Lecture 7.1: Blood Flow Flashcards
What is Haemodynamics?
It is the science that describes the association between blood flow, blood pressure and resistance
What drives the flow of blood?
- Pressure gradient drives the flow of
blood - Flow is proportional to pressure
difference, the greater the
difference the greater the flow
What is the relationship between blood flow and velocity?
- Flow along a vessel is constant
- Velocity can vary along the tube
(depending on the radius of the
tube) - For a given flow rate, velocity is
inversely proportional to cross
sectional area
What is Laminar Blood Flow?
Occurs when blood moves in ordered, parallel layers through the arteries with no obstructions to agitate the layers
What is Turbulent Blood Flow?
Occurs when blood flows in a whole host of random pathways, turbulence results in sound production that can be heard with a stethoscope
What is the gradient of velocity in a blood vessel?
Maximal at centre, theoretically zero at the edge
What factors determine resistance in a blood vessel? (3)
- Vessel Length (L)
- Blood Viscosity (η)
- Radius of the Blood Vessel (r)
How does the radius of a vessel change resistance?
- A change in radius alters resistance
inversely to the 4th power of the
radius (1/R ∝ R^4) - A twofold increase in radius.
produces a 16 fold decrease in
resistance
What 3 things does Poiseuilles Law assum?
1) Vessels are rigid
2) Viscosity remains constant
3) Flow is laminar
But blood vessels are not rigid, their walls are…? What is the effect of this?
- Distensible
- Pressure within the vessel
generates a transmural pressure
i.e. from ‘inside’ to ‘outside’ - A vessel stretches so resistance
must fall, easier for blood to flow - The opposite is that as pressure
falls, the vessel may collapse
Which type of blood vessels are the most distensible?
Veins
Distensible vessels are said to have…?
Capacitance
What is Normal BP?
Systolic – 120mmHg
Diastolic – 80mmHg
What is Viscosity?
- Viscosity is an internal friction
between the components of a liquid - It’s a measure of the extent to
which layers of fluid resist sliding
over each other
What is Blood Flow?
The volume of blood passing a given point per unit time
What is Resistance?
Collective term for the various frictional forces impeding the flow of blood
What is Velocity?
The rate of movement of fluid particles travelling along a tube
What is Q=VA?
Volumetric Flow Rate= Mean Velocity x Cross-Sectional Area of Flow Radius
What is the Blood Flow Equation?
Flow = Pressure Difference/Resistance
What is the Resistance Equation?
Resistance= [8 x viscosity x length]/ [πr^4]
Poiseuilles Law: Q= [∆Pπr^4]/[8nL]
What does each symbol stand for?
Q= volume flux
P= pressure difference
r= radius
n= viscosity
L= length
For resistances in series, simply add up the individual resistances (Equation)?
(ΣRT = R1 + R2 + R3)
For resistances in parallel, add up the reciprocals in this case (Equation)?
Σ1/RT = 1/R1 + I/R2 + 1/R3
What type of vessel is least distensible/has highest resistance?
This means terminal arteries and arterioles are the main sites of resistance
