What is Flow and what is it driven by?
Flow: the volume of blood passing a given point per unit time.
Flow of blood through blood vessels is driven by a gradient of hydrostatic pressure.
Flow is proportional to the pressure difference between the ends of a vessel (fluid moves from high pressure end to low pressure end)
The higher the pressure difference, the greater the flow
What is Flow Resistance?
The flow for a given pressure gradient is determined by the resistance of the vessel.
If the resistance is high at a given pressure gradient, flow is slow.
Resistance is determined by the nature of the fluid and the vessel.
What is Velocity?
The rate of movement of fluid particles along the tube (distance per unit time)
Flow must be the same at all points along a vessel.
Velocity can vary along the length if the radius (size) of tube changes.
At a given flow, velocity is inversely proportional to cross sectional area of the tube; the bigger the cross-sectional area, the slower the velocity (at a given flow)
NOTE: Vessels with a large overall cross sectional area have a low velocity e.g. Capillaries. Each capillary is individually small but there are millions of them.
What is Laminar flow?
In most blood vessels, flow is laminar
No sound is generated
There is a gradient of velocity from the middle to the edge of the vessel.
Velocity is highest in the centre.
Fluid is stationary at the edge (close to the walls)
Describe Turbulent flow
As the mean velocity increases, flow eventually becomes turbulent (once the velocity increases past threshold).
The velocity gradient breaks down.
Fluid tumbles over - no constant regular change in velocity.
Flow resistance is greatly increased.
Turbulent flow generates sound.
Describe the flow in a vessel with constant pressure driving flow (supplied with an infinite amount of fluid at a constant pressure)
The flow will be determined by the mean velocity - linear movement along the tube.
The mean velocity depends upon: the viscosity of the fluid and the radius of the tube.
What is viscosity?
(Stickiness of the fluid)
In laminar flow the fluid moves in concentric layers like layers of an onion- the middle layers move faster than the layers at the edge. So fluid layers must slide over one another.
The extent to which fluid layers resist sliding over one another is known as viscosity.
The higher the viscosity, the slower the central layers will flow and the lower the average velocity.
The lower the viscosity, the faster the velocity.
Describe the effects of radius on velocity
Viscosity determines the slope of the gradient of velocity At a constant gradient, the wider the tube, the faster the middle layers move.
Mean velocity is proportional to the cross sectional area of the tube.
So mean velocity is
Inversely proportional to viscosity
Directly proportional to cross sectional area (r2)
BUT flow is the product of mean velocity and cross sectional area (flow is not fixed)
What does Poiseulles Law mean?
Flow resistance obeys Ohms Law (V =IR)
Pressure = Flow x Resistance
Rearranging Poiseulles Law:
Delta Pressure = (flow x 8x viscosity x length) / (pi x radius(2) )
Therefore Resistance = (8 x viscosity x length) / (pi x radius(2) )
Resistance increases as viscosity increases
Resistance decreases with the fourth power of radius
The thicker the blood the harder it is to push blood through blood vessels
The smaller the blood vessels, the harder it is push blood through blood vessels .
The flow rate at a given pressure gradient is directly proportional to the fourth power of the radius of the tube.
What happens if the blood vessels are connected together?
If vessels are connected together in series, add the resistances together.
If vessels are connected together in parallel, the single equivalent resistance is lower than the any of the single resistances (because there are alternative pathways for blood flow)
What happens if flow is fixed?
The higher the resistance the greater the pressure changes from one end of the vessel to the other.
If the pressure is fixed, describe flow and resistance
The higher the resistance, the lower the flow
What are the three variables?
Describe flow over the whole systemic circulation
Flow is same at all points
Discuss pressure and resistance in arteries
Arteries are large tubes and have low resistance; large radius = large cross sectional area --> easy for blood to flow through
At a constant flow, pressure drop over arteries (from end to end) is small
Discuss pressure and resistance in arterioles
Arterioles are small with a thick smooth muscle layer and have high resistance
Pressure drop over arterioles (from end to end) is large.
Why do arteries have a high hydrostatic pressure within them?
They are connected to arterioles which have high resistance so high pressure is needed to drive flow through arterioles.
For a given total flow, the higher the resistance of the arterioles, the higher the arterial pressure.
If the heart pumps more blood and the resistance of arterioles remains the same, the arterial pressure will rise.
