Bloodflow Flashcards
The cardiovascular system consists of ……… pumps and circuits (systemic and pulmonary) that are connected in ………….
Both circulations are very similar - there are elastic arteries, resistance vessels and exchange manoeuvres
The cardiovascular system consists of two pumps and circuits (systemic and pulmonary) that are connected in series
Both circulations are very similar - there are elastic arteries, resistance vessels and exchange manoeuvres
Resevior- store blood
Recall five fundamental roles of the normal circulation.
- To transport blood around the body (to deliver oxygen, nutrients and signalling molecules (eg hormones), to remove carbon dioxide and metabolites, and to regulate temperature)
- Flow is achieved by the action of a muscular pump (heart) which generates a pressure gradient that propels blood through a network of tubes (blood vessels).
- The circulation consists of two such pumps (left and right ventricles) which are physically coupled and pump through the systemic and pulmonary circulations respectively.
- Diffusion is crucial for movement of materials through tissues
- Diffusion is only effective over short distances so a capillary needs to be ~10mm from every cell. This necessitates a highly branched structure
Initially when blood leaves the heart it is carried by large, thick-walled, elastic ………….. which act as ………….. vessels
Then you move into smaller ………….. and ………….. which have extensive …………… …………. in their walls which regulates their diameter and produces a ………….. to blood flow
A lot of the pressure drop in the arteries takes place in the ………….. ………….. and …………..
The veins are very stretchy and highly ………….. so they act as a ………….. for blood volume
In terms of cross-sectional area, the ………….. make up the largest cross-sectional area in the CVS - this is because it has an exchange function
Much of the blood at any one point rests in the veins and venules - this is why they are considered a reservoir for blood volume
You can load up the vessels with blood under normal (rest) conditions, but if you need to exercise you get venoconstriction meaning that you decrease the amount of stored blood and move more blood back to the heart
By shifting the blood from the reservoir to the heart you produce more venous return and more cardiac output
The relative volumes are completely different from the relative cross-sectional areas
Initially when blood leaves the heart it is carried by large, thick-walled, elastic arteries which act as dampening vessels
Then you move into smaller arteries and arterioles which have extensive smooth muscle in their walls which regulates their diameter and produces a resistance to blood flow
A lot of the pressure drop in the arteries takes place in the small arteries and arterioles
The veins are very stretchy and highly compliant so they act as a reservoir for blood volume
In terms of cross-sectional area, the capillaries make up the largest cross-sectional area in the CVS - this is because it has an exchange function
Much of the blood at any one point rests in the veins and venules - this is why they are considered a reservoir for blood volume
You can load up the vessels with blood under normal (rest) conditions, but if you need to exercise you get venoconstriction meaning that you decrease the amount of stored blood and move more blood back to the heart
By shifting the blood from the reservoir to the heart you produce more venous return and more cardiac output
The relative volumes are completely different from the relative cross-sectional areas
Which part of the circulatory system produces resistance to blood flow and state what feature of this blood vessel allows it to do this?
Where does a lot of the pressure drop in arteries take place?
Then you move into smaller arteries and arterioles which have extensive smooth muscle in their walls which regulates their diameter and produces a resistance to blood flow
A lot of the pressure drop in the arteries takes place in the small arteries and arteriole
What blood vessel is considered a reservoir for blood volume and why are they considered this way?
The veins are very stretchy and highly compliant so they act as a reservoir for blood volume
Much of the blood at any one point rests in the veins and venules - this is why they are considered a reservoir for blood volume
The relative volumes are completely different from the relative cross-sectional areas
Blood pressure – the force that drives the circulation
Blood flows not due to pressure but due to PRESSURE DIFERENCE
Recall Darceys law?
What can pressure difference be estimates as being as?
Pressure Difference = Flow x Resistance
Pressure Difference can be estimated as being mean arterial blood pressure
Q is the cardiac output
Resistance = the resistance of all the vessels - also called peripheral vascular resistance - this is an approximation because it assumes a steady flow and assumed that the vessels are rigid and that right atrial pressure is negligible
What are the haemodynamic determinants of Mean Blood pressure?
