Your heart pumps blood through a coordinated cycle of electrical signals, muscle contractions, and one-way valves that repeats roughly 60 to 100 times per minute at rest. Each beat pushes about 70 milliliters of blood out of the left ventricle, and at that pace, the heart moves 5 to 6 liters of blood through your body every minute. Here’s how the whole process works, from the signal that starts each beat to the two separate loops blood travels through.
The Electrical Signal That Starts Each Beat
Every heartbeat begins with a tiny burst of electricity from a cluster of cells in the upper right chamber of your heart called the SA node. This is your heart’s natural pacemaker. The signal spreads across both upper chambers (the atria), causing them to contract and push blood downward into the lower chambers (the ventricles).
Before the signal reaches the ventricles, it passes through a relay point called the AV node. This node deliberately delays the signal for a fraction of a second, giving the atria time to finish emptying. The signal then travels down a bundle of specialized fibers through the center of the heart and fans out into a network called the Purkinje fibers, which deliver the signal to the ventricles. Both ventricles contract almost simultaneously, squeezing blood out to the lungs and the rest of the body.
What Happens During a Single Heartbeat
Each heartbeat has two main phases: contraction (systole) and relaxation (diastole). Within those phases, the heart moves through a precise sequence driven entirely by pressure changes.
When the ventricles first start contracting, pressure inside them rises sharply. This immediately forces the valves between the upper and lower chambers to snap shut, which is what produces the first sound of your heartbeat. For a brief moment, all four valves are closed and the ventricles are building pressure against a sealed chamber. The left ventricle generates a peak pressure of about 130 mmHg during this squeeze, while the right ventricle reaches around 25 mmHg. The left side works much harder because it needs to push blood through the entire body.
Once ventricular pressure exceeds the pressure in the large arteries leaving the heart, the outflow valves pop open and blood surges out. The left ventricle ejects blood into the aorta, and the right ventricle sends blood to the pulmonary artery heading toward the lungs. This ejection phase is the moment blood actually moves forward.
After the ventricles finish contracting, they relax and pressure drops quickly. The blood in the arteries briefly pushes backward, slamming the outflow valves shut (that’s the second heart sound). Again, all four valves are momentarily closed. As ventricular pressure continues to fall below the pressure in the atria, the valves between the chambers open and blood rushes in from above. The ventricles fill passively at first, then the atria give a final squeeze to top them off. That completes one cycle, and the next electrical signal is already on its way.
Four Valves Keep Blood Moving Forward
The heart has four valves, each made of thin but strong flaps of tissue that act like one-way doors. They open only when pressure on one side exceeds the other, and they prevent blood from flowing backward.
- Tricuspid valve: sits between the right atrium and right ventricle, with three leaflets that let blood flow down into the ventricle and block it from leaking back up.
- Pulmonary valve: guards the exit from the right ventricle into the pulmonary artery, preventing blood from sliding back after it’s been pushed toward the lungs.
- Mitral valve: the only valve with two leaflets instead of three, positioned between the left atrium and left ventricle.
- Aortic valve: controls the exit from the left ventricle into the aorta, keeping high-pressure blood from falling back into the heart between beats.
These valves open and close passively based on pressure differences alone. They have no muscles of their own. When you hear a heartbeat through a stethoscope, you’re hearing valves snapping shut: first the two valves between the chambers, then the two outflow valves.
Two Circuits: Lungs and Body
Blood doesn’t travel in one big loop. It follows two separate circuits, and the heart powers both of them simultaneously.
The pulmonary circuit is the shorter loop. Oxygen-depleted blood from the body enters the right atrium, passes into the right ventricle, and gets pumped to the lungs through the pulmonary artery. In the lungs, blood picks up fresh oxygen and releases carbon dioxide. It then returns to the left atrium through the pulmonary veins.
The systemic circuit is the longer, higher-pressure loop. Oxygen-rich blood enters the left ventricle and gets pumped into the aorta, the body’s largest artery. From there, it branches into smaller and smaller vessels until it reaches capillaries in your tissues, where oxygen is delivered and carbon dioxide is collected. The now oxygen-poor blood gathers into progressively larger veins until it reaches two major vessels that drain into the right atrium, completing the circle.
The right and left sides of the heart pump the same volume of blood per beat, but at very different pressures. The left ventricle generates roughly five times the pressure of the right because it needs to push blood all the way to your fingers and toes. The right ventricle only needs to reach the nearby lungs.
How Much Blood the Heart Moves
At rest, the left ventricle ejects about 75 milliliters per beat in men and about 66 milliliters in women. Multiply that by a resting heart rate of 60 to 100 beats per minute, and the heart pumps 5 to 6 liters of blood per minute. That’s roughly your entire blood volume circulating once every minute while you’re sitting still.
During intense exercise, cardiac output can increase four to five times above resting levels. The heart achieves this by both beating faster and squeezing out more blood per beat.
What Changes Your Heart’s Pumping Strength
The volume of blood your heart ejects per beat isn’t fixed. It adjusts constantly based on three main factors.
The first is how much blood returns to the heart through the veins. When more blood fills the ventricle before it contracts, the muscle fibers stretch further, and the heart responds with a stronger contraction. This is why hydration matters: when you’re dehydrated, less blood returns to the heart, the ventricle doesn’t fill as much, and each beat pumps out less blood. It’s also why lying down can make your heart work more efficiently, because gravity helps blood return from your legs.
The second factor is how hard the heart muscle itself can contract, independent of how stretched it is. Adrenaline increases this contractile force, which is why your heart pounds harder when you’re startled or exercising. Over time, regular aerobic exercise strengthens the heart muscle so it can eject more blood per beat at rest, which is why trained athletes often have resting heart rates well below 60 beats per minute, sometimes closer to 40.
The third factor is the resistance the heart has to pump against. If your arteries are stiff or narrowed, the ventricle must generate more pressure to push blood through. This increased workload is one reason chronic high blood pressure causes the heart muscle to thicken over time, which can eventually reduce its efficiency rather than improve it.