Your heart is a muscular pump about the size of your fist that beats roughly 100,000 times a day, pushing 5 to 6 liters of blood through your body every minute while you’re at rest. It works through a tightly coordinated system of chambers, valves, electrical signals, and nerve inputs that keep blood flowing in one direction, delivering oxygen to every tissue and carrying waste away.
The Four Chambers and Their Roles
Your heart has four hollow chambers: two on top (the atria) and two on the bottom (the ventricles). The right side handles blood that needs oxygen. The left side handles blood that already has it. A muscular wall called the septum divides the two sides so oxygen-rich and oxygen-poor blood never mix.
Oxygen-poor blood from your body enters the right atrium through two large veins. From there it moves into the right ventricle, which pumps it to your lungs to pick up fresh oxygen and release carbon dioxide. The newly oxygenated blood returns to your heart’s left atrium, drops into the left ventricle, and gets launched out through the aorta to supply the rest of your body. The left ventricle is the thickest, strongest chamber because it has to generate enough pressure to push blood all the way to your feet and back.
How Valves Keep Blood Moving Forward
Four one-way valves prevent blood from flowing backward. Each valve has flaps (called cusps) that open to let blood pass and snap shut to stop it from leaking back.
- Tricuspid valve: sits between the right atrium and right ventricle
- Pulmonary valve: sits between the right ventricle and the artery leading to the lungs
- Mitral valve: sits between the left atrium and left ventricle
- Aortic valve: sits between the left ventricle and the aorta
The “lub-dub” sound of your heartbeat is the sound of these valves closing. The first sound comes from the tricuspid and mitral valves shutting as the ventricles start to squeeze. The second sound comes from the pulmonary and aortic valves closing once the ventricles finish pushing blood out.
The Electrical System That Triggers Each Beat
Your heart doesn’t wait for instructions from your brain to beat. It has its own built-in pacemaker: a small cluster of cells in the upper right atrium called the SA node. This node fires an electrical signal that spreads across both atria, causing them to contract and push blood down into the ventricles.
The signal then reaches a second relay point near the center of the heart called the AV node. Here, something important happens: the signal pauses for a fraction of a second. That tiny delay gives the atria time to finish emptying before the ventricles fire. Without it, the chambers would squeeze almost simultaneously, and blood wouldn’t move efficiently.
After the pause, the signal travels down a highway of specialized nerve fibers running through the wall between the ventricles, then fans out through a network of fibers that reach deep into the muscle of both ventricles. This triggers the powerful contraction that sends blood to your lungs and body. The whole sequence, from the SA node’s first spark to full ventricular contraction, takes less than a second and repeats continuously for your entire life.
What Happens in a Single Heartbeat
Each heartbeat has two main phases. During systole, the ventricles contract and push blood out. During diastole, the ventricles relax and refill.
Systole begins with a brief moment when the ventricles start squeezing but all four valves are closed. Pressure inside the ventricles builds rapidly with nowhere for the blood to go. Once the pressure exceeds the pressure in the arteries, the pulmonary and aortic valves pop open and blood is ejected into the lungs and body.
Diastole is when the ventricles relax. As pressure drops inside them, the outflow valves close (that’s the second heart sound). For a brief moment all valves are closed again while the ventricles continue to relax. Once ventricular pressure falls below atrial pressure, the tricuspid and mitral valves open and blood rushes in from the atria. Most of the filling happens quickly in this initial rush, with a slower trickle filling the ventricles through the middle of diastole. The atria then give one final squeeze to top off the ventricles, and the cycle starts over.
At a normal resting heart rate of about 60 to 100 beats per minute, your heart spends more time in diastole than systole. That relaxation phase is when the heart muscle itself gets most of its blood supply.
How the Heart Feeds Itself
Even though the heart is constantly full of blood, it can’t absorb oxygen from the blood passing through its chambers. It needs its own dedicated supply. That job belongs to the coronary arteries, two small but critical blood vessels that branch off the very beginning of the aorta.
The left main coronary artery supplies the left atrium, left ventricle, and about two-thirds of the septum. The right coronary artery feeds the right atrium, right ventricle, and the remaining third of the septum. It also supplies blood to the SA and AV nodes, meaning it literally fuels the heart’s electrical system. These arteries branch into smaller and smaller vessels that penetrate deep into the heart muscle. When a coronary artery becomes blocked, the section of muscle it feeds starts to die. That’s a heart attack.
How Your Nervous System Adjusts Heart Rate
Your heart sets its own rhythm through the SA node, but your nervous system constantly fine-tunes how fast and how hard it beats based on what your body needs. Two branches of the nervous system handle this.
The sympathetic branch is your accelerator. When you exercise, feel stressed, or need more blood flow, sympathetic nerves speed up the SA node’s firing rate and make the heart muscle contract more forcefully. This is the “fight or flight” response at work.
The parasympathetic branch, working largely through the vagus nerve, is your brake. It slows the heart rate by directly reducing the firing speed of the SA node and slowing conduction through the AV node. At rest, parasympathetic influence dominates, which is why a calm, healthy heart beats relatively slowly. If parasympathetic input is suddenly ramped up, it can override and suppress sympathetic activity almost instantly, a response known as accentuated antagonism. This is why techniques like slow deep breathing or cold water on the face can quickly bring down a racing heart rate.
These two systems don’t simply oppose each other like an on/off switch. They interact in complex, overlapping ways, each capable of dialing the other down. The balance between them shifts constantly throughout the day, adjusting your heart’s output to match whether you’re sleeping, sprinting, digesting a meal, or standing up from a chair.
How Much Blood Your Heart Pumps
At rest, a healthy adult heart pushes out 5 to 6 liters of blood per minute. That number, called cardiac output, is the product of two things: how many times your heart beats per minute and how much blood it ejects with each beat. During intense exercise, cardiac output can increase four to five times to meet the demand for oxygen in working muscles.
Your heart adapts over time, too. Regular aerobic exercise strengthens the heart muscle so it can push out more blood per beat. This is why well-trained athletes often have resting heart rates in the 40s or 50s. Their hearts don’t need to beat as often because each beat is more efficient.