Your heart is a muscular pump roughly the size of your fist that beats about 100,000 times a day, pushing around 2,000 gallons of blood through your body every 24 hours. At rest, it moves 5 to 6 liters of blood per minute. During intense exercise, that number can climb above 35 liters per minute. Everything about the heart’s design, from its four chambers to its built-in electrical wiring, exists to keep blood flowing in one direction: out to your body, back to your lungs, and out again.
Four Chambers, Two Pumps
The heart is divided into a right side and a left side, each with an upper chamber (atrium) and a lower chamber (ventricle). The right side handles used, oxygen-poor blood. The left side handles freshly oxygenated blood. Together, they form two pumps working in sync.
Blood returning from your body enters the right atrium, then drops into the right ventricle, which pushes it to your lungs. In the lungs, blood picks up oxygen and releases carbon dioxide. That oxygen-rich blood flows back into the left atrium, passes into the left ventricle, and gets launched out to the rest of your body. The left ventricle is the strongest chamber because it needs to generate enough pressure to push blood all the way from your chest to your fingertips and toes. Peak pressure in the left ventricle reaches 90 to 140 mm Hg during contraction, compared to just 15 to 30 mm Hg on the right side, which only needs to reach the nearby lungs.
How Valves Keep Blood Moving Forward
Four one-way valves prevent blood from flowing backward. Each valve has flaps (called cusps) that snap open to let blood through, then close tightly so it can’t leak back.
- Tricuspid valve: sits between the right atrium and right ventricle
- Pulmonary valve: sits between the right ventricle and the artery leading to the lungs
- Mitral valve: sits between the left atrium and left ventricle
- Aortic valve: sits between the left ventricle and the aorta, the body’s largest artery
When you hear a heartbeat through a stethoscope, those two sounds (“lub-dub”) are valves closing. The first sound is the tricuspid and mitral valves shutting as the ventricles begin to squeeze. The second is the pulmonary and aortic valves closing after blood has been pushed out.
The Heart’s Built-In Electrical System
Your heart doesn’t wait for your brain to tell it to beat. It generates its own electrical signals through a network of specialized cells that act like built-in wiring.
Each heartbeat begins at a small cluster of cells in the upper right atrium called the 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 down into the ventricles. The signal then reaches the AV node, a relay station that deliberately pauses the impulse for a fraction of a second. That brief delay is critical: it gives the atria time to fully empty before the ventricles start squeezing.
After the pause, the signal travels down a bundle of nerve fibers through the center of the heart and branches out into a network of fine fibers that reach every part of both ventricles. This triggers the ventricles to contract from the bottom up, wringing blood upward and out through the pulmonary and aortic valves. The entire electrical sequence happens in less than a second and repeats 60 to 100 times per minute at rest.
What Happens in a Single Heartbeat
Each heartbeat has two main phases: contraction (systole) and relaxation (diastole). Within those phases, pressure changes open and close the valves in a precise sequence.
During systole, the ventricles begin squeezing. For a brief moment, all four valves are closed because the pressure inside the ventricles is rising but hasn’t yet exceeded the pressure in the outgoing arteries. No blood moves during this instant. Once ventricular pressure climbs high enough, the aortic and pulmonary valves pop open and blood surges out. The initial burst is fast and forceful, then tapers off as the ventricles empty.
Diastole begins when the ventricles relax. Pressure inside them drops, and blood in the arteries briefly pushes backward, snapping the aortic and pulmonary valves shut. Again, there’s a moment when all four valves are closed and no blood moves. As ventricular pressure continues to fall below atrial pressure, the mitral and tricuspid valves open and blood rushes in from the atria. Most filling happens quickly at first, then slows to a trickle until the atria contract at the very end of diastole to top off the ventricles. Then the cycle repeats.
Two Loops of Circulation
Blood travels through your body in two distinct circuits that run simultaneously.
The pulmonary circuit is the shorter loop. It carries oxygen-depleted blood from the right ventricle to the lungs, where it passes through tiny capillaries surrounding air sacs. Carbon dioxide leaves the blood and oxygen enters it. The refreshed blood returns to the left atrium through the pulmonary veins.
The systemic circuit is the longer loop. It sends oxygen-rich blood from the left ventricle into the aorta, which branches into progressively smaller arteries reaching every organ and tissue. In the capillaries, oxygen and nutrients pass into cells while carbon dioxide and waste products pass into the blood. That used blood collects into small veins, which merge into larger and larger veins until it reaches two main vessels (the superior and inferior vena cava) that drain into the right atrium. From there, the cycle starts over.
How the Heart Feeds Itself
Despite being filled with blood at all times, the heart muscle can’t absorb oxygen directly from the blood passing through its chambers. It needs its own dedicated blood supply, delivered through the coronary arteries. These small arteries branch off from the aorta just above the aortic valve and wrap around the outside of the heart, feeding oxygen and nutrients to the muscle tissue that does all the pumping work. When coronary arteries become narrowed or blocked, the heart muscle is starved of oxygen, which is what causes a heart attack.
Signs the Heart Isn’t Working Well
When the heart can’t pump effectively, blood flow slows and fluid starts backing up in the vessels. That backup causes fluid to leak out of blood vessels and collect in tissues throughout the body. The most common symptoms people notice are shortness of breath during everyday activities like climbing stairs, persistent fatigue that doesn’t improve with rest, and swelling in the ankles, lower legs, or abdomen from fluid buildup.
Other signs include a persistent cough, difficulty sleeping while lying flat (because fluid shifts toward the lungs), swollen neck veins, loss of appetite, nausea, and frequent urination as the body tries to get rid of excess fluid. Shortness of breath during routine physical activity is often one of the earliest warning signs that the heart’s pumping ability has declined.