2. Electrical activity of the Heart
- The heart shows automaticity which means it functions automatically.
- Normally one region of the heart shows automatic electrical activity which
stimulated the rest of the heart this is called pacemaker. The pacemaker of
the heart is called sinoatrial node (SA node).
- SA node cells show pacemaker potentials which is
different from the action potentials produced by the
rest of the heart cells.
SA node
- Because the cells of the heart are connected by
electrical synspses (gap junctions) the SA node
affect the rest of the heart.
3. Myocardial action potential
Normal cardiomyocytes have a resting
potential of -90 mV.
When stimulated by pacemaker they
become depolarized to specific
threshold when the gated Na+ channels
open.
The quick decline to -15 mV. happens
because of the quick reversal of the
membrane polarity.
The inflow of Ca2+ and the out flow of K+
The repolarization to -80 MV.
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4. Pacemaker action potential
It is caused by spontaneous depolarization.
When these depolarization reach threshold they trigger action potential.
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5. Conducting tissue of the heart
SA node = Sinoatrial node is the
pacemaker of the heart, conduction
starts from here.
AV node = Atrioventricular node
located between atria and ventricles.
It gives a small delay before
continuation of impulses to bundle
of His.
Bundle of His = located in septum
and sends fibers to the ventricles.
Purkinje fiber = spreads the impulses
throughout the ventricles.
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7. The electrocardiogram
The potential difference generated
by the heart is conducted to the
body surface.
Electrodes placed on the skin can
measure these signal producing
electrocardiograph (ECG or EKG).
the recording device is called the
electrocardiogram.
ECG measures production and
conduction of action potential in
the heart.
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9. Cardiac output
Cardiac output is the volume of blood pumped per minute by each ventricle.
Heart rate is the number of heart beats per minute.
Stroke volume is volume of blood pumped per beat by each ventricles.
The average resting cardiac rate (heart rate) is 70 beats/minute.
The average stroke volume is 70-80 ml/beat.
Cardiac output = Heart rate (beats/min) x Stroke volume (ml/beat)
CO = HR x SV
So on average:
CO = 70 beat/min x 70 ml/beat = 4,900 ml/min or about 5 L/min
In an average person there is about 5 L blood in the body. So because the CO
is approximately 5 L/min then every minute this volume is circulated around
the body.
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10. 1- Regulation of heart Rate
CO = HR x SV
The heart rate is controlled by neural influences (sympathetic and Parasympathetics).
However, in the absence of any neural effects the heart is still automatically controlled by
The rate of SA node (pacemaker potentials).
- The SA node has a certain rate which dictates the rate of heart beating.
Sympathetic Parasympathetic
activity _ activity
+ SA node
11. 2- Regulation of Stroke Volume
CO = HR x SV
The stroke volume is controlled by:
1- End Diastolic volume (EDV):
Volume of blood in ventricles at the end of diastole. The more the ventricles
are filled with blood during diastole the greater EDV and therefore the greater
stroke volume.
2- Total Peripheral Resistance (TPR):
The resistance to blood flow in the arteries. A greater resistance to blood flow
in the arteries, such as when there is vasoconstriction (narrowing of arteries)
reduces the stroke volume.
3- Contractility:
The strength of heart muscle contraction. The greater the contractility the
greater the stroke volume.
12. 4- Venous return
CO = HR x SV
The EDV is the amount of blood filled in the ventricles at the end of diastole.
This depends on how much blood is returned to the heart by veins which is
called venous return. The greater the venous return the greater the EDV
therefore the greater stroke volume. Skeletal Muscle pump increases the
venous return.
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13. 5- Blood pressure in the arterioles
CO = HR x SV
Increased
The arterial pressure rises
pressure
during systole and falls during
diastole.
Resistance to flow in the
arterial system is big at the
arterioles because they are the
smallest arteries and they can
become narrower by
vasoconstriction.
Increase in the resistance of
the arterioles increases blood
pressure in upstream and
decrease SV.
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14. Summary of factors that regulate cardiac output
Solid lines show the stimulators (increase)
Dashed arrows shows inhibitors (decrease)
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