How Does Your Heart Know to Contract?

This is the second post in a three part series: Part 1 (How Does the Heart Work?), Part 3 (What Causes The Most Common Heart Problems?)

When you look inside the heart you see it has 4 hollow chambers separated by valves that allow blood to move through the heart as it contracts and relaxes. 

But how does your heart muscle know when to contract and when to relax? Your heart has it’s own electrical circuit! Part of your nervous system tells this system how quickly or slowly your heart should beat. Then this electrical circuit tells the cells in your heart muscle when to contract to pump blood out to your lungs and body. It then gives them time to relax so that the chambers can fill with blood all over again. Pretty amazing, right?

Your heart has a “right side” with two chambers and a “left side” with two chambers. These sides are separated by heart muscle. Inside this strip of heart muscle are some very amazing and hard working nerve fibers. These fibers travel down between the middle of your heart and then fan out into the heart muscle that surrounds the outside of your heart’s chambers. There is a small node in the first chamber of your heart, the Right Atria, that sends out the “time to contract” signal to the rest of your heart muscle. As this signal is passed along the fibers of your heart’s electrical circuit, the muscles of the heart begin to contract. As the signal starts in the top chamber of your heart this means that the top of your heart contracts first. As the electrical stimulus travels through the cells of your upper chambers (the Atrias) blood is pushed through the valves that separate your Atria and Ventricles to allow your ventricles to fill with blood. The “time to contract” stimulus then encounters a second node that slows the contraction down just a bit to give the atrias time to completely empty as they push the blood into the ventricles. The contraction stimulus is then sent down into the cells surrounding the ventricles and they contract from the bottom up. This pushes all of that blood out of the ventricles to either the lungs to get fresh oxygen (if the blood is leaving the right ventricle) or to the body to deliver that fresh oxygen (if the blood is leaving the left ventricle).

In case you needed another reason to be absolutely amazed at what your heart can do, if the first node that is supposed to send out the “time to contract” signal doesn’t send it out, the second node that slows the impulse will take over. If that second node fails to send out the signal, then the ventricles themselves will take over. Each of these options creates a slower and slower heartbeat so they are not a long term solution, but they can keep you alive and that is pretty incredible!

Have you ever seen a printout at the hospital or on a TV show that has all those squiggly lines on it? That is the reading from an electrocardiogram and it shows your healthcare team what the electrical activity in your heart is doing. By looking at this reading they can determine which of your nodes are functioning, which of the fibers are relaying the “time to contract” signal correctly and which of your chambers are being affected. This is a crucial test for determining how best to treat a heart condition. The test is performed by placing sticky leads onto the persons body (mostly on their chest) to get an accurate read of how their heart is functioning. It is very quick and does not hurt at all (well maybe a little if it is an emergency and you have a hairy chest and we need to get the leads to stick better, you may just get a free wax treatment haha). But it really is a very simple test and it provides so much valuable and amazing information!