Your heart pumps by squeezing and relaxing in a coordinated rhythm, pushing blood through four chambers in a specific sequence. Each beat sends oxygen-rich blood out to your body and routes oxygen-depleted blood to your lungs for a refill. At rest, this cycle repeats about 60 to 100 times per minute, moving roughly 2,000 gallons of blood every 24 hours.
Four Chambers, One Direction
The heart is divided into four chambers: two on the right (the right atrium and right ventricle) and two on the left (the left atrium and left ventricle). Blood always moves in one direction through these rooms, and four one-way valves keep it from flowing backward.
The journey starts when oxygen-poor blood returns from your body and enters the right atrium. From there it flows into the right ventricle, which pumps it to the lungs. In the lungs, blood drops off carbon dioxide and picks up fresh oxygen. This newly oxygenated blood travels back to the heart, entering the left atrium and then flowing into the left ventricle. The left ventricle, the strongest chamber, pumps that oxygen-rich blood out through your largest artery and into the rest of your body. After delivering oxygen to your tissues, the now-depleted blood returns to the right atrium, and the loop starts over.
Two Loops Running at Once
Your cardiovascular system runs two circuits simultaneously. The pulmonary circuit is the shorter loop: the right ventricle sends blood to the lungs and back. The systemic circuit is the longer one: the left ventricle sends blood to every organ, muscle, and tissue in your body before it circles back.
Because the systemic circuit covers so much more distance, the left ventricle generates significantly more pressure than the right. Peak pressure in the left ventricle reaches about 120 mmHg during a contraction, while the right ventricle only reaches about 25 mmHg. That’s why the muscular wall of the left ventricle is noticeably thicker than the right.
What Makes the Heart Beat: The Electrical System
Your heart doesn’t need your brain to tell it when to beat. It has its own built-in pacemaker called the SA node, a small cluster of specialized cells in the upper right atrium. The SA node fires an electrical signal that spreads across both atria, causing them to contract and push blood down into the ventricles.
That signal then reaches a second relay station, the AV node, located near the center of the heart. The AV node deliberately delays the signal by a fraction of a second. This tiny pause is critical: it gives the atria time to finish emptying before the ventricles start squeezing. After the delay, the signal travels down a bundle of specialized fibers through the center of the heart and branches out into a network of fibers embedded in the ventricle walls. These fibers deliver the signal almost simultaneously to every part of both ventricles, triggering a powerful, coordinated contraction that ejects blood into the arteries.
The Squeeze-and-Relax Cycle
Each heartbeat has two main phases: systole (contraction) and diastole (relaxation). Within those phases, several distinct events happen in rapid sequence.
When the ventricles first start to contract, all four valves are briefly closed. The ventricles are squeezing, but blood has nowhere to go yet, so pressure builds rapidly inside the chambers. This buildup phase lasts only a fraction of a second before pressure in the left ventricle exceeds the pressure in the aorta and pressure in the right ventricle exceeds the pressure in the pulmonary artery. At that point, the outflow valves pop open and blood surges into the arteries. Most of the blood is ejected in the first part of this phase, with the flow tapering off as the ventricles finish contracting.
Then the ventricles relax. Pressure inside them drops quickly, and the outflow valves snap shut (producing part of the familiar “lub-dub” sound). For another brief moment, all valves are closed again while pressure continues to fall. Once ventricular pressure drops below the pressure in the atria, the inlet valves open and blood rushes in from the atria, rapidly refilling the ventricles. The final portion of filling happens slowly, right up until the atria contract and top off the ventricles just before the next beat.
How Heart Muscle Creates Force
Heart muscle cells work differently from the muscles in your arms or legs, but they generate force through the same basic principle. Inside each cell, tiny protein filaments slide past each other, shortening the cell. When millions of cells shorten together, the chamber walls squeeze inward and push blood out.
The trigger for this contraction is calcium. When the electrical signal arrives at a heart muscle cell, calcium floods into the cell’s interior and binds to specific proteins attached to the filaments. This binding shifts the proteins out of the way, exposing connection points where the filaments can grab onto each other and pull. When calcium is pumped back out of the cell, those connection points are blocked again, the filaments release, and the muscle relaxes. This calcium-driven on-off switch repeats with every single heartbeat.
How Much Blood Each Beat Moves
Each contraction pushes out a specific volume of blood called stroke volume. In healthy adults at rest, this is roughly 75 mL for men and 66 mL for women, a little more than a shot glass per beat. Multiply that by your heart rate, and a resting heart pumps about 5 liters per minute, which is your entire blood volume circulated in roughly one minute.
During intense exercise, both heart rate and stroke volume increase, and cardiac output can rise to 20 liters per minute or more in fit individuals. The heart muscle itself is remarkably hungry for oxygen to fuel all this work. At rest, the heart extracts about 70% of the oxygen from its own blood supply, compared to about 40% for the brain. That’s one reason blocked coronary arteries are so dangerous: the heart has very little room to increase its oxygen extraction, so any reduction in blood flow hits hard.
How Your Body Adjusts the Pump
Your autonomic nervous system continuously fine-tunes heart rate and contraction strength without any conscious effort. Two opposing branches handle this. The parasympathetic branch (working through the vagus nerve) acts as a brake, slowing the heart. The sympathetic branch acts as an accelerator, speeding it up and making each contraction more forceful.
At rest, both branches are active and roughly balanced. When you stand up from a chair or start walking, your heart rate increases primarily because the parasympathetic brake eases off, not because the accelerator ramps up. At higher exercise intensities, the sympathetic system takes over as the dominant driver of heart rate, pushing it well above resting levels. This layered control system is also what responds to stress, temperature changes, and shifts in blood pressure, keeping your cardiac output matched to whatever your body needs at any given moment.