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3/19/12
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Arterialblood
pressure
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Arterial Blood PressureIt is the force exerted by the blood
against any unit area of the vesselwall. It is always measured inmillimeter of mercury (mmHg).
If the pressure in a vessel is 100mmHg, this means that the force
exerted is sufficient to push acolumn of mercury up to a level100 mm high.
varies between a maximum
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Systolic Blood Pressure
Systolic blood pressure is themaximum pressure that bloodexerts on vessels during systole
Range = 90-140 mmHg.
Average = 120 mmHg
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Diastolic blood pressuremeasures the minimum pressurein the systemic arteries during
diastole)
Range = 60-90 mmHg.
Average = 80 mmHg
Diastolic Blood Pressure
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Normal Blood Pressure
120/80
Note: it is not adivision
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Pulse Pressure
ü It is the difference betweensystolic and diastolicpressures
ü The average value is 40mmHg.
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Mean Blood Pressure
It is the averagepressure
throughout thecardiac cycle.
MAP= Pdiastole + 1/3 (Psystole -Pdiastole )
= 80 mmHg + 1/3 (40
mmHg) = mmH
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PhysiologicalVariations
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v
Correspondingarteries of both sideof body may differby 5-10mmHg
v in the femoral artery
is 10-15 mmHghigher than in thebrachial and 15-20mmHg higher thanin the popliteal
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Age
At birth the ABP isvery low (50/30mmHg).
Thereafter, it rapidlyrises during the firstfew weeks to 90/60
mmHg.
The pressure
increases gradually
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Sex
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Exercise
• Moderateexercise
→
mild in systolic↑pr.
→ little or no changein diastolic pr.
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Exercise
• Severe exercise
→ severe in systolic↑pr.
→ ↓ in diastolic pr.
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Emotions
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Build
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Meals
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Sleep
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Pressures in the systemic
circulation•
The decrease in thearterial pressure atthe end of each part
of the systemiccirculation is directly
proportional to thevascular resistance
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Aorta(93 mmHg)
Largearteries (90-93 mm Hg)
v. Smallarteries (85mm Hg)
largeveins (8-10mm Hg)
Venularend (15mm Hg)
Arteriolarend (35
mm Hg)
Rightatrium (O-2 mm Hg)
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Pressures in the systemic
circulation
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Blood pressure =COP X total peripheral
resistance
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1- cardiac output
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1- cardiac output
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it is the resistance which the
blood has to overcome while
passing through the peripheralvessels
Affected by
1. Diameter of vessels
2. Viscosity
2- Peripheral resistance
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4r
8VL
(r)Radius
(L)Length(V)ViscosityαR
4 π
=×
(r 4)
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a)Diameter of vessels
• The resistance is inversely proportional to
the fourth power of the radius of the vessel1
•
R α
R 4
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a)Diameter of vessels
diameter resistance
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a)Diameter of vessels
• The peripheralresistance in thesystemic circulation
is chiefly in thearterioles becausearterioles are narrowvessels connected in
series, while thecapillaries form anetwork of vessels
connected in parallel
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b) Blood Viscosity
Depends on:
• hematocrit
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The the length
of the vessel, theis the
resistance due tofriction betweenthe blood and
vessel walls
c) Length of the vessel
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Increase in theperipheral resistance
raises the diastolicpressure
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• Systole→→→ Distend
• Diastole→→→ Recoil
• Prevents a greatrise in the systolicpressure and agreat drop in thediastolic pressure.
• transfer the
3-Elasticity of aorta and arteries
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Arteriosclerosis
• The elastic and muscular tissuesare replaced by fibrous tissue).
• Accumulation of plaque in thearteries
• Flow of blood faces more resistance
• Blood pressure is altered.HOW???
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Arteriosclerosisreduces the elasticity
of the circulatorysystem Therefore, thesystolic pressure ismuch increased,
while the diastolicpressure is slightly
reduced (high
pulse pressure )↑↑Systolic bloodpressure
↓ diastolic pressure
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With advancedarteriosclerosis, more
cholesterol isdeposited in thearterioles narrowingtheir diameter.
