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8/11/2019 Circuitry, Hemodynamics and Cardiovascular Pressures
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Circuitry of Cardiovascular System
Cost. Fig 4-1
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3 Basic Principle of Circulatory
Function
1. The rate of blood flow to each tissue is almost
always precisely controlled in relation to the
tissues need
2. CO is controlled mainly by the sum of all localtissue flows
3. Arterial pressure regulation is generally
independent of either local blood flow control
or CO control
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Hemodynamics:Relationship of Pressure, Flow, and Resistance
Flow (Q): movement of blood; always from high pressure to lowpressure
Pressure (P): force exerted by the blood
Force: generated by the heart and varies throughout the system
Resistance (R): measure of the friction that impedes flow
Cost. Fig 4-2
Q=P/R
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Velocity of FlowCost. Fig 4-3
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Velocity of FlowCost. Fig 4-6
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Other Characteristics
Conductance:measure of the blood flowthrough a vessel for a given P. Expressed asml/sec/mmHg
Fluid Viscosity:amount of pressure requiredto force whole blood through a vessel ascompared to water.
Vascular distensibility: amount a vessel canaccommodate an increase in pressure byincreasing volume within the vessel.
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Functions and Pressures
of the Arterial System
Arteries
Arterioles
Capillaries
Cost. Fig 4-2
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Arteries
Strong vascular walls
Function as low-resistance
conduits and as pressure
reservoirs
Maintain blood flow to
the tissues during
ventricular relaxation
Compliance= vol/ P
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Arterial Blood Pressure
Systolic P (SP)= max arterial P at the peak of systole
Diastolic P (DP)= min arterial P during diastole
Arterial P=Systolic/Diastolic
Pulse P= SystolicDiastolic
Mean Arterial P (MAP)= DP + 1/3 (SP-DP)
Cost. Fig 4-9
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Arterial Baroreceptors
Baroreceptor reflex is a
homeostatic adjustment
to MAP in the short term
(seconds to hours)
Primary atrial
barorecptors:
2 carotid sinuses
Aortic arch
Firing rate is proportional
to MAP and PP
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Operation of the Arterial Baroreceptor Reflex
Cost. Fig 4-31
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Operation of the Arterial Baroreceptor
Reflex
Cost. Fig 4-32
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Other Baroreceptors
Large systemic veins, the pulmonary vessels,
and the walls of the heart also contain
baroreceptors.
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Blood Volume and Long-Term Regulation of
Arterial Pressure
Baroreceptor reflexes are short-term regulators of arterial pressure but adapt
to a maintained change in pressure.
The most important long term regulator of arterial pressure is blood volume.
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A t i lC Fi 4 2
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Arterioles
Dominant site of resistance to flow in the vascular system
Major role in determining MAP
Major role in distributing flow to organs and tissues
Since MAP is identical throughout the body, arterioles use
vasodilation and vasoconstriction of smooth muscle walls to alter
resistance to flow.
Cost. Fig 4-2
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Capillaries
Thin walled tube of endothelial
cells one layer thick No smooth muscle, no elastic
tissue
Cells are separated by
intercellular cleft
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Velocity of Capillary Blood Flow
Determined by R of arterioles and number of
open capillary sphincters
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Veins
Low resistance
conduits for venous
return
Very compliant and
contain most of the
blood in the vascular
system
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Determinants of Venous Pressure
SNS causes venoconstrictionto maintain pressure and
venous return
Skeletal muscle pump and
respiratory pump increase
venous pressure locally and
enhance venous return.
Venous valves allow pressure
to produce flow toward the
heart.
V PCost Table 4 1
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Venous Pressures:
Right Atrial (Central Venous)
Regulated by a balance
between blood pumped
out and blood flowing in
Heart weakness elevates
RAP
Strong contractions
decreases RAP
Rapid venous return
increases RAP
Cost. Table 4-1
Cost Table 4-1
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Venous Pressures:
Right Atrial (Central Venous)
Venous return can beincreased by:
Increased blood volume
Increased peripheral venouspressures
Dilation of arterioles
Normal value is 0 mmHg(equal to atmosphericpressure around body)
High RAPbacks up into
large veins
Cost. Table 4-1
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Venous Pressures:
Intra-abdominal pressures
Normal value is 6 mmHg
One key determinant of peripheral venous
pressures (legs)
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Venous valves
One way flow of blood toward the heart.
With any tension of skeletal muscle within the
legs, veins are compressed which squeezes
blood toward the heart.
Helps to regulate gravitational pressures.
With prolonged standing, blood will accumulate in
the legs and feet and increase the venous
pressure of the capillariesedema
Varicose veins
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Microcirculation
Capillaries are the sites where nutrients and waste products are
exchanged between blood an tissues
Flow is less than anywhere else in body, intermittent due to
vasoconstriction, determined by NEEDS OF THE TISSUE
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Net Filtration Pressure
(PC) (Pi)
(C) (i)
(PC) + (C) + (Pi) + (i) = net filtration P
if positive, fluid moves out of C
if negative, fluid moves into C
If positive
If negative
Cost. Fig. 4-34
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Net Filtration Pressure
(PC) + (C) + (Pi) + (i) = net filtration P
if positive, fluid moves out of C
if negative, fluid moves into C
Cost. Fig. 4 34
Cost. Fig. 4-34
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Net Filtration Pressure
(PC) + (C) + (Pi) + (i) = net filtration P
if positive, fluid moves out of C
if negative, fluid moves into C
Cost. Fig. 4 34
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Increase Lymph Flow
(PC) (Pi)
(C) (i)
Changes in pressures
Increased permeability of the capillary
If positive
If negative
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Acute Control of Tissue Blood Flow Tissue metabolism, Vasodilators (NO) and Vasoconstrictors (endothelin)
Local factors that change with metabolic activity cause arteriolar vasodilationand increased blood flow (active hyperemia)
Flow autoregulation (change in R to maintain Flow when P is changing) occurs
due to local factors and myogenic responses to stretch.
Cost. Table 4-7
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Other Controls of Tissue Blood Flow
Long term controls: Vascularity (angiogenesis)
Humoral Control: Vasoconstrictors and
vasodilators
Ionic control
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Integrative Cardiovascular Function:
Regulation of Systemic Arterial Pressure
Recommended