blood pressure regulation mechanisms

8/9/2019 Blood pressure regulation mechanisms

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Blood Pressure


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Blood Pressure

` Pressure in the reservoir is easy

` Since there is no net flow, the force on all sides of the

reservoir is equal. Thus, measuring pressure anywhere

measures pressure.


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Blood Pressure

` Pressure in a pipe is hard

` Since there is net flow, there are fewer particles hitting the

inflow area than the wall and more particles hitting the

outflow area than the wall.


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Blood Pressure

` There are two types of pressure:

` Static Pressure` Pressure from the blood distending the vessel against the

vascular smooth muscle

` LaPlace: T=Pr (tension, pressure, radius)


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Blood Pressure

` There are two types of pressure

` Dynamic Pressure` Pressure from the movement of particles along the blood

stream

` Pitot: P=Vv2/2 (density, velocity)


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Blood Pressure

in a tube or a blood vessel the total energythe sum of the kinetic energy of flow and thepotential energyis constant (Bernoulli's principle).

According to the principle, the greater the velocity of flow in a vessel, the lower the lateral

pressure distending its walls. When a vessel is narrowed, the velocity of flow in the narrowed

portion increases and in the distending pressure decreases.Therefore, when a vessel is narrowed

by a pathologic process such as an atherosclerotic plaque, the lateral pressure at the constriction

is decreased and the narrowing tends to maintain itself.


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Bernoulli Pressure Lowering

The linear drop in fluid pressure is according to Poiseuille's law, but the constriction produces

an extra drop in pressure according to the BernoulliPrinciple

The liquid column height is a measure of the fluid pressure at that point in the flow

tube. The vertical tubes act as manometers. The manometers show that the pressure

is lowered at the constriction relative to what it would have been in a uniform tube.

The pressure that drives the fluid through the tube is the static fluid pressure at the

bottom of the reservoir. The resistance to flow represented by the tube causes a drop

in pressure as you proceed along the tube.


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Blood Pressure

` TheTotal Pressure is the sum of the static and

dynamic pressures.

` This is much like Total Energy is the sum of the kinetic

and potential energies.


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Blood Pressure

` From a practical standpoint

` THE blood pressure is what we measure if we stick a

catheter into the lumen of a vessel and measure the

outflow pressure.


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If a cannula is inserted into an artery, the arterial pressure can be measureddirectly with a mercury manometer


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Arterial Blood Pressure (BP)

= The lateral pressure force generated by the pumping

action of the heart on the wall of aorta & arterial blood

vessels per unit area.

Measured in (mmHg), & sometimes in (cmH2O), where

1 mmHg = 1.36 cmH2O.

Of 2 components:

` systolic (= max press reached) = 110-130 mmHg.` diastolic (= min press reached) = 70-90 mmHg.

In normal adult } 120/80 mmHg.


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Arterial Blood Pressure (continued)

Diastolic pressure is more important, because diastolic

period is longer than the systolic period in the cardiac

cycle.

Pulse pressure = Systolic BP Diastolic BP.

Mean arterial pressure = Diastolic BP + 1/3 Pulse press.

In normal adult } 120/80 mmHg.


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Blood Pressure (BP): Measurements

Figure 15-7: Measurement of arterial blood pressure


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Factors Controlling MAP :

The Driving Pressure for Blood Flow

Figure 15-10: Factors that influence mean arterial pressure


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Factors affecting ABP:

Sex M > F due to hormones/ equal at menopause.

Age Elderly > children due to atherosclerosis.

Em

otionso due to secretion of adrenaline &

noradrenaline.

Exercise o due to o venous return.

Hormones o (e.g.Adrenaline, noradrenaline, thyroid H).

Gravity o Lower limbs > upper limbs. Race Orientals > Westerns ? dietry factors, or weather.

Sleep q due to q venous return.

Pregnancy o due to o metabolism.


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Effects ofgravity on arterial and venous pressures.

Each cm ofdistance produces a 0.77 mmHg change.

Sphincters protectcapillaries

VENOUS PUMP keeps PV

< 25 mmHg

Veins Arteries

190mmHg

100mmHg0


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Factors determining ABP:

Blood Pressure = Cardiac Output X Peripheral

Resistance

(BP) (CO)Flow

(PR)Diameter of

arterioles

BP depends on:

1. Cardiac output CO = SV X HR.2. Peripheral resistance.

3. Blood volume.


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Regulation of Arterial Blood Pressure


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Regulation of ABP:

Maintaining B.P. is important to ensure a steady blood

flow (perfusion) to tissues.

