Your heart is a muscular pump about the size of your fist that beats roughly 100,000 times a day, pushing around 7,200 liters of blood through your body every 24 hours. It manages this without any conscious effort on your part, driven by its own built-in electrical system and a muscle designed to work nonstop for a lifetime. Here’s what’s actually happening inside your chest with every beat.
Four Chambers, Four Valves
The heart is divided into four chambers. The two upper chambers, called atria, receive incoming blood. The two lower chambers, called ventricles, pump blood out. The right side of the heart handles blood that needs oxygen. The left side handles blood that already has it.
Separating these chambers are four one-way valves that open and close with each heartbeat to keep blood moving in the right direction. The tricuspid valve sits between the right atrium and right ventricle. The pulmonary valve guards the exit from the right ventricle to the lungs. On the left side, the mitral valve connects the left atrium to the left ventricle, and the aortic valve controls the final exit into the aorta, the largest blood vessel in your body. Each valve has flaps (called leaflets) that snap open to let blood through, then seal shut to prevent backflow. The clicking sounds of those valves opening and closing are what a doctor hears through a stethoscope.
The Path Blood Takes
Blood follows two distinct loops through your body. The first loop runs between your heart and lungs. The second loop runs between your heart and everything else: your brain, muscles, organs, and skin.
Here’s the full circuit, starting with blood that has already delivered its oxygen to your tissues and is heading back to the heart:
- Step 1: Oxygen-depleted blood flows into the right atrium from your body through large veins.
- Step 2: It passes through the tricuspid valve into the right ventricle.
- Step 3: The right ventricle pumps it through the pulmonary valve and into the lungs, where it picks up fresh oxygen and releases carbon dioxide.
- Step 4: Oxygen-rich blood returns to the left atrium.
- Step 5: It flows through the mitral valve into the left ventricle.
- Step 6: The left ventricle, the strongest chamber, pumps it through the aortic valve into the aorta, which branches into arteries that deliver oxygenated blood to every tissue in your body.
This entire journey takes roughly one minute. The left ventricle has thicker, more muscular walls than the right because it needs to generate enough force to push blood all the way to your toes and back. The right ventricle only needs to push blood the short distance to your lungs.
The Heart’s Built-In Pacemaker
Unlike most muscles, your heart doesn’t wait for a signal from your brain to contract. It generates its own electrical impulses through a specialized group of cells called the sinus node, located in the right atrium. This is your heart’s natural pacemaker.
Each heartbeat starts when the sinus node fires an electrical signal that spreads across both atria, causing them to contract and push blood into the ventricles. The signal then reaches a second relay station called the AV node, which sits between the atria and ventricles. Here, the impulse pauses for a fraction of a second. That brief delay is critical: it gives the ventricles time to fill completely before they contract.
After the pause, the signal travels down a pathway called the bundle of His, which splits into left and right branches that spread across the ventricles. These branches end in a network of tiny fibers that trigger the ventricles to contract almost simultaneously, squeezing blood out to the lungs and body. The whole electrical sequence, from the sinus node firing to the ventricles contracting, takes less than a second.
What Happens in a Single Beat
Each heartbeat has two main phases. During the contraction phase (systole), the ventricles squeeze and push blood out into the arteries. During the relaxation phase (diastole), the ventricles relax and fill with blood again. Your blood pressure reading reflects these two phases: the top number measures pressure during systole, and the bottom number measures pressure during diastole.
At a normal resting heart rate, diastole is actually longer than systole. Your heart spends more time filling than pumping. As your heart rate increases during exercise, diastole shortens more dramatically than systole, which means your heart has less time to fill between beats. This is one reason extremely high heart rates can actually reduce pumping efficiency rather than improve it.
Why Cardiac Muscle Never Takes a Break
Your heart muscle, called the myocardium, is unlike any other muscle in your body. Skeletal muscles fatigue after sustained effort, but cardiac muscle cells are built for endurance. They’re densely packed with mitochondria, the structures inside cells that produce energy. This gives the heart a massive capacity to generate fuel continuously.
Cardiac muscle runs primarily on aerobic energy production, burning fatty acids, carbohydrates, and even ketones to keep contracting. The cells are also branched and interconnected, which allows electrical signals to pass seamlessly from one cell to the next so the entire heart contracts as a coordinated unit rather than a collection of individual fibers twitching independently. Over an average human lifespan, this adds up to roughly one billion heartbeats.
How Your Heart Responds to Exercise
At rest, your heart pumps about 5 to 6 liters of blood per minute. During intense exercise, that number can increase four to five times over. Your heart achieves this partly by beating faster and partly by pumping more blood with each beat (a measure called stroke volume).
How much your stroke volume increases depends on your fitness level. Trained athletes tend to keep increasing the amount of blood pumped per beat even at near-maximum effort. Sedentary or moderately active people typically hit a plateau in stroke volume at about 40 to 50 percent of their maximum capacity, after which the heart relies almost entirely on beating faster to keep up with demand. This is one of the key cardiac adaptations that separates a conditioned heart from an unconditioned one: a fit heart moves more blood per beat, so it can do the same work at a lower heart rate.
Normal Heart Rate by Age
Resting heart rate varies significantly depending on age. Children’s hearts beat much faster than adults’ because their hearts are smaller and need more beats to circulate the same relative volume of blood.
- Newborns (0 to 1 month): 100 to 160 beats per minute
- Infants (1 to 12 months): 80 to 140
- Toddlers (1 to 3 years): 80 to 130
- Preschoolers (3 to 5 years): 80 to 110
- School-age children (6 to 12 years): 70 to 100
- Adolescents and adults: 60 to 100
Within the adult range, a lower resting heart rate generally indicates better cardiovascular fitness. Well-trained endurance athletes often have resting heart rates in the 40s or 50s. A consistently elevated resting heart rate, on the other hand, can signal that the heart is working harder than it should to meet the body’s needs.
When the Rhythm Goes Wrong
Because the heart depends on precise electrical signaling, problems in that system can cause irregular rhythms called arrhythmias. The most common type is atrial fibrillation, where the upper chambers quiver chaotically instead of contracting in an organized way.
Other rhythm problems include premature beats, which feel like your heart skipped a beat or fluttered. What actually happens is the signal to beat comes too early, creating a brief pause followed by a stronger-than-usual contraction. These are extremely common and often harmless. Bradycardia refers to a resting heart rate below 60 beats per minute (which can be perfectly normal in fit individuals but problematic in others). Tachycardia means a resting rate above 100. The most dangerous rhythm problem is ventricular fibrillation, where the lower chambers quiver instead of pumping, effectively stopping blood flow to the body.
Symptoms of arrhythmias range from barely noticeable to severe. You might feel a fluttering sensation, pounding in your chest, dizziness, or shortness of breath. Some arrhythmias come and go in episodes lasting seconds, while others persist and require treatment to restore a normal rhythm.