The Peripheral Circulation Flashcards
What are the main components of the CVS?
Heart, Arteries, Arterioles, Capillaries, Veins
What is the function of the heart?
Cyclic muscular pump that enables circulation of the blood
What is the function of arteries?
Gross conduction and distribution of blood supply
What is the purpose of arterioles?
Local distribution and fine control of defined tissue volume
What is the function of capillaries?
Microdiffusion and filteration
What is the function of veins?
Collection and return, and capacitance
What is compliance?
Ability to distend and increase volume due to pressure increase
What is capacitance?
Effectively the same as compliance- a measure of relative volume increase per unit in pressure
What is pressure?
A measure of mechanical energy gradient in blood that drives flow around different parts of the system
What does cyclic muscular contraction produce?
Pressure waves to move blood into circulation
How is cardiac output calculated?
Stroke volume x heart rate
What is the average cardiac output?
~5l/min
What happens with each beat of the heart?
Stroke volume is delivered to the major arterial tree
What is the resistance of the major arterial tree?
Low
What is the compliance of the major arterial tree?
~1-2% change / mmHg
Still very important
What is the pressure of the major arterial tree?
High
What needs to be true of arterial pressures?
They need to be high enough to drive cardiac output through high resistance arterioles
How much can resistance very from arteriole to arteriole?
Greatly
What is total peripheral resistance?
The sum of all arteriolar resistance
What does compliance affect?
Pulsatile pressure flow in arteries
In what manner does the heart eject blood?
Cyclically
When does blood flow into arteries?
Systole
What does compliance act to do?
Store mechanical energy of rising pressure wave during systole, and so dissipates energy more gradually over diastole
What would happen if arteries were very rigid walled?
Pressure would rise enough in systole to force whole stroke volume through TPR, but fall to 0 in diastole
What effect does aortic compliance have?
Dampens the pulsatile nature of the systolic pressure wave
Which vessels act to smooth out the pressure wave during systole?
Aorta and elastic arteries and less smooth muscle
What is the Windkessel Effect?
The capacitance effect, whereby more blood flows in than out, so pressure doesn’t rise as rapidly because elastic arteries recoil in diastole to release energy, which smooths flow through arterioles
What is systolic pressure?
The maximum arterial pressure that is reached during systole
What is diastolic pressure?
The minimum arterial pressure that is reached during diastole
What does the blood pressure gradient do?
Drives flow all all time points in the cardiac cycle
What is blood pressure measured in?
mmHg
What is systolic pressure typically said to be?
120mmHg
What is diastolic pressure typically said to be?
88mmHg
What factors affect systolic and diastolic pressure?
Cardiac output
Arterial compliance
How is cardiac output calculated?
SV x HR
What determines cardiac ouput?
How hard the heart is pumping
What is meant by arterial compliance?
The stretchiness of elastic arteries
What is the compliance of elastic arteries?
1.5-2% /mmHg
What is pulse pressure?
The difference between systolic and diastolic pressure
What is pulse pressure at rest?
About 40mmHg, but varies
How is the average pressure calculated?
Calculated as diastole + 1/3 pulse pressure
Why is average pressure calculated in the way that it is?
Because time in systole is shorter, and so diastole predominates in average pressure
What causes a decrease in pulse pressure?
Summated resistance and capacitance of arteriolar network
What happens to variation in pulse pressure as it goes down the arterial tree?
It decreases
Why does variation in pulse pressure decrease as it goes down the arterial tree?
As energy gets lost due to increased resistance as it branches out
What is the result of the decrease in difference in pulse pressure as you go down the arterial tree?
Can approach a smooth mean pressure prior to flowing to capillary circulation- the pressure gradient decreases significantly by the time it reaches capillaries
What is the pressure gradient at the end of the arteriolar branches?
40mmHg
What happens if the pressure gradient at the end of the arteriolar tree isn’t enough to push it through capillaries?
