Your heart pumps about 70 milliliters of blood with every beat, pushing 5 to 6 liters through your body each minute while you’re sitting still. It does this through a precisely timed loop: blood enters the heart, gets pumped to the lungs for oxygen, returns, then gets pumped out to the rest of your body. Each heartbeat involves electrical signals, valve openings and closings, and coordinated muscle contractions that all happen in under a second.
The Two Loops Your Blood Travels
Your heart runs two separate circuits at the same time. The first is the pulmonary loop: the right side of the heart sends oxygen-depleted blood to your lungs, where it drops off carbon dioxide and picks up fresh oxygen. That refreshed blood then returns to the left side of the heart.
The second is the systemic loop. The left side of the heart pumps that oxygen-rich blood out through your arteries to every tissue in your body. Your cells absorb the oxygen and nutrients they need, then dump carbon dioxide and other waste products back into the blood. That used blood travels through your veins back to the right side of the heart, and the whole cycle starts over. These two loops run simultaneously, so one beat of your heart moves blood through both circuits at once.
Four Chambers, Four Roles
The heart has four hollow chambers arranged in two pairs. The two upper chambers are the atria, which receive incoming blood. The two lower chambers are the ventricles, which pump blood out.
Here’s the path blood follows through all four:
- Right atrium: Oxygen-poor blood from your body enters here through large veins.
- Right ventricle: Blood flows directly from the right atrium into the right ventricle, which pumps it to the lungs.
- Left atrium: Oxygen-rich blood returning from the lungs enters here.
- Left ventricle: Blood flows from the left atrium into the left ventricle, the strongest chamber, which pumps it out to your entire body through the aorta.
The left ventricle has the thickest walls because it needs to generate enough pressure to push blood all the way to your fingers and toes. The right ventricle only needs to push blood the short distance to your lungs, so it works under much lower pressure.
How the Valves Keep Blood Moving Forward
Four one-way valves prevent blood from flowing backward. They open and close based on pressure differences, like swinging doors that only open in one direction.
Two valves sit between the atria and ventricles. The tricuspid valve is the door between the right atrium and right ventricle. The mitral valve is the door between the left atrium and left ventricle. These open when the atria contract, letting blood drop into the ventricles. They snap shut the moment the ventricles start to squeeze, which prevents blood from washing back up into the atria. That closing is what creates the first sound of your heartbeat.
Two more valves guard the exits. The pulmonary valve opens when the right ventricle pushes blood toward the lungs. The aortic valve opens when the left ventricle pushes blood into the aorta. Once the ventricles relax, pressure in those arteries pushes these valves closed again, producing the second heart sound. Together, these two closings give you the familiar “lub-dub.”
The Electrical Signal That Starts Each Beat
Your heart doesn’t wait for your brain to tell it to beat. It has its own built-in pacemaker: a small cluster of cells in the upper right atrium called the SA node. This node fires an electrical impulse that spreads across both atria, causing them to contract and push blood down into the ventricles.
The signal then reaches a second relay point called the AV node, which sits between the atria and ventricles. The AV node deliberately delays the impulse for a fraction of a second. This pause is critical. It gives the atria time to finish emptying before the ventricles fire. Without that delay, the chambers would squeeze at the same time and the heart would pump far less efficiently.
After the delay, the signal travels down a bundle of specialized fibers running through the wall that separates the two ventricles, then branches out into a network of fibers called Purkinje fibers that spread across the ventricle walls. This causes both ventricles to contract almost simultaneously from the bottom up, wringing blood upward and out through the pulmonary and aortic valves.
One Heartbeat in Three Phases
A single heartbeat, called the cardiac cycle, has three main phases. At a resting heart rate of about 72 beats per minute, the entire cycle takes roughly 0.8 seconds.
Phase 1: The Atria Contract
The electrical signal from the SA node triggers both atria to squeeze. At this point, the valves between the atria and ventricles are already open, and blood has been passively flowing downward. The atrial contraction tops off the ventricles, adding the final 25% of their filling volume. By the end of this phase, each ventricle holds about 130 milliliters of blood.
Phase 2: The Ventricles Contract
This is the power stroke. As the ventricles begin to squeeze, pressure inside them rises sharply. The mitral and tricuspid valves slam shut. For a brief instant, all four valves are closed and the ventricles are contracting against a sealed chamber, building pressure rapidly without moving any blood. This moment is called isovolumic contraction.
Once pressure in the left ventricle exceeds the pressure in the aorta (and pressure in the right ventricle exceeds the pressure in the pulmonary artery), the exit valves pop open. Blood surges out, fast at first, then more slowly as the ventricles empty. Under resting conditions, about 70 milliliters leaves each ventricle per beat. That means roughly half the blood stays behind, giving the heart a reserve it can tap during exercise.
Phase 3: The Heart Relaxes
The ventricles stop contracting and pressure inside them drops. The aortic and pulmonary valves close as blood in the arteries briefly pushes backward against them. Again, for a short moment all four valves are closed while the ventricles relax. Once ventricular pressure falls below atrial pressure, the mitral and tricuspid valves open and blood begins flowing in from the atria again, mostly by gravity and the natural suction created by the relaxing ventricle walls. The heart spends more of its time in this relaxation phase than in contraction, which is when the heart muscle itself receives most of its own blood supply.
How Your Heart Speeds Up and Slows Down
A normal resting heart rate for adults falls between 60 and 100 beats per minute. Your nervous system adjusts this rate constantly based on what your body needs.
Two branches of the nervous system pull the heart rate in opposite directions. The parasympathetic branch (sometimes called “rest and digest”) acts like a brake, keeping your heart rate low when you’re calm. The sympathetic branch (your “fight or flight” system) acts like a gas pedal. At rest, both systems are active and roughly balanced, working together to hold your rate steady.
When you start mild exercise, your body initially speeds up the heart mainly by releasing the brake, reducing parasympathetic activity. As exercise intensity increases, the sympathetic system kicks in more aggressively, releasing signals that make the SA node fire faster. Your heart rate can double or triple during intense exertion, and each beat can push out more blood than the resting 70 milliliters, dramatically increasing total output. When you stop exercising, the parasympathetic brake gradually re-engages and your heart rate drifts back down.
Pressure sensors in your major arteries also help regulate heart rate on a beat-to-beat basis. When blood pressure rises, these sensors signal the heart to slow down. When it drops, they signal it to speed up. This feedback loop keeps blood pressure stable as you change positions, stand up, or shift activity throughout the day.
Putting It All Together
Every heartbeat is the same coordinated sequence: an electrical impulse fires, the atria contract, a brief delay lets them empty, the ventricles contract, blood exits through one-way valves, the heart relaxes, and blood refills the chambers. At 70 beats per minute, your heart performs this cycle about 100,000 times a day, pumping roughly 7,500 liters of blood. The left side handles the long trip to your body, the right side handles the short trip to your lungs, and both sides work in perfect sync so oxygen-rich blood is always available where your cells need it.