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Overview of the Cardiovascular System
2 vascular (blood vessel) loops:
Pulmonary circulation: from heart to
lungs and back)
Systemic circulation: from heart to
other organs and back
Flow through systemic and
pulmonary circuits are in series
Flow within systemic (and pulmonary)
circuit is in parallel
Parallel flow allows independent
regulation of blood flow to organs
Parallel Flow in the Cardiovascular System
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Blood Vessels
Heart Arteries  Arterioles Capillaries  Venules  Veins
Arteries – relatively large, branching vessels that conduct
blood away from the heart. Major artery is aorta
Arterioles – small branching vessels with high resistance
Capillaries – site of exchange between blood and tissues
Venules – small converging vessels - drain blood to veins
Veins – relatively large converging vessels that conduct blood
to the heart. Major vein is vena cava (superior and inferior)
Closed system
Blood Vessel anatomy
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Physical Laws Governing Blood Flow
Blood flow is calculated as flow per unit time
Dictated by
Pressure Gradients in the vasculature
Resistance in the vasculature
Flow Rule
Circulatory system is a closed system
Pressure = force exerted by blood
Flow occurs from high pressure to low pressure
Heart creates pressure gradient for bulk flow of blood
A gradient must exist throughout circulatory system to maintain
blood flow
Flow = pressure gradient/resistance
Pressure gradient: Pressures throughout the vasculature
are not constant
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Effect of Resistance on Flow
The lower the resistance, the greater the flow
Total peripheral resistance (TPR) is the sum of the resistance
of all peripheral vasculature in the systemic circulation.
It decreases during exercise to allow blood flow to increase
(next lecture)
Flow =pressure gradient/resistance
Poiseuille’s Law
Resistance =
8 x length x viscosity
π x radius4
LENGTH: The longer the vessel, the greater the
resistance to flow
Capillaries tend to be short compared with larger vessels
Describes the behaviour of a perfect fluid in a rigid tube
BUT
Blood vessels are not rigid
Blood is a 2-phase system (cells and plasma)
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Regulate Blood Flow by Regulating Radius
Regulation of radius of arterioles
(and small arteries)
Vasoconstriction
– decrease radius (by
contracting smooth muscle)
 increase resistance 
decrease blood flow
Vasodilation
– increase radius (by relaxing
smooth muscle)
decrease resistance
increase blood flow
VISCOSITY
Mostly, when considering flow rate, viscosity
remains constant
Plasma almost twice as viscous as water
Whole blood 3-4 times as viscous as water
Viscosity depends on haematocrit
Red blood cells tend to flow in centre stream
(concept of plasma skimming)
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Viscosity
Branches leaving large vessels at right angles tend to
receive rbc-poor blood (plasma skimming)
Haematocrit (therefore viscosity) of capillary blood is 25%
lower than whole-body haematocrit
Polycythaemia: increased haematocrit
Large vessels (arteries and veins)
increased haematocrit increases viscosity
Smaller vessels (arterioles, venules,
veins) effect of increased haematocrit has
less impact on viscosity because flow is
different in these vessels.
cross sectional area is increased
rbc in capillaries ‘single file’
rbc tend to flow in centre stream
Blood vessels are not rigid tubes
Laplace’s Law: Wall tension = Pressure x radius
wall thickness
Wall tension on the aorta is high (pressure is high relative to
radius)
In chronic hypertension, aortic wall thickness increases in
compensation
Large arteries must have strong walls - an artery of twice the
radius needs to withstand twice the tension at any given
blood pressure
Aneurysms tend to develop in larger arteries
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