How the Heart Works

How the Heart Works

The cardiac electrical system

The heart generates its own electrical signal, or impulse. This

electrical impulse can be recorded by placing electrodes on the skin

of the chest, thus producing the electrocardiogram (ECG, or EKG).

The heart's electrical impulse arises in the upper portion of the

right atrium and spreads across. As it spreads, it triggers the heart

muscle to contract in a coordinated fashion. If the electrical signal

becomes deranged in some way, the efficient pumping action of the

heart may deteriorate, or stop altogether.

[IMAGE]

The components of the heart's electrical system are illustrated here.

From an electrical standpoint, the heart is divided into two portions

- the atria, and the ventricles. Separating the atria from the

ventricles is a non-muscular, fibrous "disc." This disc (labeled AV

disk in the figure,) prevents the passage of the electrical signal

between the atria and the ventricles.

In this figure, SN = sinus node; AVN = AV node; RA = right atrium; LA

= left atrium; RV = right ventricle; LV = left ventricle.

The next figures illustrate how the electrical impulse spreads across

the heart.

[IMAGE]The electrical impulse originates in the sinus node. From

there, it spreads across both atria, causing the atria to contract. As

the electrical impulse passes through the atria, it generates the

so-called "P" wave on the ECG. (The P wave is indicated by the solid

purple line on the ECG below.)

[IMAGE]When the wave of electricity reaches the AV disc, it is

essentially abolished (that is, it does not pass across the AV disc to

the ventricles), except in the specialized AV conduction system.[IMAGE]

The specialized AV conduction system consists of the AV node (AVN),

the His bundle, and the right and left bundle branches (RBB and LBB).

The AV node conducts the electrical impulse very slowly, and passes it

to the His bundle (pronounced "hiss"). The His bundle penetrates the

AV disk, and passes the signal to the right and left bundle branches.

The right and left bundle branches, in turn, send the electrical