The heart’s primary job is to pump blood to every cell in your body. It delivers oxygen and nutrients your cells need to function and carries away carbon dioxide and other waste so your lungs, kidneys, and liver can dispose of them. Over an average lifetime, the heart beats more than 2.6 billion times to keep this cycle running without interruption.
How the Heart Moves Blood
Your heart is a muscular pump roughly the size of your fist, divided into four chambers. The two upper chambers (atria) receive blood flowing into the heart. The two lower chambers (ventricles) push blood out. The entire system runs on a two-loop design: one loop sends blood to the lungs, and the other sends it to the rest of the body. Both loops operate simultaneously with every beat.
At rest, the heart pumps about 5 to 6 liters of blood per minute. During intense exercise, a fit athlete’s heart can push more than 35 liters per minute by beating faster and squeezing harder with each contraction. Each individual squeeze, called a stroke, ejects roughly 70 milliliters of blood from each ventricle, about the volume of a shot glass.
The Two Circulation Loops
Pulmonary Loop: Heart to Lungs and Back
Oxygen-poor blood returning from your body enters the right side of the heart through two large veins. It flows into the right atrium, then drops into the right ventricle, which pumps it to the lungs through the pulmonary artery. In the lungs, the blood picks up fresh oxygen and releases carbon dioxide (which you then exhale). Oxygen-rich blood travels back to the heart through the pulmonary veins, entering the left atrium.
Systemic Loop: Heart to Body and Back
From the left atrium, oxygen-rich blood moves into the left ventricle. This is the most muscular chamber in the heart because it generates the high pressure needed to push blood through the aorta, the body’s largest artery, and out to every organ, muscle, and tissue. Once your cells absorb the oxygen and nutrients, the now oxygen-depleted blood returns to the right side of the heart, and the cycle starts again.
Valves Keep Blood Flowing One Way
Four valves inside the heart act as one-way gates. They open to let blood through, then snap shut to prevent it from flowing backward. Two valves sit between the atria and ventricles, and two sit at the exits of the ventricles where blood leaves toward the lungs and body. When a valve doesn’t close properly (a condition sometimes called a “leaky valve”), blood can flow in the wrong direction, forcing the heart to work harder.
The rhythmic sound of a heartbeat, the familiar “lub-dub,” comes from these valves closing. The first sound happens when the valves between the atria and ventricles shut. The second sound happens when the valves at the ventricle exits close.
The Heart’s Built-In Electrical System
Unlike most muscles, your heart doesn’t wait for a signal from the brain to contract. It has its own internal pacemaker, a small cluster of cells called the SA node, located in the right atrium. This node fires an electrical impulse that spreads across both atria, causing them to contract and push blood into the ventricles.
The signal then reaches a second relay point, the AV node, near the center of the heart. The AV node deliberately delays the impulse by a fraction of a second. That tiny pause is critical: it gives the atria enough time to finish emptying before the ventricles contract. Once the delay passes, the signal travels down into the ventricles, triggering the powerful squeeze that sends blood to the lungs and body.
Your nervous system fine-tunes the speed of this pacemaker depending on what you’re doing. During stress or physical activity, your fight-or-flight response speeds up the SA node. When you’re resting or digesting food, your rest-and-digest response slows it down. A normal resting heart rate for adults falls between 60 and 100 beats per minute, though well-trained athletes often have resting rates closer to 40 beats per minute because their hearts pump more blood per beat.
What Happens During a Single Beat
Each heartbeat is a precisely timed sequence of pressure changes. It breaks into two main phases: contraction (systole) and relaxation (diastole).
During systole, the ventricles squeeze. Pressure inside them rises rapidly, slamming the valves between the atria and ventricles shut. For a brief moment, all four valves are closed and pressure builds with nowhere to go. Once pressure in the left ventricle exceeds the pressure in the aorta (and the right ventricle exceeds the pressure in the pulmonary artery), the exit valves pop open and blood surges out. The first part of this ejection is fast and forceful. As the ventricles empty, the push gradually weakens.
During diastole, the ventricles relax. Pressure inside them drops, and the exit valves close as blood briefly tries to flow backward. For another brief moment, all four valves are closed again while the ventricles continue to relax. Once ventricular pressure falls below atrial pressure, the valves between the atria and ventricles open, and blood that has been pooling in the atria rushes in. The ventricles fill quickly at first, then slowly top off before the next contraction begins.
Beyond Oxygen: What Else the Heart Delivers
Oxygen transport gets the most attention, but the blood your heart circulates carries much more. Glucose, fatty acids, amino acids, vitamins, and minerals all hitch a ride to cells that need them. Hormones produced by glands like the thyroid, adrenal glands, and pancreas depend on blood flow to reach their target organs. Without the heart’s constant pumping, chemical signals between organs would never arrive.
The return trip is equally important. Blood picks up carbon dioxide (a byproduct of cellular energy production) and routes it to the lungs for exhalation. Other metabolic waste products travel through the bloodstream to the kidneys, which filter them out and send them to the bladder. The liver handles additional detoxification work, and it too depends on a steady supply of blood delivered by the heart. In this sense, the heart doesn’t just feed the body. It also takes out the trash.