Therefore, thediastolic pressure increased because of
the increased in theperipheral resistance..↑↑Systolic bloodpressure
↑↑ diastolic pressure
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4- blood volume
decrease in the bloodvolume decreases both
the systolic and diastolic
pressure
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Ø
The systemic circulation has manyregulatory mechanisms whichmaintain the normal mean arterialpressure between 90 mmHg and 110mmHg
Control of the arterial blood
pressure
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Nervous pressurecontrol
mechanismsbegin to reactwithin seconds
and become fullyactive within a
minute
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Intermediately actingpressurecontrol
mechanisms
come intoplay withinminutes,
then becomefully active
within 30minutes to
severalhours.
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(high pressure
receptors)
carotid sinus and
aortic archreceptors
1- arterial Baroreceptors
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B a r o r
e c e p t o
r s
Stimulus
The baroreceptors arestimulated by distentionof the structures in whichthey are located, and so
they discharge at anincreased rate when thepressure in thesestructures rises
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B a r o r
e c e p t o
r s
The threshold for eliciting activity inthe carotid sinus nerve is about 50
mm Hg; maximal activity occurs atabout 200 mm Hg.
At normal blood pressure levels (about100 mm Hg mean pressure), someactivity in the baroreceptors(responsible for the vagal tone)
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MECHANISM OF baroreceptorREFLEX
PRESSURE BETWEEN
0-50mmHg
NOTSTIMULATED
AT ALL
>50
mmHg
RESPOND MORE ANDMORE UNTIL REACHMAX AT 200mmHg
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Baroreceptors
Baroreceptors are more sensitive to
variable pressure than to constantpressure. they respond much more toa rapidly changing pressure thanto a stationary pressure.
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B a r o r
e c e p t o
r s
Afferent (buffer
nerves) The afferent nerve fibers fromthe carotid sinus form adistinct branch of theglossopharyngeal nerve, thecarotid sinus nerve(Herings nerve). The fibersfrom the aortic arch form abranch of the vagus nerve,
the aortic depressor nerve.
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B a r o r
e c e p t o
r s
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Arterial Baroreflex Responses
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Arterial Baroreflex Responses
Arterialbaroreceptor
activityCardiovascular centers in
medulla
Sympathetic
nerveactivity
Vagalnerve
activity
Vascul
Strok
evolum
e
Hear
trate
Cardia
Arterial pressureAffere
ntarm
Efferentarm
Functions of baroreceptors
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To stabilize blood pressure andheart rate if they increases ordecreases above or below theirnormal levels.
To reduce the daily variation inthe arterial pressure (from
moment to moment), e.g. changesin body posture
They also have an important role in
Functions of baroreceptors
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B t
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Baroreceptors
Baroreceptors resettingBaroreceptors are of NO importancein long-term regulation of arterial
pressure. They adapt in one to twodays to whatever pressure level theyare exposed to i.e. they stop theirdischarge despite the presence of the
stimulus because they get used to it;thus they lose their responsiveness.
B t
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Baroreceptors
Baroreceptors resetting
In chronic hypertension, the
baroreceptor reflex mechanism is"reset" to maintain an elevated ratherthan a normal blood pressure.
3- Arterial Chemoreceptors
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p
Arterial Chemoreceptor Reflex
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Arterial Chemoreceptor Reflex
Chemorecept or
afferent
nerves
Stimulus:Changes
inarterial
P O2,
PCO2, pH, BP<80mmHg
Response: changes in pulmonary
ventilation & vascular
Brainstemrespiratory
&vasomotor centers
Sympatheti c
nerves Mot or neuron
s
Arterialchemoreceptor
activity
Vasoconstriction↑ heart rate
BPback to
normal
Ventilati
on
BP<80mmHg
++
Respirator y
centers
+
+Cardiovascular
centers
é heart rate
Ch t
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Chemoreceptors
This reflex helps to return the arterialpressure back toward the normal
level whenever it falls too low.However, it is not a powerful arterial
pressure controller in a normal
arterial pressure range, because itdoes not respond strongly untilthe arterial pressure falls below
80 mmHg.
