What determines the rhythm of your heart?

heart diagram
See more pictures of the heart.

When a doctor checks your heartbeat, have you ever wondered how it stays so regular? Or what's gone wrong when someone has to get a pacemaker? When it comes to the heart, timing is critical. Without a strong heartbeat, blood can't get to where it needs to go, and a heartbeat needs to be steady in order to be strong.

To understand what sets the beat of your heart, and why that rhythm is so important, it's first helpful to understand what exactly a heartbeat is and what it does.


A "beat" is a contraction of the heart. Each time a section of the heart contracts, it forces blood from one point to another. It goes like this:

Heart Pictures

  • When blood returns to the heart from the rest of the body, it flows into the right atrium (1). The blood has been supplying oxygen throughout the body and needs a refill.
  • The right atrium fills with this blood, which then flows into the right ventricle (2), as well. The right ventricle is going to send the blood into the lungs for an oxygen fill-up.
  • To get as much blood into the right ventricle as possible, the right atrium contracts, pushing all of the blood down into the ventricle.
  • Once the right ventricle is full, it contracts, forcing the blood into the lungs.
  • Once the blood has picked up oxygen, it moves from the lungs to the left atrium (3), and then down into the left ventricle (4). The atrium contracts and then the ventricle contracts, like on the right side.
  • The right and left atriums actually contract at the same time. The right atrium pushes oxygen-low blood into the right ventricle, and the left atrium pushes oxygenated blood from the previous cycle into the left ventricle.
  • When the left ventricle contracts, it sends the blood to the rest of the body.
  • The blood eventually returns to the right atrium, low on oxygen, and the process starts again.

In­ each heartbeat, the atrium has to contract first, or else the ventricles will be low on blood and their contractions won't be effective.

What exactly keeps the pace? Since your heart is your body's engine, it makes sense that it might work something like the engine in your car: It starts with a spark.



SA node and AV Node: Setting the Pace

An artificial pacemaker.
An artificial pacemaker mimics the electrical impulses normally created by the SA node.
Don Farrall/Photodisc/Getty Images

Your heart is more like a car engine that you might realize. Loosely speaking, the heart's chambers are the pistons, the contraction of those chambers is the piston stroke, and the ignited gas is the blood that keeps everything going. The heart even has a sparkplug. An electrical impulse triggers each contraction and sets the timing of the whole process.

When someone needs a pacemaker, it's usually because there's a problem with these electrical impulses, which weakens the heartbeat, causing all sorts of issues. If the heart can't get enough blood pumping through the body, the body -- and especially the brain -- suffers from lack of oxygen. An artificial pacemaker sends out electrical impulses to mimic the heart's natural pacemaker, the sinoatrial node (SA node), located in the right atrium.


The SA node is a group of cells that generates electrical current. It sends out an electrical charge at some set interval -- say, once every second, which would establish the low-end normal heart rate of 60 beats per minute (60 to 80 is a healthy heart rate). These impulses are the "sparks" that cause the right atrium to contract, starting the whole string of events that gets blood pumping in waves through your body. It's this electrical impulse that sets the rhythm of your heart. Whenever the SA node sends out a charge, your heart beats. When you need more blood pumping, like when you need more oxygen to climb steps or run a mile, the SA node shortens its electrical-discharge interval.

There are actually two pacemakers. The SA node is the primary; the atrioventricular node (AV node), located in a bundle of tissues on the border between the right atrium and the right ventricle, is the secondary. When the SA node sends out an electrical impulse, the first place it goes is to the AV node. While the SA node sets the rhythm of your pulse, the AV node sets the rhythm of your heart contractions. It delays the signal on its way to the ventricle, giving the atrium time to contract first. It holds it up for about a tenth of a second [source: Signalife]. If the atrium and the ventricle contracted at the same time, the ventricles would push out their blood before they were totally full, resulting in low blood pressure, among other problems.

When the heart's electrical system misfires, it's called atrial fibrillation. Basically, what happens is the heart starts generating electrical impulses in more than one place, not just in the SA node. This messes everything up and can result in a pulse well above the 60 to 80 range that a healthy heart generates. With too many triggers, the right atrium can't possibly contract fully each time, meaning it never gets a full pump of blood into right ventricle, and the body gets deprived of blood. An artificial pacemaker stabilizes the system by taking over the job of sending out electrical impulses, getting the heart back into a regular rhythm.

For more information on the heart, atrial fibrillation and related topics, look over the links on the next page.


Lots More Information

Related HowStuffWorks Articles

More Great Links

  • Atrial Fibrillation. MedicineNet. http://www.medicinenet.com/atrial_fibrillation/article.htm
  • The Electrical System. Heart Rhythm Society. http://www.hrspatients.org/patients/the_normal_heart/electrical_system.asp
  • Heart Rhythm Disorders. eMedicine Health. http://www.emedicinehealth.com/heart_rhythm_disorders/article_em.htm