B.P. is regulated neurally through centers in medulla

oblongata:

1.Vasomotor Center (V.M.C.), or (pressor area):

Sym

patheticf

ibers.2. Cardiac Inhibitory Center (C.I.C.), or (depressor area):

Parasympathetic fibers(vagus).


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Regulation of ABP (continued)cardiac control centers in medulla

oblongata

1. Cardiacacceleratorcenter(V.M.C)

2. Cardiacinhibitorycenter(C.I.C)

Sympathetic n. fibers Parasympathetic n. fibers Regulatory mechanisms depend on:

a. Fast acting reflexes:

Concerned by controlling CO (SV, HR), & PR.

b. Long-term mechanism:

Concerned mainly by regulat

ing the blood volume.


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Regulation of Blood Pressure

Mechanisms for controlling MAP

(a) Rapid/Short term BP Control/Nervous

Mechanisms

(b) Intermediate Bp Control Mechanisms

(c) Long Term BP Control mechanisms


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Rapid / Short term / Nervous

Mechanisms for controlling BP

Charactristics -` Act rapidly - within secs. to few mts.

` Lasts forfew hrs. to few days

` Prevents - sudden rise or fall in BP

` Operates through -

(a) Baroreceptors

(b) Chemoreceptors

(c) CNS Ischemic Response


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Baroreceptors

` Def. - Stretch receptors,mechanoreceptor` Present in - walls of heart & large blood vesels

` Structure - highly branched myelinated, knobby nerve

endings

` Stimulated by stretching of art. wall

High BP stretching discharge freq.


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Types of Baroreceptors

1.High Pressure baroreceptors Located at - carotid sinus Imp.

- aortic arch baroreceptors

- wall of LV

- root of Rt. SubclavianA.

2. Low Pressure baroreceptors

Located at - RA, LA,

- Entrance of SVC,IVC,- Pulm. trunk, pulm.A. & Veins


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1. Baroreceptors reflex:

Baroreceptors are receptors found in carotid sinus &

aortic arch.

Are stimulated by changes in BP.

o BP

+ Baroreceptors

= V.M.C ++ C.I.C

= Sympathetic

Vasodilatation & q TPR

+ Parasympathetic

Slowing of SA node (q HR)& q CO


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Baroreceptor Reflex

` Normally Baroreceptors discharge at low rate` Discharge rate - at high BP & at low BP

` Below 60 mm Hg no discharge at all

` Above 160 mm Hg no further rise in discharge

` i.e. baroreceptors are sensitive in the range of 60 -

160 mm Hg.

` Maximally sensitive at MAP 95 mm Hg.

` Respond more to rapidly changing BP than to astationary high or low levels of BP.


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Baroreceptor Reflex

` Baroreceptor resetting Baroreceptors resetthemselves in 1-2 days, to whatever they are exposed.

` So they have no role in long term regulation of BP

(only in short term control)

` Applied Carotid sinus massage to reduce HR in PAT

(vagally mediated slowing of HR)


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Baroreceptor Reflex

Atrial stretch receptors & pulm. Baroreceptors` Present in atria, pulm trunk & its divisions

` Imp. Role in reducing arterial pr. Changes due tochanges in blood vol.

all receptors art. Baroreceptors all receptorsintact denervated, denervated

atrial receptors intact

BV BP BP BP

atrial receptors make total reflex system much more potent for controlof MAP.


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Baroreceptor Reflex

Atrial stretch receptors & pulm. Baroreceptorsmechanism

BV venous return discharge from atrialreceptors

BP


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MOTOR CORTEX

HYPOTHALAMUS

VASOMO

TOR

CEN

TER

PRESSORAREA

DEPRESSORAREA

CARDIOINHIBITORYAREA

Vagus

HEART

Arterioles

VeinsAdrenalMedulla

BaroreceptorsCarotid SinusAorticArch

ChemoreceptorsCarotid BodiesAortic Bodies

Bainbridge Reflex (o Heart Rate)Atrial Receptors Volume Reflex (o Urinary OUTPUT)

a.qVascularSympathetic Toneb.qADHSecretionc. q Aldosterone Secretion

ChemosensitiveArea

GlossopharyngealNerve

SympatheticNervousSystem


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Chemoreceptor Reflex

` Respond to - Po2, Pco2 , H+ conc.` Present in Carotid andAortic Bodies.

