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Cell Bio 15, 16- Hemodynamics
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Galen proposed that arterial blood originated in the heart, venous blood in the liver, and that it moved out to
the organs for consumption
– Would require 500 lbs of blood production/day
William Harvey established the central premise of blood flow
– Blood flows away from the heart in arteries and returns to the heart in veins
– He was not able to establish how blood moved from arteries to veins
The term hemodynamics describes the physical factors governing blood flow within the cardiovascular system
Blood Flow
Flow is the volume of a liquid that passes a specific point in a tube (vessel) per unit time
Flow= (change in volume/change in time)
Blood flow to the entire body is equal to cardiac output
In a resting individual, cardiac output of the heart is ~5 L/min
– Thus blood flow at the aorta is 5 L/min
• Note that blood flow at the aorta can remain very constant at rest, while at the same time
regional flow may be varying from minute to minute
At rest, the entire blood volume is pumped by the heart in one minute
– CO = HR*SV = 75 bpm*70 ml/beat = 5.25 L/min
Any given red blood cell could travel the entire circulation in one minute
Perfusion
• Perfusion is the flow per given volume/mass of tissue
 mL/min/gram
• Thus a large organ such as the femur could have a greater flow but less perfusion than an ovary, because the
ovary receives much more blood per gram of tissue
• Unlike a gas, a liquid cannot be compressed
• If pressure is applied to a liquid it must go somewhere
– It travels to an area of lower pressure
Factors of Flow (required for blood to flow)
• Pressure must be applied to a liquid in order for it to overcome resistance in the blood vessels, and flow
– Specifically a pressure differential is required between the two ends of the vessel
• Resistance is created by blood contacting the vessel walls, and represents an impediment to flow
– However, resistance is a contributing factor to pressure and is required in order to build pressure
In mathematical terms, flow is governed by a pressure differential over resistance (Ohms Law of hydrodynamics)
 Flow= (change in pressure/resistance)
 Flow increases with increasing pressure differential
 Flow decreases with increasing resistance
Velocity of Flow
• The same volume of blood must flow through each segment of the circulation each minute
– Principle of continuity/conservation of mass
• In order to reach the entire body, the vasculature is necessarily a branching system of vessels
• A fundamental law of vessel branching is that at each branch point, the combined cross-sectional area of
daughter vessels exceeds the cross-sectional area of the parent vessel
• Mathematically, the velocity of blood flow must change in order to maintain constant flow (~5L/min) in each
segment of the vasculature
• Flow velocity is the distance that a particle of liquid travels with respect to time
– It is expressed in units of distance per time (cm/sec)
• Note that flow measures the bulk movement of liquid
Cell Bio 15, 16- Hemodynamics
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As mentioned previously, the same volume of blood must pass (flow) through each segment of the circulation
each minute
– However, the total cross-sectional area of the vasculature changes dramatically
Thus, the velocity of blood flow (v) is inversely proportional to vascular cross-sectional area (A)
– The larger the cross-sectional area the slower the velocity of flow
Pressure
• Pressure is a stress, or force acting in any direction against resistance
– Stedman’s
• Hydrostatic pressure is the physical force exerted by a liquid against a surface
– Blood pressure (a hydrostatic pressure) is the force per unit area exerted on a vessel wall by the blood
• Blood pressure (generally arterial) is routinely measured in patients and provides a useful indicator of their
cardiovascular health
• Blood pressure is always measured as a pressure difference between two points
– We can only define a movement, or distortion of a mechanical system by the difference between two
forces
Types of Pressure differences
• There are two main types of pressure differences that are of interest in CV physiology
– Clinical: The pressure difference between a particular site within a vessel and the barometric pressure
– “Blood pressure”
– Physiological: The pressure difference between two intravascular sites
– Driving pressure, which causes blood flow
Practical Driving Pressure
• In practice, the pressure gradient, or driving pressure of interest is the arterial minus the venous pressure
– ~85-95 mm/Hg
Cell Bio 15, 16- Hemodynamics
Origins of Pressure (2 categories)
1. Physical
• By definition, blood pressure is the force per unit area exerted on a vessel wall by the blood
• Thus, in the strictest sense, in order to have blood pressure we only need two physical factors
– Blood
– Blood vessel
• The volume (thus the weight) of six quarts of blood against vessel walls alone creates blood pressure
– How much pressure is created depends on compliance of the vessel wall
• As mentioned previously, a vessel wall is necessary for a volume of blood to exert a pressure
• The compliance of the vessel walls dictates the degree to which changing blood volume changes blood pressure
