Blood Pressure and Vascular System

8/14/2019 Blood Pressure and Vascular System

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DR Sherwan Shal

May, 2007

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Vessel internal Diameter

Wall thickness

Elasticity

amount of Elastin-stretchability Contractility

amount of Smooth Muscle

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Artery -- much elastin & smooth muscle

can expand and recoil Arteriole-- smooth muscle, no elastin less pressure related expansionCapillary - single layer of endothelial cells

allows exchange with interstitial fluidVenule -- no elastin or smooth muscle,

but has venous valvesVein --elastin & smooth muscle

affect venous return to heart

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= force of blood against vessel wall Depends on:

volume of blood

compliance (stretchability) of vessel

resistance of vessel to flow BP would be constant if blood volume enterin= blood volume leaving arteries BUT it isnt

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Arteries = transport from heart to bodywalls with some muscle & much elastin

aorta & other arteries like balloon--> mapressure head till next ventricular contr

Elastic recoil maintains pressure fluctucoming from ventricles

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Stroke Volume enters artery in Systole.

Only 1/3 of this leaves artery --> arterioles During Diastole

no more blood enters arteries but blood continues to leave due to artery elastic recoil

Pulse Pressure =Systolic Pressure - Diastolic Pressure

120 mmHg - 80 mmHg = 40 mmHgfelt as pulse indicative of changes in art

volume

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Average pressure driving blood flow closer to diastolic pressure, not midpoindiastole is longer (at rest)

MAP = Diastolic Pr. + 1/3 Pulse Pr.

e.g. MAP = 80 mmHg + 13.3 = 93

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Due to their compliance, arteries have loresistance (recall the elastin?)

thus all arterial pressures ( Systolic, DiaPulse & MAP) remain constant thru arte

tree Thus,Arterial Blood Pressure useful in progood indicator of CV health

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Flow = Pressure Gradient / Vascular Resista

flow is directly proportional to pressure g

change in pressure along vessel

directly proportional to vessel diameter flow is inversely proportional to vascular r

vascular resistance = friction

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Vessel Radius (r)Vessel Radius (r)

flow varies directly as 4th power of ra

Vessel Length (L)Vessel Length (L)

longer vessel, more frictional surface a

Blood Viscosity (n)Blood Viscosity (n) frictional properties of cell or molecul

surfaces as they slide over each other

e.g. polycythemia/blood doping/proteins

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Blood Flow = r4P / 8nLr = vessel radius

P = pressure

n = viscosity

L = vessel length ( doesnt cha

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Arterioles flow thru target tissues Decrease diameter--> Increase resistanc

resistance in arteriole Increase BP; this InBP maintains flow direction

pulsatile waves disappear--> BP steady arterioles

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Compared with arteries, arterioles haveelastin & more smooth muscle.

Resistance accomplished by effects on SMuscle

smooth muscle relaxed = vasodilation (mblood flow)

smooth m contracted = vasoconstriction blood flow)

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accomplished by changing vessel diam

Arterioles responsible for over 60% of thresistance to flow of the whole circulatosystem

called Total Peripheral Resistance

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Two types of control of vascular tone

1. Intrinsic-

locally produced substances/conditions

Chemical or Physical

2. Extrinsic- blood-borne regulatory substances not

released locally

Neural & Hormonal

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1. Metabolic Products resulting from local metab

active tissue drop in O2 rise in CO2, H+, lactic & carbonic acid rise in K+ (due to incr. APs) rise in adenosine (dilates coronary arteries)

increased prostaglandins change in osmolarity

ALL these indicate need for more O2ALL these indicate need for more O2

----> VASODILATION----> VASODILATION

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Vessel endothelial cells secrete regulator substa

(vasoactive chemical mediators) Nitric Oxide - relaxes smooth m . EDRF = End

Derived Relaxing Factor Endothelin -most potent vasoconstrictor, smo

contraction

other Growth Factors cause angiogenesis (proof new vessels)

Histamine- relaxes sm. M., but really part ofinflammatory response

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Heat or Cold heat vasodilates, cold vasoconstricts

environmental or therapeutic

Myogenic response to stretch causes sm

contraction, results from stretch-sensitivchannels--> contraction

resists change occurring w/stretch

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SNS most important

SNS causes arteriole contraction

arterioles always under some SNS effect sympathetic tone

thus, can increase or decrease SNS activichange arteriole radius

Neural (SNS) & Hormonal

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Rearrange to:Rearrange to:

Pressure = Flow Rate X Resistance Pressure = Flow Rate X ResistanceMean Arterial Pressure = CO X ResistaMean Arterial Pressure = CO X Resista

Result:Result:1. SNS arterial stimulation increases1. SNS arterial stimulation increases

resistanceresistance2. downstream (arteriole--> capillary) blood2. downstream (arteriole--> capillary) blood

flow to the tissue is decreasedflow to the tissue is decreased

3. upstream (arterial) pressure is elevating3. upstream (arterial) pressure is elevating

E.g. dam in riverE.g. dam in river

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Thus SNS maintains the mean driving pressu(MAP) to all organs (except brain) , but indivorgans control local flow

intrinsic vasodilation factors can locally overeffect

Skeletal & Cardiac Mmost sensitive to local factors greatest need for dramatic changes in blood supto activity and buildup of metabolic products

SNS effects on heart itself increase both HR a(increased venous return)-->

Thus, heart overcomes weaker SNS vasoconseffects

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Little PNS innervation of vessels penile & clitoral blood flow controPNS

Require rapid & diffuse vasodilatioerection

mechanism via NO synthesis & rel

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Adrenal Medulla releases Epinephrine durin

STRESS Epi binds to both and -2 adrenergic

binding reinforces SNS vasoconstriction

But -2 adrenergic receptors --> vasodilati

B-2 receptors ONLY found in smooth muscl Heart, Skeletal muscle & liver

facilitates ability to mobilise energy during(flight or fight) response

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Vasopressin (hypothalamic)

Angiotensin II (kidney)

both vasoconstrictors

impt in regulation of water & salt balanc

controlling BP

impt during hemorrhage in trying to maplasma volume & osmolarity

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Total CO can remain constant, but % CO delieach tissue varies

Flow to tissue regulated at 2 points

3. Arterioles

Controlled through Intrinsic (Local) & Extrinsic 4. Capillaries

Controlled by cuffs of smooth muscle called presphincters

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In skin -shunts to cut flow thru cap beds (arteri

anastomoses) some capillary beds connected by Metarteriolearteriole & venule

these assted with spiraling smooth muscle callePrecapillary sphincters

control opening to bed at rest only 10% PC sphincters open flow through cap bed depends on metabolic a

of tissue

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Blood Pressure and Vascular System

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