Transcript Ch 14: Cardiovascular Physiology

Cardiovascular Physiology
Dr. Faten
zakareia
Associate. Prof.
Physiology
KSU
The cardiovascular system
consists of
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1-The heart which acts as a
pressure pump.The heart is a
muscular organ enclosed in a fibrous
sac, the pericardium, and located in
the chest
The wall of the heart is composed of
cardiac muscle cells “myocardium”
The heart is divided to 2 halves each
has one atrium and one ventricle
2- a group of blood vessels (arteries,
arterioles, capillaries, venules, and
veins) all contain blood which is ever
circulating throughout life.
Valves of the heart :
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There are 4 sets of valves
1- Tricusped valve (between right atrium &
right ventricle).
2- Mitral valve (between left atrium & left
ventricle).
Tricuspid and mitral valves are called
Atrioventricular valves ( A-V valves).
*They prevent backflow of blood from
ventricle to atria during ventricular systole.
They close when ventricular pressure >
atrial pressure and produce 1st heart sound.
*They open when the atrial pressure >
ventricular pressure.
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3- Pulmonary valve (between right ventricle
& pulmonary artery).
4- Aortic valve (between left ventricle &
Aorta).
Pulmonary and aortic valves are called
semilunar valves. They prevent backflow of
blood from aortic & pulmonary vessels into
ventricles during diastole.
They close when pressure inside Aortic and
pulmonary vessels is > ventricular pressure
and produce the 2nd heart sound.
They open when ventricular pressure > Aortic
& pulmonary pressure
Cardiac properties
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The properties of cardiac muscle
are:
*Excitability
*Rhythmicity
*Conductivity
*Contractility
Excitability
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It is the ability of cardiac muscle to
respond to a threshold stimulus by
developing an action potential followed
by contraction(or the amount of inward
current required to bring a myocardium
to the threshold potential)
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It has 2 phases
absolute refractory period ( during the
depolarization- systole)
relative refractory period ( repolarization-
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Refractory Periods:
- Absolute refractory period (APR): For most of the duration of
AP, the myocardium refractory to fire another action potential
-It includes: upstroke, plateau and a portion of repolarization
(to -50mv)
Cardiac Action Potential
The Spreading of Action Potential
SA node is the pacemaker of the heart where the AP is
initiated
Atrial internodal tracts and atria helps distributing of AP to atria
AV node has the slowest conduction velocity
Bundle of His and purkinje system distribute the action
potential to the ventricles very fast to perform an efficient
contraction
Rhythmicity
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It is the ability of cardiac muscle to contract
regularly independent of nerve supply ( myogenic).
It is inherent spontaneous rhythmcity.
1-Sino-atrial node ( S-A node):
It posses the greatest rhythmicity & initiates the
beat of heart.
It is the main pace maker of the heart.
It lies in posterior wall of right atrium near opening
of superior vena cava (S.V.C).
it develops action potential without stimulation.
Origin of Cardiac Action
Potential & its Spreading
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2-Atrial internodal tracts help
distributing of AP to atria.
3-AV node has the slowest
conduction velocity where
delay of the impulse occur
(why?).
4-Bundle of His and purkinje
system distribute the action
potential to the ventricles very
fast to perform an efficient
contraction
Regulation of Pacemaker Activity )
regulation of heart rate)
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The SA node displays intrinsic automaticity
(spontaneous pacemaker activity) at a rate
of 100-110 action potentials ("beats") per
minute.
This intrinsic rhythm is primarily influenced
by autonomic nerves,
A- vagal influences being dominant over
sympathetic influences at rest. This "vagal
tone" reduces the resting heart rate down to
60-80 beats/min.
The rate of SA nodal firing can be
altered by
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changes in autonomic nerve activity (sympathetic
and vagal).
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circulating hormones (thyroxine, adrenaline)
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serum ion concentrations
cellular hypoxia
drugs
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Rhythmicity in different cardiac
fibers:
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S-A node is the normal pace maker of
heart i.e. it initiates the excitation wave that
drives whole heart and makes the pace
(speed) of heart.
. S-A node generates the highest
frequency of impulse and discharge
spontaneously at a rate of 75
impulse/min(due to vagal tone)
If the S-A node is damaged, A-V node
becomes the new pace maker and heart
follow it but at a slower rate ( 40
impulse/min).( A-V nodal rhythm).
Cont..
