Children's Heart Institute | How the Heart Works: The Heart Electric System

The Children's Heart Institute - Hasan Abdallah, M.D.

Heart House | Heart Electric System | Blood Flow Diagram
Blood Flow Slide Show | How the Heart Pumps | The Aorta
How Blue Blood Becomes Red

How the Heart Works: The Heart Electric System

We will now add electricity to the Heart House to demonstrate how the heart's electrical system works. The Sinus Node generates an electric impulse and sends it through the heart causing it to beat. Located at the entrance to the Right Atrium, it is similar to a light switch at the entrance to your home except that the Sinus Node can switch itself on and off automatically.

The heart has four pumps, two little ones (right and left atrium), and two large ones (right and left ventricles). The main role of the left and right atriums is to squeeze their walls when they are full with blood so they can fill the ventricles, which are downstream from them.

You can think of the atrial pump as a priming pump. By pouring its blood into the ventricle, it prepares it for pumping similar to pouring gasoline into a carburetor to prepare the engine for action.

When the ventricles are full of blood their walls contract, and it imparts high pressure to the blood inside them. The blood from the right ventricle flows to the lungs and the blood from the left ventricle flows to the body.

Electricity has been added to the Heart House below:

Now that we have a good general idea about the electrical system of the heart we would like to discuss it further. Learning more about the electrical system of the heart is very important. This will make you under your child's condition or your condition if you have irregular heart beats or a condition related to very fast heart beat or a slow heart beat.

The Sinus Node Makes the Heart Beat
Your heart beat is basically an electric signal which is initiated by the Sinus Node (the on and off switch). The Sinus node is the blue light in the heart house diagram. The Sinus Node is considered to be the main switch of the heart which inititates the electric impulse of the heart and terminates it. When the Sinus Node is turned on (electric switch is on) an electric current spreads to activate both right atrium and left atrium and then the AV Node (Atrial Ventrical Node). The AV Node is the yellow light in the heart house diagram.

The AV Node is located at the junction or common point between the front heart rooms and the back heart rooms. The AV Node works like a delay station. When an electric impulse comes to the AV Node it is a little bit delayed before the electric impulse goes down to activate the ventricles. This is very important as during this time the Right Atrium and Left Atrium close their doors.

The Electric Impulse
The heart generates the electrical impulse that in turn makes the heart beat happen. The electric impulse originates high in the right atrium at the Sinoatrial Node (SA Node), a group of special cells. The impulse leaves the SA Node to spread across both the left and right atria causing them to contract simultaneously.

The SA Node is also known as the normal heart pacemaker, as it sets the pace of the heartbeat. It is responsible for the quickness and swiftness of the heartbeat. Like an electric turbine engine, the SA node generates electric impulses spontaneously all the time, however the rate of its electric impulse discharge is determined by the whole body's needs.

The signals then converge at the atrioventricular AV node, which is the relay station or electrical gate between the otherwise electrically isolated atria and ventricles. The delay provided by the AV node enables both atria to empty completely before the electrical impulse reaches the ventricles. The AV node slows down the impulse.

Bundle of His
The AV node connects at it's lower end to specialized nerve fibers known as the His bundle (Bundle of His), that works like a tiny electric cable, carrying the electric impulse over the middle heart wall (septum).

The Bundle of His then divides into two branches; the right bundle branch and the left bundle branch. The right bundle branch goes to the right ventricle, and comes very close to the surface of the right ventricle muscle and then divides into a tremendous net work of tiny nerve fibers known as Purkinje fibers. The left bundle similarly, terminates into Purkinje fibers in the wall of the left ventricle.

Purkinje Fibers
The Purkinje fibers allows the electrical impulse to directly activate the ventricular muscle to contract, and so a heartbeat is generated, the blood from the right ventricle is pumped to the lungs, and the blood from the left ventricle is pumped to the body. The process of electric induction of the heart contraction is known as Electric-Mechanical Coupling.

After activating the heart muscle to contract, the electric impulse disappears, the heart muscle, then relaxes, and is waiting for the sinus node to generate a new electric impulse before it contracts again.

