Your heart is a muscular pump about the size of your fist, beating roughly 100,000 times a day to push blood through your entire body. Over an average lifetime, it beats more than 2.5 billion times. At rest, it pumps 5 to 6 liters of blood per minute, delivering oxygen and nutrients to every organ and tissue while carrying waste products away. Understanding how it accomplishes this starts with its structure, then follows the blood on its journey.
The Four Chambers
Your heart is divided into four hollow chambers: two on top (the atria) and two on the bottom (the ventricles). The right and left sides do different jobs.
The right atrium collects oxygen-poor blood returning from your body through two large veins. One brings blood from your upper body, the other from your lower body. The right atrium pushes that blood down into the right ventricle, which squeezes it out toward your lungs to pick up fresh oxygen. Meanwhile, the left atrium receives oxygen-rich blood coming back from the lungs and passes it to the left ventricle. The left ventricle is the strongest chamber. It generates enough force to push oxygenated blood out through your largest artery, the aorta, and into the rest of your body.
How Valves Keep Blood Moving Forward
Four one-way valves prevent blood from flowing backward. They open and close based on pressure differences between chambers: when pressure builds on one side, the valve swings open, and when the pressure reverses, it snaps shut.
- Tricuspid valve: sits between the right atrium and right ventricle.
- Pulmonary valve: opens when the right ventricle pushes blood toward the lungs.
- Mitral valve: sits between the left atrium and left ventricle.
- Aortic valve: opens when the left ventricle pushes blood into the aorta.
The classic “lub-dub” sound of a heartbeat comes from these 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 shutting after blood has been ejected.
The Path Blood Takes Through Your Heart
Blood follows a single loop through the heart in a precise sequence. Oxygen-poor blood from your body enters the right atrium. The tricuspid valve opens, and blood flows into the right ventricle. When the right ventricle is full, it contracts, closing the tricuspid valve and opening the pulmonary valve. Blood travels through the pulmonary artery to the lungs, where it picks up oxygen and releases carbon dioxide.
Now oxygen-rich, the blood returns to the left atrium through the pulmonary veins. The mitral valve opens, filling the left ventricle. When the left ventricle contracts, the mitral valve closes and the aortic valve opens. Blood surges into the aorta and out to the rest of your body, completing the circuit.
This means blood passes through your heart twice during each full loop: once on the right side heading to the lungs, and once on the left side heading to the body.
The Layers of the Heart Wall
The heart wall has three layers. The outer layer is a thin coating of connective tissue and fat that protects the heart from friction as it moves inside the chest. This outer layer also houses the coronary blood vessels that feed the heart muscle itself. The middle layer is the thickest and consists of the actual muscle tissue that contracts with each beat. The inner layer is a smooth membrane lining the inside of the chambers and valves, creating a frictionless surface so blood flows freely without clotting.
How the Heart Feeds Itself
The heart muscle needs its own blood supply to keep working. Two coronary arteries branch off from the aorta right at the top and wrap around the heart’s surface. The right coronary artery feeds the right side of the heart, and the left main coronary artery feeds the left side. These arteries divide into smaller and smaller branches that penetrate deep into the muscle. When one of these arteries becomes blocked, the section of heart muscle it supplies is starved of oxygen, which is what happens during a heart attack.
What Triggers Each Beat
Your heartbeat starts with an electrical impulse, not a signal from your brain. A small cluster of specialized cells in the upper right atrium acts as the heart’s natural pacemaker. This cluster 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 cluster of cells near the center of the heart, which introduces a brief delay, just a fraction of a second. This pause is critical: it gives the atria time to finish emptying before the ventricles fire. After the delay, the signal travels down through a network of specialized fibers that branch into both ventricles, triggering them to contract almost simultaneously and eject blood into the lungs and body.
This entire electrical sequence happens with each heartbeat, producing the rhythmic pattern you can see on a heart monitor.
Squeezing and Relaxing: The Cardiac Cycle
Each heartbeat has two main phases. During the contraction phase, called systole, the ventricles squeeze. At first, all four valves are briefly closed while pressure builds rapidly inside the ventricles. Once ventricular pressure exceeds the pressure in the arteries, the outflow valves pop open and blood is ejected, quickly at first, then more slowly as the ventricles empty.
During the relaxation phase, called diastole, the ventricles release and pressure inside them drops. The outflow valves snap shut as arterial blood briefly pushes backward against them. Once ventricular pressure falls below atrial pressure, the inlet valves open and blood pours in from the atria, rapidly at first, then tapering to a trickle until the next contraction starts the cycle over.
At a normal resting heart rate of 60 to 100 beats per minute, your heart actually spends more time relaxing than contracting. That relaxation period is when the heart muscle itself receives most of its blood supply through the coronary arteries.
How Your Body Adjusts Heart Rate
Your heart’s built-in pacemaker sets a baseline rhythm, but your nervous system constantly adjusts the speed. Two opposing branches of your nervous system work like a gas pedal and a brake. The “brake” branch slows your heart through the vagus nerve, which runs from your brainstem to your heart. The “gas pedal” branch speeds things up by releasing adrenaline and related chemicals.
At rest, both systems are active and roughly balanced, keeping your heart rate steady. When you start light exercise, your body initially speeds up the heart mainly by releasing the brake, reducing the calming signals from the vagus nerve. At higher exercise intensities, the accelerating system kicks in more aggressively, releasing adrenaline to drive the heart faster and harder. This is why your heart rate climbs gradually during a warm-up but can spike quickly during intense effort.
Pressure sensors in your major arteries also play a role. They detect changes in blood pressure and send signals that fine-tune heart rate moment to moment, helping keep blood flow stable whether you’re standing up from a chair or sprinting for a bus.
Normal Heart Rate and Output
A healthy resting heart rate for adults falls between 60 and 100 beats per minute. Well-trained athletes often have resting rates below 60 because their hearts pump more blood per beat, so fewer beats are needed. A resting rate consistently above 100 or below 60 (in someone who isn’t athletic) is worth discussing with a doctor.
The amount of blood your heart pumps per beat is called stroke volume. Multiply stroke volume by heart rate and you get cardiac output: the total volume pumped per minute. At rest, that’s typically 5 to 6 liters. During vigorous exercise, cardiac output can increase four or five times as both heart rate and stroke volume rise to meet your muscles’ demand for oxygen.