 If S-A node & A-V node are damaged, or if
their conduction is blocked, A-V bundle &
purkinje fibers become the pacemaker with
a rhythm of 25 impulse /mn ( idioventricular
rhythm).
 Any pace maker other than the S-A node is
called an ectopic focus.
 Rhythmicity is high in S-N node> A-V node>
purkinje fibers & ventricles.
Conductivity
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It is the ability of the cardiac muscle
fiber to conduct cardiac impulses that
are initiated in the S-A node &
transmitted in a specialized conducting
system formed from specialized
muscle fibers
The conducting system of the
heart is composed of
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S-A node, internodal pathway, A-V node,
bundles of Hiss& purkinje fibers.
I. S-N node
It is located at the posterior wall of right atrium near
opening of the SVC. It consists of modified cardiac
muscle fibers. It is normal pacemaker.
II. The internodal pathway.
-Action potential travels from S-A node through
atrial muscle to A-V node.
-Action potential can travel through small bundles of
atrial muscle fibers - internodal pathway- (anterior,
middle, posterior) at higher velocity.
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III. A-V node & bundle of Hiss
-A-V node is situated in right atrium at posterior
part of interatrial septum.
-Receives impulses from S-A node & transmits
it to ventricles through A-V bundle.
-At the A-v node, impulse is delayed
briefly to give the atria time to finish
contraction before ventricular contraction
starts.
IV- Right & left bundle branches.
They pass along sides of interventricular
septum to reach apex, then reflect to
ventricular wall.
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V.Purkinje fibers
The fibers penetrate about 1/3 of
muscle wall & terminate on muscle
fibers of the ventricle.& When impulses
reaches end of purkinje fibers they are
transmitted through ventricular
muscle fibers.
This allows both ventricles to contract
at the same time for their effective
pumping (synchronous contraction).
Contractility
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It is the ability to convert chemical
energy into mechanical form of energy.
Contraction is called systole and
relaxation is called diastole.
Factors affecting
myocardial contractility
Are called to have inotropic effect.
 That increase contractility +ve
inotropic factors
 That decrease myocardial contractility
–ve inotropic factors. These as follow
-Starling Law of the heart.
-Cardiac innervations ( sympathetic ,
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parasympathetic.
Cont.. Factors affecting contractility
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Hormonal and chemical factors (
adren. ,NE, , alkali , digitalis, ca++
increase. While Ach, acids, ether,
bacterial toxins, chloroform decrease.
Physical factors ( warming increase,
cooling decrease).
Oxygen supply ( hypoxia
decrease.).sympathetic(decrease)
&parasymparhetic(increase)
Cardiac Cycle
Is the mechanical and electrical events
occurring from the beginning of one
heart beat to the next one
 Consist of a period of
- Relaxation “diastole”
- Contraction “systole”
 Normal Cardiac cycle time:
60 / heart rate = 60/75 = 0.8 sec
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The cardiac cycle
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Each cycle is initiated by depolarization
of S-A node, followed by contraction of
atria. Then atria starts to relax, the
signal is transmitted to ventricles
through A-V node & hiss bundle to
cause ventricular contraction.
Cardiac cycle time
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*This
is time required for one complete
cardiac cycle. When heart rate is 75
beats/min, the time will be 0.8 sec.
*The time is inversely proportional with the
heart rate.
- Cardiac cycle starts by systole of both atria
(0.1 sec), followed by systole of both
ventricles (0.3 sec), then diastole of whole
heart (0.4 sec).
N.B Atrial and ventricular systole do not
occur at same time, but their relaxation
occurs at the same time during diastole of
the whole heart which lasts foe 0.4 Sec.
Events occurring
during the cardiac
cycle are
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1-Mid to late diastole:
-Here the heart is in complete relaxation. Blood is
accumulating in the atria.
-Pressure in the atria becomes higher than
pressure in the ventricles.
- Blood flow passively from atria to ventricles
through A-V valves which are open.
-Semilunar valves are closed.
-The atria contract and force the blood remaining in
their chambers into the ventricles.
2- Ventricular systole
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-The ventricles contract and the pressure
within them increase rapidly, closing the A-V
valves.
-When intraventricular pressure becomes
higher than aortic and pulmonary pressure,
the semilunar valves are forced to open.
And blood rushes through them out of
ventricles.
- During ventricular systole, the atria are
relaxed and their chambers are again filled
with blood.
3- Early Diastole.
