Physiology Exam 8: Arterial and Venous Function
What 3 factors can cause aortic compliance to decrease?
1. Advanced age 2. Arteriosclerosis 3. Hypertension
What 2 factors determine diastolic pressure?
1. HR which determines the time interval for blood transfer from the arterial system to the venous system. 2. Total peripheral resistance which determines the rate of volume transfer. Both of these determine DBP because they are related to the residual arterial volume in the aorta after ejection.
What are 2 reasons why the elasticity of arteries is important?
1. Helps transfer kinetic energy to blood after LV has relaxed 2. Prevents massive pressure increase during LV contraction
What 4 factors increase SV to increase pulse pressure?
1. Increased preload 2. Increased inotropy 3. Decreased afterload 4. Decreased heart rate
What are the 7 short-term control mechanisms that increase venous return?
1. Passive fluid shift from ISF into plasma volume 2. Venous valves 3. Cardiac suction effect (decreased pressure in the heart) 4. Increased cardiac contraction (increases Ppv) 5. Increased SNS stimulation 6. Skeletal muscle pump 7. Respiratory pump (exhalation increases venous return)
What are the 2 long-term control mechanisms that increase venous return?
1. Salt and water retention 2. Increased blood volume Both of these are facilitated by the kidneys.
What 2 things influence mean systemic pressure?
1. Total blood volume 2. Distribution of blood in stressed and unstressed volume
At rest, what percent of CO goes to the brain?
15%
By what factor can blood flow to the skin increase to reach maximal blood flow?
15x
By what factor can cerebral blood flow increase to reach maximal blood flow?
2.7x
By what factor can skeletal muscle blood flow increase to reach maximal blood flow?
20x (greatest vasodilatory reserve)
At rest, what percent of CO goes to the GI tract?
25%
At rest, what percent of CO goes to the kidneys?
25%
At rest, what percent of CO goes to the skeletal muscle?
25%
At rest, what percent of CO goes to the coronary arteries?
5%
At rest, what percent of CO goes to the skin?
5%
What is the average pressure drop caused by arterioles?
55 mmHg
By what factor can coronary blood flow increase to reach maximal blood flow?
5x
What percent of SVR determined by the arterioles?
60-70%
If total blood volume is 5 L, and 4 L are unstressed volume and 1 L is stressed volume, what is MSP usually?
7 mmHg
By what factor can splanchnic (abdominal organs) blood flow increase to reach maximal blood flow?
8x
What vessel type is involved in pulse dampening?
Aorta
What vessel types are involved in distribution?
Aorta, large arteries, and small arteries?
Stressed volume refers to blood volume in which vessel type?
Arteries
Which blood vessels are considered a "pressure reservoir?
Arteries
Which vessel type is innervated and sensitive to metabolic milieu?
Arterioles
Which vessel type is the most muscular?
Arterioles
Which vessel type regulates pressure and flow?
Arterioles
Why does the aorta and the vena cava have no contained blood volume?
Blood in the aorta quickly leaves to the more peripheral arteries because you want to perfuse the tissues. Blood in the vena cava almost instantly drains into the RA.
What is the formula for compliance? What is its normal value?
C = ∆V/∆P. Normal value = 2 mL/mmHg
At what RAP values does CO max out? What is the max CO that can be reached?
CO maxes out around 9 L/min when RAP is 4-5 mmHg.
What does pulse wave velocity (PWV) a good predictor of?
CV mortality in patients with hypertension.
What is Pcv?
CVP
In moderate HF, what happens to CVP and CO?
CVP is elevated, but CO could be normal due to the fact that the kidneys have induced a hypervolemic state to compensate for the loss of inotropy.
In severe HF, what happens to CVP and CO?
CVP is even more elevated, and CO is depressed. The hypervolemic state is not able to fully compensate for the loss of inotropy.
Which vessel types are involved in exchange?
Capillaries and venules
An increase in total peripheral resistance (TPR) does what to the cardiac function and vascular function curves? What does this mean for the steady state operating point?
Cardiac function curve is rotated clockwise. Vascular function curve is rotated counter-clockwise. The steady state operating point is shifted downward. CO and venous return are decreased at a given RAP.
