Flashcards in CVS S5 - Blood Flow Deck (38)
What drives flow?
A gradient of pressure
The volume of fluid passing a given point per unit time
Rate of movement of fluid particles along the tube
What is laminar flow?
There is a gradient of flow from the middle to the edge of a blood vessel
Velocity is highest at the centre
Fluid is stationary at the edge
Most blood flow is laminar
What is turbulent flow?
When layers of fluid try to move over one another faster than physics will allow
As velocity increases, eventually flow will become turbulent
Turbulence greatly increases resistance
The extent to which fluid layers resist flowing over one another
What are the effects of viscosity on flow?
The higher the viscosity, the lower the difference between the velocity of flow in the middle and the edge of a vessel
The higher the viscosity, the lower the velocity
What is the effect of tube diameter on velocity?
Velocity is proportional to cross sectional area of a tube
How does viscosity affect resistance?
The greater the viscosity, the greater the resistance
Describe the relationship between pressure and resistance if flow is fixed
If flow is fixed, then the higher the resistance, the greater the pressure change from one end of the vessel to the other.
Describe the relationship between flow and resistance if pressure is fixed
If pressure is fixed then the higher the resistance, the lower the flow.
What is the relationship between radius of vessel and resistance?
Resistance decreases with the 4th power of the radius
This means small changes in radius can have a great effect on resistance
How is resistance combined for vessels in series?
The resistances for each vessel are added
How is resistance combined for vessels in parallel?
The resistance for each is reduced as there is another path for the blood to go
Describe flow over the systemic circulation
Flow remains the same at all points
Describe resistance and pressure changes in arteries
Resistance is low in arteries
There is a low pressure drop from one end of an artery to the other
Describe resistance and pressure changes in arterioles
Resistance is high
Pressure change from one end of an arteriole to the other is high
Describe resistance in veins and venules
Resistance is low
Pressure change from one end to the other is low
Why are arteries high pressure?
Because the arterioles directly after them are high resistance
It's hard to push blood into them
Therefore pressure increases
What is transmural pressure?
Generated by pressure in distensible vessels
Describe blood flow through distensible vessels
The higher the pressure in the vessel, the more stretched the vessel so the wider the lumen so more laminar flow so lower the resistance so flow increases
If pressure falls to zero, vessel walls collapse and flow ceases
Describe the capacitance property of distensible vessels
As vessels widen under increasing pressure, more blood transiently flows in than out
This allows distensible vessels to store blood - conferring capacitance
Veins are the most distensible (67% of their blood is at rest)
Describe systolic arterial pressure and what factors affect it
Maximum arterial pressure
Usually ~120 mmHg
-How hard the heart pumps
-Total peripheral resistance (TPR)
-Compliance (stretchiness) of arteries
Describe diastolic arterial pressure and what factors affect it
Minimum arterial pressure
What is pulse pressure?
The difference between systolic and diastolic pressure
What is the average arterial pressure?
Diastolic pressure plus 1/3rd of systolic pressure
(Because systole is shorter than diastole)
Total peripheral resistance
The sum of the resistance of all the peripheral vasculature of the systemic circulation
What is the pulse wave?
A pulse wave is brought about by contraction of the ventricles
Travels faster than blood
Can be felt at various locations where arteries may be pushed against a reasonably hard surface
What is the dicrotic notch?
The slight dip in the pulse wave caused by slight back flow as blood flows back through the aortic valve as pressure in the LV goes below that in the aorta