What makes your heart beat?

An average adult heart is a little bigger than a clenched fist, weighs about 11 ounces (roughly the same weight as one large turnip) and despite what we see on greeting cards, looks more like an upside-down pear than a Valentine. According to the American Heart Association the heart beats an average of 100,000 times a day -- anywhere from 60 to 100 beats a minute -- which adds up to more than 2.5 billion beats by the time we turn 70-years-old.

What makes the heart beat? Electricity. But before we can understand what electricity is doing in the body, we first need to understand the parts of the heart and how they work together.

The heart has four chambers, two upper and two lower. The upper chambers are called the right and left atria, and the two lower are the right and left ventricles. A valve connects each atrium with its corresponding ventricle. The tricuspid valve connects the right atrium and ventricle, and the mitral valve connects the left atrium and ventricle. Two additional valves complete the set: the pulmonic valve connects the right ventricle with the pulmonary artery and the aortic valve connects the left ventricle with the aorta. These four valves function like gates, allowing blood to flow in one direction with each heartbeat -- about 2,000 gallons of blood every day.

So what's the power source to keep this system running - and your heart beating?

The heart beats because of a small electrical current generated by the the cardiac conduction system. The cardiac conduction system is a group of muscle cells in the walls of the heart. It's comprised of five major components:

The sinoatrial node (SA node): Known as the heart's "pacemaker", this node fires at regular intervals, causing the heart to beat.
The atrioventricular node (AV node): the electrical "relay station" between the upper and lower heart chambers.
The bundle of His (also known as the atrioventricular bundle): muscle fibers that conduct the electrical impulses that regulate heartbeat.
Bundle branches: Connected to the bundle of His, these lead to the lower ventricals.
Purkinje fibers: Embedded in the subendothelial tissue, these fibers conduct impulses through the heart.

When all these components work together as they should, you have a healthy heart with a beat rhythm of about 60 to 70 beats per minute, or higher, depending on your age and other factors.

"Pacemaker" Cells and the SA Node

Special cells often referred to as "pacemaker" cells produce electricity in the body by rapidly changing their electrical charge from positive to negative and back again. When the heart muscle is relaxed the cells are electrically polarized, meaning the inside of each cell has a negative electrical charge. The environment outside the cells is positive. Cells depolarize as some of their negative atoms are allowed through the cell membrane, and it's this depolarization that causes electricity in the heart. Once one cell depolarizes it sparks a chain reaction and electricity flows from cell to cell. When cells return to normal it's called repolarization, and the process is repeated with every heartbeat.

The SA node is regulated by the autonomic nervous system, which controls all of the automatic functions of the body including heartbeat, breathing and digestion. The sympathetic nervous system and the parasympathetic nervous system are part of the autonomic nervous system and work together to control how fast the pacemaker cells spontaneously depolarize and increase and decrease the rate the SA node sends out electrical signals.

"The sympathetic nervous system is responsible for increasing the heart beat during exercise, while the parasympathetic nervous system lowers the pulse during periods of rest," explains Marshal Fox, M.D., electrophysiology cardiologist at Baystate Health in Massachusetts.

When the SA node fires off an electrical impulse the pulse of electricity first travels through the top chambers of the heart and continues through the AV node where it's slowed down. By slowing down the electrical signal the AV node allows time for the upper chambers of the heart to contract first, before the ventricles. Once through the AV node's gate the pulse continues to move through the bundle branches and Purkinje fibers and then finally ends in the ventricles, which contract and pump blood through the body.

"People may have different intrinsic resting pulse rates and the reason for this is due to the balance between the sympathetic and parasympathetic nervous systems," explains Fox. "Athletes, for instance, develop higher parasympathetic tone with continued training and therefore while at rest will have a lower pulse than their counterpart couch potatoes."

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