CVS Session 5 (Lecture 5.1) Flashcards
Define flow through a tube.
Volume of fluid passing a given point per unit time
Define velocity through a tube.
The rate of movement of fluid particles along the tube
What is the relationship between flow and pressure?
Flow is proportional to pressure difference between ends of a vessel
If flow is constant, what is the relationship between velocity and the radius of a tube?
Velocity is inversely proportional to cross sectional area
same no. of particles to move a given distance
In most blood vessels what is the flow described as?
Laminar
Describe laminar flow?
Smooth flow of blood generating no noise
Gradient of velocity from the middle to the edge of vessel with particles in the centre moving fastest and fluid being stationary at the edge
Fluid moves in concentric layers so slide over each other.
Describe turbulent flow? How can it be generated?
Flow through a vessel that generates noise (bruit).
As mean velocity increases flow eventually becomes turbulent
Velocity gradient breaks down
Fluid tumbles over
Fluid resistance greatly increased
If the driving pressure is constant, what is the flow determined by?
Mean velocity
What is the mean velocity dependent on?
Viscosity of fluid
Radius of tube
What is viscosity? What is the relationship between viscosity and velocity?
Extent to which fluid layers resist sliding over one another
Velocity is inversely proportional to viscosity
At a constant gradient, what happens to the mean velocity as the radius of a tube increases?
Velocity increases as it is proportional to C.S.A.
What is the formula that relates flow, velocity and C.S.A?
Q=V . CSA
What is Poiseulles Law?
Flow = Pr^4 / (Viscosity.Length)
What is the formula relating pressure, flow and resistance?
P=QR
Rearrange Poiseulles law with P=QR to equate pressure change.
P = Flow * (8.Viscosity.Length/pi r^4)
What is the relationship between resistance and viscosity and radius?
Resistance is proportional to viscosity and inversely proportional to the fourth power of radius
What is the formula for resistance for blood vessels in series? What is the significance of this?
R = R1 + R2
Resistances add for vessels in series
What is the formula for resistance for blood vessels in parallel? What is the significance of this?
R = (R1.R2)/R1 + R2
Resistance is lower in parallel vessels
What resistance are arteries and arterioles? What is the pressure change across these vessels?
Low (arteries) resistance - pressure change is low
High (arterioles) resistance - pressure change is high
What is the resistance of a) individual capillaries and b) capillaries in parallel ?
a) High resistance
b) Overall resistance is low - pressure drop is smaller
What resistance are venules and veins?
Low resistance
For what reason is the pressure in arteries high?
High resistance of arterioles to ensure adequate flow through them
Which two variables affect arterial pressure?
Arteriolar resistance and stroke volume
What is name of the ability of a blood vessel to stretch?
Distensibility
What happens as vessels stretch?
Resistance falls so flow increases for a given pressure.
What happens when the pressure in a distensible vessel falls?
Flow is reduced and eventually can lead to vessel collapse (below critical closing pressure) and blood flow =0 with infinite flow resistance.
What does an abrupt increase in pressure do to the flow in a distensible vessel?
Transient increase in flow in than out of vessel.
What do distensible vessels store? What is this property called?
Store blood.
Capacitance
What do blood cells and plasma do in vessels?
RBCs congregate in middle so travel fastest whilst plasma accumulates at edges so is stationary.
Apparent viscosity increased.
