Your heart beats because a small cluster of specialized cells generates its own electrical impulses, roughly 60 to 100 times per minute in adults, without any signal from the brain. These cells sit in a region called the sinoatrial (SA) node, located in the upper wall of the right atrium. The SA node is your heart’s natural pacemaker, and it fires automatically for your entire life.
The Built-In Pacemaker
The SA node is a crescent-shaped patch of tissue just a few square millimeters in size. What makes it remarkable is that its cells never truly rest. Unlike ordinary muscle cells, which sit quietly until they receive a signal, pacemaker cells begin recharging the moment they finish firing. As soon as one electrical impulse ends, the cell immediately starts building toward the next one. This self-starting behavior is what keeps your heart beating even when it’s completely disconnected from the nervous system, which is why transplanted hearts (with all their nerve connections severed) still beat on their own.
How the Signal Travels
Once the SA node fires, the electrical impulse spreads across both upper chambers (the atria), causing them to contract and push blood down into the lower chambers. The signal then reaches a second checkpoint called the AV node, which sits between the upper and lower chambers. The AV node briefly delays the signal, giving the lower chambers time to fill with blood before they contract.
From the AV node, the impulse travels down a pathway of specialized fibers that branch out across the walls of both lower chambers (the ventricles). These fibers deliver the signal almost simultaneously to all parts of the ventricle walls, triggering a powerful, coordinated squeeze that pushes blood out to the lungs and the rest of the body. The entire sequence, from SA node firing to ventricular contraction, takes less than a second.
What Actually Makes the Muscle Contract
Electricity alone doesn’t squeeze blood. The electrical signal is really just the trigger for a cascade involving calcium. When the impulse reaches a heart muscle cell, it causes a small amount of calcium to flow in from outside the cell. That initial trickle of calcium triggers a much larger release of calcium from storage compartments inside the cell, a process sometimes called “calcium-induced calcium release.” The calcium concentration inside the cell surges roughly a hundredfold during this process.
All that calcium latches onto a protein woven into the muscle fibers. This binding causes a shape change that allows the muscle’s thick and thin filaments to grab onto each other and slide together, shortening the cell. Millions of cells shortening in unison is what produces the physical squeeze of a heartbeat. When the calcium is pumped back into storage, the filaments release each other and slide apart, and the muscle relaxes. This cycle of calcium flooding in and being swept away repeats with every single beat.
The Energy Behind Every Beat
Each heartbeat consumes a significant amount of energy in the form of ATP, the molecule cells use as fuel. ATP powers the ion pumps that move calcium, sodium, and potassium in and out of heart cells. It also fuels the sliding of muscle filaments that produces the actual contraction. Recent research using real-time measurements has shown that ATP levels fluctuate beat to beat, meaning your heart is constantly generating fresh energy in lockstep with each contraction. Mitochondria, the energy-producing structures inside cells, are physically tethered to the calcium storage compartments so they can sense each calcium surge and ramp up ATP production to match demand. The heart consumes more energy per gram than almost any other organ, and it relies heavily on a steady supply of oxygen to keep that production running.
How Your Body Speeds Up or Slows Down the Beat
Although the SA node can fire on its own, your nervous system constantly adjusts its rate. Two opposing branches of the autonomic nervous system act like a gas pedal and a brake. The sympathetic branch (the “fight or flight” system) releases adrenaline and a closely related chemical called noradrenaline. These molecules bind to receptors on SA node cells and speed up the rate at which those cells recharge between impulses. Your heart rate climbs, and the force of each contraction increases. This is why your heart pounds when you’re frightened or exercising.
The parasympathetic branch works through the vagus nerve, which releases a different chemical that slows the recharging of pacemaker cells. At rest, the vagus nerve is the dominant influence, which is why a healthy resting heart rate sits at the lower end of the 60 to 100 range. During sleep, vagal activity increases further and your heart rate drops even more. The balance between these two systems shifts constantly throughout the day based on activity, stress, temperature, and even breathing patterns.
Normal Heart Rate by Age
The SA node doesn’t fire at the same rate throughout life. Younger hearts beat considerably faster. Newborns have a normal resting heart rate of 100 to 160 beats per minute. Infants settle into 80 to 140, toddlers 80 to 130, and school-age children 70 to 100. By adolescence, the adult range of 60 to 100 beats per minute takes hold. Well-trained athletes often have resting rates below 60 because their hearts pump more blood with each beat, so fewer beats are needed.
What a Single Beat Looks Like Mechanically
Each heartbeat has two main phases. During diastole, the heart muscle relaxes and the chambers fill with blood. Pressure inside the ventricles drops low enough that the valves between the upper and lower chambers swing open, and blood flows in. During systole, the ventricles contract. Pressure builds briefly while all the valves are closed (a moment called isovolumic contraction), then the exit valves pop open and blood is ejected into the arteries. The familiar “lub-dub” sound of a heartbeat comes from these valves snapping shut: the first sound when the valves between the atria and ventricles close at the start of systole, and the second when the exit valves close at the end.
When the Electrical System Misfires
An arrhythmia occurs when the heart’s electrical signals don’t work properly. The heart may beat too fast (tachycardia, over 100 beats per minute), too slow (bradycardia, under 60), or in an erratic, uncoordinated pattern. Atrial fibrillation, the most common sustained arrhythmia, happens when chaotic electrical signals cause the upper chambers to quiver instead of contracting in an organized way. Ventricular fibrillation is far more dangerous: the lower chambers quiver uselessly and can’t pump blood at all, making it a medical emergency.
When the SA node or the conduction pathways are damaged or diseased, an artificial pacemaker can take over. Modern pacemakers include devices small enough to be implanted directly inside the heart through a vein, with no wires connecting to an external generator. These leadless pacemakers are roughly the size of a large vitamin capsule and deliver electrical pulses that mimic the SA node’s natural rhythm, keeping the heart beating at an appropriate rate when the built-in system can no longer do so reliably.