Session 5 - Blood flow Flashcards
Define velocity
-The rate of movement of fluid particles along a tube (distance/time)
At a given flow, what is velocity inversely proportional to?
-Cross sectional area
At a given flow, which vessels have high velocity?
-Those with a small cross-sectional area, eg aorta compared with high cross-secional area eg capillaries
What is lamina flow?
- Flow of blood which has a gradient velocity from the middle to the edge of the vessels as
- The fluid slides over one another in concentric layers as the velocity is highest in the centre and stationary at the edge
What is turbulent flow?
-Gradient velocity has broken down and the fluid tumbles over each other increasing resistance requiring a higher pressure
What is a bruit?
-The noise which turbulent flow makes
What drives the flow of blood through BVs?
-The pressure gradient between the two ends of the vessel
What determines flow at a given pressure?
-Velocity
What determines velocity at a given pressure?
- Viscosity of blood
- Radius of tube
What is viscosity?
-The extent to which the concentric layers of blood resist sliding over one another in laminar flow
What is the relationship between velocity and viscosity?
- Velocity is inversely proportional to viscosity, ie
- The higher the viscosity (the greater the resistance of the particles) the lower the average velocity
-At a constant pressure, what is velocity proportional to?
- Cross-sectional area
- Wider the tube, faster the flow
If pressure is fixed, increasing the cross-sectional area has what effect on velocity?
-Increases
If flow is fixed, increasing cross-sectional area has what effect on velocity?
-Decreases velocity
What is the equation to calculate flow?
-mean velocity x cross-sectional area
What is resistance in a vessel determined by?
- Viscosity of fluid
- Radius of vessel
What is the relationship between resistance and flow?
- Flow is inversely proportional to resistance
- ie The higher the resistance the lower the flow
What is ohms law?
-Pressure= flow x resistance
How do changes in radius effect flow?
-Small changes in radius have a large effect on resistance and thus flow
If two vessels flow in series, how do you calculate the total resistance?
-Add the resistance of the tubes together?
-If vessels flow in parallel, what effect does this have on resistance?
-The total resistance is lower than any of the individual resistances
What effect does pressure gradient have on flow?
-The higher the pressure gradient, the faster the flow
If flow is fixed, what effect does resistance have on the pressure gradient?
-The higher the resistance, the greater the pressure change from one end of the vessel to the other (ie the higher the pressure gradient)
If pressure is fixed, what effect does resistance have on flow?
-The higher the resistance the lower the flow
Is flow constant in the systemic circulation?
-Yes
Why do arteries have a low pressure change?
-There is low resistance as the tube is relatively wide
Why is there a high pressure drop in arterioles?
-Resistance is high as the lumens narrow
At a given flow, resistance is inversley proportional to cross-sectional area, so why do capillaries have a very low reistance?
-Because there are many connected in parallel
Why is the pressure drop low across the veins?
-There is low resistance
What effect does resistance of arterioles have on arterial pressure?
-The higher the resistance, the higher the arterial pressure
What happens to arterial pressure, if CO increases but resistance remains the same?
-Arterial pressure will rise
What causes turbulent flow?
-Increased resistance to flow
What is transmural pressure?
-The pressure generated between the inside and outside of a vessel wall
Because blood vessels are distensible, what effect does this have on resistance if the lumen widens?
-Drops
How does distensibility of veins allow them to have capacitance?
- When veins are collapsed there is no flow, as pressure increased the lumen widens and resistance decreases
- As the vessel widens with increasing pressure, more blood transiently flows in than out -> thus they act as a store of blood
Define systolic pressure
-The pressure of blood against the walls of the arteries when the ventricles are coontracting
Define diastolic pressure
-The pressure of blood against the walls of the arteries when the ventricles are relaxing and refilling
What is pulse pressure?
-The difference between the systolic and diastolic pressure (usually about 40mmHg at rest
What is mean arterial pressure? How is it calculated?
- The average arterial pressure across the cardiac cycle
- 2/3 diastolic + 1/3 systolic (heart spends 2/3 time in diastole and 1/3 systole)
What is total peripheral resistance?
-The total resistance opposing blood flow in the systemic circulation
What does arterial pressure need to be high enough to do?