Discuss pressure and resistance in capillaries
Individual capillaries have high resistance but there are so many of them connected in parallel that the overall (combined) resistance is low - pressure drop over capillaries is small.
Describe the pressure across the systemic circulation (in mmHg)
Arteries: 100mmHg (low resistance)
Arterioles: 100mmHg (high resistance)
Capillaries: 35mmHg (high resistance individually but low effective resistance overall)
Venules: 10mmHg (low resistance)
Veins: 8mmHg (low resistance)
Back to the heart from veins: 3 mmHg
From heart to arteries: 100mmHg
Discuss pressure and resistance in venules and veins
Large vessels with low resistance
Pressure drop over venules and veins is low
Low pressure within veins
Describe how flow may become turbulent in some vessels
E.g. In the aorta, if the resistance increases to an extent, aorta flow may become turbulent.
Flow is also turbulent if a vessel is narrowed so flow is restricted e.g.. In atherosclerosis.
Sound generated could be 'bruit' - swooshing sound
Describe floppy vessels
Blood vessels have distensible walls.
The pressure within the vessel generates a transmural pressure (pressure difference across the wall) between the inside and outside.
The transmural pressure tends to stretch the tube.
The higher the transmural pressure, the more the wall will stretch.
As the vessel stretches, so resistance falls.
So the higher the pressure in a vessel, the easier it is for blood to flow through (can be useful)
As the pressure within a distensible vessel falls, the walls eventually collapse --> blood flow ceases BEFORE the driving pressure falls to zero (lumen of tube shrinks so much that there is no blood flow).
Pressure has to increase before walls stretch again and allow flow.
As vessels widen with increasing pressure more blood transiently flows in than out (and vice versa) so vessels expand.
Distensible vessels 'store blood' - they have capacitance.
Veins are the most distensible.
Release blood by dropping pressure. Therefore floppy vessels can regulate the amount of flow for effective functioning of the CVS.
Describe cell distribution in blood vessels
Blood cells congregate in the middle of the flow (in the vessel) so cell distribution is not even in the blood vessels.
So blood flowing down the middle has higher viscosity than blood flowing at the edges
And cells in the middle travel faster than the plasma cells at the edges.
Describe cardiac output
CO = stroke volume (amount of blood pumped each beat) x heart rate (number of beats per minute)
Heart ejects intermittently; pulsatile flow
In systole blood flows into arteries (~1/3 time) in diastole (remaining 2/3 times) blood is not pushed out.
At rest it pumps ~5L/min in typical adult
Around 80ml of blood is pushed into arteries at each stroke
Why is there high pressure in the arteries?
Due to the difficulty of blood leaving the arteries (through the arterioles and precapillary sphincters - the resistance vessels).
Arterial pressure must rise high enough to drive the cardiac output through the resistance of the arterioles in order for the same amount of blood entering the arteries to leave the arteries.
Describe the arterial pressure
If CO stays the same and total peripheral resistance increases, arterial pressure must increase.
Pressure is determined by cardiac output (amount of blood entering blood vessel) and total peripheral resistance (difficulty of blood leaving the vessel)
Arteries have to cope with an intermittent supply of blood (no pumping during diastole )
What if arteries had rigid walls?
Pressure would rise high enough in systole to force the whole stroke volume through the total peripheral resistance (nowhere else for the blood to go).
During systole pressure and flow is very high.
Pressure falls to zero in diastole as there is no flow during diastole
Flow would be intermittent (only occur during systole) at rest.
The system could not tolerate this intermittent blood flow and high arterial pressure inside arteries.
But arteries had distensible 'stretchy' walls. What does this mean?
In systole arteries stretch as they expand when pressure increases.
During the period of expansion, more blood flows in than out (blood is accommodating extra volumes)
So pressure does not rise so much (because less blood has to leave than enter therefore pressure does not have to push out as much blood so pressure is lower)
Once heart stops pushing blood through the arteries, extra blood (accommodated in the vessel) continues to flow out.
As arteries recoil in diastole, flow continues through the arterioles (even though there is no blood being pushed into the arteries by the heart).
