Download Cardiovascular physiology

Cardiovascular physiology
The cardiovascular system consists of:
The heart which acts as a pressure pump and
a group of blood vessels (arteries, arterioles,
capillaries, venules, and veins) All contain
blood which is ever circulating throughout
life.
The heartis made of 2 halves (right & left)
(Each half is made up of an atrium and a
ventricle with A-V valve in between (it allows
blood to pass in one direction, from atrium to
ventricle).
The right heart:-
The right atrium receives venous blood from body
through the superior vena cava (SVC)→pumps it to right
ventricle→ pumps it into the pulmonary artery→ lungs
(pulmonary or lesser circulation).
The left heart:
Left atrium receives arterial blood from the pulmonary
veins → pumps it to left ventricle → to the body through
aorta. Arterial blood is changed into venous blood which
returns back to the Right heart (systemic or greater
circulation).
One way movement of blood
To avoid admixture between arterial and venous blood ,
the circulation must be strictly one way direction.
*From Rt atrium → RT ventricle →Pulmonary artery.
*From Lt atrium → Lt ventricle → Aorta.
This is done by the action of the valves.
1
Valves of the heart :
There are 4 sets of valves
1- Tricusped valve (between right atrium & right
ventricle).
2- Mitral valve (between left atrium & left ventricle).
Tricusped 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.
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
The properties of cardiac muscle are:
*Excitability
*Conductivity
*Rhythmicity
*Contractility
Excitability
2
It is the ability of cardiac muscle to respond to a
threshold stimulus by developing an action potential
followed by contraction
Rhythmicity
It is the ability of cardiac muscle to contract regularly
independent of nerve supply. It is inherent spontaneous
rhythmcity.
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 is made of modified cardiac muscle.
Its membrane potential is unstable and it develops
action potential without stimulation.
Sympathetic nerves accelerate the onset of self excitation
and increases heart rate in contrast to parasympathetic
nerves e.g. vagal nerves which slow the heart rate
Rhythmicity in different cardiac fibers:
 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.
 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).
 If S-A node & A-V node are damaged, or if their
conduction is blocked, A-V bundle & purkinje fibers
3
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
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 not nerve fibers.
The conducting system of the heart is composed of:
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.
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.
4
-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.
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
It is the ability to convert chemical energy into
mechanical form of energy.
Contraction is called systole and relaxation is called
diastole.
The cardiac cycle
It is the cardiac events that occur during one complete
heartbeat (from the beginning one heart beat to the
beginning of the next one).
- 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.
Systole and diastole means contraction & relaxation.
- Cardiac cycle time
5
*This is time required for one complete cardiac cycle.
When heart rate is 75 beats/min, the time will be 0.8.
*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:
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
-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.
-At the end of systole, the ventricles relax the
intraventricular pressure drop.
6
- 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
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.
The electrocardiogram (ECG)
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:
-Body fluids are good conductors, so that when cardiac
impulses pass through the heart, electrical currents
7
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.
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
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.
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.
Cardiac output
8
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
There are 2 major factors which determine CO. These are
the stroke volume and the heart rate
Regulation of stroke volume
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.
Within physiological limits, the heart pumps all blood
that comes to it without allowing stasis of blood in veins.
*The important factor stretching the cardiac muscle is the
venous return, the amount of blood entering the heart are
distending the ventricles.
*The heart has built up mechanism that allows it to pump
automatically whatever amount of blood flows into right
atrium from veins.
*When increased quantities of blood flow into right
atrium as during exercise, this stretches the walls of heart
9
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:




1- Physical factors
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.
2- Autonomic nervous system
*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.
10
Arterial blood pressure (ABP)
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.
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.
The venous return depends on:1-blood pressure gradient
The blood flows from the ventricle into large, thickwalled elastic arteries expand as blood is pushed into
them, then blood flows into smaller arteries, arterioles,
11
capillaries, venules, veins and finally back to the large
vena cava entering the right heart.
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).
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
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:1- Diameter of arterioles, their narrowing by
sympathetic stimulation and atheroscelerosis
increases PR.
12
2- Blood volume. Increase blood volume will
increase PR.
3- Blood viscosity. Increase blood viscosity also
raises PR.
Factors affecting blood pressure
1- 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:
a- 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).
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.
13
c- During vigorous exercise, there is generalized
vasoconstriction except blood vessels of skeletal
muscles, heart and brain.
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.
 * When blood pressure falls, the kidneys retain body
water, increasing blood volume and blood pressure
rises. In addition when ABP falls the kidney release
rennin 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.
 As sodium moves into blood, water follows. Thus both
volume and pressure rises.
2- Temperature
Cold has a vasoconstricting effect. Heat has a
vasodilator effect.
3- Chemicals
Adrenaline increases heart rate and blood pressure.
Nicotine increases blood pressure by causing
vasoconstriction.
Alcohol and histamine cause vasodilatation and
decreases blood pressure.
4- Diet
A diet low is salts, saturated fats and cholesterol help
to prevent hypertension.
14
**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.
15