Your heart beats because a tiny cluster of cells in its upper right chamber generates an electrical impulse, roughly 60 to 100 times per minute, that ripples through the heart in a precise sequence. That impulse triggers muscle contractions that push blood forward, then the muscle relaxes and refills. The entire cycle, from one beat to the next, takes less than a second.
The Electrical Spark That Starts Each Beat
Every heartbeat begins in a small patch of specialized tissue called the sinoatrial (SA) node, tucked into the wall of your right atrium (the upper right chamber). This node acts as your heart’s natural pacemaker. It fires an electrical signal on its own, without any instruction from your brain, at a steady rate of 60 to 100 times per minute when you’re at rest.
Once that signal fires, it fans out across both upper chambers, causing them to contract and squeeze blood downward into the lower chambers. The signal then reaches a second checkpoint called the atrioventricular (AV) node, which sits between the upper and lower chambers. The AV node deliberately delays the signal for a fraction of a second. That tiny pause is critical: it gives the upper chambers time to finish emptying before the lower chambers start contracting.
After the delay, the signal travels down a bundle of specialized fibers through the center wall of the heart, then branches out into a network of fine fibers that reach every part of the lower chambers (ventricles). When these fibers fire, the ventricles contract together in a powerful, coordinated squeeze that sends blood out to your lungs and the rest of your body.
What Happens Inside the Muscle Cells
When that electrical wave reaches a heart muscle cell, it triggers a rapid exchange of charged particles (ions) across the cell membrane. First, sodium rushes into the cell in less than a millisecond, flipping its electrical charge from negative to positive. This is what makes the impulse travel so quickly through heart tissue, at roughly one meter per second.
Next comes something unique to heart cells. Calcium floods in through special channels, creating a long plateau phase that keeps the cell activated much longer than a typical nerve or skeletal muscle cell would stay active. That sustained calcium signal is what actually makes the muscle fiber shorten and contract. Without it, the electrical impulse would pass through without producing any squeeze at all. Finally, potassium flows back out of the cell, resetting it to its resting state so it’s ready to fire again on the next beat.
Contraction, Relaxation, and the Sounds You Hear
Each heartbeat has two main phases. The contraction phase (systole) is when the ventricles squeeze and push blood out. The relaxation phase (diastole) is when they refill. Within those two phases, your heart moves through a surprisingly detailed sequence.
During systole, the ventricles first tighten with all valves closed, building pressure like squeezing a sealed water balloon. Once pressure inside the ventricles exceeds the pressure in the arteries, the outflow valves pop open and blood surges out. A healthy heart ejects 50% to 70% of the blood in the left ventricle with each contraction.
When the ventricles relax, the outflow valves snap shut to prevent blood from flowing backward. Pressure drops quickly, and once it falls below the pressure in the upper chambers, the inlet valves open and blood rushes in. Most of the filling happens passively and rapidly in the first moment of diastole. The upper chambers then give a final squeeze at the end to top off the ventricles, and the cycle starts again.
The familiar “lub-dub” sound comes from valves closing. The “lub” (called S1) is the inlet valves shutting at the start of contraction. The “dub” (S2) is the outflow valves closing at the start of relaxation. When a doctor listens with a stethoscope, they’re checking that these sounds are clean and appropriately timed, since extra or unusual sounds can signal valve problems.
How Your Body Speeds Up or Slows Down the Beat
Although the SA node sets its own rhythm, your nervous system constantly adjusts the pace. Two branches of your autonomic nervous system pull in opposite directions. The parasympathetic branch (carried mainly by the vagus nerve) acts as a brake, slowing the heart. The sympathetic branch acts as an accelerator, speeding it up. At rest, both systems are active in roughly equal measure, fine-tuning your rate from moment to moment.
When you start moving, something interesting happens. Mild exercise speeds the heart mainly by releasing the brake: parasympathetic activity drops, and the heart naturally drifts faster. As intensity increases, the sympathetic system kicks in more aggressively, dumping adrenaline-like signals that push the rate higher and make each contraction stronger. This layered system lets your heart respond smoothly across a huge range of demands, from sleep to sprinting.
Highly trained athletes often have resting heart rates in the 40s or 50s because their hearts pump more blood per beat, so fewer beats are needed to deliver the same amount of oxygen. The SA node is still capable of firing faster, but the body’s regulatory system keeps it dialed down because the demand is already met.
Why the Sequence Matters
The heart’s electrical and mechanical systems are tightly linked, and the order of events is just as important as the events themselves. If the upper chambers and lower chambers contracted at the same time, blood wouldn’t flow efficiently from top to bottom. If the AV node didn’t create that brief delay, the ventricles would try to squeeze before they were full. If calcium didn’t sustain the contraction long enough, the heart wouldn’t generate enough pressure to push blood through miles of blood vessels.
Disruptions to any step in this sequence are what produce arrhythmias. The SA node might fire too fast, too slow, or irregularly. The AV node might delay the signal too long or let it pass too quickly. Rogue electrical circuits can form in damaged tissue and override the normal pathway. In each case, the underlying mechanics are the same: an electrical signal that’s supposed to travel a specific route at a specific speed has gone off course, and the coordinated squeeze-and-fill cycle breaks down.