Your heart pumps blood through two separate loops, one to your lungs and one to the rest of your body, by contracting its four chambers in a precisely timed sequence roughly 60 to 100 times every minute. At rest, this process moves about 5 to 6 liters of blood per minute. Every contraction is triggered by an electrical signal that originates inside the heart itself, making it one of the few organs that generates its own rhythm.
Four Chambers, Two Circuits
The heart is divided into a right side and a left side, each with an upper chamber (atrium) and a lower chamber (ventricle). The right side handles the pulmonary circuit: it receives oxygen-depleted blood returning from your body and pumps it to your lungs, where it picks up fresh oxygen and releases carbon dioxide. The left side handles the systemic circuit: it receives that newly oxygenated blood from the lungs and pumps it out to every tissue in your body.
The path looks like this: blood from your body enters the right atrium through two large veins, then moves into the right ventricle, which pushes it to the lungs. Oxygenated blood returns from the lungs into the left atrium, drops into the left ventricle, and gets launched into the aorta to supply the entire body. From there it travels through progressively smaller arteries and capillaries, delivers oxygen and nutrients to cells, collects waste products, and loops back to the right atrium to start again.
The left ventricle has the thickest muscular wall of all four chambers because it generates the highest pressure. It needs enough force to push blood all the way to your toes and back. The right ventricle, by contrast, only needs to push blood the short distance to your lungs, so it operates at much lower pressure.
The Electrical System That Keeps It Beating
Your heart doesn’t wait for instructions from your brain. A small cluster of specialized cells in the upper right atrium, called the sinoatrial (SA) node, spontaneously generates an electrical impulse that sets your heart rate. These cells depolarize on their own in a repeating cycle, acting as the heart’s natural pacemaker.
Each impulse spreads across both atria, causing them to contract and push blood down into the ventricles. The signal then reaches a second relay point, the atrioventricular (AV) node, which sits between the atria and ventricles. The AV node introduces a brief delay, about a tenth of a second, so the ventricles have time to fill before they contract. After the delay, the signal travels rapidly down a specialized pathway into the ventricle walls, causing both ventricles to squeeze almost simultaneously from the bottom up. This bottom-up contraction is what efficiently ejects blood upward into the arteries leaving the heart.
What Happens in a Single Heartbeat
Each heartbeat is a cycle of contraction (systole) and relaxation (diastole), and it happens in distinct phases that take less than a second combined.
First, the ventricles begin to contract with all their valves closed. Pressure builds inside the chambers but no blood moves yet. This brief pressure-building phase accounts for about 6% of the cardiac cycle. Once ventricular pressure exceeds the pressure in the arteries, the outlet valves pop open and blood rushes out in a rapid ejection phase lasting about 13% of the cycle. Ejection then tapers off as the ventricle muscle begins to relax, though some blood continues flowing forward on momentum alone.
Next comes relaxation. The outlet valves snap shut (producing the second heart sound you hear through a stethoscope), and the ventricles relax with all valves closed again. Pressure drops quickly. Once ventricular pressure falls below atrial pressure, the inlet valves open and blood pours in from the atria. This filling phase is the longest part of the cycle, taking up about 44% of each beat. Near the very end, the atria give a final squeeze that contributes roughly 10% of the total blood volume entering the ventricles, topping them off just before the next contraction begins.
How the Heart Adjusts Its Own Strength
The heart has a built-in mechanism for matching its output to your body’s needs, even without signals from the nervous system. When more blood flows back to the heart (during exercise, for example), the ventricles stretch more as they fill. That extra stretch causes the muscle fibers to contract more forcefully on the next beat, ejecting the larger volume of blood. This relationship, known as the Frank-Starling mechanism, means the heart automatically pumps out whatever volume it receives. If you stand up suddenly and less blood returns to the heart, the next beat is a little weaker. If you start jogging and your muscles send more blood back, the next beat is stronger.
On top of this, your nervous system and hormones like adrenaline can increase both heart rate and contraction strength during stress or exercise, pushing cardiac output well above the resting 5 to 6 liters per minute. Trained endurance athletes can reach outputs of 30 liters per minute or more at peak effort.
How the Heart Feeds Itself
The heart muscle works nonstop, so it needs its own dedicated blood supply. This comes from the coronary arteries, which branch off the aorta right at its base and wrap around the outside of the heart. Two main coronary arteries do the job. The left main coronary artery splits into branches that feed the front and back of the left side, including the thick-walled left ventricle and the septum (the wall dividing the two sides). The right coronary artery supplies the right side of the heart, including the SA and AV nodes that control rhythm.
Coronary blood flow has an unusual timing. Because the heart muscle squeezes so tightly during contraction, blood flow through the coronary arteries actually drops during systole and peaks during diastole, when the muscle relaxes. This is why a faster heart rate, which shortens diastole, can reduce the time available for the heart to feed itself.
Blood Pressure and the Pumping Cycle
The blood pressure reading you get at a doctor’s office directly reflects these two phases. The top number (systolic pressure) measures the force on your artery walls when the left ventricle contracts and ejects blood. The bottom number (diastolic pressure) measures the residual pressure in your arteries while the heart is relaxing and refilling. A normal resting heart rate falls between 60 and 100 beats per minute, and each beat ejects roughly 70 milliliters of blood from the left ventricle. Multiply those together and you get the 5 to 6 liters per minute that keeps oxygen flowing to every cell in your body.
When arteries stiffen or narrow, the heart has to generate more force to push the same volume of blood through, which raises systolic pressure. Over time, this extra workload can thicken the heart muscle and eventually weaken it, which is why sustained high blood pressure is one of the most significant risk factors for heart failure.