Hemodynamics Purpose of control mechanisms of blood flow? Maintain homeostasis Purpose of blood...
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Hemodynamics
Purpose of control mechanisms of blood flow?
Maintain homeostasis
Purpose of blood flow?
Nutrient and waste exchange
Blood flow to brain and heart must be maintained
Insufficient blood volume to perfuse all tissuesimultaneously
Blood flow must match metabolic needs of tissue
AJ Davidoff
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MAP = CO x TPR
Important to maintainadequate perfusion pressure in order to control blood flow
Sherwood Fig 10-1
HR x SV
MAP = mean arterial pressureTPR = total peripheral resistanceCO = cardiac output
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Sherwood
Capillary exchange is the sole purpose of the circulatory system
Blood flow depends on pressure gradients and vascular resistance
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Relationship between blood flow, pressure and resistance
Ohm's Law: V = I*R or I = V/R
V = voltage (potential difference)I = currentR = resistance
Blood Flow: P = Q*R or Q = P/R
Q = flow (mL/min) P = pressure gradient (mm Hg)R = resistance (mm Hg/mL/min)
The major mechanism for changing blood flow is by changing arterial resistance (e.g., TPR or in a single
organ)
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Pressure gradients
Pressure difference affects flownot absolute pressure
Sherwood Fig 10-3
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Resistance to Blood Flow
Poiseuille equationR = 8L r4
R = resistance = viscosity of bloodL = length of blood vesselr4 = radius of blood vessel raised to the fourth power
If radius decreases by one half, resistance increases by 16-fold (= 24)!!!
(r4 = area)
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Radius profoundly affects blood flow
Sherwood Fig 10-4
R~ 1/r4
Q = P/R
Flow ~ r4
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Costanzo Fig 4-5
Q = P/R
Total resistance equals the sum of the individual resistances
Total flow is the same at each level, but pressure decreases progressively
(93 mm Hg) (4 mm Hg)
Why?
Series Resistance
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Parallel Resistance
Flow in aorta is equal to the flow in the vena cave (steady state)Flow to each organ is a fraction of the total blood flowTotal resistance is less then any of the individual resistances,therefore no significant loss of arterial pressure to each organ
5 L/min 5 L/min
Needs work
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Velocity of Blood Flowv = Q/A
v = velocity of flow (cm/sec)Q = flow (ml/sec)A = cross-sectional area (cm2)Costanzo Fig 4-4
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Costanzo Fig 4-3
Total cross sectional area of systemic blood vessels
v = Q/A
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Laminar flow and Turbulence
Laminar flow is parabolic, highest velocity in center (least resistance), lowest adjacent to vessel walls
Turbulent flow is disoriented, no longer parabolic, energy wasted, thus more pressure required to drive blood flow.
quiet
noisy
Costanzo Fig 4-6
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Ganong Fig 30-8
Turbulence is velocity of blood flowdiameter of blood vessel1/ viscosity of blood
Mohrman and Heller Fig 6-6
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Bernouilles Principle (in a single vessel)
Total energy = distending pressure (PD) + kinetic energy (KE)
Higher velocity through a constriction
PDKE
Bad for plaque regionsWhy?
Total energy is actually not conserved completely because of heat loss
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Bad for aneurysmsWhy?
KE
PD
Bernouilles Principle
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Cardiovascular Physiology Conceptshttp://www.cvphysiology.com/Blood%20Pressure/BP004.htm
Compliance of blood vessels
C = compliance (mL/mm Hg)V = volume (mL)P = pressure (mm Hg)
C = V/ P
• Compliance is a slope
• At low pressures, veins have a greater compliance than arteries
• At high pressures, compliance is similar in veins and arteries (but volume is much greater in veins)
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Compliance changes related to vasocontraction or aging
With vasocontraction:• Venous volume
decreases and pressure increases
• Venous compliance decreases
Similar effects in arteries with aging
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Martini Fig 21-2
Arteries
Conduits
BloodVessels
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Pressure reservoir
Sherwood Fig 10-6 & -7
Elastic recoil continues to drive blood toward arterioles during diastole
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B&B Fig 17-11
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MAP = diastolic pressure + 1/3 pulse pressure(at rest)
2/3 time in diastole1/3 time in systole
80 mph for 40 min120 mph for 20 min
Sherwood Fig 10-7
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G&H Fig 15-6
Dampening pulse pressures
Arterial pulse pressureinfluenced by:
elasticityrigidityresistance
resistance, pulse pressure
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What does systolic pressure tell you?
What does diastolic pressure tell you?
CO & TPR
TPR
Cardiac Output (CO) = MAP TPR
Sherwood Fig 10-9
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G&H Fig 15-4 and B&B
Aortic pressure changes
rigid
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G&H Fig 15-4
Aortic pressure changes
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G&H Fig 23-4
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Mean arterial pressure (MAP) is the main driving force for blood flow through capillaries
G&H Fig 14-2
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Basis of auscultatory method for measuring BP(Sounds of Korotkoff)
Mohrman and Heller Fig 6-9Turbulent flow is noisy
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Why should cuff be placed at heart level?
What effects on BP measurement wouldthe presence of obesity cause?