Hemodynamics Flashcards
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
hematocrit
vessel radius
linear velocity
temperature
shear stress
resistance to movement between laminae (pressure)
sheer rate
relative velocities between laminate (velocity of blood flow)
viscosity
shear stress/shear rate
(pressure/velocity)
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?
arterioles
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
