How the Heart Works – the Electrical System
The heart beats about 100,000 times a day. The rate and rhythm of the heart beat are orchestrated by a specialized electrical system. Understanding this system is the basis of understanding what happens when someone has an arrhythmia (irregular heart beat), or a conduction abnormality (such as a left or right bundle branch block), and whether any of these conditions are clinically serious. The topics of these conditions or how to exercise to possibly improve symptoms of such heart conditions are too big to discuss here. However hopefully you will find this blog entry helpful in understanding a little more about the heart's electrical system.
The Anatomy of the Electrical System of the Heart
The anatomy of this electrical system is shown in the two figures below. The heart has two nodes which are referred to as the sinoatrial node (SA node) and the atrioventricular node (AV node). These are specialized parts of heart muscle involved in initiating the electrical signal in the heart. The SA node is situated up in the right atrium (for more background on heart anatomy click here). The AV node also lies in the upper part of the heart but is more central and close to the ventricles. Extending down from the AV node there are specialized fibers called the “bundle of His” (named after the person who discovered them), which split into right and left bundle branches. The right bundle branch will spread the signal through the right ventricle and the left bundle branch spreads it through the left ventricle. The left bundle branch also splits into anterior and posterior branches called fascicles (not depicted). Extending from the bundle branches are numerous Perkinje Fibers which spread out though of the heart.
The upper part of the heart is electrically isolated from the bottom. This means that normally the electrical signal in the atria (upper part of the heart) cannot spread to the lower part of the heart, except through the AV node and Bundle of His.
How it Works
To illustrate the electrical signal, the parts of the electrical system in the animation below are coloured yellow. The white depicts the electrical signal. However, the heart does not light up in real life.
All the muscle cells that make up the heart are able to 1) activate themselves (auto-rhythmic), and 2) spread the signal to other heart muscle cells. Once activated, the cells forcefully contract. So as the signal spreads through a muscle, the contraction also spreads. In the normal heart, the signal usually begins in the SA node because that area activates itself a little quicker than other parts of the atria. This allows the signal and therefore the contraction to start in one place and spread across the top of the heart, allowing a unified contraction of the atria. The right and left atria therefore contract basically at the same time. These two atria give that final squeeze of blood into the ventricles which have been filling since the previous heart beat. The AV node is also activated. However it is unique in that it delays the signal before sending it down the bundle of His. This slight pause lasts a fraction of a second and allows both atria to finish squeezing blood to the ventricles before the ventricles are activated.
During the activation of the ventricles, the Bundle of His, the subsequent bundle branches and Purkinje fibers will allow the signal to spread extremely rapidly from the AV node, down the wall (septum) between the ventricles, and then up the outer walls. This sequence through the ventricles allows the septum to contract first creating a rigid middle wall. Then the quick contraction of the right and left outer walls squish the blood out of the ventricles against a rigid septum. The speed of the signal as it spreads through the ventricles makes for a quick, coordinated contraction. This contraction lasts less than 1/10 th of a second, and drives blood out of the heart.
Once the electrical signal is finished, the heart beat is finished and the ventricles relax which allows them to refill. The SA node starts again for the next beat. Whether you are resting or active or stressed, the body will modify the speed of the electrical system to match the demand for blood flow.
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