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Path of blood flow through the CVS
Cardiovascular system = closed system
Flow through systemic and pulmonary
circuits are in series
Flow within systemic (and pulmonary)
circuit is in parallel
Parallel flow (below) allows independent
regulation of blood flow to organs
Physical laws governing blood flow
Pressure Gradients in the Cardiovascular System
Resistance in the Cardiovascular System
Flow = ΔP/R = pressure gradient/resistance
Pressure is 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
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Pressure gradient across
pulmonary circuit
Pressure gradient = pressure in
pulmonary arteries minus pressure in
pulmonary veins just before they empty
into left atrium
Pulmonary arterial pressure = 15 mm Hg
Pulmonary venous pressure = 0 mm Hg
Pressure gradient = 15 – 0 = 15 mm Hg
Pressure gradient across
systemic circuit
Pressure gradient = pressure in aorta
minus pressure in vena cava just before
it empties into right atrium
Pressure in aorta = mean arterial
pressure (MAP) = 90 mm Hg
Pressure in vena cava = central venous
pressure (CVP) = 0 mm Hg
Pressure gradient = MAP – CVP = 90 –
0 = 90 mm Hg
Resistance in the Cardiovascular System
Systemic circuit: high P, high R; Pulmonary circuit: low P, low R
Effect of Resistance on Flow
Remember
Flow = ΔP/R
For any given
pressure, the lower
the resistance, the
greater the flow
Factors affecting resistance to flow: Poiseuille’s Law
R=
length x viscosity
radius4
Length of vessel (normally doesn’t change)
Viscosity of fluid (normally doesn’t change)
Radius of vessel: Arterioles (and small arteries) - can regulate radius
RADIUS IS THE MOST IMPORTANT FACTOR
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Regulate blood flow by regulating radius
Radius dependent on contraction state
of smooth muscle in arteriole wall
Vasoconstriction: increased contraction
(decreased radius)
Vasodilation: decreased contraction
(increased radius)
Functions of Varying Arteriole Radius
Controlling blood flow to individual capillary beds
Regulating mean arterial pressure
The combined resistance of all blood vessels is termed
TPR: total peripheral resistance. Regional changes in
vasodilation and vasoconstriction can change TPR
Control of blood flow distribution to organs
Regulation of blood flow to organs based on need
(eg to skeletal muscles during exercise)
Regulated by varying radius (and therefore resistance)
Organ blood flow = MAP / organ resistance
ie driving force for blood flow
resistance to flow in that organ
For any given P gradient,
blood flow changes when
resistance changes
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Factors that influence vasodilation and
vasoconstriction
Autonomic nerves (sympathetic constricts)
Hormones (eg adrenaline constricts)
Metabolism (eg. decreased O2 causes dilation)
These factors therefore influence blood flow
Blood pressure: Mean Arterial Pressure
MAP = driving force for blood flow
F = ΔP/R
Regulating MAP critical to normal function
MAP < normal
Hypotension
Inadequate blood flow to tissues
MAP > normal
Hypertension (scholarship topic this year)
Stress on heart and walls of blood vessels
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Regulation of MAP
Flow = pressure gradient
resistance
CO = MAP
TPR
Therefore MAP=CO x TPR = HR x SV x TPR
This means that MAP is completely determined by HR, SV & TPR
Short- and long-term regulation of MAP
Short-term regulation
Seconds to minutes
Involves heart &blood vessels
Primarily neuronal control
Long-term regulation
Minutes to days
Regulate blood volume
Involves kidneys
Primarily hormonal control
This is a classic example of the different modes of action of
the 2 key regulatory systems (nervous & endocrine systems)
Short-Term Regulation of MAP
The baroreceptor reflex:
A negative feedback loop that helps maintain
normal blood pressure
Baroreceptors = stretch receptors
(mechanoreceptos)
Arterial baroreceptors
High pressure baroreceptors
Sinoaortic baroreceptors
Location
Carotid sinus
Aortic arch
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Cardiovascular Control Centre
Medulla oblongata
Integration center for blood pressure regulation
Output
Sympathetic nerves
Parasympathetic nerves
Sympathetic:
SA node (increase HR)
Ventricles (increase contractility)
Arterioles (increase resistance)
Veins (increase venomotor tone)
Parasympathetic:
SA node (decrease HR)
Major neural pathways in the control
of cardiovascular function
Baroreceptor Reflex in response to a decrease
in MAP
Components of
Baroreceptor Reflex
Detectors = baroreceptors
Afferents = nerves
Integration center =
cardiovascular control center
Efferents = autonomic nervous
system
Effectors = heart, arterioles,
veins
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Physiology Research Focus: hypertension
(scholarship topic 2012/13)
Hypertension and diabetes
are frequent comorbidities
Factors associated with hypertension
Serious problem in developed
countries, increasing problem
in developing countries
Figures taken from scholarship special reading journal articles
Physiology Research Focus: hypertension
(scholarship topic 2012/13)
Drugs & medical devices are in development. We can only develop
effective treatments if we understand the Physiology underlying the disease
Figures taken from scholarship special reading journal articles
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