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Define vascular distensibility
- The ability of a vessel to STRETCH
- ALL blood vessels have this characteristic
What is characteristic of vascular distensibility?
- Accommodates pulsatile flow
- Provides continuous flow through even the smallest vessels
What are the most distensible vessels?
Veins - 8x more distensible than arteries
What pathological condition will decrease distensibility?
Atherosclerosis -> BP will increase due to decreased distensibility
Describe vascular tone as it relates to distensibility?
- Vascular smooth muscle normally in a semi-contracted state
- Vasoconstrictors increase vascular tone and decrease distensibility to increase BP
Describe the relationship between driving pressure and flow in a rigid tube
It is linear
Describe the relationship between driving pressure and flow in a distensible tube
- It is nonlinear
- As pressure inside the tube rises, the size (radius) of the tube increases
- In other words, the walls are distended
In a distensible tube, how is resistance affected by subsequent increases in flow?
It is reduced
Describe the passive characteristic of distensible vessels that is INDEPENDENT of the presence or absence of vascular smooth muscle.
- Small, distensible blood vessels exposed to high pressures will exhibit less resistance to flow than at normal pressures
- This is a PASSIVE characteristic
How is it explained that the majority of blood volume is in large and small veins?
Veins are 8x more distensible
What does the law of LaPlace state?
- For cylindrically-shaped structures, wall tension is proportional to radius
- For spherically-shaped structures, wall tension is proportional to ½ the radius
- T= ½ (PxR)
- T=wall tension
- P=pressure of fluid within the cylinder
- R=radius of the tube
To what is the law of LaPlace applicable?
- Blood vessels
- Left ventricle
- Aortic aneurysm
- Ability of capillary vs. vein to withstand pressure
What is tension as stated in the law of LaPlace?
- The internal force generated by a structure
- Example: blood vessel or alveolus
Why is it important to maintain a balance between pressure caused by smooth muscle and elastic tissues? (the wall of the structure and fluid pressure within a tube or sphere)
To prevent progressive distension or collapsing
What may be considered cylinders when considering the law of LaPlace?
- Blood vessels
- Left Ventricle
As a structure expands, what happens to the wall tension?
What are the consequences of the law of LaPlace?
To withstand the same internal pressure, large-diameter arteries must have THICKER walls than small-diameter arteries (cylinders)
What is the difference in application of the law of LaPlace to a cylinder and a sphere?
For a given vessel radius and internal pressure, a spherical vessel will have ½ the wall tension of a cylindrical vessel
Why do large arteries have thicker walls?
For a given pressure, arteries of twice the radius must be able to withstand twice the wall tension
What reinforcement do arteries have to withstand wall tension?
Fibrous bands reduce the risk of aneurysm
Why do capillaries not require fibrous bands?
They rely on their small size (r) to reduce wall tension
Why are larger vessels more likely to rupture than smaller ones?
The larger the radius, the higher the tension, and more likely to rupture
Describe the vicious cycle of aneurysm formation.
- The bulging vessel subjects the weakened wall to even more tension, which may continue to expand and may rupture.
- With the aneurysm approaching a spherical shape, there is less tension than the same radius cylinder, BUT continued expansion produces tension exceeding that of the ORIGINAL cylinder
How does the law of LaPlace apply to the left ventricle? Frank-Starling?
- The greater the filling pressure of the LV, the greater the tension on the ventricular wall (radius does not change much)
- The greater the tension in the ventricular wall at end-diastole, the greater the stroke volume
Describe the relationship between vascular distensibility and changes in volume and pressure.
- If there is an increase in pressure that causes an increase in volume, the distensibility of that vessel is expressed as a % per mmHg of pressure
- 1mmHg pressure increase that causes 1mL volume increase = distensibility of the vessel is 10% per mmHg
What is the relationship between systemic and pulmonary arterial pressures?
Pulmonary pressures are about 1/6 the systemic pressure
How does pulmonary distensibility relate to systemic distensibility?
Nearly 6x that of systemic arteries
What is the significance of pulmonary distensibility being so different than systemic?
Allows full CO and gas exchange in a small area
Define vascular capacitance (compliance)
The TOTAL QUANTITY of blood that can be stored in a given portion of circulation for each mmHg of pressure
- Compliance is the ability of a vessel to stretch and hold volume
- Compliance = distensibility x volume
What is the compliance relationship between veins and arteries
- Veins are 8x more distensible and hold 3x more volume
- Veins are 24x more compliant
During surgery, is a venous or arterial bleed worse? Why?
Venous because there is a large amount of blood VOLUME being lost at a low pressure
Describe the slopes of the compliance curve
- X-axis is increasing pressure
- Y-axis is increasing volume
- Veins have highest slope meaning they hold large volumes at low pressures
- Arteries have a low slope meaning they hold less blood at a given pressure
- Aged arteries have a very low slope meaning they hold much less volume than younger arteries at a given pressure
Describe the volume-pressure relationship between arteries and veins
- Any increase in volume in arterial vessels results in larger increases in pressure in the veins
- When the veins are constricted, large quantities of blood are transferred to the heart, thereby increasing CO
What is another name for stress-relaxation?
- A vessel exposed to increased volume INITIALLY experiences a large increase in pressure
- As the vessel smooth muscle progressively stretches, the pressure returns back to normal OVER A PERIOD OF MINUTES TO HOURS
What is the purpose of stress-relaxation
- Allows circulation to accommodate a large amount of extra blood when needed.
