L.10 - viscosity/poiseulle’s law Flashcards
What is viscosity?
- Viscosity of a fluid has a very serious effect on the ability of the fluid to flow.
- Viscosity is a measure of the ease with which a food can flow (its mechanical equivalent is friction).
Where is viscosity present?
In all fluids
How is viscosity measured?
In pascal-seconds (Pa s) and is usually denoted by the symbol n
(In physiology textbooks, viscosity is quoted in the unites poise where 1 Pa s = 10 poise)
n water (viscosity water) at 20 degrees is what in Pa s?
10^-3 Pa s
n blood (viscosity blood) is what in Pa s?
~ 5 • 10^-3 Pa s
n blood (viscosity blood) is directly proportional to what?
- to the haematocrit ratio
What is polycythaemia Vera?
- a Desiree where there is an overproduction of red blood cells.
What happens in the polycythaemia Vera disease?
- haematocrit increases
- viscosity of blood increase
- results in blood circulation problems such as thrombosis, emboli, and heart problems
What is Anaemia?
- an underproduction of RBC and while the blood is easier to move, anaemia results in poor O2 transport?
What happens to n (viscosity) when you have anaemia?
- n increases as blood temperature decreases
- as the temp of blood decreases from 37 C to 0 C, the viscosity of blood can increase by 2-3 times, resulting in poorer circulation.
What effect does viscosity have on the blood flow?
- Imagine blood as a series of finite cylindrical layers trying to move
- The frictional forces (viscosity) mean that the blood immediately next to the vessel wall is at rest (v = 0 m s^-1)
- the blood at the centre of the artery is moving with the max velocity (= Vmax)
- Hence the average velocity of the fluid (v) is equal to Vmax / 2
- from the continuity principle , the flow rate is given by:
Q = AV max / 2
What will affect the distribution of red blood cells within a blood vessel?
- this conical velocity (Q= Av max /2)
What is the pressure gradient across the artery? how will this effect the distribution of rbc?
The pressure gradient across the artery it is as follows:
- the maximum pressure is at the edges
- the minimum pressure is at the centre due to the Bernoulli effect
This creates a pressure gradient towards the center of the artery
- This will result in the red blood cells not being distributed evenly within the blood vessels.
- They will accumulate in the centre of the arteries.
- Pressure gradient forces RBC towards the centre of the artery.
- This can have serious implications within the CVS.
What happens I’d you have a smaller vessel branching from a main artery?
- Since the region being fed by the main artery has a higher red blood cell count, then that haematocrit will be high
- However, the region fed by the smaller branching vessels contain blood with few red blood cells, and hence the haematocrit (and hence viscosity) in these vessels will be low.
- This has implications for blood flow rate in regions supplied by these smaller blood vessels.
For viscous fluids, what happens to the pressure?
- It was shown that for viscous fluids, the pressure drops as the fluid (a.k.a. blood) moves through the length of the tube.
- This is simply due to the fact that work must be expended to overcome the viscous forces (strong analogy with simple electrical circuit.)
