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DR.Saad Kleman Abd
The circulatory System
BLOOD PRESSURE
Blood pressure: pressure generated by the ejection of blood into the vasculature by
ventricular contraction.
Pressure gradient between the left ventricle and right atrium provide the driving force
for blood flow.
MAP = CO x TPR
CO
TPR
MAP = DP + 1/3 (SP-DP)
DP = diastolic pressure  80 mm Hg
SP= systolic pressure  120 mm Hg
(SP-DP) = pulse pressure  40 mm Hg
MAP = 80 mm Hg + 1/3 (40 mm Hg)
= 93 mm Hg
Factors determining blood pressure :
In order to have BP there must be a cardiac out put and a resistance to blood flow in the
systemic circulation.
BP = cardiac out * peripheral resistance
But at rest cardiac out put is constant and as a result the BP will determine principally by
the peripheral resistance, which lies mainly in the small arteries of
the body (arterioles) which is the greatest resistance to blood flow.
The resistance of flow depends upon;
 Diameter of blood vessels, especially arterioles.
 Partially to the blood viscosity (hematocrit).
Vascular resistance affected by autonomic nervous system,
adrenalin
&
angiotensin
cause vasoconstriction
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DR.Saad Kleman Abd
The circulatory System
(they are called vasopressors).
The rennin-angiotensin-aldosterone mechanism is concerned with the long –term regulation
of BP
The capillaries are even smaller vessels than arterioles
but although each individual capillary will offer
a higher resistance than arterioles.
There is a large number of capillaries in parallel
supplied by each arteriole, as a result there is a large alternative pathways for the blood to
take in to passage from an arteriole to the veins. So capillary net work does not offer such
resistance to blood flow as arterioles
BLOOD PRESSURE DISTRIBUTION THROUGHOUT SYSTEMIC CIRCULATION
o Left ventricular pressure
fluctuates between 0-120 mm Hg
o ABP fluctuates between 120-80
mm Hg during diastole.
o ABP does not fall to 0 mm Hg
during diastole because of elastic
recoil
o ABP remains relatively constant
throughout the arterial system
o ABP drops suddenly in the
arterioles due to the high
resistance
o In the arterioles the systolic-todiastolic pulse is converted to non-pulsatile pressure
o Pressure continues to decline throughout the capillaries and venous system.
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DR.Saad Kleman Abd
The circulatory System
Blood Pressure Regulation
Pgs 510-512, Silverthorn, 3rd ed
MAP = CO x TPR
Dynamics of blood flow:
Blood flow through blood vessels which are in the form of closed system due to:
1- The pumping action of the heart.
2- Recoil of the walls of the arteries.
3- Compression of the veins by skeletal muscle during exercise.
4- Negative pressure in the thorax during inspiration.
The blood flow to each tissue is regulated by locals chemical and general neural mechanism
that dilates or constricts the vessel’s of the tissues.
Anatomic considerations of blood vessels:
The wall of aorta and large arteries contain large mount of elastic tissue, they stretched
during diastole.
While the wall of arteriole contain smooth muscle which innervated by noradrenergic nerve
fiber leading to constriction.
There are also some cholinergic nerve fibers which cause dilation.
The arterioles are the major site of resistance of blood flow, and small change in
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DR.Saad Kleman Abd
The circulatory System
their diameter causes large changes in the peripheral resistance.
The arterioles attached to capillaries and at its beginning there is pre-capillary sphincter
(smooth muscle), they respond to local or circulating vaso constrictor substances.
The endothelial cells of capillary may be tied as in muscle or fenestrated as intestinal villi,
kidneys& liver.
Lymphatic vessels have similar structure to capillaries but they contain valves and the
endothelial cells are close together.
Veins and venules have slightly thicker wall than capillaries and contain little smooth
muscle noradrenergic nerve fiber lead to considerable vaso constriction in their wall.
Venous tone play important role in circulatory adjustment.
Components of the circulatory system
Arteries
Lumen diameter Wall thickness
4.0 mm
1.0 mm
30 m
6.0 m
-elastic and muscular
-conductance vessels
-pressure reservoir
Arterioles
MICROCIRCULATION
-highly muscular
-resistance vessels
-controls blood flow to tissues
Capillaries
8.0 m
0.5 m
-thin walled (endothelium only)
-exchange between blood and tissues
Venules & small veins
20 m
1.0 m
5.0 mm
0.5 mm
-thin-walled
-return blood from capillaries to veins
Veins
-highly distensible
-capacitance vessels
-serve as blood reservoir
 60% of total blood volume
Arterial blood pressure :
It refers to the pressure in aorta and large arteries and the term Bp, if unqualified refers to
the arterial BP
The organs of the body require blood flow and the arterial blood pressure (100 mmHg is
needed to push the blood through the arterioles capillaries and veins in order to achieve this
flow (systemic circulation).
In pulmonary circulation a low pulmonary artery BP 16 mm Hg is adequate to maintain
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DR.Saad Kleman Abd
The circulatory System
L. / min. blood flow (R. V. pressure in diastole is 0 mm Hg and in systole 25 mm Hg.)
There are two further factors to be taken into account concerning BP :
1- The brain is higher than heart in sitting or standing position and BP needed to
pump blood up – hill from heart to brain.