2- Atrial stretch receptors
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p
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Bainbridge Reflex
Atrial Stretch Receptors
Medullary cardiovascular centersActivation
IncreasedHeart Rate
andcontractility
IncreasedSympatheticActivity to SA
Node
Increased IntravascularVolume
DirectStretchingof SA Node
I d bl d
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Increased bloodvolume causing atrial
stretch
Atrial Stretchreceptors stimulation
• Decreasedperipheral resistance• Increased venous
capacity• VD of the afferentrenal arterioles →increasedUrine Output
Generalizedvasodilatation
• Increased UrineOutput• Decreased Water
and Na+ Reabsorption• vasodilatation
DecreasedVasopressin
IncreasedAtrial Natriuretic
Peptide
• Increased
Natriuresis• Increased urineOutput• vasodilation
Decreased blood volume and blood pressure
Atrial reflexes
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Atrial reflexes
Atrial reflexes are pressure
controllers as well as volumecontrollers.
Central ischemic response
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Central ischemic response
4- Central ischemic response
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4 Central ischemic response
The CIR is stimulated by cerebral ischemia.Blood pressures bellow 50-60 mmHg will
initiate this response.The CIR is extremely powerful; it can
elevate blood pressure to 200 mmHg.Blood flow through peripheral organs is
stopped or greatly reduced as a result of
generalized vasoconstriction. (exceptbrain ,heart and lungs)
The CIR maintains brain blood flow whenarterial pressure falls to critically low levels.
4- Central ischemic response
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4 Central ischemic response
Despite the extremely powerful nature of thecentral ischemic response, it does not becomevery active until the arterial pressure falls farbelow normal, down to levels below 50 mmHg,reaching its greatest degree of stimulation at apressure of 15 to 20 mmHg.
Therefore, it is not one of themechanisms for regulatingnormal arterial pressure.
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Intermediately actingpressurecontrol
mechanisms
come intoplay withinminutes,
then becomefully active
within 30minutes toseveralhours.
Epinephrine
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Sympatheticstimulation
Ø These hormones can reach some parts of the circulation that haveno sympathetic nervous supply at all, including the very minutevessels such as the metarterioles.
Ø
They have especially potent actions on some vascular beds,
EpinephrineNorepinephrine
Renin-Ang II system
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Increased
BP
Decreased extracellular fluid volume
Renin-Ang II system
Renin-Ang II system
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• It requires approximately 20 minutesbecoming fully active.
• However, it has a correspondingly
longer duration of action.
Renin-Ang II system
Vasopressin
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stimulation of atrial B receptorsis reduced (e.g.hypovolemia).
Ang II
(ADH)
Vasopressin
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• The vasopressin (ADH) system wasshown to return the blood pressureabout 75% of the way back toward
normal within a few minutes afteracute hemorrhage has decreased thearterial pressure to as low as50mm.Hg.
• It is the most potent vasoconstrictor.
(ADH)
The intrinsic mechanism
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The intrinsic mechanism
Capillary fluid shift
Long term Renal-body
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gregulation
of bloodpressure
HOW KIDNEYREGULATS
ABP???
yfluid-
pressurecontrolmechanismHormonal
controlmechanism
(vasopressin,AngII and
aldosterone)erythropoiesi
s
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The special value of thismechanism is that it will notstop increasing the blood
volume until the pressure rises
B l o o d S u p p l y
• A p p r o x im a t e l y o n e-f o u r th ( 1 2 0 0 m l ) o f s y s t e m i c c a r d i a co u t p u t f lo w s t h r o u g ht h e k i d n e y s e a c hm i n u t e
• A r t e r i a l fl o w i n t oa n d v e n o u s f l o w o u to f t h e k i d n e y s f o l l o ws i m i l a r p a t h s
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Thank you for yourattention!
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Bloodpressuresand the
vascularsystem
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Capillary Network:
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p y
• Blood flows from
arterioles throughmetarterioles, thenthrough capillarynetwork
• Venules drainnetwork
• Smooth muscle inarterioles,metarterioles,precapillarysphincters
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Cross-Sectional Area
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Cross Sectional Area
• As diameter of vesselsdecreases, thetotal cross-
sectional areaincreases andvelocity of bloodflow decreases
• Much like astream that flowsrapidly through anarrow gorge but
flows slowl
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Blood Vessel Comparison:
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p
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