The heart is the organ that pushes blood through your body. It’s a muscular pump about the size of your fist, located slightly left of center in your chest, and it beats roughly 60 to 100 times per minute at rest. Over the course of a single day, the average heart pumps about 2,000 gallons of blood.
How the Heart Pumps Blood
The heart has four chambers that work in a coordinated sequence. The two upper chambers, called atria, collect incoming blood. The two lower chambers, called ventricles, do the heavy lifting of pushing blood out to the lungs and the rest of the body.
Each heartbeat follows a two-phase cycle. During the contraction phase (systole), the ventricles squeeze and force blood into the arteries. During the relaxation phase (diastole), the ventricles relax and refill with blood from the atria. This cycle repeats continuously from before birth until the moment you die.
The contraction phase itself happens in stages. First, the ventricles begin to squeeze while all four valves are closed, rapidly building pressure inside the chambers. Once that pressure exceeds the pressure in the outgoing arteries, the exit valves pop open and blood surges out. Initially the flow is fast and forceful, then it tapers as the ventricles empty. During relaxation, the process reverses: the exit valves snap shut, the ventricles relax, and once their pressure drops low enough, the inlet valves open and blood rushes in from the atria to refill them.
The Heart’s Two Circulation Loops
Your heart actually runs two pumping circuits at the same time, one on each side.
The right side handles pulmonary circulation. Oxygen-poor blood from your body enters the right atrium through two large veins, passes into the right ventricle, and gets pumped to the lungs. There it picks up fresh oxygen and releases carbon dioxide.
The left side handles systemic circulation. Oxygen-rich blood returns from the lungs to the left atrium, flows into the left ventricle, and gets pumped out through the aorta (the body’s largest artery) to supply every organ, tissue, and cell. The left ventricle has the thickest walls of any chamber because it needs to generate enough force to push blood all the way to your fingers and toes.
Valves Keep Blood Moving Forward
Four one-way valves prevent blood from sloshing backward between beats. Each valve has a set of flaps (called leaflets) that open to let blood through, then snap shut to seal the chamber behind it. The tricuspid valve sits between the right atrium and right ventricle. The pulmonary valve guards the exit from the right ventricle to the lungs. On the left side, the mitral valve separates the left atrium and left ventricle, and the aortic valve separates the left ventricle from the aorta.
These valves open and close in precise time with the pumping action. When they work properly, blood flows in one direction only. The familiar “lub-dub” sound of a heartbeat is the sound of these valves closing in sequence.
What Triggers Each Heartbeat
Unlike most muscles in your body, the heart generates its own electrical signals. A small cluster of cells in the upper right atrium, called the SA node, acts as the heart’s natural pacemaker. It fires an electrical impulse that spreads across both atria, causing them to contract and push blood into the ventricles.
The signal then reaches a relay point near the center of the heart called the AV node, which pauses it for a fraction of a second. That brief delay is important: it gives the atria time to fully empty before the ventricles fire. From there, the signal travels down a bundle of specialized fibers that branch out across both ventricles, triggering them to contract almost simultaneously. This entire sequence, from the initial spark to the full ventricular squeeze, happens in less than a second.
How the Heart Feeds Itself
The heart muscle works nonstop, so it needs its own dedicated blood supply. Two coronary arteries branch off the aorta and wrap around the outside of the heart, delivering oxygen-rich blood directly to the heart muscle. The left coronary artery feeds the left side (including the powerful left ventricle), while the right coronary artery supplies the right side. When one of these arteries gets blocked, the heart muscle downstream is starved of oxygen. That’s a heart attack.
Blood Pressure and the Heart’s Force
The blood pressure reading you get at a doctor’s office is a direct measurement of your heart’s pumping force. The top number (systolic pressure) reflects the pressure inside your arteries during each contraction. The bottom number (diastolic pressure) reflects the pressure during the resting phase between beats, and it’s always lower because the heart isn’t actively pushing at that moment.
Your Heart Gets Help
While the heart does the primary work of pushing blood, it doesn’t work entirely alone. Two other mechanisms assist with returning blood from the lower body back up to the heart, working against gravity.
The first is the skeletal muscle pump. When your leg muscles contract during walking, running, or even standing, they squeeze the veins running through them. Because veins contain small one-way valves, this squeezing pushes blood upward toward the heart while preventing it from pooling in your feet. This is one reason prolonged sitting or standing still can cause swollen ankles: without regular muscle contractions, blood return slows down.
The second is the respiratory pump. Every time you breathe in, pressure drops inside your chest cavity. That pressure drop pulls blood from the veins in your abdomen upward into the chest and toward the heart. When you exhale, the rising pressure in the chest speeds that blood into the heart’s atria. Together, these two pumps supplement the heart’s effort and keep blood circulating efficiently throughout the entire body.