Hemodynamics 2 Flashcards
What are the types of blood flow?
- laminar flow
- tubular flow
What is laminar flow?.
Parabolic. Profile:
Concentric rings of equal flow rates;
Slowest at the edges(friction with the walls) highest at the center
Flow is silent
What is turbulent flow?
Occurs when laminar flow is disrupted
Requires an increased pressure to maintain flow (flow is not as efficient)
NOISY- heard by auscultation. Called bruit. Eddies are chaotic
Reynolds number is used to predict whether blood flow is laminar or turbulent
What is the significance of laminar flow?
In laminar flow adjacent layers of blood slide past each other. The layers in the center of the tube have a greater velocity than those nearer the walls; giving a parabolic shape of the flow. Shear is the sliding motion of one lamina past another. Shear causes the red cells to orientate preferentially in the direction of flow and move a little towards the central axis. This leaves a thin, cell - deficient layer of plasma next to vessel wall. This marginal plasma layer is very important since it facilitates blood flow since turbulent flow requires since turbulent flow requires much more energy to move the blood. Laminar flow is silent
When does laminar flow become tubular flow?
Turbulence increase resistance when the Reynolds number exceeds 2000
What factors increase turbulence?
High flow velocity (v)
Large vessel diameter (D)
High blood density (p)
What factors decrease turbulence?
Increased viscosity (n)
What is Reynolds equation?
Reynolds number (Re)= pro-turbulent factors: anti-turbulence factors
Re= vDp/n
Critical value for Re= 2000
When Re > 2000 laminar flow becomes turbulent
Velocity most important as: viscosity, density usually constant
Why does vessel diameter D increase turbulence ?
Blood flow is slower at the surface of the vessels: the larger the vessel the smaller the surface area: volume ratio. Contact with the vessel wall (surface area contact) slows velocity (v) because of resistance
Decreased SA / volume ratio occurs with increased diameter
-reduced blood in contact with a surface area
- increased turbulence
- think a bigger tube- more area in the middle for turbulence to occur
Re= vDp/n
Although- increased velocity of blood favors turbulence
What are the clinical conditions promoting turbulence?
Heart: defective valves
Blood vessels: narrowed blood vessels
Large diameter vessels favor turbulence expected in small diameter vessels.
High velocity flow flavors turbulence
Bit high velocity glow is found in narrow diameter vessels
Apparent contradiction. Question is which has the most effect on promoting turbulence ? The increased velocity of flow in narrowed vessels outweigh the benefit that narrowed vessels give.
So, narrowed blood vessels, have a high velocity of flow, and blood flow tends to be turbulent,..
Describe single line flow as a type of blood flow
- occurs in capillaries
- RBC diameter > capillary diameter
- RBC flexes as passes through the capillary
- RBC actually flows faster than plasma (less friction in axial flow, vs plasma at boundary flow)
How does sickle cell blood flow affect single line blood flow?
Red blood cell (RBC) is rigid, hemoglobin Hb rod-like and RBC sickle shape. Do not pass easily through capillary —> tissue ischemia and painful “sick line crisis”
Low blood flow: RBCs may stick together and to endothelial lining
What is the math behind Laplace’s law?
Transmural pressure (🔼P)= Pi - Pt
T= tension in vessel wall to counteract pressure
Wall thicknesse excluded- You will learn about this later
Laplace’s law= 🔼P= 2T/r
If the pressure inside this wall is constant , the larger the radius (diameter) of the vessel, the greater the wall tension needed
For same pressure: larger vessel, requires greater wall tension
What is the medical example of Laplace’s law?
Aneurysm; wall stress= pressure* tension/radius
- frequent in large arteries (aorta) due to Laplace law
- Aorts distension
- aorta radius large- requires more tension to offset given blood pressure
Reaches a point where vessel cannot generate more wall tension
Eventually aneurysm can rupture
Describe the compliance and elastic properties of arteries and veins
Large arteries and veins contribute very little to overall resistance to blood flow. Therefore changes in their diameter have minimal effect on blood flow. However, the compliance and elastic properties of arteries and veins are important because it determines how much blood can be stored with them. Veins, especially, act as reservoirs of blood
What is compliance ?
Compliance describes how the volume of a compartment changes (🔼V )in response to a given change in the pressure within (🔼P)
Compare compliance of veins and arteries
Veins are more compliant. For a small change in pressure they can increase their volume greatly. Compliance =🔼V/🔼P
Compliance is a measure of “distensibility”
Veins are more distensible than arteries
Why are veins highly compliant at operating pressures?
at their operating pressures (0-10 mmHg), veins are highly compliant
Veinous pooling
So if the distending pressure is high then the veins will distend and accommodate the blood.
