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Define vascular distensibility
- The ability of a vessel to STRETCH
- ALL blood vessels have this characteristic
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What is characteristic of vascular distensibility?
- Accommodates pulsatile flow
- Provides continuous flow through even the smallest vessels
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What are the most distensible vessels?
Veins - 8x more distensible than arteries
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What pathological condition will decrease distensibility?
Atherosclerosis -> BP will increase due to decreased distensibility
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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
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Describe the relationship between driving pressure and flow in a rigid tube
It is linear
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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
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In a distensible tube, how is resistance affected by subsequent increases in flow?
It is reduced
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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
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How is it explained that the majority of blood volume is in large and small veins?
Veins are 8x more distensible
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What does the law of LaPlace state?
- For cylindrically-shaped structures, wall tension is proportional to radius
- T=PxR
- 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
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To what is the law of LaPlace applicable?
- Blood vessels
- Left ventricle
- Aortic aneurysm
- Ability of capillary vs. vein to withstand pressure
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What is tension as stated in the law of LaPlace?
- The internal force generated by a structure
- Example: blood vessel or alveolus
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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
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What may be considered cylinders when considering the law of LaPlace?
- Blood vessels
- Left Ventricle
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As a structure expands, what happens to the wall tension?
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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)
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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
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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
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What reinforcement do arteries have to withstand wall tension?
Fibrous bands reduce the risk of aneurysm
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Why do capillaries not require fibrous bands?
They rely on their small size (r) to reduce wall tension
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Why are larger vessels more likely to rupture than smaller ones?
The larger the radius, the higher the tension, and more likely to rupture
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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
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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
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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
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What is the relationship between systemic and pulmonary arterial pressures?
Pulmonary pressures are about 1/6 the systemic pressure
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How does pulmonary distensibility relate to systemic distensibility?
Nearly 6x that of systemic arteries
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What is the significance of pulmonary distensibility being so different than systemic?
Allows full CO and gas exchange in a small area
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Define vascular capacitance (compliance)
The TOTAL QUANTITY of blood that can be stored in a given portion of circulation for each mmHg of pressure
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Define compliance
- Compliance is the ability of a vessel to stretch and hold volume
- Compliance = distensibility x volume
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What is the compliance relationship between veins and arteries
- Veins are 8x more distensible and hold 3x more volume
- Veins are 24x more compliant
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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
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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
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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
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What is another name for stress-relaxation?
Delayed compliance
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Describe 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
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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.
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What is the difference between systolic and diastolic pressure?
Pulse pressure
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What causes the dicrotic notch (incisura)?
Retrograde flow just before AV closes causing brief period of aortic pressure less than SBP
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What is the formula for MAP?
(SBP+2(DBP))/3
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Why is DBPx2 used to calculate the MAP?
2/3 of cardiac cycle spent in diastole
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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
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What is a reflection of stroke volume with all other factors equal?
Pulse pressure
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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
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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
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What happens with intensity of pulsations in smaller arteries?
Becomes progressively less
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How does compliance affect blood pressure?
The more compliant a vessel, the more blood needed to cause an increase in pressure
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What 2 factors dampen pulse pressure of peripheral arteries?
- 1) Resistance
- 2) Compliance
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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
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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
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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
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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
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Describe the pulse pressure in arteriosclerosis
- Pulse pressure increases to nearly TWICE normal
- Vessels are less compliant
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Describe the pulse pressure in aortic stenosis
- Significantly decreased
- Diminished flow through the aortic valve's decreased opening
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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
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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
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What is suggested of a wide pulse pressure?Causes?
- Reduced large artery vascular compliance
- Causes:
- Isolated systolic hypertension
- Aortic regurgitation
- Thyrotoxicosis (thyroid storm)
- PDA
- Aortic coarctation
- Anemia
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What are causes of a narrowed pulse pressure?
- Tachycardia
- Severe aortic stenosis
- Constrictive pericarditis
- Pericardial effusion
- Ascites
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What constitutes a widened pulse pressure?
>40mmHg
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What constitutes a narrowed pulse pressure?
<30mmHg
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How does age affect arterial compliance?
- Vessels have decreased compliance with age
- Increased BP with smaller volume due to decreased compliance
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How does pulse pressure change with age?
Pulse pressure increases secondary to reduced compliance
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What is the minimum MAP to perfuse coronaries, brain, and kidneys? Recommended normal?
- 60mmHg minimum
- 70-110mmHg normal
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Besides the venous system, what else functions as blood reservoirs?
- Spleen
- Liver
- Large abdominal veins
- Venous plexus
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What is special about the splenic reservoir?
Stores large amount of RBCs
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What tends to cause resistance to flow in large peripheral veins?
Compression factors
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Where does abdominal pressure increase venous pressure?
In the legs
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Where does cor pulmonale notably increase venous pressure?
In the liver (hepatic congestion)
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What does right atrial pressure represent?
Balance of blood leaving right atrium and flow of blood from the large veins into the right atrium.
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What are the normal, upper, and lower limits of CVP?
- Normal: 0mmHg
- Lower: -5 to -3mmHg
- Upper: 20-30mmHg
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What are factors that increase RAP?
- 1) Increased blood volume
- 2) Increased venous tone
- 3) Dilation of arterioles
- 4) Decreased cardiac function
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Above what RAP does blood begin to back up into peripheral system?
0mmHg
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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
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What is RAP when standing?
0mmHg
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Where does excess blood go from RA when standing?
Arteries
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What is the venous pressure in legs when standing?
+90mmHg because of gravitational weight of blood in veins between heart and feet
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What do neck veins do when standing?
Collapse
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What do veins inside skull do when standing?
Negative pressure because they cannot collapse within a non collapsible chamber
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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.
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How do veins mitigate low pressure in legs and still promote forward flow?
1-way valves
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What may result due to faulty venous valves?
Varicose veins
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