So flow is more continuous and less pulsatile, and pressure is less (than if walls were rigid)
Discuss arterial pressure values and waveform
Rises to a maximum during the middle of systole: the systolic pressure (typically 120mmHg)
Falls to a minimum (does not fall to zero - pressure is maintained by the recoil of the arteries so 'extra' blood is continued to be ejected during diastole) at the end of diastole- the diastolic pressure (typically 80mmHg)
Waveform is not symmetrical - systole is only a 1/3 of the time, diastole occupies the other 2/3
What factors affect the systolic pressure (how high the pressure rises)?
How hard the heart pumps
Total peripheral resistance: if TPR increased, pressure has to increase in order to eject blood through the TPR
Stretchiness ('compliance') of the arteries.
Due to their elasticity they can swell up and accommodate more blood so less blood has to flow out ( so pressure doesn't have to rise as high because it is ejecting a smaller volume of blood).
Arteries lose their elasticity with age.
What are the factors affecting diastolic pressure?
Total peripheral resistance - how hard it is for blood to leave during diastole due to recoil of the wall.
Diastolic pressure is more sensitive than systolic pressure.
As diastole is a fixed length of time and if the TPR is high, pressure falls more slowly so diastolic pressure is higher.
(Variations in diastolic pressure are a good indication of TPR) - more directly affected
What is the pulse pressure?
The difference between systolic and diastolic (typically about 40mmHg) - the extent to which the pressure changes during the beat.
If the heart is beating very hard, pulse pressure increases
What is the average pressure?
Drives the total blood flow through the total peripheral resistance and beyond.
Calculated as diastolic pressure + 1/3 pulse pressure because systole is shorter than diastole.
Pressure waveform is not uniform because diastole (twice as long) and systole are not the same.
Describe how we can control resistance in resistance vessels
Arterioles and pre-capillary sphincters (in skeletal muscle) have high resistance- blood flow is difficult because of narrow lumen.
They control the distribution of blood and ensure the different parts of the body receive what they need.
We can control blood flow by controlling resistance; lumen diameter is controlled by state of contraction of smooth muscle in walls.
Resistance vessels have high smooth muscle content.
Lumen is narrowed by tonic contraction of smooth muscle in the vessels' walls.
Discuss vasomotor tone
Muscle is partially contracted all the time - this is tonic contraction.
Tonic Contraction of smooth muscle is known as vasomotor tone (smooth muscle constantly contracting all the time).
This means the lumen is normally small and resistance is normally high.
Increased tonic contraction is known as vasoconstriction --> higher resistance (blood flow more difficult)
Vasodilatation decreases resistance to flow.
What are the factors affecting the contraction of vascular smooth muscle?
Vasomotor tone is mostly produced by the sympathetic branch of the ANS (ganglia are close to the CNS with long post-ganglionic fibres to tissues).
NA acts on smooth muscle constantly to keep contraction constant.
This tone is antagonised by vasodilator factors.
The actual state of contraction (resistance) is determined by balance between the sympathetic system and vasodilator factors.
What is Reactive Hyperaemia?
If the circulation e.g. To an arm is cut off for a minute or two, blood flow is temporarily restricted to 0 e.g. when blood flow temporarily stops completely.
When blood flow is restored, amount of blood flow into the arm is hugely greater than before - blood flow is much easier.
This is due to the accumulation of chemicals - vasodilator metabolites during the period of occlusion.
What are Vasodilator Metabolites?
Metabolically active tissues produce vasodilator metabolites.
They accumulate around blood vessels and act on smooth muscle, causing unusual relaxation (almost completely) after a minute of so.
As soon as the blood flow is returned, arterial pressure meets a low resistance so you get a very rapid blood flow into the arm.
As that surge of blood flows through the arm, metabolites are washed away.
As concentration of metabolites fall, smooth muscle constricts again.
Amount of extra blood flow matches the amount of blood flow that did not flow during the duration of occlusion.
System controls itself - it makes sure over time that the total blood flow is what it should be (autoregulation)
Give some examples of vasodilator metabolites
H+ - produced during anaerobic glycolysis (lactic acid is produced)
K+ - leak from cells when metabolic activity is compromised
Adenosine - also leak from cells been metabolic activity is compromised.
They act to relax vascular smooth muscle.
Their effect depends on the balance between the rate at which they are produced and the rate at which the blood flow washes them away (provided blood is supplied at a constant pressure)
What happens when metabolism increases?
More metabolites are produced so vasodilation occurs (smooth muscle relaxes)
Blood flow washes away the extra metabolites.