On standing, the pressure (distending) in the veins of the foot increases, and since the veins are so compliant these veins will accommodate a large volume of blood. This results in “veinous pooling”
Note that venous pooling will result in a fall in VR to the heart
What are the effects of venom obstruction?
Venockbstrictionn squeezes blood out of the veins and shifts it towards the heart
I.e. increased VR note: venoconstriction is akin to decreased compliance (sympathetic activity decreased venous compliance)
What are the effects of venodilation?
Conversely, venodilation allows more blood to pool in the veins and decreased VR
Note: venodilation is akin to increased compliance (sympathetic activity increased venous compliance)
Describe vena cava compliance
At high pressures-all fibers stretched, once the vein has circular profile, it does not increase its diameter because it has little elastin in its walls
Very compliant
Vascular tone (SNS) shifts compliance curve down and to the right
Steep slope at low pressures:
- low pressure/volume, large veins collapse
- increased pressure/volume, increased circular shape
- until vessel vessels attain circular shape, walls not stretched much
- small changes in pressure with large changes in volume
Summarize compliance as well as contrast in veins and arteries
Compliance is the change in volume with distending pressure
Veins are more compliant than arteries
Flip the axis:
Slope= 1/ compliance= elastance
Arteries are more elastic than veins
What is elasticity?
Ability of a vessel to recoil to a distending pressure
Describe aortic compliance and elastic properties
Not as compliant as veins greater elasticity
In early phase of cardiac ejection, the aortic volume increases because there is more blood entering the aorta than leaving it. So the elastic aorta is stretched.
Towards the end of systole and during diastole, the stretched aortic and arterial walls recoil and in the process give up their stored potential energy. This reconverted energy. This reconverted energy drives blood around the circulation
What are the main functions of aorta and arteries?
Aorta (and arteries) develop and withstand high pressures. Stable resistance. Therefore changes in their diameter have minimal effect on blood flow. They convert the pulsatile flow from heart into steady flow through the vascular beds
Decreased compliance in artery (stiffness) comes with an increase in pressure. Thus pressures higher in arteries
Describe arterial compliance & age
Aortic compliance is clinically important in the elderly
Old artery less compliant/stiffer than young artery
At a given MAP, old arteries hold less blood than young
For an old artery to hold the same volume of blood as a young artery, the pressure in the old artery must be higher.
Systolic BP and pulse pressure in elderly> young
Systolic hypertension of the elderly (diastolic can be normal)
How does the reduced compliance of the old aorta affect the blood pressure?
Normally, when about 70 ml of blood (I.e. typical SV value) is ejected into the aorta, the aortic pressure rises from about 80 mmHg to about 120 mmHg which is the systolic BP
Ejection of 70 ml of blood into an aorta which is less compliant means that the systolic BP rises to much greater values e.g. up to 160 mmHg
Diastolic BP MSY be normal (at about 80 mmHg)
Therefore, elderly person BP = 160/80 mmHg
Condition is called “systolic hypertension of the elderly”
What are the effects of altered compliance?
Decreased ventricular compliance leads to increased LVEDPr and vice versa
LV hypertrophy leads to decreased compliance and increased LVEDPr
Dilated cardiomyopathies lead to increased compliance and decreased LVEDPr
What are the consequences of reduced ventricular compliance?
LV hypertrophy—> increased wall thickness —> decreased compliance —> increased LV End diastolic Pr —> decreased filling —> decreased stroke volume —> decreased cardiac output
Describe the measurement of blood pressure
Sphygmomanometry
Inflate cuff to > 180 mmHg
Auscultate brachial artery
Reduce cuff pressure slowly
1st korotkoff sound is when cuff pressure is just below systolic BP - due to transient spurt of blood into artery = systolic BP
Korotkoff sounds grow louder
As cuff pressure approaches diastolic BP - artery is open most of the time and sounds get quieter
At diastolic BP- sounds disappear
Describe automated BP measurements
Place cuff on upper arm
Automatically inflated to above systolic BP
Cuff pressure is released
Arterial pulsation causes cuff to oscillate
Oscillations
Digital reading of systolic BP, diastolic BP and mean arterial pressure in the brachial artery
Describe blood pressure in the aorta during one cardiac cycle
The ventricle ejects blood into aorta (during systole) faster than it can flow away into the periphery. This causes a steep rise in aortic pressure. About 70% of the stroke volume is stored initially in the elastic arteries during systole. Remaining 30% runs off through the runs off through the peripheral vessels.
Max. Pressure in the aorta is the systolic BP = 120 mmHg
As blood runs into the peripheral circulation the aortic BP falls. Minimum pressure in aorta is the diastolic = 80 mmHg
Pulse pressure= SBP- DBP= 120-80= 40 mmHg
How do we calculate the mean arterial pressure?