MBP = Cardiac output (CO) x Resistance (PVR)
P difference = Q x R
P difference = pressure difference, Q = volumetric flow, R = resistance
Pressure falls across the circulation due to …………….. (frictional) pressure losses. …………….. …………….. and …………….. present most resistance to flow.
Pressure falls across the circulation due to viscous (frictional) pressure losses. Small arteries and arterioles present most resistance to flow.
Distinguish the components of Poiseuille’s equation.
What is the main determinant of resistance?
Inner radius of the tube
Halving the radius decreases the flow 16 times
Poisuille’s equation (stated above) emphasises the importance of artery diameter to alter resistance
What sort of flow occurs in vessels generally?
Blood generally exhibits laminar flow:
the flow of a fluid when each particle of the fluid follows a smooth path flowing in layers or streamlines, paths which never interfere with one another. Velocity of the fluid is constant at any one point.
The pathophysiological changes to the endothelial lining of the blood vessels occur because of changes in what?
In parts of vessels blood can exhibit turbulent flow:
this is irregular flow characterized by tiny whirlpool regions and associated with pathophysiological changes to the endothelial lining of the blood vessels. The velocity of this fluid is not constant at every point.
The pathophysiological changes to the endothelial lining of the blood vessels occur because of changes in shear stress
Where does blood flow its quickest?
Blood flows quickest in the middle and slowest on the sides
This is because there are adhesive forces which attach the blood to the vessel walls
Why is velocity less nearer to the endothelium for blood?
Adhesive forces between fluid and surface
Velocity…………… as distance from wall increases
What is shear rate?
When shear rate is MULTIPLIED by viscosity you get …………….. ………………..
Velocity increases as distance from wall increases
Shear Rate = the velocity gradient that is established - the difference between the highest velocity blood in the middle of the lumen and the lowest velocity blood that adheres to the blood vessel walls
When shear rate is MULTIPLIED by viscosity you get SHEAR STRESS
What happens when sheer stress is high?
What happens if sheer stress is low?
Shear stress disturbs endothelial function which is important for laminar flow and the production of various transmitter substances which give rise to vessel dilation and constriction
High shear stress, as found in laminar flow, promotes endothelial cell survival and quiescence, cell alignment in the direction of flow, and secretion of substances that promote vasodilation and anticoagulation.
Low shear stress, or changing shear stress direction as found in turbulent flow, promotes endothelial proliferation and apoptosis, shape change, and secretion of substances that promote vasoconstriction, coagulation, and platelet aggregation.n
Turbulent flow- degeneration of endothelail lining causing coronary atheroma
Shear stress is important because it determines how happy the endothelial cells are and determines their function
Recap of effects of shear stress on endothelial function:
Normal Shear Stress = the endothelial cells align themselves normally and produce substances normally
Low Shear Stress = the endothelial cells are all mixed up and don’t behave in a normal way and don’t produce their substances normally
You measure the changes in sound that you hear
You put a cuff around the upper arm and you increase the pressure until the cuff pressure exceeds arterial pressure
You place the stethoscope distal to the cuff - initially you won’t hear anything because the blood flow is occluded
As you let the cuff down, you eventually get to a point where the pressure in the cuff is just overcome by the pressure in the artery
At this point, blood starts to squirt through the occlusion and sets up turbulent flow - you hear a LIGHT TAPPING SOUND
If you continue to reduce the pressure in the cuff, you reach a point where you have no occlusion in the artery and so blood will start to flow in a laminar fashion again which you CAN NOT HEAR - this is the DIASTOLIC BLOOD PRESSURE
Summary:
Sounds APPEARS = Systolic Blood Pressure
Sound DISAPPEARS = Diastolic Blood Pressure
The difference between systolic and diastolic blood pressure = PULSE PRESSURE
Mean Blood Pressure = Diastolic + 1/3 of pulse pressure (this is lower than you’d expect
- Systolic blood pressure (SBP)
- Diastolic blood pressure (DBP)
- Pulse pressure (PP) = SBP - DBP
Mean blood pressure ≈ DBP + 1/3 PP
What is the windkessel effect?
