Your heart pumps blood through two separate loops, one to your lungs and one to the rest of your body, by squeezing and relaxing in a precisely timed rhythm. At rest, it pushes 5 to 6 liters of blood per minute through roughly 60,000 miles of blood vessels. Every beat involves electrical signals, pressure changes, and four one-way valves working in sequence to keep blood flowing in the right direction.
The Four Chambers and Their Jobs
The heart is divided into four hollow chambers. The two upper chambers, called atria, receive incoming blood. The two lower chambers, called ventricles, do the heavy pumping. A muscular wall down the middle keeps oxygen-rich blood on the left side completely separate from oxygen-poor blood on the right.
The right side handles the lung loop. Oxygen-poor blood returning from your body enters the right atrium through two large veins, then passes into the right ventricle, which pumps it to the lungs. The left side handles the body loop. Freshly oxygenated blood from the lungs enters the left atrium, drops into the left ventricle, and gets launched out through the aorta to supply every organ and tissue. The left ventricle has the thickest walls because it generates the most pressure, peaking at about 120 mmHg to push blood through the entire systemic circulation. The right ventricle only needs about 25 mmHg to reach the nearby lungs.
Two Circuits Working Together
Blood travels in a figure-eight pattern through two distinct circuits. The pulmonary circuit is the short loop: the right ventricle sends oxygen-depleted blood to the lungs, where it releases carbon dioxide and picks up fresh oxygen, then returns to the left atrium. The systemic circuit is the long loop: the left ventricle sends oxygen-rich blood out through arteries to capillaries throughout your body, where cells absorb oxygen and nutrients and dump waste products. The now oxygen-poor blood collects in veins and drains back to the right atrium, completing the cycle.
One counterintuitive detail: arteries don’t always carry oxygenated blood. The pulmonary artery carries oxygen-poor blood from the heart to the lungs, and the pulmonary veins carry oxygen-rich blood back. The naming is based on direction of flow (away from or toward the heart), not oxygen content.
How Valves Keep Blood Moving Forward
Four valves act as one-way doors, opening and closing based on pressure differences on either side. When pressure behind a valve is greater than pressure in front of it, the valve swings open. When pressure reverses, the valve snaps shut to prevent backflow.
- Tricuspid valve: sits between the right atrium and right ventricle
- Pulmonary valve: sits between the right ventricle and the pulmonary artery
- Mitral valve: sits between the left atrium and left ventricle
- Aortic valve: sits between the left ventricle and the aorta
The classic “lub-dub” sound of a heartbeat comes from these valves closing. The first sound (“lub”) is the tricuspid and mitral valves shutting as the ventricles start to squeeze. The second sound (“dub”) is the pulmonary and aortic valves shutting after the ventricles finish ejecting blood.
The Electrical System That Triggers Each Beat
Your heart doesn’t wait for instructions from the brain. It generates its own electrical impulses through a built-in conduction system, which is why a heart can keep beating even when removed from the body.
Each beat starts at the sinoatrial node, a small cluster of cells in the upper right atrium that acts as the heart’s natural pacemaker. This node fires an electrical signal that spreads across both atria, causing them to contract and push blood down into the ventricles. The signal then reaches the atrioventricular node, which sits between the atria and ventricles. This node deliberately delays the signal for a fraction of a second, giving the atria time to empty completely before the ventricles fire.
After the delay, the signal travels down a bundle of specialized fibers through the wall separating the ventricles, then branches out into a network of fibers that spread across the bottom of both ventricles. This causes the ventricles to contract from the bottom up, wringing blood upward and out through the pulmonary and aortic valves. The whole sequence, from the first electrical spark to a completed pump, takes less than a second.
What Happens in a Single Heartbeat
Each heartbeat has two main phases: systole (when the heart squeezes) and diastole (when it relaxes and refills). Within those phases, several distinct steps happen in rapid sequence.
During systole, the ventricles first contract with all four valves closed. This is a brief moment of buildup where pressure inside the ventricles skyrockets but no blood moves yet, like pressing on a sealed water balloon. Once ventricular pressure exceeds the pressure in the arteries, the outflow valves pop open and blood surges out. The initial rush is fast and forceful, then tapers off as the ventricles empty.
During diastole, the ventricles relax and their internal pressure drops quickly, approaching zero. Once ventricular pressure falls below atrial pressure, the inlet valves open and blood pours in rapidly from the atria. Most ventricular filling happens passively during this phase, driven by pressure differences alone. At the very end of diastole, the atria give a final squeeze to top off the ventricles before the next cycle begins.
For most adults at rest, this entire cycle repeats 60 to 100 times per minute.
How the Heart Adjusts During Exercise
When you start exercising, your muscles demand more oxygen, and your heart responds by increasing its output. It does this two ways: pumping more blood per beat (stroke volume) and beating faster (heart rate).
During light exercise, both stroke volume and heart rate increase. The heart fills with more blood between beats because returning blood flow speeds up, and it squeezes more forcefully to empty more completely. As exercise intensity climbs higher, stroke volume plateaus and further increases in output come almost entirely from a faster heart rate. A well-trained endurance athlete’s heart can pump significantly more blood per beat than an untrained heart, which is why athletes often have lower resting heart rates. Their hearts accomplish the same output with fewer, more powerful beats.
What Keeps the Whole System Running
The heart’s pumping ability depends on several factors working together: the strength of the muscle contractions, the timing of the electrical signals, the integrity of the valves, and the pressure in the blood vessels. A problem with any one of these can reduce the heart’s efficiency. Leaky valves allow blood to flow backward, forcing the heart to pump the same blood twice. Electrical misfires can make the chambers contract out of sync. Chronically high blood pressure forces the heart to work harder with every beat, eventually thickening and stiffening the muscle walls.
What makes the heart remarkable is its endurance. It beats roughly 100,000 times a day without rest, adjusting its output moment to moment based on what your body needs, whether you’re sleeping, sprinting, or standing up from a chair.