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Flashcards in Hemodynamics Deck (23):

Where is the blood volume in the body?


transmural pressure

pressure difference between the outside and inside of athe vessel wall


hydrostatic pressure

pressure difference between one height and another in the body

no matter the position of the body, the pressure differential from artery to vein is equivalent in each region


pressure gradient

the pressure difference between two locations

in normal circulation, pressure favors going back to the heart

may be reversed in the capillaries


components that determine flow

driving pressure, resistance, and hydrostatic influences


relationship between velocity and vascular cross secional area

flow = (cross-sectional area) x (blood velocity)

Q = A x V


Poiseuille's Law

Q = [(P1-P2)*pi*r4]/(8*eta*L)

P = pressure

r = radius

L = length

eta = viscosity

Q = flow

R = resistance


Bernoulli Principle

total fluid energy (pressure) in flowing blood: Etotal = Epotential + Ekinetic + Egravity

Epotential: potential energy from cardiac contractionl stored in vessel walls

Ekinetic: kinetic energy in direction of blood flow; increases in proportion to blood velocity

Egravity: gravity can increase or decrease pressure depending on position relative to heart


What is viscosity of blood dependent on?

fibrinogen concentration


vessel radius

linear velocity



shear stress

resistance to movement between laminae (pressure)


sheer rate

relative velocities between laminate (velocity of blood flow)



shear stress/shear rate


unit = Poise (dyne sec/cm.sq.)


laminar flow

flow in blood vessels occurs in longitudinal, concentric layers

central layers move faster than those near the vessel wall (parabolic velocity profile)

fluid elements remain in a given layer as they move along


viscosity, hematocrit, and blood velocity

apparent viscocity of blood dcreases as the shear rate or velocity increases

the higher the hematocrit, the higher the viscosity

"shear thinning"


turbulent flow

irregular flow with lateral components producing eddies and vortices - dissapates energy, may be accompanied by audible vibrations (murmurs or bruits)

predisposing factors for turbulence - sharp bends or obstructions


viscosity effect of vessel size

blood viscosity is relatively insensitive to changes in vessel radius for large vessels

but decreases steeply with decreases in radius for smaller vessels


Reynold's number

index for turbulence

dimensionless number indicating propensity for turbulent blood flow

the higher the Reynold's number (>3000), the greater the chance for turbulent blood flow to develop

NR = (rho*D*v)/(eta)

D = vessel diameter

v = blood velocity

eta = blood viscosity

rho = blood density


axial streaming

high velocity flow causes red cells to move toward the center of the stream leaving "plasma rich-RBC poor" fluid near the vessel wall


plasma skimming

refers to the tendency of branching blood vessels to have relatively less RBCs

problem is solved by arterial cushions that slow down and push them into the branching vessels


pressures in the systemic circulation

resistance is found precapillary

from the veins back toward the right heart, there is an even smaller pressure gradient and lower resistance


where does the largest pressure drop occur?


arteriolar constriction increases pressure in the proximal arterial system and increases the pressure drop

arteriolar dilation decreases proximal arterial pressure and decreases the pressure drop


CV Pressure-Flow/Resistance relationships

in the CV system, there is no flow at a positive driving pressure, this varies with activation of the sympathetic nervous system

sympathetic activation (constriction) alters the pressure-resistance relationship


critical closing pressure

the amount of pressure necessary just before flow is seen in a vessel