The heart is a muscular pump that moves blood through every part of your body, delivering oxygen and nutrients to your cells and carrying waste products away. At rest, it pushes about 5 to 6 liters of blood per minute through a network of blood vessels, and it does this continuously, every minute of every day, for your entire life. Roughly the size of your fist and weighing between 230 and 340 grams depending on sex, it’s one of the hardest-working organs you have.
How Blood Moves Through the Heart
The heart has four chambers. The two upper chambers, called atria, receive incoming blood. The two lower chambers, called ventricles, pump blood out. Blood flows through the heart in a strict one-way path, guided by four valves that open and close with each beat to keep everything moving in the right direction.
Here’s the sequence: blood returning from your body, now low in oxygen, enters the right atrium. It passes through the tricuspid valve into the right ventricle, which pumps it through the pulmonary valve and into the lungs. In the lungs, the blood picks up fresh oxygen and releases carbon dioxide. That oxygen-rich blood flows back to the heart’s left atrium, passes through the mitral valve into the left ventricle, and gets pumped out through the aortic valve to the rest of your body.
The valves work like one-way doors. Each has thin flaps (called leaflets) that snap shut after blood passes through, preventing backflow. When a doctor listens to your heartbeat with a stethoscope, the sounds they hear are these valves closing.
Two Circulation Loops
Your heart essentially runs two pumps side by side. The right side powers the pulmonary circuit, a short loop that sends oxygen-depleted blood to the lungs and brings it back freshly oxygenated. The left side powers the systemic circuit, which is far larger: it sends oxygen-rich blood out through arteries to every tissue in the body, from your brain down to your toes. After delivering oxygen and picking up carbon dioxide and other waste, the blood returns through veins to the right side of the heart, and the cycle starts over.
The left ventricle has thicker, more muscular walls than the right because it needs to generate enough pressure to push blood through the entire systemic circuit. The right ventricle only needs to push blood the short distance to the lungs.
What Happens in a Single Heartbeat
Each heartbeat has two main phases. During systole, the ventricles contract and force blood out into the arteries. During diastole, the ventricles relax and fill with blood again. A healthy ventricle ejects more than 60% of its blood volume with each contraction.
The process is more precise than it sounds. When a ventricle starts to contract, pressure builds inside it before any blood actually leaves. This brief moment, called isovolumic contraction, is like squeezing a water balloon before the cap pops off. Once the pressure inside the ventricle exceeds the pressure in the artery beyond it, the valve opens and blood rushes out. After ejection, the valve snaps shut, the ventricle relaxes, and pressure drops low enough for blood from the atrium above to flow in and refill it.
At a resting heart rate of about 70 beats per minute, this entire cycle takes less than a second. Over a typical human lifespan, the heart beats roughly 700 million to one billion times.
The Heart’s Built-In Electrical System
Unlike most muscles, which need a signal from your brain to contract, the heart generates its own electrical impulses. A small cluster of cells in the right atrium acts as a natural pacemaker, firing electrical signals that spread across both atria and make them contract. The signal then pauses briefly at a relay point between the atria and ventricles. That pause is important: it gives the atria time to finish squeezing blood into the ventricles before the ventricles fire. Once the signal passes through the relay, it races along specialized fibers in the walls of the ventricles, triggering a powerful, coordinated contraction from the bottom up that efficiently pushes blood out.
This electrical system is why the heart can keep beating even when removed from the body, as long as it has oxygen. It doesn’t depend on the brain to initiate each beat.
How Your Body Speeds Up or Slows Down the Heart
While the heart sets its own rhythm, your nervous system adjusts the pace to match what your body needs. Two branches of the involuntary nervous system share this job. The sympathetic branch, your “fight or flight” system, releases hormones like adrenaline and norepinephrine that speed the heart up. The parasympathetic branch releases a different chemical signal that slows it down.
At rest, the parasympathetic system is dominant, keeping your heart rate calm. When you start exercising, your body first pulls back on that braking signal, allowing the heart to gradually speed up. If you push harder, the sympathetic system kicks in and actively accelerates it. This is why your heart rate rises smoothly during a workout rather than jumping instantly to its maximum.
How the Heart Feeds Itself
The heart pumps blood to every organ, but it also needs its own supply. About 5% of the blood the heart pumps gets routed right back to the heart muscle itself through a dedicated set of blood vessels called the coronary arteries. These arteries branch off from the very beginning of the main artery leaving the heart and spread across the surface of the heart before diving into the muscle tissue.
Two main coronary arteries divide the workload. One supplies the right side of the heart along with the electrical pacemaker and relay areas. The other splits into two branches that feed the left atrium and the thick-walled left ventricle. Smaller vessels branch deeper into the muscle, providing fine-tuned control over blood flow to meet the heart’s constantly shifting oxygen demands. When these arteries become narrowed or blocked, the heart muscle can’t get enough oxygen, which is what causes a heart attack.
More Than Just a Pump
Beyond circulating blood, the heart plays a role in several other body functions that aren’t always obvious. It helps regulate blood pressure through the force and rate of its contractions. It distributes hormones released by glands elsewhere in the body. It carries immune cells to sites of infection. And it moves heat around the body, routing warm blood from your core to your skin when you need to cool down.
The heart also produces its own hormone. When blood volume gets too high and the atrial walls stretch, they release a peptide that signals the kidneys to excrete more sodium and water, helping bring blood pressure back down. So the heart isn’t just responding to the body’s needs. It’s actively communicating with other organs to keep conditions stable.