During ejection, blood enters the aorta and other downstream elastic arteries faster than it leaves them.
~40% of the stroke volume is stored by the elastic arteries.
When the aortic valve closes, ejection ceases and pressure falls slowly and there is diastolic flow in the downstream circulation.
Why is this?
- When the aortic valve closes, ejection ceases but due to recoil of the elastic arteries, pressure falls slowly and there is diastolic flow in the downstream circulation.
- This damping effect is sometimes termed the “Windkessel”
- If arterial compliance decreases (arteries become stiffer), eg with age, the damping effect of the Windkessel is reduced and the pulse pressure increases.
Once the aortic valve closes, ventricular pressure falls rapidly but aortic pressure only falls slowly in diastole
This is explained by the elasticity of the aorta which buffers changes in pressure and so it doesn’t drop to zero like the ventricular pressure - the pressure is maintained by the elasticity of the vessel
NOTE: this elasticity changes through life
Dichrotic Notch - when blood enters the aorta faster than it leaves the aorta, about 40% of the stroke volume is stored by the elastic arteries
When the aortic valve closes, the ejection of blood stops but there is a recoil because the arteries and the aorta are very elastic which produces the dichrotic notch
Pressure falls slowly downstream of the aorta hence showing that the elasticity allows it to act as a buffer
The pressure inside the vessel (TRANSMURAL PRESSURE) determines the distension of the vessel wall
The relationship between transmural pressure and wall tension is determined by Laplace’s Law
Circumferential stress also depends on vessel wall thickness
The relationship between the transmural pressure and vessel volume is called the COMPLIANCE
Compliance is dependent on vessel elasticity
If the muscle round the blood vessel isn’t contractile enough we get pressure pushing out from the inside of the tube onto the walls. This causes circumferential stress
Transmural=existing or occurring across the entire wall of an organ or blood vessel.
If the wall thickness enough pressure isn’t great enough we cant support enough pressure development in the pressure and the pressure causes the vessel to distend and this become worse if the muscle cells around the vessel aren’t strong enough (age) to develop enough force. Prolong pressure changes and wall distention causes an aneurysms.
If an aneurysm forms in the blood vessel, this means that for the same internal pressure, the inward force exerted by the muscular wall must also ………………
However, if the muscle fibre is ……………… and the compliance isn’t great, the force needed to withstand the internal pressure cannot be produced and so the aneurysm will continue to expand
If an aneurysm forms in the blood vessel, this means that for the same internal pressure, the inward force exerted by the muscular wall must also increase
However, if the muscle fibre is weakened and the compliance isn’t great, the force needed to withstand the internal pressure cannot be produced and so the aneurysm will continue to expand
Complience- Not resistive
Venous compliance is about 10 to 20 times greater than arterial compliance
When you change the nervous supply to the smooth muscle causing contraction, you decrease the venous volume and increase venous pressure
This allows you to change the volume of blood in the reservoir
Explain how standing (gravity) affects the circulation.
When you stand up quickly, gravity makes the blood pool in the legs which is due to the venous volume/capacitance
When it pools in the legs, this reduces venous return to the heart which means that cardiac output falls and you get less blood going to the brain
Why we (generally) don’t faint on standing?
•Standing causes:
–Activation of the sympathetic nervous system to:
- constrict venous smooth muscle and ‘stiffen’ the veins.
- constrict arteries to increase resistance and maintain blood pressure
- increase heart rate + force of contraction and maintain cardiac output
–Myogenic venoconstriction (in response to elevated venous pressure) to ‘stiffen’ veins
–Use of muscle and respiratory ‘pumps’ to improve venous return
- Nevertheless cerebral blood flow falls on standing
- Failure of these mechanisms causes fainting (syncope)
How does the skeletal pump and respiratory pump work?
Skeletal Pump - the contraction of the muscle squeezes blood back through the veins to the heart
This assists the movement of blood back to the heart and decreases venous capacitance
Respiratory Pump - as we breathe in, we expand our chest and our intrathoracic pressure decreases which allows blood to come back to the right atrium and increase venous return
These two simple mechanisms allow us to be able to stand up for long periods of time without fainting
Name 2 conditions due standing?
