Blood flow Flashcards
Describe the terms flow and velocity, with reference to the movement of fluids through tubes. What is the relationship between them?
Flow = volume/ time Velocity = distance/ time (rate of movement)
Describe what is meant by laminar and turbulent flow
Turbulent Flow
As the mean velocity increases, flow eventually becomes turbulent. The velocity gradient breaks down as layers of fluid try to move over each other faster than physics will allow. The fluid tumbles over, greatly increasing flow resistance.
Laminar Flow
In laminar flow, there is a gradient of velocity from the middle to the edge of the vessel. Velocity is highest in the centre, and lowest for fluid at the edge. The flow in most blood vessels is laminar.
Describe what is meant by viscosity and the effect of viscosity on flow
The extent to which fluid layers resist sliding over one another.
High viscosity: thick, slow velocity
Low viscosity: thin, high velocity
Describe the effects of changing tube diameter on flow rate
Vessels with a small cross sectional area have a high velocity
Vessels with a high cross sectional area have a low velocity
What is meant by resistance to flow and what factors affect it?
Resistance = pressure/ flow (pressure = flow x resistance)
- resistance increases as viscosity increases
- vessel diameter: the larger the less resistance
Describe the effects of combining flow resistance in series and in parallel
For vessels in series, resistances add together.
- R1 + R2
For vessels in parallel, the effective resistance is lower, as there is more than one path for the current to flow down.
(R1 x R2) / (R1 + R2)
Describe the pattern of flow resistance and pressure over the systemic circulation
o Over the whole circulation, flow is the same at all points
o Arteries are low resistance
Pressure drop over arteries is small
o Arterioles are high resistance
Pressure drop over arterioles is large
o Venules and veins are low resistance
Pressure drop over venules and veins is small
Describe how the distensibility of blood vessels affects the relationship between flow and pressure
Blood vessels have distensible walls, and the pressure within the vessel generates a transmural pressure across the wall. This stretches the vessel.
As the vessel stretches, the diameter of the lumen increases, so resistance falls and flow increases. So the higher the pressure in a vessel, the easier it is for blood to flow through it.
As the pressure within a distensible vessel falls, the walls eventually collapse, and blood flow ceases before the driving pressure falls to zero.
Describe how the distensibility of blood vessels produces the property of capacitance
As vessels widen with increasing pressure, more blood transiently flows in than out.
This allows distensible vessels to ‘store’ blood – They have capacitance.
Veins are the most distensible vessel, with 67% of the blood in them at rest.
Define: Systolic pressure, diastolic pressure, pulse pressure and average pressure
systolic: arterial pressure during contraction of the ventricles. This is the highest pressure.
diastolic: arterial pressure between systolic contraction
pulse: the difference between systolic and diastolic
average: diastolic + 1/3 pulse, as systole is shorter than diastole
What is total peripheral resistance?
Total Peripheral Resistance – The sum of the resistance of all of the peripheral vasculature in the systemic circulation.
Describe how the elastic nature of arteries acts to reduce arterial pressure fluctuation
If arteries had rigid walls, the pressure in them would rise enough in systole to force the whole stroke volume through the total peripheral resistance, and fall to zero in diastole.
But arteries have distensible walls, allowing them to stretch in systole. More blood flows in than out, so pressure does not rise so much. The arteries recoil in diastole and flow continues through the arterioles.
Draw a typical arterial wave form
Systolic diastolic wave + dicrotic wave and dicrotic notch
Describe the pulse wave
Contraction of the ventricles generates a pulse wave, which propagates along the arteries faster than blood. This is felt at a variety of locations where arteries come close to the surface and can be pushed against a reasonably hard surface.
Dicrotic Notch
The slight dip seen in the pulse wave is known as the Dicrotic Notch. This is due to pressure in the left ventricle falling below aortic pressure and the subsequent backflow of blood (this backflow is responsible for closing the aortic valve)
Dicrotic Wave
The slight rise seen in the pulse wave directly after the dicrotic notch is the Dicrotic Wave. This slight increase in pressure is due to the recoil of blood off the closed aortic valve.
Describe the role of arterioles as resistance vessels
Arterioles control blood flow to tissues by variable flow restriction. Their walls contain much smooth muscle, and it’s state of contraction determines lumen diameter and therefore flow resistance (see above).
Vasoconstriction Decrease in Flow
Vasodilatation Increase in flow