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-At the end of systole, the ventricles relax the
intraventricular pressure drop.
- The semilunar valves close to prevent backflow of
blood, and for a moment the ventricles are
completely closed chambers (A-V and semilunar
valves are closed).
-When the interventricular pressure drops below
atrial pressure (which has been increasing as blood
is filling their chambers), the A-V valves are forced
open and the ventricles again begin refill rapidly
with completing the cycle.
Heart sounds
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Using a stethoscope, 2 heart sounds can be
heard during each cardiac cycle.
First heart sound:
Longer and louder than the second heart
sound, like the word ( LUP). And is caused
by closure of the A-V valves at the
beginning of ventricular systole.
Second heart sound:
Short and sharp like the word (DUP) and is
caused by closure of semilunar valve at the
beginning of ventricular diastole.
The sequence is like that Lup-dup, pause,
lup –dup, pause and so on.
Cardiac output
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Stroke volume:- It is the volume of blood pumped
by each ventricle per bear( each cardiac cycle). It=
70 ml.
Cardiac output (CO): It is the volume of blood
pumped by each ventricle per minute
It = HR x Stroke volume
= 75 beats/min x 70 ml ≈5L/min.
This volume will vary with the size of individual.
*Women have smaller CO than men.
*Children have smaller CO than adults.
Since the normal adult blood volume is 5L, the
entire blood supply passing through the body each
minute is 5L.
Factors affecting cardiac output
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There are 2 major factors which
determine CO. These are
The stroke volume and
The heart rate
Regulation of stroke volume
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According to starling's law,
the more the cardiac muscle fibers are stretched
the stronger will be the contraction. Provided that
the fibers are not excessively stretched.
*The important factor stretching the cardiac muscle
is the venous return, the amount of blood entering
the heart are distending the ventricles.
C.O.P
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*When increased quantities of blood flow into
right atrium as during exercise, this stretches
the walls of heart chambers. As a result,
cardiac muscle contracts with increased
force and extra blood flowing into heart is
automatically pumped into aorta and flows
again through systemic circulation.
*On the other hand, low venous return such
as result from severs blood loss or extremely
rapid heart rate, decreases stroke volume
causing the heart to contract less forcefully.
Regulation of heart rate
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Physical factors
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Age, sex, body temperature and exercise
influence heart rate
Resting HR is faster in fetus and then gradually
decreases throughout life.
HR is faster in females (72-80 beats/min) than in
males (64-72 beats/min).
Heat increases HR as occur in high fever. Cold
has the opposite effect.
Exercise acts through sympathetic nervous
system to increase HR.
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Autonomic nervous system
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*Sympathetic nerves stimulate SA node and
cardiac muscle itself and increase HR as occur
during stress.
* Parasympathetic nerves (vagus nerve) slow HR.
3-Hormones and drugs
Adrenaline and thyroxin increase HR
Increased calcium level in blood causes prolonged
contraction that the heart stops entirely. Reduced
calcium level in blood depresses the heart. Excess
or reduced sodium and potassium modify heart
activity.
Arterial blood
pressure (ABP)
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Blood pressure is the pressure the blood
exerts against the inner walls of the blood
vessels, and is the force that keeps blood
circulating continuously even between heart
beats.
Normal values of blood pressure:
In normal adults at rest, blood pressure
rises and falls during each beat.
Systolic BP is the pressure in the arteries at
the peak of ventricular contraction. It varies
between 110-140 mmHg.
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The diastolic BP is the pressure in the
arteries when ventricles relax. It varies
between 75-80 mmHg.
Blood pressure 120/80 refers to systolic
pressure of 120 mmHg and diastolic
pressure of 80 mmHg. It can be measured
by auscultatory method using the
stethoscope.
Variations in blood pressure
Blood pressure varies with age, weight,
race, mood, physical activity, posture.
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The venous return depends on:1-blood pressure gradient
The blood flows from the ventricle into
large, thick-walled elastic arteries
expand as blood is pushed into them,
then blood flows into smaller arteries,
arterioles, capillaries, venules, veins
and finally back to the large vena cava
entering the right heart.
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The pressure is highest in large
arteries and continuously drop
throughout the pathway, reaching zero
or –ve pressure at venae cava
High pressure in the arteries force the
blood to continually move into where
pressure is low (blood flows along a
pressure gradient from high to low
pressure).
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Continual blood flow depends on the
stretching of a large arteries and their
ability to recoil and keep the pressure
on the blood as it flow in circulation.
