CVS Session 5: flow Flashcards
Describe the CVS in terms of series and parallel organisation
Systemic and pulmonary vascular system
Describe the vessels through which blood flows
Heart Large (elastic/conducting) arteries Medium (muscular/distributing) arteries Arterioles Metarterioles Capillaries Post-capillary venules Venules Medium veins Large veins Heart
Define flow
The volume of fluid passing a given point per unit time. It is proportional to the pressure difference between the ends of a vessel: higher pressure difference=higher flow
Define velocity
The rate of movement of fluid particles along the tube
What is resistance to flow determined by?
Nature of the fluid and vessel
Relationship between flow and velocity
Flow is constant at all points along a vessel
Velocity can vary if the radius of the tube changes
At a given flow, velocity is inversely proportional to cross sectional area:
-vessels with a small cross sectional area have a high velocity (e.g. aorta)
-vessels with a large cross sectional area have a low velocity (e.g. capillaries)
Where does blood flow fastest and slowest
Fastest where cross sectional area is least, e.g. aorta
Slowest in capillaries-allows time for gas and nutrient exchange
Laminar flow
In most vessels
Gradient of velocity from middle to edge: fastest flow in the centre, fluid is stationary at the edge
So cells flow down the middle and plasma down the outside
Turbulent flow
As mean velocity increases, turbulence eventually results
Velocity gradient breaks down: fluid tumbles down, resistance greatly increased
This occurs when there is:
-high flow velocity
-low viscosity
-irregular lumen of a vessel e.g. atherosclerosis
What determines flow in a vessel with constant pressure?
Mean velocity
What determines mean velocity?
- VISCOSITY of fluid
- mean velocity inversely proportional to viscosity
- in laminar flow fluid moves in concentric layers that slide over one another, with middle moving fastest. Viscosity is the extent to which fluid layers resist sliding over
- high viscosity=slower flow of central layers=lower velocity - RADIUS of tube
- mean velocity directly proportional to radius
- viscosity determines the slope of the velocity gradient. At a constant gradient, wider tube=faster flow in middle
Describe Poiseuille’s Law
Describes how flow is the product of mean velocity and cross sectional area
When flow is laminar and steady in blood vessels larger than arterioles, flow is:
-proportional to the difference between inflow vs. outflow, and to the fourth power of the radius
-inversely proportional to the length of the vessel and to the viscosity of the blood
Cause of hyperviscosity syndrome
- High plasma protein levels (treated by plasmapheresis)
- High rbc/wbc count (treated by phlebotomy)
Underlying cause is often neoplasia
Why can severe anaemia lead to functional murmurs?
High blood flow velocity
Redcued viscosity of blood due to low rbc
Relationship between pressure, flow and resistance
PRESSURE=FLOW X RESISTANCE
Resistance increases as viscosity increases; resistance decreases to the fourth power of an increase in radius
therefore
it is much harder to push blood through small vessels and to push thicker blood
If FLOW is fixed: the higher resistance, the greater the pressure change from one end of the vessel to the other
If PRESSURE is fixed: the higher resistance, the lower the flow
Resistances in series vs parallel
Vessels in SERIES: resistances ADD (total resistance=sum of individual resistances)
Vessels in PARALLEL: effective resistance is LOWER (reciprocal of total resistance=sum of reciprocals of individual resistances)
Why do capillaries offer little resistance when they are very narrow?
Although narrow, offer little collective resistance due to their parallel arrangment
Describe flow across the whole circulation
The same at all points
Describe resistance across the whole circulation
Arteries-low (pressure drop over arteries is small)
Arterioles-high (pressure drop over arterioles is large
Individual capillaries-high
Capillaries in system-low (as many arranged in parallel)
Venules/veins-low (pressure drop low)
Describe pressure across the whole circulation
Within arteries is high due to high resistance of arterioles: for a given flow, higher resistance of arterioles=high arterial pressure
If heart pumps more and resistance stays the same, pressure will increase
When would turbulent flow generate a bruit (vascular murmur)?
