What is the equation for blood flow
Change in pressure / resistance
Resitance in vessels is governed by what low
Poiseuilles law
State Poiseuilles law
Flow = [(pi r^4) / 8] X [delta P / n l
In poiseulles law what do each of the terms represent
r = radius
n = viscosity
l = length of the vessel
delta P = pressure difference
What is critically important when determining the resistance of a vessel
r
What are the first vessels where blood meets high resistance
Small arteries - arterioles
The minimum pressure required for flow in a blood vessel is called - why is this?
Crtical opening/closing pressure (dep on which way)
Biological vessels are not rigid and will therefore collapse
What law is the minimun opening pressure governed by
Law of Laplace
What does transmural pressure represent
The pressure across the wall of the blood vessel
What is the equation for transmural pressure
Rearrange to give tension
TP = Tension / Radius
Tension = (Pressure X Internal Radius) / Wall Width
What is the implication of transmural pressure in aneurysms
In a smaller anuerysm there is a greater pressure across the wall - as it gets bigger more likely to burst
What is the equation used to define the compliance of a vessel
delta V / delta P
What type of vessel are the most compliant - what are the implications of this
Veins more compliant
They store the largest fraction of the blood - why post mortem blood pooling occurs in the veins
Because of their compliance and ability to store more blood veins can also be known as
Capacitance vessels
What would occur in a vessel if it was completely non elastic
There would be flow in systole, but no flow during diastole as pressure would fall too low this would give a discontinuous pulsatile flow
What is the effect of having elastic in the arteries
The elastic recoil gives a constant pulsatile flow in what is known as the WINDKESSEL EFFECT
Describe laminar flow
Streamines are straight and non overlapping
Describe turbulent flow
Eddie currents
Common in biological vessels as not perfect cyilinder
What equation is used to determine what type of flow will be seen
Reynolds number
What is the reynolds equation
pDV/n
What are the terms in the reynolds equation
p = density D = diameter v = velocity n = viscosity
A reynolds number less than 2000 means
Laminar flow
A reynolds number greater than 3000 means
Turbulent flow
Where does BP fall the most
Across the arterioles as this is where high resistance is encountered
Where does blood velocity decrease the most
At the arterioles
What vessels have the largest S/A
Capil.
Where is the greatest fraction of blood stored at any one time - what is this fraction
In the venous system
54%
What controls the blood flow to a capillary bed
The precapillary sphincter
How do pre capillay sphincters control blood flow
They allow blood to be diverted to where it most needed
How much of the total blood volume is in the capil.
5%
What are capillaries sites of
Major nutrient exchange
Describe how tissue fluid is formed
Plasma is pushed through the gaps of the capillary endothelial cells to form the tissue fluid
Ammount formed is a balance between filtration and absorption
The net is the tissue fluid
What are the three factors which tissue fluid formation is dependent on
The pressure difference between capillary and intersitial fluid
Difference in colloidal osmotic pressure (exerted by the proteins which are found in the blood)
CFC - capillary filtration coefficient - essentially a constant rate of flow across the vessel
Describe the effect of a high BP on tissue fluid formation
Hydrostatic pressure increases so more tissue fluid is formed
Describe the effect of a low colloidal osmotic pressure on tissue fluid formaiton
More tissue fluid formed as not enough osmotic potential to reabsorb the filtrate
What system returns the tissue fluid back to the CVS
Lymphatic vessels