A decrease in total peripheral resistance (TPR) does what to the cardiac function and vascular function curves? What does this mean for the steady state operating point?
Cardiac function curve is rotated counter-clockwise. Vascular function curve is rotated clockwise. The steady state operating point is shifted upward. CO and venous return are increased at a given RAP.
Where are the pulse tonometer probes placed for PWV measuring?
Carotid artery pulse and femoral artery pulse. Distance between the two is determined and then divided by the time between detecting the pulse wave in each location.
A decrease in RAP indicates what about SV?
Decrease in RAP correlates to increased SV.
Besides increasing SV, how can pulse pressure be otherwise increased?
Decreasing aortic compliance, but this is not interventionally possible!
True or false: at higher pressures, veins have much higher compliance (20x more) than arteries.
False, arteries and veins have similar compliance at high pressures.
True or false: velocity of blood flow is identical through all levels of the vasculature.
False, velocity of blood flow is greatly decreased in the smaller blood vessels (arterioles, capillaries, and venules)
What is the equation for blood flow?
Flow = mean velocity x cross sectional area
In what part of the vasculature do swings in pressure start to disappear? In other words, when does the arterial waveform transform to a flat, single pressure reading?
In the arterioles right before reaching the capillaries
What do high PWV values indicate?
Increased arterial rigidity or decreased compliance.
What happens to internal radii as you go from the larger arteries to the smaller arteries?
Internal radii decreases.
What is the function of collagen fibers in blood vessels?
It allows for stretching (compliance)
What effect does decreased contractility have on the cardiac function curve? What does this mean for the steady state operating point?
It causes a clockwise rotation, and the slope is decreased. The steady state operating point is shifted downward and to the right, so CO is decreased and RAP is increased. This is what HF looks like.
What effect does increased contractility have on the cardiac function curve? What does this mean for the steady state operating point?
It causes a counter-clockwise rotation, and the slope is increased. The steady state operating point is shifted upward and to the left, so CO is increased and RAP is decreased.
What happens to venous return in severe HF?
It decreases because CVP becomes extremely high.
What happens to the amount of collagen as you go from the larger arteries to the smaller arteries?
It decreases.
What happens to the amount of elastic fibers as you go from the larger arteries to the smaller arteries?
It decreases.
What effect does venoconstriction have on stressed volume? What does this do to the vascular function curve?
It increases stressed volume which increases MSP. Venodilation decreases stressed volume which decreases MSP. Venoconstriction shifts VFC to the right; venodilation shifts VFC to the left.
What happens to the amount of smooth muscle as you go from the larger arteries to the smaller arteries?
It increases.
In HF patients, what happens to CVP? What does this mean for venous return?
It is increased which means that venous return decreases.
What is the steady state operating point?
It is the RAP value where the venous return curve and cardiac function curves intersect. Basically, it is the RAP that satisfies both curves (where CO=VR)
What does pulse wave velocity (PWV) measure, and how is it measured?
It measures the velocity of the pulse wave generated by LV contraction and is an index of aortic stiffness and/or elasticity. It is measured using noninvasive ultrasound techniques.
What are the 2 equations for MAP?
MAP ≈ ((SBP-DBP)/3) + DBP MAP = (CO x SVR) + CVP Note: the second equation is just a rearrangement of Ohm's Law where ∆P = MAP-CVP, Q = CO, and R = SVR.
When total blood volume is between 0-4 L, what is MSP and where is the volume located?
MSP is 0, and all the volume is unstressed volume
When total blood volume is ≥ 4 L, what is MSP and where is the volume located?
MSP is some value greater than 0 because the excess volume will now be in the stressed volume.
What happens to MSP with increased TPR?
MSP stays the same because no blood volume has been lost or gained. MSP only responds to volume changes.
How is vasodilatory reserve calculated?
Maximal flow - Basal flow
Do arterioles have more elastin or more smooth muscle?
More smooth muscle. They have little elastin.
What effect does increased contractility have on the vascular function curve?
No effect.
Does pulmonary circulation exhibit large pressure swings?
No, they are much smaller than what is seen in systemic circulation.
What is the formula for PWV? Give units.
PWV = ∆x/∆t where x = distance and t = time. Units are meters/second
What pressure gradient dictates venous return?
