Your heart is a muscular pump roughly the size of your fist that beats about 100,000 times a day, pushing blood through a closed loop of vessels that delivers oxygen to every cell in your body and carries waste back out. At rest, most adults have a heart rate between 60 and 100 beats per minute, though well-trained athletes can sit comfortably in the 40s or 50s. Each beat involves a precisely timed sequence of electrical signals, valve openings, and muscle contractions that all happen in under a second.
Four Chambers, Two Circuits
The heart is divided into four hollow chambers. The two upper chambers, called atria, receive incoming blood. The two lower chambers, called ventricles, pump blood out. A muscular wall down the middle keeps oxygen-rich blood on the left side completely separate from oxygen-poor blood on the right.
This separation creates two distinct circuits. The right side of the heart handles pulmonary circulation: it receives used, oxygen-poor blood returning from your body and sends it to your lungs to pick up fresh oxygen and drop off carbon dioxide. The left side handles systemic circulation: it receives that freshly oxygenated blood from the lungs and pumps it out to every organ, muscle, and tissue in your body. The left ventricle does the heaviest lifting because it needs to generate enough pressure to push blood all the way to your toes and back. That’s why it has the thickest walls of all four chambers.
The Path Blood Takes
Blood follows the same route through your heart every single beat. Oxygen-poor blood flows back from your body through two large veins (the superior and inferior vena cava) and empties into the right atrium. From there it passes directly into the right ventricle, which contracts and pushes it through the pulmonary artery toward the lungs.
In the lungs, blood releases carbon dioxide and absorbs oxygen. It then returns to the heart through the pulmonary veins, entering the left atrium. Blood flows from the left atrium into the left ventricle, which contracts powerfully and sends it out through the aorta, the body’s largest artery. From the aorta, blood branches into smaller and smaller vessels until it reaches the tiniest capillaries, where oxygen and nutrients pass into surrounding tissues. Then the cycle starts again.
With each contraction, the left ventricle ejects roughly 70 milliliters of blood. That may not sound like much, but multiply it by every beat over a full day and your heart moves thousands of liters of blood from sunrise to sunrise.
Valves Keep Blood Moving Forward
Four one-way valves prevent blood from sloshing backward. Two of them sit between the atria and ventricles: the tricuspid valve on the right side and the mitral valve on the left. These snap shut when the ventricles start to contract, stopping blood from being pushed back up into the atria. The other two, the pulmonary valve and the aortic valve, guard the exits of the right and left ventricles respectively. They close once the ventricles finish squeezing, preventing blood in the arteries from draining back in.
Those valve closures are what produce the familiar heartbeat sound. The “lub” you hear through a stethoscope (called S1) is the mitral and tricuspid valves shutting at the start of a contraction. The “dub” (S2) is the aortic and pulmonary valves closing right after.
The Heart’s Built-In Electrical System
Your heart doesn’t wait for instructions from your brain to beat. It generates its own electrical impulses through a specialized group of cells in the upper right atrium called the SA node, often referred to as the heart’s natural pacemaker. The SA node fires a signal that spreads across both atria, causing them to contract and push blood down into the ventricles.
That signal then reaches a second relay point, the AV node, which sits between the atria and ventricles. The AV node introduces a brief delay, about a tenth of a second, giving the ventricles time to fill completely before they contract. After the pause, the signal travels down a network of specialized fibers that branch through the walls of both ventricles. These fibers trigger the ventricles to squeeze from the bottom up, efficiently pushing blood out through the pulmonary artery and the aorta.
This entire electrical sequence, from the SA node firing to the ventricles finishing their contraction, takes less than a second at a resting heart rate.
One Heartbeat in Detail
Each complete heartbeat is called a cardiac cycle, and at a typical resting rate of 75 beats per minute, one cycle lasts about 0.8 seconds. It has two main phases: contraction (systole) and relaxation (diastole). Roughly one-third of the cycle is spent contracting and two-thirds relaxing, which means your heart actually spends more time resting than working during each beat.
During systole, the ventricles squeeze. For a brief moment at the very start, all four valves are closed and pressure builds rapidly inside the ventricles without any blood moving. Once that pressure exceeds the pressure in the arteries, the exit valves pop open and blood is ejected into the aorta and pulmonary artery.
During diastole, the ventricles relax. Pressure inside them drops, the exit valves close (producing the “dub”), and once ventricular pressure falls below atrial pressure, the inlet valves open. Blood rushes in from the atria, first quickly, then more slowly. At the very end of diastole, the atria give one final contraction to top off the ventricles, contributing about 25% of the total filling volume. Then the cycle repeats.
How Your Nervous System Adjusts Heart Rate
While the SA node sets the baseline rhythm, your autonomic nervous system constantly fine-tunes it based on what your body needs. Two branches work in opposition to each other, like a gas pedal and a brake.
The sympathetic branch is the gas pedal. When you exercise, feel stressed, or sense danger, it releases signaling chemicals that speed up the SA node’s firing rate, make each contraction stronger, and increase the amount of blood pumped per beat. This is the system behind your racing heart during a sprint or a jump scare.
The parasympathetic branch is the brake. Acting primarily through the vagus nerve, it slows the SA node’s firing rate and reduces the force of contraction. This branch dominates when you’re relaxed, digesting food, or sleeping. The balance between these two systems shifts constantly throughout the day, which is why your heart rate rises and falls without you ever thinking about it.
How the Heart Feeds Itself
The heart muscle works nonstop, so it has an enormous appetite for oxygen. Even though blood-filled chambers sit right inside it, the heart can’t absorb oxygen from that blood directly. Instead, it has its own dedicated blood supply through the coronary arteries, which branch off the aorta just above the aortic valve. These arteries wrap around the outside of the heart, diving into the muscle tissue to deliver oxygen and nutrients. If a coronary artery becomes narrowed or blocked, the section of heart muscle it feeds starts to starve. That’s what happens during a heart attack.
Interestingly, the coronary arteries fill mostly during diastole, when the heart muscle is relaxed. During contraction, the muscle squeezes so tightly that it compresses its own blood vessels. This is one reason why the longer diastole phase matters: it’s when the heart does most of its own refueling.