2-BP is needed to enable the kidney to make urine.
Flow, Pressure and Resistance:
Blood flow from region of high pressure to region of low pressure except in certain
situations.
The relation between mean flow, mean pressure and resistance in blood vessels is:Flow (F) = pressure (P) / Resistance(R)
Laminar Flow:
With in the blood vessel an infinitely thin layer of blood in contact with the wall of blood
vessel dose not move, the next layer has a small velocity, the next has a higher velocity and
so on.
Velocity being greatest in the center of the stream, laminar flow occurs at velocities up to
critical velocity. At or above this velocity the flow is turbulent and creates sound, While
laminar flow is silent.
(Probability of turbulence is also related to the diameter of the vessels and the viscosity of
the blood. It increases in Anemia due to low viscosity of the blood leading to systolic
murmur).
Regulation and control of cardio vascular system:Controlled: - means that some variables are kept steady .
Regulated: - means that variables are attempt appropriately according to requirements.
Controlling and regulation mechanism are located both centrally in C.N.S. and locally in
tissues and organs them self.
Central or systemic mechanism affect heart and blood vessels to:5
DR.Saad Kleman Abd
The circulatory System
1- Maintain arterial blood pressure with narrow range which provides a adequate
perfusion of the brain and heart it self with out excessive cardiac work.
2- Maintain appropriate filling pressure for the heart.
Systemic and local mechanism causing circulatory adjustment by:1- Change the caliber of the arterioles and other resistance vessels.
2-Alter blood storage in venous reservoirs.
3-Vary the rate and stroke out put of the heart.
Local mechanisms cause variation in vascular resistance to regulate local blood
flow which subjected to systemic and local factors.
The local mechanism operate to maintain blood flow in active tissues (kidney, intestine,
liver, heart, brain), while systemic mechanism synergize with local mechanism and adjust
vascular response throughout the body.
I- Local Regulatory Mechanism:Auto regulation: - Is the capacity of tissue to regulate their own blood flow it may be due
to:o Intrinsic contractile response of smooth muscle to stretch (myogenic theory of
auto regulation).
o As the pressure rises in the blood vessels, they are distended and the vascular
smooth muscle fibers that surround the vessels contract.
o Due to auto regulation, When blood flow decrease vasodilators substances
tend to accumulate in blood leading to vasodilation.
o increased flow lead to washing of vaso dilators and then constriction occur.
Local Vaso Dilators Metabolite:
Metabolites that produce vaso dilation in most tissues are: Decrease O2 tension and PH lead to relaxation of arterioles and per capillary
sphincters.
 Increase in CO2 tension and osmolality , can affect brain and skin especially.
 Rise in temperature.
 Potassium ions accumulation (epically in skeletal muscles)
 Lactic acid.
 Histamine in injured tissues lead to increase permeability &swelling.
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The circulatory System
Local Vaso Constrictors:
1- Injured arteries and blood vessels constrict strongly due to local liberation of serotonin
from platelets.
2- Decreased temperature cause vaso constriction.
II-Systemic Regulatory Mechanism:
General Control of Blood Vessels:
o
Smooth muscle constriction in the blood
vessels.
o The effect of sympathetic nervous system on
arterioles →vaso constriction.
o Vaso motor center (VMC) in medulla keep the
arterioles in a state of partial vaso constriction.
o Increased activity of VMC lead to vaso constriction and then increased blood pressure.
o While decreased activity of VMC lead to vaso dilatation and then decreased BP.
o Norephinephrine and angiotensin II is generalized vaso constrictor.
o Also ephenephrine but except on skeletal muscles it cause vaso dilation.
Baroreceptors :
There are sensory receptors found in the wall of the blood
vessels in the region of carotid sinus, the brachio cephalic
trunk and the aortic arch.
It is sensitive to blood pressure changes and it maintains
the blood pressure constant.
The barroreceptor activity acts as brake on the VMC
activity (Negative feedback mechanism).
If there is increase in BP this will increase baroreceptor
activity and this will lead to decrease VMC activity which
decrease sympathetic tone to arteriole
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The circulatory System
leading to vasodilation and then decreased
BP,
due to decreased peripheral resistance and also by
action of baroreceptor on cardiac center in medulla
to slow heart and respiratory center by decreasing
breathing.
Other Factors affecting VMC :
 Higher centers which are the higher parts of
the CNS, it includes a region of cerebral
cortex.
 Emotional
excitement
and
stress
are
accompanied by stimulation of VMC by higher centers leading to Increased vaso
constriction and then increased BP (as in the case of recording BP in a clinic).
 Over breathing of O2 cause washing of CO2 which is needed to the function of VMC
so it cause decreased BP
 Decreased O2 will stimulate VMC, chemo receptors affected by hypoxia.

Hypercapnia also stimulates VMC following hemorrhage.
 Many other nerves in the body affects VMC, moderate pain will stimulate VMC,
while sever pain will inhibit VMC
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The circulatory System
BARORECEPTOR REFLEX
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Pgs. 510-12, Silverthorn