(and vice-versa)
• In theory, the compliance of a vessel can take three forms
– Zero compliance
• Increasing volume creates unlimited increase in pressure
– Infinite compliance
• No amount of volume can increase pressure
– Finite compliance
• Dictates the degree to which increasing volume increases pressure
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Compliance= slope of the relationship between pressure and volume
Compliance= (change in volume/change in pressure)
Less compliant vessels cause greater increases in pressure as volume increases, more compliant vessels cause
lesser increases in pressure as volume increases
Cell Bio 15, 16- Hemodynamics
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The arterial side of the circulation must be under high pressure in order to overcome vascular resistance
– Arteries are less compliant; increases in volume lead to greater increases in pressure
The venous side of the circulation, which acts as a blood reservoir must be under low pressure
– Veins are more compliant; increases
The posture of the body has a significant influence on blood pressure
– In CV physiology blood pressures must be given relative to a reference h level (usually the heart)
Because both arterial and venous pressures are elevated when standing, the driving pressure that governs blood
flow is not affected
After cardiac arrest the entire vascular system has a blood pressure of ~7 mm/Hg
– Absence of pumping activity or gravity (i.e., subject is recumbent)
2. Physiological
• Several physiological factors determine blood pressure
• These may be viewed in terms of arterial and venous sides of the systemic circulation
• On the arterial side, there are two principal physiological factors influencing the physical factors, and thus
determining blood pressure
• Cardiac output
• Peripheral resistance
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On the venous side, cardiac output and peripheral resistance also contribute to venous pressure
As well, pressure is influenced by additional physiological factors
– Respiratory cycle, Contraction of skeletal muscle, Suction of heart
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Blood flow in the circulation is not steady, as the heart imparts its energy in a pulsatile manner, which affects
the velocity of blood flow
– Changes in the velocity of blood flow lead to compensatory changes in intravascular pressure
• Bernoulli effect
It is accepted that blood flows from an area of higher pressure to an area of lower pressure
– However, it is more accurate to state that blood flows from an area of higher total energy to an area of
lower total energy
Total energy consists of potential energy, or (blood) pressure, and kinetic energy, or velocity (of blood flow)
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Cell Bio 15, 16- Hemodynamics
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Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in
pressure
– If there is more kinetic energy (velocity), there must be less potential energy (pressure), and vice-versa
– This concept is best demonstrated with a tube of varying radius
Catheter Blood Pressure
• The Bernoulli effect has important practical implications for measurement of blood pressure with an opentipped catheter
• The catheter opening must be placed perpendicularly to the flow of blood for the most accurate reading of
blood pressure
– In other words, only using this catheter placement is a true pressure reading obtained, without
confounding effects of kinetic energy
Atherosclerosis
• In a vessel narrowed by atherosclerosis, flow is maintained by increasing flow velocity
• However, an increase in flow velocity compromises pressure, which affects perfusion of distal segments
– Because of the resistance created by the stenosis, and the likelihood of turbulence, the post-stenosis
potential energy (pressure) and total energy both fall
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ΔP, as we have seen, drives blood flow through the vasculature
However, arterial and venous pressures are kept within a narrow range by many feedback mechanisms
– The pressure gradient across the body is quite constant
Therefore, the effect of ΔP in regulating blood flow through organs (perfusion) is relatively minor
Cell Bio 15, 16- Hemodynamics
Resistance
As mentioned previously, the effect of ΔP in regulating blood flow through organs is relatively minor
Indeed, vascular resistance is a much more significant factor in regulating the flow of blood through the
circulation
Total Peripheral Resistance
• Total peripheral resistance is the resistance of the entire systemic circulation
– This resistance can vary twenty-fold
• PRUs between 0.2 and 4
• Resistance in the pulmonary circuit is about one seventh that in the systemic circulation
• Resistance cannot be measured by direct means—it must be calculated from measurements of blood flow and
pressure differential
• Resistance (R)= (change in Pressure (P)/flow (F))
100 mmHg (pressure differential from Arteries to veins)/100 ml/sec (cardiac output/flow through circ
system)= 1 PRU
Poisseuille’s Law
• The most fundamental law that governs the flow of fluids through cylindrical tubes was derived by the French
physician/physiologist Poiseuille
