CVS Session 5 Flashcards
What is flow proportional to?
Pressure difference b/w ends of the vessel
What is flow?
The volume of fluid (or gas) passing a given point per unit time
What limits flow?
Resistance of the vessel
What determines resistance of the vessel?
Interaction of the nature of the fluid and the vessel
What is velocity?
The rate of movement of fluid particles along a tube
What must remain constant and what can vary if the radius changes?
Flow must be constant
Velocity can vary
At a given flow, what is the relationship between velocity and cross sectional area?
Inversely proportional
How does the velocity of blood in the aorta compare to that of blood in the capillaries?
High in aorta
Low in capillaries
What is kept constant by having high velocity in narrow vessels and low velocity in wide vessels?
Number of particles delivered
What is laminar flow?
Gradient of velocity from centre to edge - velocity highest in centre, 0 immediately adjacent to wall
What is another name for laminar flow?
Silent flow
What is turbulent flow caused by?
Increased mean velocity causing velocity gradient to break down
What is turbulent flow?
Fluid tumbles over itself and flow resistance increases - net flow forwards but particle move randomly
What is a bruit?
Noise caused by turbulent flow
When is a bruit normal and when is it pathological?
Normal in heart
Pathological in BV
If a vessel has a constant pressure, what will determine flow?
Mean velocity
What is the relationship between mean velocity and viscosity?
Inversely proportional
What is flow a product of?
Mean velocity and cross sectional area
What two factors determine mean velocity?
Tube radius
Viscosity
What is Poiseulle’s law?
Flow is proportional to pressure difference divided by viscous resistance = (Pr^4)/(viscosity x length)
What two things is mean velocity proportional to?
Cross sectional area
r^2
Why does increasing the cross-sectional area of a tube increase velocity?
A wide tube allows for faster middle layers as the velocity gradient remains constant
What is viscosity?
The extent of resistance to layer sliding in laminar flow
How does fluid move in laminar flow?
In concentric layers w/middle faster than edges
How does increasing viscosity cause average velocity to decrease?
Causes central layers to move more slowly –> decreased velocity gradient therefore decreased average velocity
What does viscosity determine graphically in relation to velocity?
Slope of gradient of velocity
What law does flow resistance obey?
Ohm’s law
What two things is flow resistance proportional to?
Viscosity
1/(r^4)
What makes blood harder to push?
Small BV
Thicker blood
How does resistance compare in BV in series to those in parallel?
Series = resistances add Parallel = lower effective resistance as alternative pathways available
If flow is fixed and resistance is increased, what happens to the pressure change from one end of the vessel to another?
Increases
If pressure is fixed and resistance is increased, what happens to flow?
Decreases
Why do cells travel round the circulation faster than plasma?
They bounce off walls and congregate in the middle of flow so the apparent viscosity increases and they travel faster than the surrounding plasma
What can be said about the flow in the whole circulation system?
Is it 5 l per minute at all points
How does the pressure change across arteries?
Small drop
Why is BP measured in the arm?
There is only a small drop in pressure between here and the heart therefore it is the most accurate and practical place to measure
What is the magnitude of the pressure drop across arterioles?
Large
Compare the resistance of single capillaries to their overall resistance.
Singularly are high resistance but in parallel so overall resistance is low
Are venules and veins high or low resistance?
Low resistance
What determines arterial pressure?
Cardiac output and TPR
What will tip laminar flow into turbulent flow when on the edge of transition?
Stress
How can distensible BV walls decrease resistance?
Transmural pressure changes –> stretches tube –> increases lumen
How is capacitance achieved?
Vessels widen w/ increasing pressure causing more blood transiently in than out
What happens to the lumen of BV with distensible walls in cases of sudden pressure changes?
Sudden increase = increase lumen –> more blood in than out
Sudden decrease = more blood out than in –> decrease lumen
What is the critical closing pressure of a BV?
Pressure inside vessel
Why must arteries have distensible walls?
So that blood can continue to flow in diastole and the entire cardiac output does not have to be pushed past TPR during systole
How does the distensibility of artery walls allow blood to flow during diastole even though no blood is entering the arteries?
In systole: arteries stretch –> more blood in than out –> pressure does not increase much –> arteries recoil in diastole
What is systolic pressure?
Maximum arterial pressure at the end of systole
What is diastolic pressure?
Minimum arterial pressure at the end of diastole
What determines systolic pressure?
How hard the heart pumps
TPR
Compliance (stretchiness) of arteries
Why does systolic pressure increase with age?
Compliance of arteries decreases with age
What is the approximate value of systolic pressure?
~120 mmHg
What is the approximate value of diastolic pressure?
~80 mmHg
What determines diastolic pressure?
TPR
Systolic pressure - determines start point of diastolic
What is the better indicator of diastolic pressure and why?
TPR, it is not affected by compliance
What is pulse pressure?
Systolic - diastolic
What does pulse pressure tell you about?
Force of contraction of the heart e.g. adrenaline or exercise effects
How is average pressure calculated?
Diastolic + 1/3 pulse
2/3 diastolic + 1/3 systolic
(Systole is shorter than diastole)
What are the approximate values of pressure in the arteries, arterioles, capillaries, venules and veins?
Arteries = 100 mmHg Arterioles = 35 mmHg Capillaries = 10 mmHg Venules = 8 mmHg Veins = 3 mmHg
Why are arterioles coupled with pre-capillary sphincters in skeletal muscle?
It’s requirement for a variable blood flow
What are pre-capillary sphincters?
Smooth muscle around arterioles which can contract/relax to alter resistance to blood flow
What is vasomotor tone?
Tonic (continuous) contraction of smooth muscle
How is vasoconstriction achieved?
NA on alpha 1 receptors in virtually every tissue causing increased resistance to flow
How is vasodilation achieved?
Vasodilator factors antagonise vasomotor tone - BV do not actively relax, they contract less
What is reactive hyperaemia?
Circulation cut off for 1-2 mins –> vasodilator metabolites not washed away –> transient large increase in bloodflow on reperfusion
What does the effect of vasodilator metabolites depend on?
Balance b/w rate of production and rate of removal by blood flow
Why does resistance usually increase without a decrease in blood supply resistance?
Due to metabolites
What is autoregulation?
Constant metabolism –> constant [metabolite] –> supply pressure increases –> transient increase in blood flow –> metabolites washed away –> smooth muscle contracts increasing vasomotor tone
What must occur for a tissue to automatically take what blood it needs irrespective of arterial supply?
Supply pressure to remain within certain limits
What determines how much blood is pumped around the body?
If all tissues in the body alter the resistance of their arterioles to match their metabolism
What determines the pressure of blood in the veins?
Capacitance - the volume of blood they contain
What does capacitance depend on?
Balance b/w flow in from the body and out via the heart
What is central venous pressure?
Pressure in the great veins that fills the heart in diastole
What determines venous and arterial pressure?
Venous = capacitance of veins Arterial = resistance vessels
What does central venous pressure depend on?
Return of blood from the body
Pumping of the heart
Gravity
Muscle pumping
How does muscle pumping increase venous return?
Skeletal muscle compresses vein pushing blood out of veins to heart
Inhaling ‘sucks’ blood back to heart due to pressure changes
What is central venous pressure balanced by?
Inflow and outflow of blood
What must arterial pressure be high enough to ensure?
Tissues get what blood they need
What happens as more blood is needed?
TPR decreases
What fills the heart?
Central venous pressure
What 4 variables can the CVS be reduced to?
Arterial pressure
Pressure in veins
Cardiac output
TPR
