law
. Driving force for flow results from the difference in total fluid energy btw ant 2 points
. Total energy of fluid measured as pressure
. Blow flow directly proportional to pressure
. Blood flow is inversely proportional to resistance
. Resistance directly proportional to vessel length
. Resistance inversely proportional to radius
. F = deltaP/R
Types of pressure
. Force applied to surface area
. Pascal
. MmHg (133.3 Pa/1.36 cm H2O)
. CmH2O (98 Pa)
Gravitational pressure
. Form of potential energy due to gravity
. Pressure that results from gravity is hydrostatic pressure
. Pressure anywhere along column depends on product of its density, height, and gravitational acceleration
Hydrostatic pressure
. Exerted by a stationary (static, not moving) column of fluid in a container
. Change in pressure between 2 different heights
Dynamic or kinetic pressure
. Pressure generated by blood movement
. Generated by heart and elastic recoil of large aa.
Driving pressure
. Sum of dynamic and hydrostatic pressures
Transmural pressure
. Change in pressure between inside (hydrostatic) and outside of the vessel
Vascular resistance in series
. Circuit of resistors arranged in chain
. Corresponds to progressive circulation through different vessels in circulatory tree
. LV -> aorta -> small artery
. Total resistance = sum of individual resistances
Parallel vascular resistance
. Resistors arranged w/ heads and tails connected together
. Corresponds to circulation through different organs/regions of body
. Aorta to head, intestines, kidneys, and pelvis
. Total conductance = sum of individual conductance (1/R)
. Total resistance less than any individual resistance
. Flow is directly proportional to conductance
Poiseuille’s Law
. Blood flow is directly proportional to axial pressure gradient
. Blood flow directly proportional to vessel radius raised to the 4th power
. Blood flow inversely proportional to the vessel resistance
. Blood flow inversely proportional to vessel length and blood viscosity
Shear rate
. Velocity gradient between the moving sheets
. Relative velocities btw laminae (velocity of blood flow)
Shear stress
. Force that must be applied to 2nd sheet to make it move faster
. Resistance to movement btw laminae (pressure)
Viscosity
. Shear stress required to produce a particular shear rate
. Measure of internal friction
. Units: Poise
. Viscosity = shear stress/shear rate
Laminar flow
. Shearing laminae are in concentric cylinders that move w/ different velocities
. Inner layer moves w/ highest velocity
. Outermost cylinder moves at slowest velocity
. Resulting velocity profile is a parabola w/ max velocity at central axis
. The lower the viscosity the sharper the parabolic distribution
Effect of hematocrit on viscosity
. Relative viscosity inc. as hematocrit inc.
. Anemia: dec. hematocrit, dec. viscosity
. Polycythemia: inc. hematocrit, inc. viscosity
Axial streaming
. Tendency of RBCs to accumulate in the center of the lumen of small vessels as flow velocity inc.
. Reduces viscosity and therefore resistance to flow
. Not important in larger vessels so it is assumed blood viscosity is independent of velocity
Turbulent flow
. Disorderly pattern of fluid movement
. Non-laminar
. Occurs as driving pressure inc. and flow reaches critical velocity
. Streams mix radially and axial producing eddies and vortices
. Dissipates energy which causes resistance to flow
. Underlies heart murmurs, bp, damage to endothelial lining, and thrombi
Reynold’s number
. Indicates propensity for turbulent blood flow
. The higher the reynold’s number (>3000) the greater chance for turbulent blood flow to develop
. Determinants: diameter, velocity, density, viscosity
Bernoulli principle
. 2 forms of energy (potential and kinetic) are interconvertible
. Hemodynamic and hydrostatic pressure can be converted into kinetic energy to cause a liquid to flow
Total energy, potential energy, and kinetic energy in vascular system
. Total: constant flow system, total energy remains constant
. Potential: hydrostatic pressure
. Kinetic: velocity of a fluid equals the distance the fluid has traveled per unit time, velocity varies inversely w/ cross sectional area
T/F velocity will vary inversely w/ cross-sectional area
T
Blood for in aortic stenosis
. Abrupt dec. in vessel cross sectional areacauses potential energy to convert to kinetic
. As results the transmural pressure dec. and velocity of flow inc.
Blood flow in capillaries
. Small radius = low wall tension
. Reason why capillaries in feet don’t burst
Arteriolar vasoconstriction
. Greater wall thickness/ lumen diameter ratio = low wall tension
. Provides greater control of vessel diameter and blood flow
Aneurysm blood flow
. Large radius = high wall tension
. Can’t withstand transmural pressures and therefor can eventually rupture
Blood flow in ciliated hearts
. Large radius= high wall tension
. Causes more work (higher oxygen consumption) required during systole to overcome the higher wall tension
Elasticity in blood vessels
. Elastic properties of blood vessels are major cause of nonlinear pressure-flow relationship in vascular beds
Type of tissues that make up vascular wall of blood vessels
. Endothelial cells: form single continuous layer in direct contact w/ blood
. Elastic tissue: cant stretch more than 100%
. Vascular smooth muscle cells: in all segments but capillaries, exert tension through active contraction, as smooth muscle contraction inc., elasticity dec.
. Fibrous tissue: collagen w/ high tensile strength but low elasticity
The higher the wall thickness/diameter, the greater the control of ___
Vessel diameter and blood flow
LaPlace relationship
. Precapillary sphincter has highest ratio and of thickness/lumen diameter and therefore the greatest control
. Veins have small wall thickness/lumen diameter ratio (thin wall, large diameter) and therefore regulate volume more than flow or pressure
Non-linear pressure-flow relationship
. As driving pressure inc., the transmural pressure also inc. causing vessel to distend lowering the resistance
. As results blood flow inc. at higher pressures more than it would in rigid tube
. At low pressure the resistance inc.
Resistance vessels
. Arteries
. Large aa. Have smaller esistance
. Small aa. Moderate resistance
. Arterioles have max resistance, pressure drop is greatest there, change from pulsatile flow to steady flow
Why is pulsatile flow damped out at capillary level?
Distensibility of large aa.
. Frictional resistance in small aa. And arterioles
Compliance
. Distensibility of elastic surface
. Defined as change in volume caused by given change in pressure
. Ability of a blood vessel to undergo deformation ad a result of a change in pressure
. Higher compliance the more easily the vessel can be stretched
What determines the compliance properties of a vessel wall?
Physical arrangement of the vascular components
. Load on vascular wall is borne by elastin and smooth muscle and lastly by collagen
. Elastic properties on vessel wall, vascular smooth muscle tone, and vessel geometry are determinants
Age-related changes in aortic compliance
. Distensibility of aa. Dec. w/ age
. Slope of curve at physiological pressures dec. (lowers compliance) in older groups
. Aa. Have less elastin and more collagen
. As compliance dec. a given inc. in volume elicits a larger inc. in pressure
. Results in wider pulse pressure ad more cardiac work, independent risk factor of developing heart disease
. Regular exercise slows and reverse age-related decreases in aortic compliance
