CVS S5 - Blood Flow Flashcards
Define flow
The volume of fluid passing a given point per unit time
What drives flow?
A gradient of pressure
Define velocity
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
Define viscosity
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?
Stretches vessels
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 Affected by: -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 Usually ~80mmHg Affected by: -Systolic pressure -TPR
What is pulse pressure?
The difference between systolic and diastolic pressure
Usually ~40mmHg
What is the average arterial pressure?
Diastolic pressure plus 1/3rd of systolic pressure
Because systole is shorter than diastole
Define TPR
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
What is the dicrotic wave?
The slight rise seen in the pulse wave directly after the dicrotic notch
Caused by the recoil of blood as the aortic valve closes
Describe the importance of smooth muscle in arteriolar walls
Smooth muscle can be contracted or relaxed so can increase or decrease resistance and blood flow to tissues
What is vasomotor tone?
The degree to which vascular smooth muscle is contracted
If more contracted, vasoconstriction
If less contracted, vasodilation/vasodilatation
In all but maximum flow conditions, muscle is at least partially contracted
What determines vasomotor tone?
Mainly a sympathetic branch of the ANS
Alpha 1 adrenergic receptors
Antagonised by vasodilator factors, so tone is determined by a balance
What are vasodilator metabolites and what do they do?
Produced by metabolically active tissues
Eg K, H, adenosine
Cause relaxation of local smooth muscle
So increase flow and decrease resistance
What is reactive hyperaemia?
If a limb has blood flow cut off for a few minutes then when restored, the local arterioles will have dilated maximally (due to buildup of vasodilator metabolites) so blood flow will be very high
What is auto regulation?
At most levels of metabolic activity, organs automatically take the amount of blood they require so long as the pressure of the arteries remains within a certain range
Define central venous pressure
The pressure in the great veins
Define venous return
The rate of flow of blood back to the heart (limits cardiac output)
