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Electrical Activity of the Heart And Cardiac Output

- 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

- SA node cells show pacemaker potentials which is different from the action potentials produced by the rest of the heart cells.

- Because the cells of the heart are connected by electrical synspses (gap junctions) the SA node affect the rest of the heart.

Electrical activity of the Heart

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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Pacemaker action potential

It is caused by spontaneous depolarization.

When these depolarization reach threshold they trigger action potential.

Conducting tissue of the heart

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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 impulsesthroughout the ventricles.

How does conduction occur

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The electrocardiogramThe 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 inthe heart.

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Electrocardogram (ECG)

ECG

Cardiac outputCardiac output is the volume of blood pumped per minute by each ventricle.

Heart rate is the number of heart beats per minute.

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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 COis approximately 5 L/min then every minute this volume is circulated around the body.

Stroke volume is volume of blood pumped per beat by each ventricles.

1- Regulation of heart RateCO = 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 byThe rate of SA node (pacemaker potentials).

- The SA node has a certain rate which dictates the rate of heart beating.

SA node

Sympathetic activity

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Parasympathetic activity_

2- Regulation of Stroke VolumeCO = 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.

4- Venous return

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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.

CO = HR x SV

5- Blood pressure in the arterioles

The arterial pressure rises 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 canbecome narrower by vasoconstriction.

Increase in the resistance of the arterioles increases blood pressure in upstream and decrease SV.

Increased pressure

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CO = HR x SV

Summary of factors that regulate cardiac output

Solid lines show the stimulators (increase)Dashed arrows shows inhibitors (decrease)

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