-Drive the total CO through the total peripheral resistance
-What effect does TPR have on arterial pressure?
-The higher the TPR, the higher the arterial pressure
How does distensibility of arteries limit the pressure change between systole and diastole?
-The stretch of arteries allows more blood to flow into the arteries than out which limits pressure fluctuations as diastolic pressure does not drop as much
Why is recoil of stretched arteries important?
-Allows the ‘stored’ blood to flow through the arterioles during diastole as the pressure is maintained by the recoil of the stretched arterial wall
What determines the stroke volume?
-How hard the ventricles contract
What is compliance of arteries, and how does it affect systolic pressure?
-Stectchiness of arteries -> the higher the degree of stretch, the more blood arteries can accommodate so bp does not rise as much during systole
What is the link between age and compliance?
-With age, systolic pressure rises as vessels loose their stetchiness
Why is diastolic pressure a better measure of TPR then systolic?
-Blood flows at a fixed rate during diastole as it is not effected by the stretch of arterial walls
What determines diastolic pressure?
- TRP
- Systolic pressure
Define flow
-The volume of fluid passing a given point per unit time (vol/time)
Draw the typical arterial waveform
-Waved line between systolic and diastolic pressure with a notch caused by closing of valve
How do arterioles control distribution of flow?
- Variable flow restriction by changing lumen diameter
- Pre-capillary sphincters
What is meant by tonic contraction?
-Sustained contraction
What controls skeletal muscle blood flow?
-Precapillary sphincters which rapidly open and close as needed
Define vasoconstriction
-Redi=uciton in arterial diameter by contraction of smooth muscle in vessel walls -> increases resistance and reduces flow
Define vasodilatation?
-Increase in arterial diameter by less contraction of smooth muscle in vessel walls -> decreased resistance and increased flow
What is vasomotor tone?
-The continuous contraction of smooth muscle in BVs
What controls vasomotor tone?
-NA release from sympathetic NS acting on a-1 receptors
What antagonises vasomotor tone?
-Vasodilator metabolites
How do vasodilator metabolites help to couple supply to demand
Increased metabolic activity -> increased vasodilator factors -> increased lumen-> decreased resistance-> increased blood flow -> decreased vasodilator as it is washed away -> vasoconstriction
Cycle starts again
What determines the effect of vasodilator metabolites?
-The balance between the the rate at which they are produced and the rate at which bloodflow carries them away
What is reactive hyperaemia?
- Bloodflow to organ/limb temporarily prevented
- Metabolising tissues still producing vasodilator factors
- Accumulation as there is no flow
- Arterioles dilate maximally
- When perfusion is resotred, blood flow is very high as resistance is very low -> high blood flow needed for tissue demand
- High blood flow washes away metabolites and smooth muscle contracts to normal
What does autoregulation do?
-Ensures each organ only receives the supply it needs provided that supply pressure stays within limits
What is autoregulation?
Supply pressure changes without a change in metabolic demand-> change in blood flow-> change in concentration of metabolites-> altered resistance of arterioles to match supply pressure -> bloodflow returns to appropriate level for metabolising tissue
Why is TPR inversely proportional to the bodys need for blood?
- All the tissues of the body alter the resistance of their own vessels in order to make bloodflow match metabolism
- Thus the TPR is high if the bodys demand for blood is low
What is central venous pressure?
-Pressure in the great veins supplying the heart
What is venous return?
-The rate of blood flow back to the heart
What determines the pressure in the veins?
-The volume of blood they contain, ie their capacitance
What determines capacitance?
-The rate at which blood flows into and leaves the veins
What controls the filling of the heart during diastole?
-Central venous pressure
What determines CVP?
- Return of blood from the body
- CO of the heart
- Gravity and muscle pumping
What is muscle pumping?
-As muscles contract they compress the veins and drive the blood back to the heart
How are superficial veins effected by gravity more than deep veins?
- Deep veins are surrounded by fluid which is also effected by gravity to an equal extent so transmural pressure stays the same
- Superficial veins are surrounded on one side by air which is less effected by gravity causing an increased transmural pressure resulting in swelling and pooling
If blood pools in the legs, what effect does this have on CVP?
-Decreases it