Look at the heart animation below to see how the Sinus Node and AV Node look in a normal healthy heart:

Compare the difference between the Heart House with Electricity and the Sinus Node in your heart:

Heart House with Electricity	Sinus Node
Every time the light switch in the Heart House is turned on, it sends an electric impulse throughout the house turning on the lights.	Every time the Sinus Node in your heart fires, it sends an electric impulse throughout the heart, causing it to beat.
Located at the entrance to the Right Atrium from the Superior Venacava.	Located at the entrance to the Right Atrium from the Superior Venacava.
You have to switch it on.	Always stays on.
The more you get in and out the more you need to switch it on and off.	The more the heart has to work, the more it has to fire.

Heart Beat and Heart Rate
We call this process of filling and emptying a heart beat or a cycle, and the number of times it occurs per minute is a heart rate, e.g. an adult male heart beats 70 times per minute, or we can say his heart rate is 70/min.

In a newborn baby human, this process of filling and emptying occurs 140 times every minute, if the baby cries it can go up to 200 cycles or beats per minute, and when he is calm or sleeping it goes down to 100 beats per minute.

Fast Heart Beat:
As long as we are alive our heart will continue to beat. As soon as you feel scared your heart starts beating faster and harder preparing you to confront or to run, whatever you desire! When your body gets sick it beats faster to meet your extra energy needs.

Slow Heart Beat:
If you are relaxing it will adjust itself to beat gentler and slower. At the time of sleep it slows.

Your heart is so responsive to your needs; it is the only friend that can read your mind and body. The heart is the most alert and greatest computer operating system.

What a genius design!
Your car engine also works in cycles, during one phase of each cycle the cylinder fills, and during the next phase of the cycle it empties.

As you can see, timing and responsiveness to changes is critical. When the atrium is full, it should empty. It should not empty before that or any time longer than it's expected time. Also, the time it takes to empty is important and should be adjusted to meet the demands imposed on the heart at any time and with the shortest notice. This also applies to the ventricles, so the pumping function of the heart is efficient and dependable.

If you are in a deep sleep with a heart rate around 50 beats/minute, and your parents wake you up, telling you there is a fire in the house, your heart rate instantaneously jumps to above 100 beats/minute, and you get up in seconds.

How does the heart accomplish this task?
The heart electric system plays the major role, by generating and conducting an electric signal (electric impulse), it provides the instructions needed, for the pumps to time their actions precisely, as when to empty and when to close.

How does the body communicate its needs to the Sinus node?
The central nervous system conrols the SA node via two kinds of nerves, the symathetic and parasymathetic.

What happens if the sinus node doesn't generate an impulse?
The Sinus node is known as the pacemaker of the heart. If the heart fails to produce an impulse, other heart tissues are able to produce impulses as well. If any other site of the conduction system assumes that function, it will be called subsidiary or secondary pacemaker. However, the spontaneous firing rate of these subsidiary pacemakers is slower than the SA node's rate. Failure of the SA node to generate an impulse may result in decreased cardiac output and blood flow.

How does the Central nervous system control the conduction of the electric impulse?
The AV Node is responsible for transmitting the impulse from the atria to the ventricles. It limits the number of impulses that can pass through it in a given time period, preventing the ventricles from being driven at excessive rates when atrial disease produces abnormal impulses.

The AV Node's delaying and screening effects are under the control of the autonomic nervous system and hormones. At rest, under the influence of "decelerator" nerves (parasympathetic, or "vagal" nerves), inhibitory effects dominate, resulting in delaying and screening of impulse transmission. During exercise, "accelerator nerves" (sympathetic or adrenergic nerves) are activated, and "decelerator" nerve activity is suppressed, which facilitates AV Node signal transmission necessary for achieving rapid heart rates.

If the atrial rate is excessive, however, impulses entering the AV Node from the atria are extinguished and do not continue on down to the ventricles. This feature is important in atrial disease states.

Electrical signals rate of travel depends upon the heart tissue through which they are passing. These signals, although electrical, travel at a rate much slower than electricity through copper wires. The heart's specialized conduction tissues transmit signals relatively rapidly; heart muscle conducts signals more slowly.

Wolf-Parkinson White Syndrome (WPW Syndrome)
In this condition extra connections or "accessory pathways" are present and cross the cardiac skeleton. These pathways can allow the inappropriate transmission of signals from the atria to the ventricles or the ventricles to the atria, resulting in "short circuits".

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Next on the tour we will see how blood flows through a healthy heart.

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