The MAP is the mean pressure in the aorta during a single cardiac cycle
Rough approximation:
MAP= P(Dias)+ 1/3P(sys-dias)
More accurately, at the line where area A= area B
Value is about 95 mmHg
The MAP is a critical variable Because it is the average effective pressure that drives blood around the circulation
Describe the arterial pressure (pulse) wave
Apex beat is hard almost simultaneously with feeling the radial pulse!
When blood is ejected at high pressure into the aorta- a pressure wave is set up which is transmitted along the walls of the arteries
Shape of the pressure wave becomes narrower and taller as it proceeds down the arterial tree
Pressure wave velocity is faster in stiffer (older) arteries
Note: the velocity of blood flow itself is much SLOWER than the pressure pulse wave
What factors affect pulse pressure?
- Stroke volume (pulse pressure inversely proportional to SV)
- Arterial compliance: pulse pressure inversely proportional to compliance
Aorta becomes stiffer and less compliant in old age
Elderly: BP 160/80 mmHg
“Systolic hypertension of the elderly”
Don’t confuse arteriosclerosis and atherosclerosis. Arteriosclerosis is a consequence of ageing. Atherosclerosis is plaque formation.
Atherosclerosis = stiffness of the arteries
What are the physiological factors affecting blood pressure?
Age
Sleep
Exercise (dynamic- increase by 10-40 mmHg, isometric can increase by 100 mmHg)
Emotion/stress (increase BO)
Pregnancy (BP falls)
What is the effect of gravity on blood pressure?
Pressure foot > heart
Is this in defiance of Darcy’s law Q is inversely proportional to P?
Blood flows against pressure gradient
What is the effect of posture on arterial and venous pressures?
In the supine position- the entire body is at the level of the heart
Pressure drop of about 5 mmHg between aorta and dorsal is pedis artery in foot. Pressure in foot= 90 mmHg
🔼P = driving pressure
Pressure in veins draining feet is 5 mmHg
Pressure falls to 2 mm Hg at RA
On standing: add the weight of the column of blood (hydrostatic pressure) to BOTH arterial supply of the feet AND to the veins draining the feet
130 cm (in a 1.8 m individual) of blood is = to an extra pressure of 95 mmHg. (This is a hydrostatic pressure)
Arterial pr (in foot)= 90+ 95= 185 mmHg Venous pr(in foot)= 5+95=100
Driving pressure= 185-100=85(same as in feet of a supinevperson )
As long as there is a pressure difference between arteries and veins- blood will flow
On standing: subtract the weight of the column of blood (hydrostatic pressure) from BOTH the arterial supply of the head AND from the veins draining the head
Subtract 37 mm Hg from the arterial and the venous side
The driving pressure is the same as when recumbent
Describe venous pressure
Pressure in the venous compartment
Central venous pressure= pressure in thoracic vena cava near RA
CVP is important because it determines filling pressure if RV, and thereby SV through the Frank-Starling mechanism
RA pr is just slightly below CVP
What are the factors which affect RAP and CVP?
- RV contraction is strong: RAP decreased
- RV contraction weak: RAP increased
- Increased venous return leads to increased RAP
RAP depends on balance between ability of heart to pump blood out into circulation and tendency of blood to flow back into heart
Since venous return affects right atrial pressure, what factors affect venous return?
Factors that affect CVP and VR
-increased blood volume
-increased venomotor tone
Both increase RAP
Decreased CVP & VR
-hemorrhage
$venidilation
Both decrease Right atrial pressure
How does sympathetically induced venkconstriction affect venous return?
Sympathetically induced venoconstriction displaces blood from the peripheral veins into central veins and into RA, thus increased venous return
How does cardiac failure promote edema?
RAP increases (20- 30 mmHg). CVP then increases as well as capillary hydrostatic pressure. Favoring edema formation
What mechanical factors that affect venous return?
- Skeletal muscle pump- contraction of skeletal muscles compresses veins rhythmically
This displaces blood forward toward heart
CVP is maintained, maintaining venous return and EDV
Venous Pr in feet is decreased, because blood drains quickly into the muscle veins (pumping action)
Capillary Pr is decreased in feet and ankles and tendency to form edema is decreased
Remember valves in veins prevent back flow of blood
- Respiratory pump
Explain how respiratory pump assists in veinous return
Respiratory pump-like bellows
Moves chest blood into thorax *physiologically splitting of S2 during deep inspiration
During inspiration: venous return increase to right atrium
On inspiration decreased intrathoracic pressure leads to thoracic veins being expanded. This causes blood from head fills the vena cava. This causes increased flow to vena cava and increased venous return
Another way inspiration increases venous return would be contraction of the diaphragm, leading to increased abdominal pressure, increasing venous return
During expiration: decreased venous return
How is blood flow measured?
Fick principle
Oxygen extraction
Law of conservation of mass= [O2] pulmonary flow in artery- [O2] pulmonary flow out (veins) come back for exampke