Get ischaemia
What is the role of arterioles?
Control flow to capillary beds
What are the resistance vessels?
Arterioles and pre-capillary sphincters
What is the resistance of arterioles?
High
Why are arterioles high resistance?
Because the lumen is narrow, due to proportionally large amount of smooth muscle in tunica media
What governs flow to capillary beds?
Arteriolar vasomotor tone
How does arteriolar vasomotor tone govern flow to capillary beds?
By vasoconstriction and vasodilation
What do vasoconstriction and vasodilation work together to do?
Finely regulate very small tissue volumes, and so can precisely match substrate supply to metabolic demand
How does metabolic demand differ?
From tissue to tissue through the body
Is vasomotor tone high or low at rest?
High
What causes high vasomotor tone at rest?
Tonic contraction of muscle
Why is vasomotor tone being high at rest good?
Modest resource demand, as only need low blood flow
No need to employ large functional reserve
Increased SNS activity driving α1-GPCRs leads to further vasoconstriction
What is vasomotor done most centrally controlled by?
The autonomic SNS
How does the SNS control vasomotor tone?
Release of NA acts on α1-GPCRs, causing an increase in intracellular [Ca], and so contraction
What modulates vasomotor tone?
Circulating hormones
What impact can local factors on vasomotor tone?
They can reduce it by increasing vasodilation
What effect does vasodilation have on flow?
It reduces resistance to flow
How can local factors reduce vasomotor tone?
Achieved by local vasodilator factors
What produces local vasodilator factors?
Metabolically active tissues produce vasodilator metabolites
Give 5 examples of vasodilator metabolites
H+ CO2 K+ Adenosine Lactate
How do vasodilator metabolites act?
To relax vascular smooth muscle
What can be said of decreased vasomotor tone due to metabolic activity and local release of vasodilator factors?
It is usually acute
What causes a return to vasomotor tone dominated by SNS after local mediators have caused vasodilation?
Increased blood supply removes metabolic factors, so there is a gradual return
What locally mediates vasomotor tone?
Myogenic factors
Endothelial factors
What myogenic factors locally mediate vasomotor tone?
Arteriolar smooth muscle exposed to rapid intraluminal pressure rise
What happens when arteriolar smooth muscle is exposed to a rapid intraluminal pressure rise?
It causes a pressure spike of >200mmHg
What is the purpose of the reaction of arteriolar smooth muscle to a rapid intraluminal pressure rise?
Acute contraction protects from excess pressure, as this would damage tissue if allowed to persist
Where are endothelial factors released from?
Arteriolar endothelium
What do autacoids do?
Modulate an increase or decrease in vasomotor tone
What are blood flow changes to tissue governed by?
Acute metabolic demands in local tissue
What does increased blood flow due to acute metabolic demands lead to?
Decreased metabolite concentration (positive control signal to dilate offsets SNS)
What happens once the increased blood flow has decreased metabolite concentration?
Resistance in arterioles returns to default to supply baseline level of metabolism (negative control signal to constrict)
What is the total peripheral resistance?
The summation of all arteriolar resistances in the body
What is the total peripheral resistance inversely proportional do?
The total body demand for blood flow
What is the result of veins being very stretchy?
The provide a large circulatory reservoir
They have a very high compliance
How much of the blood volume is held in the veins at rest?
65%
What is the compliance of the veins?
10%+ in volume /mmHg
What is the result of the veins having high compliance and being a reservoir?
The can accommodate change in blood volume very quickly
What is the resistance of veins?
Total total resistance relative to arterial system
What is the pressure in veins determined by?
The volume of blood they contain
Depends on balance between flow in from body and out via heart
What does the central venous pressure range between?
-10 to +10 mmHg
Where is central venous pressure measured?
Great veins
What is central venous pressure required for?
Filling in diastole
Usually, what is central venous pressure?
2-6mmHg
What does central venous pressure need to be enough to do?
Get blood into RA
