The heart is a muscular pump that moves blood through your entire body, delivering oxygen and nutrients to every cell and carrying waste products away. It beats roughly 60 to 100 times per minute at rest, pushing 5 to 6 liters of blood through your circulatory system every minute. That continuous pumping is what keeps every organ and tissue alive.
How Blood Moves Through the Heart
The heart has four chambers: two upper chambers called atria and two lower chambers called ventricles. Blood flows through these chambers in a specific sequence that separates oxygen-poor blood from oxygen-rich blood.
The right side of the heart handles blood that has already delivered its oxygen to the body. This oxygen-depleted blood 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. This loop is called pulmonary circulation.
The freshly oxygenated blood then returns to the heart’s left atrium through the pulmonary veins, flows into the left ventricle, and gets pumped out to the rest of the body through the aorta, the body’s largest artery. This second loop, called systemic circulation, is what supplies oxygen-rich blood to your brain, muscles, digestive organs, and everything else. The left ventricle is the most muscular chamber because it needs enough force to push blood all the way to your toes and back.
Valves Keep Blood Flowing One Way
Four valves inside the heart act like one-way doors, opening to let blood through and snapping shut to prevent it from flowing backward. The tricuspid valve sits between the right atrium and right ventricle. The pulmonary valve guards the exit from the right ventricle into the lungs. On the left side, the mitral valve separates the left atrium from the left ventricle, and the aortic valve controls the exit into the aorta.
Each valve opens when blood pressure on one side exceeds the pressure on the other, then closes tightly once the blood has passed through. The “lub-dub” sound of a heartbeat is actually the sound of these valves closing in sequence. When a valve doesn’t close properly or becomes stiff, blood can leak backward or struggle to pass through, forcing the heart to work harder.
The Heart’s Built-In Electrical System
Your heart doesn’t wait for a signal from your brain to beat. It generates its own electrical impulses through a specialized conduction system. The process starts in a small cluster of cells in the right atrium called the sinoatrial (SA) node, often referred to as the heart’s natural pacemaker. This node fires an electrical signal that spreads across both atria, causing them to contract and push blood into the ventricles.
The signal then reaches a second relay point called the atrioventricular (AV) node, which sits between the atria and ventricles. The AV node deliberately delays the signal for a fraction of a second. That brief pause gives the atria time to finish emptying before the ventricles contract. From there, the signal travels down a bundle of specialized nerve fibers through the center of the heart, splits into left and right branches, and fans out through a network of fibers that trigger both ventricles to contract almost simultaneously. This coordinated squeeze is what pushes blood out to your lungs and body with each beat.
The Cardiac Cycle: Squeeze and Relax
Each heartbeat consists of two main phases. During systole, the ventricles contract and eject blood into the arteries. During diastole, the ventricles relax and refill with blood from the atria. At a normal resting heart rate, each complete cycle takes less than a second, with the relaxation phase lasting slightly longer than the contraction phase. That longer relaxation period is important because it gives the heart muscle time to receive its own blood supply and allows the chambers to fill adequately before the next squeeze.
During exercise, your heart rate increases and both phases shorten, but diastole shortens more dramatically. This is one reason extremely high heart rates can become problematic: the heart doesn’t get enough time to fill between beats, which reduces the amount of blood it can pump.
What Happens at the Capillary Level
The heart’s pumping force doesn’t just move blood through large arteries. It drives blood all the way to the capillaries, the smallest blood vessels in your body. Capillary walls are only about one micrometer thick, thin enough for oxygen and nutrients to pass through and reach individual cells. At the same time, carbon dioxide and other waste products move from cells back into the capillaries to be carried away.
This exchange is the entire point of the heart’s work. Without sufficient pumping pressure, blood can’t reach the capillary beds efficiently, and tissues start to suffer from oxygen deprivation. Every organ depends on this delivery system, from the brain (which consumes a disproportionate share of the body’s oxygen) to the kidneys (which filter waste from the blood).
The Heart Also Produces Hormones
Beyond pumping blood, the heart functions as a hormone-producing organ. When the heart’s chambers stretch due to increased blood volume or pressure, the muscle cells in the atria and ventricles release hormones that help regulate blood pressure. These hormones cause the kidneys to excrete more sodium and water, which reduces blood volume and lowers blood pressure. They also counteract other hormones in the body that would otherwise constrict blood vessels and retain fluid.
This hormonal role gives the heart a feedback mechanism: when blood pressure gets too high and the chambers stretch more than normal, the heart itself releases signals to bring that pressure back down. It’s a self-protective system built directly into the organ.
How Heart Rate Varies by Age and Sex
Resting heart rate is one of the simplest indicators of cardiovascular fitness, and it naturally varies depending on age and sex. Data from the Apple Heart and Movement Study found that men averaged about 65.5 beats per minute in their late teens, with a slight peak in the 30 to 49 age range, followed by a steady decline to around 57.8 bpm by the 80s. Women consistently had slightly higher resting heart rates across most age groups, starting at 66.8 bpm in the late teens, peaking near 69 bpm in the 30 to 49 range, and declining to about 61.6 bpm in the oldest group.
A lower resting heart rate generally reflects a stronger, more efficient heart that can pump the same amount of blood with fewer beats. This is why endurance athletes often have resting heart rates well below 60. The 5 to 6 liters per minute the heart pumps at rest stays roughly constant across individuals. A fit heart achieves that output with larger, more forceful contractions and fewer beats, while a less conditioned heart compensates with a faster rate.