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33 Cards in this Set
- Front
- Back
Hemodynamics
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Blood flow is driven by a constant pressure along varying resistances
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Elements of CV system
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Pump, vessels, blood
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Blood pressure
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Measured as pressure difference between two points
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Blood flow
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Blood flow = cardiac output = stroke volume x heart rate
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Types of pressure difference
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Axial, radial, hydrostatic
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Circulation evolutionary consequence of:
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body size
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Circulatory system integrates three organs:
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heart, blood vessels, blood
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The heart is a dual pump that drives the blood that drives blood in two serial circuits of blood vessels:
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systemic and pulmonary circulations
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Driving pressure difference
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= Flow x Resistance (viscous)
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Flow
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= delta(V) / delta(t)
= Av (cross section x velocity) |
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Blood flow is driven by constant pressure head across variable resistances
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Left and right heart each maintain a constant pressure head that drives blood flow across a branched system of blood vessels acting as resistors
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Total peripheral resistance to blood flow is result of combination of blood vessels in parallel and in series
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Parallel:
1/R_total = 1/R_1+1/R_2+1/R_3+... Series: R_total=R_1+R_2+R_3+... |
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Three types of pressure difference in blood vessel:
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Driving pressure between arterial and venous ends of the circulation:
(between 2 points along x axis) Transmural pressure between intravascular and tissue space (between two points along r axis) Hydrostatic pressure between two points in a vertical column of blood (between two points along h axis) |
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Total flow of blood delivered by the heart (total mean flow in the circulation) is called the cardiac output
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Total flow of blood = Cardiac output = Heart Rate x Stroke volume.
Example: 4.9 L/min = 70 beats/min x 70 mL/beat |
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Flow in idealized vessel increases with the fourth power of radius:
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Hagen-Poiseuille Equation
F = delta(P) * pi*r^4 / (8*nu*l) |
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Viscous resistance to flow is proportional to the viscosity of blood, but does not depend on properties of the blood-vessel walls
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R = 8*nu*l/ (pi*r^4)
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Viscosity of blood
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Measure of the lack of "internal slipperiness" between layers of fluid
viscosity = nu = shear stress / shear rate =(Force/A)/(delta(v)/delta(x)) |
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Blood flow is laminar
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Distinction between laminar and turbulent flow is clinically very important
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Critical quantity concerned in the generation of turbulence
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Reynolds number (dimensionless)
Re = v_bar * 2r*rho / nu v_bar = mean linear velocity r = vessel radius pho = density of fluid nu = viscosity of fluid |
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Pressure and flow oscillate with each heartbeat
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Maximum: systolic
Minimum: diastolic |
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Gravity
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When there is a difference in height, there will be a hydrostatic pressure difference
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Low compliance of vessel
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Causes transmural pressure to increase when the vessel blood volume is increased
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Viscous resistance of blood
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Causes axial pressure difference when there is flow
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Inertia of blood and vessels
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Causes pressure to decrease when velocity of blood flow increases
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Whole blood
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Whole blood has an anomalous viscosity. Although the viscosity of water and blood plasma are Newtonian, whole blood is non-Newtonian (viscosity varies with rate of shear or flow)
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Influence on blood viscosity:
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i) Increases with fibrinogen concentration
ii) Increases with hematocrit iii) Decreases as vessel radius falls belows a certain threshold iv) Increases at low flows v) Increases at low temperatures |
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With progressive branching of vascular tree, several parameters change at each arborization:
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i) Number of vessels
ii) Vessel radius iii) Aggregate cross-sectional area of vessels iv) mean linear velocity v) flow through a vessel vi) relative blood volume vii) circulation (or transit) time viii) pressure profile |
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Physical properties of vessels
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Closely follow level of branching in the circuit
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Blood volume
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Mostly resides in systemic veins
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Intravascular pressures
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Intravascular pressures along the systemic circuit are higher than those along the pulmonary circuit
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Steepest drop in systemic circulation
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Under normal conditions, occurs in arterioles, the site of greatest vascular resistance.
Vasoconstriction and vasodilation can steepen or flatten the pressure profile |
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Local intravascular pressure
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Depends on distribution of vascular resistance. Midpoint capillary pressure
Pc = [(Rpost/Rpre)Pa+Pv] / [1+(Rpost/Rpre)] |
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Blood vessels are elastic tubes
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Distensible elastic materials in blood vessels (elastin and collagen) allow blood vessels to distend with increasing pressure, thereby decreasing resistance
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