Ppv-Pcv or Ppv-CVP
What is Ppv?
Pressure of the peripheral venous system and is usually around 7 mmHg but subject to change.
What effect does increased aortic compliance have on pulse pressure? Is DBP or SBP changed for this to occur? (assume everything else kept constant)
Pulse pressure is decreased because SBP is decreased
What effect does decreased SV have on pulse pressure? Is DBP or SBP changed for this to occur?
Pulse pressure is decreased because SBP is decreased.
What effect does decreased aortic compliance have on pulse pressure? Is DBP or SBP changed for this to occur? (assume everything else kept constant)
Pulse pressure is increased because SBP is increased.
What effect does increased SV have on pulse pressure? Is DBP or SBP changed for this to occur?
Pulse pressure is increased because SBP is increased.
What is mean systemic pressure?
RAP value where venous return is 0. It is the pressure measured in the CV system if the heart stopped pumping.
An decrease in total blood volume does what to the vascular function curve? What does this mean for the steady state operating point?
Shifts vascular function curve and the steady state operating point down and to the left.
An increase in total blood volume does what to the vascular function curve? What does this mean for the steady state operating point?
Shifts vascular function curve and the steady state operating point up and to the right.
Why does pulse pressure become 0 in the arterioles?
Significant resistance in the arterioles eliminates pressure swings, so you only get a single pressure reading (MAP), and the flow becomes laminar instead of pulsatile.
What vessel types are involved in resistance?
Small arteries and arterioles
Which veins are the major site of venous compliance changes?
Smaller veins outside of the thorax
What is the peripheral venous pool?
The entire venous system excluding the central venous pool.
What happens to venous blood flow when skeletal muscle contracts?
The venous valves open as blood venous blood travels back to the heart.
What happens to venous blood flow when skeletal muscle relaxes?
There is no retrograde flow of venous blood because of the venous valves, so venous blood effectively stays in the same place.
As venous return increases, what happens to RAP, EDV, and CO?
They all increase.
Past 4 L of total blood volume, what is the relationship between total blood volume and MSP?
They are directly related.
What is the relationship between PWV and age?
They are directly related.
What is the relationship between HR and DBP?
They are directly related. As HR increases, there is less time for diastolic runoff, so there is a greater arterial volume at the end of diastole which increases DBP.
What is the relationship between TPR and DBP?
They are directly related. TPR opposes blood flow, so increased TPR means more arterial volume at the end of diastole which increases DBP.
What is the relationship between stressed volume and MSP? What does this do to the vascular function curve?
They are directly related. Vascular function curve shifted to the right if stressed volume increased and shifted to the left if stressed volume decreased.
What is the relationship between venous return and CVP?
They are inversely related (Don't let Darban get it twisted)
What is the relationship between venous return and RAP?
They are inversely related.
In HF patients, what do the kidneys do to try and maintain adequate CO? What does this mean for CVP?
They increase total blood volume by fluid retention mechanisms. This causes an increase in CVP.
What causes the partially constricted state of arterioles?
Tonic sympathetic activity
True or false: SV affects pulse pressure and MAP.
True
True or false: blood flow is identical through all levels of the vasculature.
True
True or false: compliance only affects pulse pressure.
True
True or false: veins are stimulated to venoconstrict by the SNS.
True, but SNS response is not as profound as that of arteries.
What happens to unstressed volume when CO needs to be increased or an organ requires more blood flow?
Unstressed volume decreases.
Unstressed volume refers to blood volume in which vessel type?
Veins
What happens at the flat portion of the venous return vs RAP graph?
Veins collapse which impedes flow to the RA. Happens at negative RAP values
Which has more smooth muscle and collagen fibers: veins or the vena cava?
Vena cava
Which vessel types are involved in collection?
Venules and the vena cava
Which vessel types are involved in capacitance?
Venules and veins
What is the central venous pool?
Volume enclosed by the right atrium and the great veins in the thorax
What happens to wall thickness (relative to the diameter of the vessel) as you go from the larger arteries to the smaller arteries?
Wall thickness increases
What is on the x and y axes of the cardiac function curve?
x axis: RAP y axis: CO in L/min
What is on the x and y axes of the vascular function curve?
x axis: RAP y axis: Venous return in L/min