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Unit 12
12.1
Control of Blood Flow
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Define diastolic pressure.
Lowest arterial pressure
measured during a
cardiac cycle
Pressure
in the
arteries
during
ventricular relaxation
when
no blood
is being
ejected
from
left ventricle
Define systolic pressure.
Highest arterial pressure
measured during a
cardiac cycle
Pressure in
arteries
after
blood
has been
ejected
from the
left ventricle
during
systole
'blip'
in
arterial pressure curve
(in image) or
dicrotic notch
- produced when
aortic valve closes
-> produces
brief period
of
retrograde flow
Define pulse pressure.
Difference
between
systolic
&
diastolic
pressure
Define mean arterial pressure.
The
average pressure
over a
complete cardiac cycle
What is the calculation for mean arterial pressure?
Pa
=
DP
+
1/3 pulse pressire
Pa
=
mean arterial pressure
DP =
diastolic pressure
What is the normal range of mean arterial pressure?
70
-
100
mmHg
What is the distribution of the blood?
85
% in
systemic circulation
10
% in
pulmonary
5
% in
cardiac chanbers
at
end
of
diastole
What does velocity depend on?
flow
cross sectional area (of
vessel
s)
What is the relevance of this image?
The
cross sectional area
of
vessel
type (collectively)
increases
from
aorta
->
arteries
->
capillaries
This
decreases
the
velocity
, and vice versa
What does flow depend on?
Pressure
difference (between
2
ends of
vessel
)
Resistance
of
vessel
What is the equation for blood flow?
Q
= ΔP/
R
Q
=
flow
ΔP =
pressure difference
R
=
resistance
Define total peripheral resistance.
Resistance
of the
entire systemic vasculature
(
TPR
) or the
systemic vascular resistance
(
SVR
)
Where is series resistance seen in the body?
Arrangement of
blood vessels
within a given
organ
Where is parallel resistance seen in the body?
Distribution
of
blood flow
among various
major arteries
branching off
aorta
What is Poiseuille's Law?
Resistance
is directly
proportional
to
1/r^4
-> if you
halve
the radius, resistance
increases 16
fold
This means that
small
changes in the
radius
of the
lumen
of
blood vessels
can have
significant effects
on the
resistance
of the vessels
What is the flow in the CVS?
Laminar
Describe laminar flow.
Velocity
of blood flow is
highest
in the
centre
of the vessel &
lowest towards
vessel walls
Smooth parabolic
(shape)
Describe turbulent flow.
Irregularity
occurs in
blood vessel
->
laminar stream
is
disrupted
&
blood flow
may become
turbulent
Laminar flow
is
silent
,
turbulent
flow is
audible
What are the major influences on Reynolds number?
Changes in
blood viscosity
Changes in
velocity
of
blood flow
What is the clinical significance of Reynolds number?
If Reynolds number (NR) is less than
2000
, blood flow will be
laminar
If NR is more than
3000
, blood flow will be
turbulent
What are the different vascular controls?
Local
(
metabolic
or
myogenic
)
Central
(
neural
)
Hormonal
Endothelial
What is the local metabolic control of blood flow?
Intrinsic
factors (high K+, adenosine, lactate, H+, CO2)
Increased
metabolic activity ->
increased
release of metabolic byproducts ->
increase
resistance ->
vasodilation
->
increase
blood flow -> increase supply of
O2
&
nutrients
CHECK
What is the local myogenic control of blood flow?
Vessels constrict
reflexively
when
intraluminal
pressure
increases
mediated by stretch activated
Ca
+
2
channels in smooth muscles
protects
capillaries
from
surges
in
arterial
pressure
What are hormones that can control blood flow?
ADH
Angiotensin II
ANP
Adrenaline
How does ADH impact blood flow?
Released
from
posterior pituitary
Binds
to
V1a receptors
Cause
vasoconstriction
How does ang II impact blood flow?
Acts on
AT1
receptors
Causes
vasoconstriction
How does ANP impact blood flow?
From
atrial myocytes
Binds
to
NPR1
receptor
Causes
vasodilation
How does adrenaline impact blood flow?
Released from
adrenal medulla
Binds to…
a1
adrenergic receptors ->
vasoconstriction
b2
adrenergic receptors ->
vasodilation
What is blood flow to tissues driven by?
Difference in
pressure
between the
arterial
&
venous
sides of the circulation
What is the central control of blood flow?
Sympathetic
adrengeric nerve fibres
a1
adrenergic receptors (
noradrenaline
acts) ->
vasoconstriction
->
decreases
diameter of arteriole ->
increases
resistance to blood flow
b2
adrenergic receptors (adrenaline) ->
vasodilation
->
increases
diameter ->
decreases
resistance to blood flow
What is active hyperaemia?
Normal
vasodilatory
response to
increased tissue
activity &
rising metabolic
levels
What is post-exercise hyperaemia?
A period of
increased
blood flow that
persists
even after
activity
has
ceased
What is reactive hyperaemia?
Period of
increased blood
flow that follows
transient ischaemia
What are the physiologic consequences of local control of blood flow?
Autoregulation
(
intrinsic ability
of an organ to
maintain stable blood flow
in changing
perfusion pressures
)
Hyperaemia
(
increased blood flow
)
What is the process of active hyperaemia?
Increased
tissue
metabolism -> increased release of metabolic
vasodilators
into
ECF
->
vasodilation
->
decrease
resistance -> increase
blood flow
->
O2
&
nutrient supply
to tissue (as long as metabolism is
increased
)
What is the process of reactive hyperaemia?
Decreased
tissue blood flow due to
occlusion
-> metabolic
vasodilators
accumulate in
ECF
->
dilation
of arterioles but occlusion prevents
blood flow
Remove occlusion ->
decreased
resistance ->
increase
blood flow
As vasodilators wash away, arterioles
constrict
& blood flow returns to
normal
Fill in the blanks
A)
Tyrosine
B)
L-dopa
C)
Dopamine
D)
Noradrenaline
E)
Adrenaline
5
What molecules cause contraction/constriction of blood vessels by acting on endothelium?
Endothelin
PGF
Thromboxane
What molecules cause relaxation/dilation of blood vessels by acting on endothelium?
PGI2
PGE
Trauma
NO
Toxins
from
bacteria
&
stress
Where is the vasoconstrictpr centre (C1)?
Upper medulla
,
lower pons
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