Blood pressure Cross sectional area Velocity of flow and Blood volume

1
Blood pressureCross sectional areaVelocity of
flowandBlood volume for the vessel types
2
All vessels act as conduits to move blood from
the heart out to the peripheral
organs. Especially, the large arteries and veins
3
Blood pressure is highest in the aorta, and
monotonically falls to near zero in the vena
cava.
4
Pressure is pulsatile in the arteries but
becomes damped in the microcirculation and veins
5
?P
R ?P/F
The biggest pressure drop is across the
arterioles. IF ?P is large then the resistance
must be large
6
Arterioles control blood flow
?P
R ?P/F
Because arterioles have a large resistance and
can change their resistance, they act as control
valves to control flow
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Capillaries are the site of exchange
Surface area is highest in capillaries which
promotes exchange of nutrients and waste products
with tissue.
8
The very thin wall (only one endothelial cell
thick) helps exchange.
9
Velocity is low in the capillaries. That allows
blood to stay in the capillary long enough to
equilibrate with the tissues by diffusion.
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Velocity is highest in the aorta.
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Volume is highest in veins causing them to act as
reservoirs.
12
Smooth muscle in the small veins and venules can
contract and squeeze blood out of the venous
reservoir raising the venous filling pressure in
the heart and thus the cardiac output
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The properties of the various vessel types in the
cardiovascular system. A. distribution of
pressure (highest in the aorta and lowest in the
veins) B. distribution of velocity (highest in
the root of the aorta lowest in capillaries) C.
distribution of surface area (highest in
capillaries which aids exchange) D. distribution
of resistance (highest in arteriole which is
important for blood flow control) E. distribution
of blood volume highest in veins (making them
act as reservoirs).
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The aorta and large arteries act as energy
storage devices
The aorta is compliant and its pressure is a
function of its volume. Compliance ?volume /
?pressure
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The heart pumps in short spurts. The compliant
aorta stores this energy during ejection and
releases it during diastole so that flow into the
periphery continues throughout the cardiac cycle
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The bagpipe player blows into the bag in short
spurts. That energy is stored in the bag and the
air escapes through the pipes in a continuous
stream thanks to the bag's compliance.
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If the vessels were rigid pipes then all forward
flow would have to occur during the ejection
period which is only about 1/3 of the cardiac
cycle. Blood pressure would have to be 3 times
higher during that period to maintain the cardiac
output.
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Why is aortic pressure pulsatile?
With each ejection the aortic volume increases by
one stroke volume. To maintain a steady state
one stroke volume of blood must leave the aorta
before the next beat.
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If aortic compliance were to decrease, pulse
pressure will increase.
20
Pulse pressure stroke volume / compliance
21
Aging reduces aortic compliance
120/80 (40)
130/80 (50)
150/80 (70)
22
Blood pressure is measured by listening for
sounds made when the cuff pressure is between
systolic and diastolic pressure. The opening and
closing causes turbulence when the arterys
lumen is very narrow (just as it opens or as it
closes).
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The mean aortic pressure is determined only by
the cardiac output and the peripheral
resistance. aortic pressure CO x peripheral
resistance Peripheral resistance does not
affect the pulse pressure pulse pressure
SV/compliance
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Intrinsic Regulation
Autoregulation matches flow to requirements

• Intrinsic to the organs
• Does not require nerves
• May use adenosine

Set Point
Heart, brain, kidney, muscle, intestine
autoregulate Skin does not
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Intrinsic Regulation
Other aspects of intrinsic regulation 1. Active
hyperemia 2. Reactive hyperemia
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Extrinsic Regulation
Sympathetic nerves constrict blood vessels
(epinephrine and norepinephrine)

• The CNS uses sympathetic nerves to constrict the
  periphery and raise blood pressure.
• The sympathetic innervation is not paired with
  Parasympathetic nerves. To dilate the periphery
  the sympathetics simply withdraw.
• Sympathetic nerve are always active (tone).

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Gs coupled receptors are vasodilators
Receptors on the blood vessel that are coupled
with Gs stimulate adenylyl cyclase, increase
cAMP, and relax the vessel. cAMP increases Ca
but also inhibits myosin light chain kinase which
controls smooth muscle contraction.
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Gq coupled receptors are constrictors
Receptors coupled with Gq activate phospholipase
C (PL-C) causing the formation of inositol
triphosphate (IP3) from phosphatidylinositol
(PIP2).  The IP3 then stimulates the sarcoplasmic
reticulum (SR) to release calcium. The formation
of diacylglycerol (DAG) activates protein kinase
C (PK-C) further contributes to vascular smooth
muscle contraction via protein phosphorylation.
29
Nitric Oxide is a locally produced dilator.
Acetylcholine is a direct constrictor of VSM but
in the body it is actually a dilator because it
stimulates NO production by eNOS in the
endothelium. NO overrides the constrictor
activity of the acetylcholine and dilates the
vessel.
Before NO was identified it was referred to as
Endothelial Derived Relaxing Factor or EDRF
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Circulating vasconconstrictors include
angiotensin, endothelin, vasopressen and
catecholamines.
Targets for anti-hypertensive drugs
31
There are no widespread neuro dilators. In many
cases an active hyperemia follows neuronal
activation of an organ by a substance that also
happens to be a dilator e.g. acetylcholine in the
gut or bradykinin in salivary glands. The primary
effect was to stimulate the organ and the
dilation that occurred was an active hyperemia.
32
capillary
Pc
pc
pt
Pt
Hydrostatic pressure (Pc) forces water out of the
capillary and that is balanced by osmotic pull of
plasma proteins (albumin) that pulls water back
in. The osmotic pressure (pc) of plasma in the
capillaries is 25 mmHg and that of interstitial
fluid (pt) is near zero. Fluid is filtered at
the arterial end and reabsorbed at the venous end
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capillary
capillary
Pc
pc
pt
Pt
The four forces controlling fluid balance at the
capillary are called the Starling Forces after
Ernest Starling
Jv Kfc ( Pc pt Pt pc)
Kfc fluid filtration coefficient (tells how
fast fluid flows for any pressure gradient)
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capillary
Raising capillary pressure causes edema
(swelling).
35
capillary
Loss of plasma proteins due to starvation causes
edema (swelling).