hemodynamics Flashcards by Emily Mosher

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

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Types of pressure

. Force applied to surface area
. Pascal
. MmHg (133.3 Pa/1.36 cm H2O)
. CmH2O (98 Pa)

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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

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Hydrostatic pressure

. Exerted by a stationary (static, not moving) column of fluid in a container
. Change in pressure between 2 different heights

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Dynamic or kinetic pressure

. Pressure generated by blood movement

. Generated by heart and elastic recoil of large aa.

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Driving pressure

. Sum of dynamic and hydrostatic pressures

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Transmural pressure

. Change in pressure between inside (hydrostatic) and outside of the vessel

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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

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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

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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

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Shear rate

. Velocity gradient between the moving sheets

. Relative velocities btw laminae (velocity of blood flow)

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Shear stress

. Force that must be applied to 2nd sheet to make it move faster
. Resistance to movement btw laminae (pressure)

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Viscosity

. Shear stress required to produce a particular shear rate
. Measure of internal friction
. Units: Poise
. Viscosity = shear stress/shear rate

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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

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Effect of hematocrit on viscosity

. Relative viscosity inc. as hematocrit inc.
. Anemia: dec. hematocrit, dec. viscosity
. Polycythemia: inc. hematocrit, inc. viscosity

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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

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