By: Rebecca Xu
Electricity flows from outlets in walls, powers various mechanisms, strikes down from the skies, and … powers your heart? The heart actually has its own electrical system that keeps it running, just like how computers need electricity to run. Shocking, right? (Pun absolutely intended.)
Have you ever thought about how your heart continues to beat, even without thinking about it? In fact, there is an electrical pathway that travels throughout the different sections of your heart, contracting and loosening the muscle so that blood continues to rush in your vessels and throughout your body. Despite the importance associated with it, the heart is just a simple pump. If the pump were to stop, however, all the cells in your body, especially the brain, would promptly die due to the lack of oxygen. Waste would build up, and the body would stop functioning.
Obviously, we need something to control and regulate this vital organ. This is where the electricity comes from. Electricity literally runs through your heart every second, causing the “lub dub” noise that reverberates throughout our body. So how does this work, exactly? First, let’s discuss the structure of the heart to get a better understanding of what it does, and how electricity plays a role. There are four main chambers. From the perspective of facing the front of someone else’s heart: the upper left chamber is called the Right Aorta; the upper right chamber is called the Left Aorta; the bottom left chamber is called the Right Ventricle, and the bottom right chamber is called the Left Ventricle, as labeled in the photo below. Following the white arrows, oxygenated blood that just came from the lungs travels through the pulmonary veins, into the left atrium, through the mitral valve, into the left ventricle, through the aortic valve, and into the aorta, that leads to the rest of the body. The blood circulates through the arteries and capillaries, delivering oxygen to each and every cell. After the blood is finished delivering the blood, it becomes deoxygenated and needs to travel back to the heart to get to the lungs, in order to get re-oxygenated. This deoxygenated blood travels from the superior or inferior vena cava, into the right atrium, through the tricuspid valve, into the right ventricle, through the pulmonary valve, and into the pulmonary artery to reach the lungs. This cycle continues each time you take a breath and your heartbeats.
Where the electricity comes in is in the movement of the blood. After all, Newton’s first law states that an object at rest remains at rest unless an outside force acts on it. This “outside force” is the very cardiac electrical system. In your heart, two different types of cells in your heart enable the electrical signal to control your heartbeat: conducting cells that carry your heart's electrical signal, and muscle cells that enable your heart's chambers to contract, an action triggered by your heart's electrical signal. The electrical signal travels through a network of conducting cell pathways that stimulate the upper and lower chambers to contract in an alternation fashion. The signal is able to travel along these pathways by means of a complex reaction that allows each cell to activate one next to it, stimulating it to "pass along" the electrical signal in an orderly manner. As cell after cell rapidly transmits the electrical charge, the entire heart contracts in one coordinated motion, creating a heartbeat.
The order of the electrical signal is as follows: the SA node (called the pacemaker of the heart) sends out an electrical impulse; the upper heart chambers (atria) contract; the AV node sends an impulse into the ventricles; the lower heart chambers (ventricles) contract or pump; the SA node sends another signal to the atria to contract, which starts the cycle over again. Referring to the image on the left, the SA node, or the sinoatrial node, sends the signal through the anterior internodal tract, the middle internodal tract, and Bachmann’s bundle, in order to stimulate the atria to contract, pumping blood into the ventricles. This signal then goes to the AV node, or the atrioventricular node, and down the Bundle of His located directly in the middle of the heart, in between the ventricles. Splitting into the left bundle branch and the right bundle branches, the signal then separates even further into the conduction pathways, also called Purkinje fibers, that stimulate the ventricles to contract and pump blood into the lungs or the body.
It is crazy how an outside electrical shock can kill you, but your heart has an electrical system of its own, controlling the “lub dub” rhythm and providing the circulation of your blood. Take a deep breath, feel your pulse, and think of all the complex bodily processes that just happened in that very second.
What Did You Learn?
1. Briefly describe the steps in the electrical pathway in the heart.
First, the SA node acts as the pacemaker and sends an electrical signal in intervals. This signal travels through the atria, causing the cardiac muscles to contract, and the blood to flow into the ventricles. Then the signals travel to the VA node, and through the ventricles, causing these chambers to contract, and for the blood to rush out of the heart and into the lungs or rest of the body.
2. Why does the heart need an electrical system?
The electrical pathway in the heart allows the heart to contract and loosen the cardiac muscles of each chamber. These chambers redirect blood from the body to the lungs and back in order for body cells to receive oxygen. Without oxygen, body cells quickly die, rendering them nonfunctional. These chambers need to be carefully organized and in sync in order to keep the flow of blood undisrupted - - this is the job of the electrical system. The electrical system allows the heart to contract at the right times, pumping the blood to where it needs to go in the body.