Narrowed cardiac valves (stenosis)
Blockage of arteries due to atherosclerosis: carotid, hepatic, renal, femoral
In brachial artery when measuring blood pressure
Distensible walls
Pressure within vessel generates a TRANSMURAL PRESSURE between inside and outside which stretches the tube
Stretch decreases resistance, so easier for blood to flow through
Vessels widen with increasing pressure, so more blood transiently flows
Distensible vessels store blood: have CAPACITANCE i.e. veins are the most distensible and carry most blood
Role of each component of the CVS
Heart-cyclic muscular pump enabling blood circulation
Arteries-gross collection and distribution of blood supply; elastic and muscular
Arterioles-local distribution and fine control to defined tissue volume
Capillaries-microdiffusion and filtration
Veins-collection, return and capacitance
Define resistance, compliance, capacitance and pressure
RESISTANCE: inverse relation to diameter, as it increases pressure will also increase
COMPLIANCE: ability to distend and increase volume due to pressure increase. Stores mechanical energy of rising pressure wave during systole and dissipates energy more gradually over diastole
CAPACITANCE: a measure of the relative volume increases per unit increase in pressure. Capacitance=volume/pressure
PRESSURE: a measure of the mechanical energy gradient in blood that drives its flow around different parts of the system
Define cardiac output
The volume of blood pumped by the heart per minute
Cardiac output=stroke volume x heart rate
To what level does arterial pressure rise and why?
To a level determined by cardiac output and total peripheral resistance
Because it needs to be high enough to drive cardiac output through high resistance arterioles
What is total peripheral resistance?
The sum of all the arteriolar resistances in the body
TPR is inversely proportional to blood flow demand
so as demand increases, TPR falls
Aortic compliance
Lessens pulsatile nature of systolic pressure wave: in systole in the aorta and elastic arteries, less smooth muscle/more elastin act to “smooth out” the pressure wave
Causes a CAPACITANCE effect: more blood flows in than out, pressure rise is less rapid, elastic arteries recoil in diastole and release energy smoothing flow to arterioles. This is known as the WINDKESSEL EFFECT
Major arterial pressures
Max in systole-systolic pressure- ~ 120mmHg
Min in diastole-diastolic pressure- ~ 80mmHg
Blood pressure gradient drives flow at all time points in the cardiac cycle
Factors that affect major arterial pressures
Cardiac output
Arterial compliance
TPR
What can reduced compliance lead to?
Essential hypertension
Calculation of mean arterial blood pressure
MAP=(CO x TPR) +CVP
CVP often small enough to be left out of equation
Pulse pressure and average arterial pressure (from systolic and diastolic readings)?
Pulse pressure: difference between systolic and diastolic pressure. At rest should be ~40 mmHG.
MABP=diastolic pressure + 1/3 pulse pressure
Usually ~93mmHg
Time spent in systole vs diastole?
Diastole predominates-0.55s
Systole-0.3s
Variation in pulse pressure?
- decreases with summated resistance and capacitance of arteriolar network
- variation decreases down arterial tree
- approaches smoothed mean prior to entering capillaries
- pressure gradient decreases by the time capillaries are reached. Allows substrate exchange
Why are arterioles known as the high resistance vessels?
Narrow lumen-high proportion of of smooth muscle in tunica media
How does arteriolar vasomotor tone govern blood flow to capillary beds?
Vasoconstriction and vasodilation: opposing control elements work together to finely regulate very small tissue volumes
Allows precise matching of substrate supply to metabolic demand
Mostly controlled by sympathetic nervous system, also modulated by circulating hormones
Why is there a high vasomotor tone in arterioles at rest?
Modest resource demand-only need low blood flow
Tonic contraction of smooth muscle
How is arteriolar vasomotor tone reduced?
By increased vasodilatation so decreased resistance to flow
This is due to vasodilator metabolites which relax vascular smooth muscle: H+, CO2, lactate, adenosine, K+
Increased blood supply removes factors, with return to vasomotor tone dominated by the SNS. Muscles don’t actively relax except under max flow conditions, so there is always some vasoconstriction
Describe autoregulation of blood flow
Changes in flow are governed by acute metabolic demands in local tissues
- this decreases [metabolites] +ve control signal to dilate offsets the SNS
- resistance in arterioles returns to default to supply baseline level of metabolism -ve control signal to contract
Reactive hyperaemia
Transient increased in organ blood flow following a period of ischaemia (e.g. arterial occlusion)
Occurs after removing a tourniquet, unclamping an artery or restoring flow to a coronary artery
Peak flow gradually increases back to normal
What factors affect vasomotor tone?
Hormones-systemic contribution
Myogenic-local effects. Arteriolar smooth muscle exposed to rapid pressue rise, acute contraction protects from excess pressure
Endothelial-local effects. Autacoid release from arteriolar endothelium modulate vasomotor tone
Describe the features of veins
Very stretchy so high compliance
Low total resistance
Low pressure: determined by the volume of blood contained
VENOUS PRESSURE: depends on balance of flow in from body and out via the heart, so is determined by cardiac output, metabolic demand and “back pressure” from pressure waves
CENTRAL VENOUS PRESSURE: -10 to +10 mmHg. The pressure in great veins required for filling RA in diastole. In normal health depends on return of blood from body, pumping of heart and gravity