- In the other direction, allows circulation to adjust in the presence of hemorrhage.
What is the difference between systolic and diastolic pressure?
What causes the dicrotic notch (incisura)?
Retrograde flow just before AV closes causing brief period of aortic pressure less than SBP
What is the formula for MAP?
Why is DBPx2 used to calculate the MAP?
2/3 of cardiac cycle spent in diastole
What 2 major factors affect pulse pressure?
- 1) Stroke volume output
- 2) Compliance of arterial tree (circulation)
- Anything affecting these 2 factors will affect pulse pressure
What is a reflection of stroke volume with all other factors equal?
How does stroke volume affect pulse pressure?
- Increases in stroke volume increase pulse pressure, the opposite is also true
- SV is DIRECTLY related to PP
How does arterial compliance affect pulse pressure?
- Increases in arterial compliance decrease pulse pressure, the opposite is also true
- Arterial compliance is INDIRECTLY related to PP
What happens with intensity of pulsations in smaller arteries?
Becomes progressively less
How does compliance affect blood pressure?
The more compliant a vessel, the more blood needed to cause an increase in pressure
What 2 factors dampen pulse pressure of peripheral arteries?
- 1) Resistance
- 2) Compliance
How does resistance dampen pulsations?
- Small amount of blood must flow forward at the pulse wave front to distend the next segment of the vessel
- Greater the resistance, the more difficult it is for forward movement to occur
How does compliance dampen pulsations?
More compliant the vessel, the greater quantity of blood needed at pulse wave front to cause an increase in pressure
Why are pulsations in large arteries greater than those in the aorta?
- SBP higher and DBP lower in large arteries than aorta
- Greater driving force lowers DBP
- Driving force for blood flow is MAP, which is more influenced by DBP
- 1) Although the PP is greater, MAP is lower because of the lower DBP
- 2) Pressure wave after ejection of blood from LV travels at higher velocity than blood itself travels
- a. this augments downstream pressure in the large arteries
- b. wave fronts help "pave the path" for MAP
What happens to pulse pressure in smaller arteries? Arterioles? Capillaries? Venules? Veins?
- Decreased in smaller arteries
- Virtually absent in arterioles
- Completely absent in capillaries, venules, and veins
Describe the pulse pressure in arteriosclerosis
- Pulse pressure increases to nearly TWICE normal
- Vessels are less compliant
Describe the pulse pressure in aortic stenosis
- Significantly decreased
- Diminished flow through the aortic valve's decreased opening
Describe the pulse pressure in patent ductus arteriosus (PDA)
- DBP falls very low before next heartbeat
- Major portion of blood pumped into aorta from LF flows backward through the PDA into the pulmonary artery and lung circulation
Describe the pulse pressure of aortic regurgitation
- Aortic pressure can fall all the way to zero between heartbeats, NO dicrotic notch
- AV incompetent - does not close completely or may be missing
- Blood pumped from LV flows immediately back into LV
What is suggested of a wide pulse pressure?Causes?
- Reduced large artery vascular compliance
- Isolated systolic hypertension
- Aortic regurgitation
- Thyrotoxicosis (thyroid storm)
- Aortic coarctation
What are causes of a narrowed pulse pressure?
- Severe aortic stenosis
- Constrictive pericarditis
- Pericardial effusion
What constitutes a widened pulse pressure?
What constitutes a narrowed pulse pressure?
How does age affect arterial compliance?
- Vessels have decreased compliance with age
- Increased BP with smaller volume due to decreased compliance
How does pulse pressure change with age?
Pulse pressure increases secondary to reduced compliance
What is the minimum MAP to perfuse coronaries, brain, and kidneys? Recommended normal?
- 60mmHg minimum
- 70-110mmHg normal
Besides the venous system, what else functions as blood reservoirs?
- Large abdominal veins
- Venous plexus
What is special about the splenic reservoir?
Stores large amount of RBCs
What tends to cause resistance to flow in large peripheral veins?
Where does abdominal pressure increase venous pressure?
In the legs
Where does cor pulmonale notably increase venous pressure?
In the liver (hepatic congestion)
What does right atrial pressure represent?
Balance of blood leaving right atrium and flow of blood from the large veins into the right atrium.
What are the normal, upper, and lower limits of CVP?
- Normal: 0mmHg
- Lower: -5 to -3mmHg
- Upper: 20-30mmHg
What are factors that increase RAP?
- 1) Increased blood volume
- 2) Increased venous tone
- 3) Dilation of arterioles
- 4) Decreased cardiac function
Above what RAP does blood begin to back up into peripheral system?
What is normal intra-abdominal pressure? What happens with increased pressure?
- 6mmHg normal.
- Increased pressure requires the venous pressure in the legs to be increased to overcome abdominal pressure and allow blood flow from legs
What is RAP when standing?
Where does excess blood go from RA when standing?
What is the venous pressure in legs when standing?
+90mmHg because of gravitational weight of blood in veins between heart and feet
What do neck veins do when standing?
What do veins inside skull do when standing?
Negative pressure because they cannot collapse within a non collapsible chamber
What is a unique surgical consideration regarding veins during craniotomy or opening of sagittal sinus?
Because the veins have a negative pressure, they can easily entrain air causing a VAE.
How do veins mitigate low pressure in legs and still promote forward flow?
What may result due to faulty venous valves?
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