2-The presence of valves in the
large veins
3- The skeletal muscle pump
activity that squeeze the blood
vessels between muscle fibers.
4- Pressure changes in the thorax
5- effect of gravity.
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Arterial blood pressure= cardiac output x
peripheral resistance.
So, ABP is directly related to cardiac output and
peripheral resistance.
Peripheral resistance
It is the amount of friction encountered by the blood
as it flows through blood vessels especially the
arterioles. It depends on:Diameter of arterioles, their narrowing by
sympathetic stimulation and atheroscelerosis
increases PR.
Blood volume. Increase blood volume will increase
PR.
Blood viscosity. Increase blood viscosity also raises
PR.
Factors affecting blood
pressure
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Neural factors (autonomic nervous
system)
Parasympathetic system has little or no
effect on blood pressure.
Sympathetic system is important and its
major action is to cause vasoconstriction 9
narrowing of blood vessels) which increases
blood pressure.
Examples:
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During standing up suddenly after
lying down, the effect of gravity
cause blood to pool in the vessels of
legs and blood pressure drops.
-This activates the baroreceptors in
large arteries of the neck and chest.
-They send off signals that result in
reflex vasoconstriction which
increases blood pressure to normal
this is called (Baroreceptor reflex).
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B- When blood volume suddenly
decreases as in hemorrhage, blood
pressure drops and the heart begins to
beat rapidly. But because the venous
return reduced by blood loss, the heart
beats weakly. In such cases, sympathetic
nerves cause vasoconstriction to
increase blood pressure so that venous
return increase.
c- During vigorous exercise, there is
generalized vasoconstriction except blood
vessels of skeletal muscles, heart and
brain.
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2- Renal factors
The kidneys play a major role in regulating
ABP by changing blood volume.
*As blood pressure or volume increases
above normal, the kidney allows more water
to leave the body in urine. This decrease
blood volume which in turn decreases blood
pressure to normal.
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When blood pressure falls, the
kidneys retain body water, increasing
blood volume and blood pressure
rises. In addition when ABP falls the
kidney release renin enzyme that
helps the formation of angiotensin II, a
potent vasoconstrictor. Angiotensin
also stimulates the release of
aldosterone hormone that enhances
sodium ion reabsorption by the kidney
into blood.
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As sodium moves into blood, water follows.
Thus both volume and pressure rises.
Temperature
Cold has a vasoconstricting effect. Heat has
a vasodilator effect.
Chemicals
Adrenaline increases heart rate and blood
pressure.
Nicotine increases blood pressure by
causing vasoconstriction.
Alcohol and histamine cause
vasodilatation and decreases blood
pressure.
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4- Diet
A diet low is salts, saturated fats and
cholesterol help to prevent hypertension.
**A brief elevation in blood pressure is a
normal response to fever, physical exertion
and emotions upset such as fear and anger.
Persistent hypertension, or high blood
pressure, is pathological, and defined as a
condition of sustained elevated arterial
blood pressure of 140/90 or higher.
Electrocardiogram ECG (EKG)
Fig
14 20
The electrocardiogram
(ECG)
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It is the record of action potential
generated by myocardial fibers during
cardiac cycle.
It can be recorded by placing
electrodes on body surface on
opposite sides of the heart.
The principle:
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-Body fluids are good conductors, so that
when cardiac impulses pass through the
heart, electrical currents spread into tissues
surrounding the heart & to surface of the
body.
-If electrodes are placed on body surface on
opposite sides of heart we can record the
potential differences, the record is ECG.
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Normal ECG
It shows at least five different waves, 3
above the isoelectric line (positive
waves) & 2 are below it (negative).
These waves are called P, Q, R, S,
and T.
Analysis of normal ECG
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The P-Wave
-It represents atrial depolarization.
It starts 0.02 Sec before atrial contraction.
The QRS complex
It represents the process of ventricular
depolarization. It includes 3 waves
Q-wave: It starts 0.02 sec before
mechanical response of ventricles (systole).
It is caused by depolarization of ventricular
septum.
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R- wave: It is the most constant wave having the
tallest amplitude. It represents depolarization of
main ventricular muscles.
S- wave: It represents depolarization of base of
ventricles.
T- wave:
It represents process of ventricular repolarization.
N.B: The manifestations of atrial repolarization
are not normally seen because they are
obscured by the more powerful QRS complex.