Your heart is a muscular pump, roughly the size of your fist, that pushes blood through every part of your body. It beats around 60 to 100 times per minute at rest and moves about 2,000 gallons of blood every day. Everything your body does, from thinking to digesting to healing a cut, depends on the steady delivery of oxygen and nutrients that your heart makes possible.
How Blood Moves Through the Heart
The heart has four chambers: two smaller ones on top (the right and left atria) and two larger, more powerful ones on the bottom (the right and left ventricles). Blood flows through these chambers in a specific order, controlled by four one-way valves that open and close to keep blood moving forward and prevent it from leaking backward.
The process starts when oxygen-poor blood returning from your body enters the right atrium through two large veins. From there it passes through the tricuspid valve into the right ventricle, which pumps it out through the pulmonary valve toward the lungs. In the lungs, blood drops off carbon dioxide and picks up fresh oxygen. That newly oxygenated blood flows back into the left atrium, passes through the mitral valve into the left ventricle, and gets pumped out through the aortic valve into the aorta, the body’s largest artery. From there, it travels to every organ and tissue.
Two Loops, One Pump
Your heart actually runs two circulatory loops at the same time. The pulmonary loop carries oxygen-poor blood from the right side of the heart to the lungs and back. The systemic loop carries oxygen-rich blood from the left side of the heart out to the rest of the body and returns it. The right and left sides of the heart work in unison, pumping the same volume of blood with every beat, but the left ventricle generates much more pressure because it needs to push blood all the way to your toes and back. Peak pressure in the left ventricle reaches about 120 mmHg, compared to only about 25 mmHg in the right ventricle.
What Happens in a Single Heartbeat
Each heartbeat has two phases. During systole, the ventricles contract and force blood out into the arteries. During diastole, the ventricles relax and refill with blood. At rest, the contraction phase takes up about one-third of the cycle and the relaxation phase takes up the other two-thirds, giving the heart more time to fill than to pump.
With each contraction, a healthy heart ejects roughly 70 milliliters of blood from each ventricle. That number, called stroke volume, multiplied by your heart rate gives your cardiac output: the total volume of blood your heart moves per minute. For most people at rest, that works out to about 5 to 6 liters per minute. During intense exercise, cardiac output can increase several times over as both heart rate and stroke volume rise.
These two phases also create the numbers in a blood pressure reading. The top number (systolic pressure) reflects the force in your arteries when the heart contracts. The bottom number (diastolic pressure) is the residual pressure between beats, when the heart is relaxed and refilling.
The Heart’s Built-In Electrical System
Your heart doesn’t wait for your brain to tell it when to beat. It has its own electrical system that fires automatically. A small cluster of cells called the sinoatrial (SA) node, located in the right atrium, acts as the heart’s natural pacemaker. It generates an electrical signal that spreads across both atria, causing them to contract and push blood into the ventricles.
The signal then reaches a relay station called the atrioventricular (AV) node near the center of the heart. The AV node pauses the signal for a fraction of a second, a critical delay that ensures the atria finish emptying before the ventricles fire. After the pause, the signal travels down a bundle of specialized fibers through the center of the heart and fans out into a network of fibers embedded in the ventricle walls. When these fibers deliver the signal, both ventricles contract simultaneously, ejecting blood into the arteries.
This entire sequence, from the initial spark in the SA node to the full contraction of the ventricles, happens in less than a second and repeats continuously for your entire life.
What the Heart Is Made Of
The heart wall has three layers. The innermost layer lines the chambers and provides a smooth surface for blood to flow against. The middle layer is thick cardiac muscle, and it does the actual work of squeezing. Unlike the muscles in your arms or legs, cardiac muscle contracts rhythmically on its own and never fatigues the way skeletal muscle does. The outer layer is a thin protective coating. The entire organ sits inside a fluid-filled sac that cushions it and reduces friction as it beats.
Heart Rate by Age
A normal resting heart rate varies significantly with age. Newborns have the fastest hearts, beating 100 to 205 times per minute. By preschool age, the range narrows to about 80 to 120 bpm. Adolescents and adults share the same range of 60 to 100 bpm at rest, though well-trained athletes often have resting rates in the 40s or 50s because their hearts pump more blood per beat. Your heart rate drops further during sleep and climbs during physical activity, stress, or illness.