• Poiseuille’s law applies to the steady (non-pulsatile) laminar flow of newtonian fluids through rigid, cylindrical
tubes (Ohm’s law applies to all vessels)
• Though the vascular system does not satisfy all aspects of the above criteria, Poiseuille’s law does
provide key insights into the determinants of blood flow
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Cell Bio 15, 16- Hemodynamics
Vessel Length
• According to the equation, the longer the vessel the greater the resistance
• However, blood vessel length is not subject to rapid change
Vessel Radius
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The narrower the blood vessel, the greater the proportion of blood therein that is in contact with the vessel wall
– The outer fluid lamina has a flow velocity of 0
• Thus overall, the narrower the blood vessel the lesser the flow of blood
Viscosity
• Fluid, including blood, tends to flow through long, smooth-walled tubes in concentric layers (laminae)
• Viscosity is a measure of the slipperiness between layers
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The outermost concentric layer of flowing blood is in contact with the wall and thus has a flow velocity of 0
Moving inward, each successive layer flows more rapidly
– Moving inward within the blood vessel, the area of contact between each successive layer decreases
thus increasing the flow velocity of each successive layer
A more viscous fluid experiences greater shear (more resistance to sliding) between the fluid layers
This greater resistance (or greater viscosity) reduces flow at any given pressure
The relatively high viscosity of blood is almost entirely related to the hematocrit (proportion of red blood cells)
– The RBCs contact vessel walls and themselves thus increasing viscosity at an exponential rate as their
numbers increase
Cell Bio 15, 16- Hemodynamics
Turbulent Flow
• When flow is laminar, all elements of the fluid move in streamlines that are parallel to the axis of the tube
• In turbulent flow the elements of the fluid move irregularly in axial, radial, and circumferential directions
• Though blood flow is generally laminar, it can become turbulent under certain conditions
• High flow velocity
• Large tube diameter
• Low fluid viscosity
• High fluid density
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Turbulence is generally encountered in predictable locations, or circumstances
Turbulent blood flow is common in vessels of a large radius, such as the aorta and the pulmonary trunk
Turbulence results from high flow velocities, which may occur in different situations
• During increased cardiac output
• Turbulence is most likely to be observed within the heart chambers or the vessels that enter and
leave the heart
• Arterial stenosis, which causes a decrease in vessel diameter, accordingly increasing flow velocity
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Anemia is associated with decreased hematocrit, and as a result, decreased viscosity
– This increases Reynolds number, thus predicting turbulence
Turbulence is energetically inefficient as kinetic energy is squandered on chaotic motion
– Resistance is vastly increased (Poiseuille’s law does not hold with turbulent flow), and as a result, the
heart must do more work—to produce greater pressure—in order to maintain blood flow
As well, blood clots are more likely to develop in turbulent flow
Laminar flow is silent, whereas turbulent flow causes vibrations, which may be detected as murmurs with a
stethoscope
– Valvular defects in the heart
– Diseased (generally atherosclerotic) arteries
Turbulent flow is beneficial in certain instances
– Blood pressure is measured indirectly by listening for resumption of blood flow (sounds of Korotkoff)
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Cell Bio 15, 16- Hemodynamics
Circuit Resistance
• From the aorta to the venae cavae blood flows through arteries, veins, and capillaries
– These vessels are collectively arranged in series
• However, all of the above vessels branch extensively
– The branches are arranged in parallel
• Vessels arranged in series have a total resistance that is the sum of each individual vessel (in the series)
• Total peripheral resistance is thus the sum of the resistances of all the arteries, arterioles, capillaries, venules,
and veins
• Rtotal = R1 + R2…
– Assume a hypothetical R of 10 (units unimportant) and 4 vessels
– Rtotal = 10 + 10 + 10 + 10 = 40
• Vessels arranged in parallel have a lower total resistance than those arranged in series
• In fact, total resistance for all vessels is far less than the resistance of any single blood vessel
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It may seem paradoxical that adding more blood vessels (in parallel) reduces total resistance
However, each additional vessel provides another pathway making it easier for blood to flow through the circuit
– Greater conductance
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Most of our tissues contribute to the overall conductance of the systemic circulation
Amputation of a limb, or removal of a kidney eliminates a parallel circuit
– Increased TPR
When many parallel vessels exist, changing the resistance of a small number of these vessels has little effect on
total resistance
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