` N.SupplySinus &Aortic N.

` Concernedmainly withResp.Regulation

` Discharge at low freq.in Normal person.

` No role in BP regulation if>60 mm.Hg.


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Role ofChemoreceptors

` In Hypoxia & Hypotension(Hemorrhage)Chemoreceptors stimulated (ifBP


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2.Chemoreceptors reflex:

Chemoreceptors are receptors found in carotid &aortic bodies.

Are stimulated by chemical changes in blood mainlyhypoxia (q O2), hypercapnia (o CO2), & pH changes.

q BP

+ Chemoreceptors++ V.M.C = C.I.C

+ Sympathetic

Vasoconstriction

& o TPR

= Parasympathetic

o HR

Haemorrhage

Hypoxia

+ Adrenalmedulla


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CNS Ischemic Response

Seen i

fBP

falls


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Intermdiate BP Control Mechanisms

CHARACERSTICS` Becomes active after several minutes

` Remain active forfew days to few

weeks` By altering Blood Volume they control

BP

St R l ti


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Stress Relaxation

ReverseStress

Relaxation Fluid load BP BP

S

tretching of

vessels

tone of smooth m. tone of smooth m.

Reflex in tone Reflex in tone

BP BP

Capillary Fluid Shift


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CapillaryFluid Shift

Mechanism

BP BP

mean cap. Pressure mean cap. Pressure

Fluid enter into IS Comp. Fluid enter into vessels

BV BV

BP BP

2 times more effective than barorecepters but slow.


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Regulation of Blood Volume:

A long-term regulatory mechanism.

Mainly renal:

1. Renin-Angiotensin System.

2. Anti-diuretic hormone (ADH), or

vasopressin.

3. Low-pressure volume receptors.


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1.Renin-Angiotensin System:

Most important mechanism for Na+ retention in

order to maintain the blood volume.

Any drop of renal blood flow &/or q Na+, willstimulate volume receptors found in juxtaglomerular

apparatus of the kidneys to secrete Renin which will

act on theA

ngiotensinS

ystem

leading toproduction

ofaldosterone.


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Renin Angiotensin Vasoconstrictor

Mechanism

` Main function(i) Control ofBP

(ii)Regulation ofECFVolume

Renin Secreted from JG Cells

Stimulus Low BPFunction convertATG toAT-I

ACE

AT-I AT-II (in lungs endo cells)


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Renin

Aldosterone

Adrenalcortex

Corticosterone

Angiotensinogen

(Lungs)

q renal blood flow &/or q Na+

++ Juxtaglomerular apparatus of kidneys(considered volume receptors)

Angiotensin I

Convertingenzymes

Angiotensin II

(powerfulvasoconstrictor)

Angiotensin III

(powerfulvasoconstrictor)

Renin-Angiotensin System:

N.B. Aldosterone is the main regulator ofNa+ retention.


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Functions of Angiotensin-II

` Vasoconstriction BP

` Na+ & Water retention by Kidney BP

` Stimulate thirst BV BP


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Functions of Angiotensin-II


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2. Anti-diuretic hormone (ADH), or

vasopressin: Hypovolemia & dehydration will stimulate the

osmoreceptors in the hypothalamus, which will lead

to release ofADH from posterior pituitary gland.

ADH will cause water reabsorption at kidney

tubules.


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3.

Low-pressure volume receptors:

Atrial natriuritic peptide (ANP) hormone, is secreted

from the wall of right atrium to regulate Na+

excretionin order to maintain blood volume.


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3. Other Vasomotor Reflexes:

1. Atrial stretch receptor reflex:

oVenous Return ++ atrial stretch receptors

reflex vasodilatation & q BP.2. Thermoreceptors: (in skin/or hypothalamus)

` Exposure to heat vasodilatation.

` Exposure to cold vasoconstriction.

3. Pulmonary receptors:

Lung inflation vasoconstriction.


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blood pressure regulation mechanisms

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