Your heart is a muscular pump about the size of your fist that beats roughly 100,000 times a day, pushing around 7,500 liters (2,000 gallons) of blood through your body every 24 hours. It does this by coordinating a precise sequence of electrical signals, muscle contractions, and one-way valves that keep blood flowing in a single direction. Here’s how all those pieces fit together.
Four Chambers, Two Pumps
The heart is divided into four hollow chambers. The two upper chambers are called atria, and the two lower chambers are called ventricles. A muscular wall down the middle separates the left side from the right, so oxygen-rich blood and oxygen-poor blood never mix.
Think of the heart as two pumps sitting side by side. The right side handles used, oxygen-poor blood coming back from your body and sends it to the lungs to pick up fresh oxygen. The left side receives that freshly oxygenated blood from the lungs and pumps it out to the rest of your body. The left ventricle is the strongest chamber because it has to generate enough pressure to push blood all the way to your head, your toes, and everywhere in between.
The Path Blood Takes
Blood follows a continuous loop, and the easiest way to understand it is to start where oxygen-poor blood returns to the heart. Two large veins, the superior and inferior vena cava, deliver this blood into the right atrium. From there it passes into the right ventricle, which pumps it to the lungs. This short trip between the heart and lungs is called pulmonary circulation.
In the lungs, blood drops off carbon dioxide and picks up oxygen. The now oxygen-rich blood flows back to the heart, entering the left atrium. It then moves into the left ventricle, which pumps it out through the aorta, the body’s largest artery, to supply every organ and tissue. This longer loop is called systemic circulation. Once cells throughout the body have used the oxygen, the blood returns through veins to the right atrium, and the whole cycle starts again.
How Valves Keep Blood Moving Forward
Four valves act as one-way doors, opening to let blood through and snapping shut to prevent it from flowing backward. Each valve has a set of thin flaps called leaflets that open and close in time with each heartbeat.
- 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 “lub-dub” sound you hear through a stethoscope comes from these valves closing. The first sound is the tricuspid and mitral valves shutting as the ventricles begin to contract. The second sound is the pulmonary and aortic valves closing once the ventricles finish pumping.
The Heart’s Built-In Electrical System
Unlike skeletal muscles, which need a signal from your brain to move, the heart generates its own electrical impulses. A small cluster of cells in the upper right atrium, called the sinus node (or SA node), fires an electrical signal that starts each heartbeat. This is your heart’s natural pacemaker.
The signal spreads across both atria, causing them to contract and push blood into the ventricles. It then arrives at a relay station between the atria and ventricles called the AV node. Here the signal pauses for a fraction of a second. That brief delay is critical: it gives the ventricles time to fill with blood before they contract.
After the pause, the signal travels down a pathway called the bundle of His, which splits into left and right branches that spread through the walls of each ventricle. This branching design ensures both ventricles contract in a coordinated, bottom-to-top squeeze that efficiently pushes blood out through the valves and into the arteries.
Systole and Diastole: The Two Phases
Every heartbeat has two phases. During systole, the ventricles contract and eject blood. During diastole, the ventricles relax and refill. At a typical resting heart rate of about 75 beats per minute, one complete cycle takes roughly 0.8 seconds. Contraction takes up about one-third of that time, while relaxation takes the other two-thirds. Your heart actually spends more time resting than working during each beat.
With each contraction, the left ventricle ejects about 70 milliliters of blood, a little over a shot glass worth. That adds up quickly. At rest, the heart pumps roughly 5 to 7 liters per minute, enough to circulate your entire blood volume (about 5 liters) in under a minute. During intense exercise, cardiac output can increase four- or fivefold.
What Makes Heart Muscle Special
Cardiac muscle is unlike any other muscle in your body. It contracts involuntarily, meaning you don’t have to think about it. The cells are densely packed with mitochondria, the tiny structures inside cells that produce energy. This is because the heart can never take a break; it needs a constant, enormous supply of fuel to keep beating around the clock.
Cardiac muscle cells also have a unique way of handling calcium, the mineral that triggers contraction. In your arm or leg muscles, an electrical signal directly triggers calcium release inside the cell. In the heart, a small amount of calcium enters the cell first, and that initial burst triggers a much larger release of calcium from internal stores. This “calcium-triggered calcium release” gives the heart fine-tuned control over the strength of each contraction.
How the Heart Feeds Itself
Even though blood constantly passes through the heart’s chambers, the heart muscle can’t absorb oxygen directly from that blood. Instead, it has its own dedicated blood supply through the coronary arteries. These arteries branch off the aorta just above the aortic valve and wrap around the outside of the heart, delivering oxygen and nutrients to every layer of heart muscle. When a coronary artery becomes blocked, the portion of heart muscle it feeds is starved of oxygen, which is what causes a heart attack.
How Your Body Adjusts Heart Rate
Your nervous system constantly fine-tunes how fast and hard your heart beats to match what your body needs at any given moment. Two opposing branches handle this. The sympathetic branch acts like a gas pedal. When you exercise, feel stressed, or sense danger, it releases chemical signals that speed up the SA node’s firing rate and strengthen each contraction. The parasympathetic branch acts like a brake. When you’re resting or sleeping, it slows the heart down by reducing the SA node’s firing rate.
At rest, the brake is slightly dominant, which is why a normal resting heart rate for most adults falls between 60 and 100 beats per minute. Well-trained athletes often have resting rates in the 40s or 50s because regular cardiovascular exercise makes the heart more efficient, pumping more blood per beat so it doesn’t need to beat as often. Children’s hearts are smaller and beat faster to compensate: a newborn’s resting rate can be as high as 205 bpm, gradually slowing as the child grows until it reaches adult levels by the teenage years.
Putting It All Together
Every heartbeat is a tightly choreographed event. The SA node fires. Electrical signals sweep across the atria, which contract and fill the ventricles. The signal pauses at the AV node, then races down through the ventricle walls. Both ventricles contract together, squeezing blood through the pulmonary and aortic valves. The valves snap shut, the ventricles relax, refill, and the cycle repeats. All of this happens in less than a second, tens of thousands of times a day, without a single conscious thought from you.