Smooth muscle contract again (returns to original state)
What is autoregulation?
If supply pressure changes, blood flow to a tissue will change which will change metabolite concentration which will alter (reduce) the resistance of arterioles so blood flow returns to an appropriate level for metabolism.
Every tissue in the body is able to control its own blood flow - it sends a local chemical signal to its arterioles telling them how much blood the tissue needs.
The arterioles are able to relax sufficiently - change vasomotor tone- to allow extra blood flow(if needed) so long as the pressure in the arteries remains constant.
If the arterial supply changes, the body change the resistance to keep the blood pressure in a range - within certain limits.
As long as we have the mechanisms to keep the pressure constant, every part of the body can take card of itself - all the tissues can look after themselves individually and locally.
Tissues will automatically take what blood they need.
What is an exception to autoregulation?
When we stand up, brain is above the heart and pressure supplying the brain is less than the pressure emerging from the heart because of the effects of gravity.
Describe autoregulation when arterial pressure falls
If the arterial pressure falls, blood flow reduces in the tissues.
Metabolites produced by the tissue will not be washed at the rate they were removed before - metabolites accumulate locally.
This caused the smooth muscle to relax which reduces resistance and therefore restored the blood flow despite the arterial pressure falling.
This is autoregulation. Tissues alter individual resistance to control their blood flow.
What is the basic rule of autoregulation?
Provided supply pressure remains within certain limits, tissues will automatically take what blood they need.
What is Total Peripheral Resistance?
Collective activity of all the resistance vessels is effectively an integrated measure of the blood flow the body needs
If all the tissues in the body alter the resistance of their arterioles to match metabolism then a 'collective signal' from all tissues sending local signals to their individual resistance vessels results.
Total peripheral resistance will be inversely proportional to the body's need for blood flow.
When the body needs less blood, resistance increases.
When the body needs more blood, resistance decreases.
Describe how the pressure and flow is determined in Veins and Venules
Pressure is determined by the volume of blood they contain (pressure is highest at the capillary end and lowest at the heart end)
Pressure depends on the balance between flow in from the body and out via the heart
Flow is driven by a low pressure gradient but volume of blood is large - most of the blood in our body is in our veins.
Storing blood in veins (large, stretchy) is very important.
Because they are so stretchy, pressure in veins is determined by the storage of blood in the veins rather than the pressure pushing blood through the veins.
What is the Central Venous Pressure?
Very variable, this is the pressure in the great veins which fills the heart during diastole (pressure in the RA is the same as in the IVC and SVC).
Pressure is determined by the amount of stretch the veins undergo rather than hydrostatic pressure.
Depends on volume of blood the veins undergo rather than hydrostatic pressure.
Squeezing a vein downstream of a valve ejects blood upstream of a valve but it is not able to return downstream of a valve (one direction flow).
What does the volume of blood in veins depend on?
Rate of return of blood from the body
Pumping of the heart - rate of blood removal by heart
Gravity and muscle pumping
What is muscle pumping?
Many veins run close to skeletal muscle.
When skeletal muscle contracts, they compress the vein and push the blood back towards the heart (one direction only because of valves).
Increased venous return increases the central venous pressure (transiently) due to muscle pumping such as during exercise.
How does gravity affect return of blood to the heart?
When we' re standing, blood flows uphill (tends to be drawn back by gravity) so blood accumulates in the lower veins of the body - blood pools in legs so there is less venous return and less central venous pressure.
What is Venous return?
Amount of blood returning to the central venous space
What are 3 things that matter when considering the systemic circulation?
Arterial pressure must be high enough to ensure tissues get what blood they need.
Total peripheral resistance falls as more blood is needed
Central venous pressure fills the heart
What does the volume of flow depend on?
Q = Volume of the flow (for a given pressure gradient)
R = Resistive forces in the circulation (namedly the viscosity of the blood, the shear forces of the vessel walls (vessel length) and vessel diameter
In simple form: Q = P/R
To keep flow rate constant, the velocity must vatry inversely with cross sectional area
Q = Velocity x Area
Velocity = Q / Area
How would your estimate of arterial pressure differ if you measured it in the arteries of the lower leg of a person sitting or standing up?
higher due to addition of hydrostatic pressure on standing.
The hydrostatic pressure results from the action of gravitational force on a column of fluid.