Your heart is a muscular pump about the size of your fist that beats roughly 100,000 times a day, pushing 5 to 6 liters of blood through your body every minute while you’re at rest. It works by coordinating perfectly timed electrical signals, muscle contractions, and one-way valves to keep blood flowing in a single direction: picking up oxygen in the lungs, delivering it to every tissue in your body, and cycling back again.
Four Chambers, Two Jobs
The heart is divided into four chambers. The two upper chambers, called atria, receive blood coming into the heart. The two lower chambers, called ventricles, pump blood out. A muscular wall called the septum runs down the middle, keeping oxygen-rich blood on the left side completely separate from oxygen-poor blood on the right.
The right side of the heart handles one job: sending used, oxygen-depleted blood to the lungs for a fresh supply of oxygen. The left side handles the bigger job: pumping newly oxygenated blood out to the entire body. That’s why the left ventricle has the thickest muscle wall of all four chambers. It needs the extra force to push blood through thousands of miles of blood vessels, from your brain down to your toes.
Four valves act as one-way doors between these chambers and the major blood vessels. They open to let blood through, then snap shut to prevent it from flowing backward. The two sounds of a heartbeat, the familiar “lub-dub,” are actually the sounds of these valves closing in sequence.
The Path Blood Takes
Blood follows a figure-eight loop through your body, passing through the heart twice on each complete circuit. Here’s the full sequence:
Oxygen-poor blood from your body drains into two large veins (the superior and inferior vena cava) and empties into the right atrium. From there it flows down into the right ventricle, which pumps it through the pulmonary artery to the lungs. This leg of the trip is called pulmonary circulation.
Inside the lungs, blood passes through tiny capillaries wrapped around millions of air sacs called alveoli. Carbon dioxide moves out of the blood and into the air sacs, where you exhale it. At the same time, oxygen from the air you just inhaled moves in the opposite direction, crossing into the blood. This gas swap happens almost instantly.
Now oxygen-rich, the blood travels back to the heart through the pulmonary veins and enters the left atrium. It drops into the left ventricle, which contracts powerfully and sends it surging into the aorta, the body’s largest artery. From the aorta, blood branches into progressively smaller arteries and capillaries, delivering oxygen and nutrients to every organ and tissue. Once the oxygen is used up, the blood collects in veins and returns to the right atrium to start the loop over again. This body-wide leg is called systemic circulation.
The Heart’s Built-In Electrical System
Your heart doesn’t wait for instructions from your brain to beat. It generates its own electrical impulses through a specialized conduction system that fires automatically, keeping the rhythm steady whether you’re awake, asleep, or not thinking about it at all.
Each heartbeat starts at a small cluster of cells in the upper right atrium called the SA node, often referred to as the heart’s natural pacemaker. The SA node sends an electrical signal that spreads across both atria, causing them to contract and push blood down into the ventricles. The signal then reaches a second relay point called the AV node, located near the center of the heart. The AV node deliberately delays the signal for a fraction of a second. This brief pause is critical: it gives the atria time to finish emptying before the ventricles fire.
After the delay, the signal travels down a bundle of specialized nerve fibers through the septum, then fans out into a network called the Purkinje fibers, which deliver the signal rapidly to both ventricles. The ventricles contract almost simultaneously, pushing blood to the lungs and the rest of the body. Then the whole system resets, and the SA node fires again.
What Happens in a Single Heartbeat
Each heartbeat has two main phases. Systole is the contraction phase, when the ventricles squeeze and eject blood. Diastole is the relaxation phase, when the ventricles refill. A single cycle at a normal resting heart rate takes less than a second.
The contraction phase begins with all four valves closed. The ventricles generate pressure rapidly without changing volume, like squeezing a sealed water balloon. Once pressure inside the ventricles exceeds the pressure in the arteries, the outflow valves pop open and blood surges out. This rapid ejection accounts for the bulk of blood leaving the heart. The rate of ejection then slows as the ventricles start to relax.
During diastole, the ventricles relax and their internal pressure drops. The outflow valves close (producing the second heart sound), and the ventricles begin to fill with blood flowing in from the atria. Most filling, about 90%, happens passively as blood simply flows downhill from the atria into the relaxed ventricles. The atria then contract at the very end, topping off the ventricles with the remaining 10% just before the next beat begins. Diastole takes up the majority of each cardiac cycle, roughly 44% of the total time is spent in the rapid filling phase alone. This is why a faster heart rate can be a problem: it shortens diastole and gives the heart less time to fill.
How Your Body Controls Heart Rate
While the SA node sets the heart’s baseline rhythm, your nervous system constantly adjusts the pace to match what your body needs. Two branches of the autonomic nervous system pull the heart rate in opposite directions. The parasympathetic branch acts like a brake, slowing the heart during rest and calm. The sympathetic branch acts like an accelerator, speeding it up during stress, excitement, or physical effort.
At rest, these two systems share roughly equal influence, fine-tuning heart rate in response to signals from pressure sensors in your blood vessels. When you start exercising, the first thing that happens is the brake lifts: parasympathetic activity decreases, and your heart rate rises. As exercise intensity increases, the sympathetic system kicks in more aggressively, further accelerating the heart and making each contraction more forceful. This dual system is why your heart can go from 60 beats per minute on the couch to 150 or more during a hard run, then gradually settle back down when you stop.
How the Heart Feeds Itself
The heart works harder than any other muscle in the body, and it needs its own dedicated blood supply to keep going. The coronary arteries branch directly off the aorta, right at the point where blood leaves the left ventricle. Two main coronary arteries, the left and the right, divide into smaller branches that wrap around the heart’s surface and penetrate into the muscle.
The left coronary artery feeds the left side of the heart, including the thick-walled left ventricle and the septum. It splits into two important branches: one supplies the front of the heart, and the other wraps around to the side. The right coronary artery supplies the right atrium and parts of both ventricles, with branches reaching the bottom of the heart. When one of these arteries becomes narrowed or blocked, the heart muscle downstream is starved of oxygen. That’s what a heart attack is: death of heart tissue due to interrupted blood supply.
What Protects the Heart
The heart sits inside a double-walled sac called the pericardium. The tough outer layer is made of connective tissue that anchors the heart in place inside the chest and prevents it from overfilling with blood. The inner layer produces a thin film of lubricating fluid that lets the heart beat with minimal friction against surrounding tissues. The pericardium also serves as a physical barrier against infections spreading from nearby organs like the lungs.
Heart disease remains the leading cause of death in the United States, responsible for more than 683,000 deaths in 2024. Understanding how the heart works helps explain why conditions like high blood pressure, blocked coronary arteries, and electrical rhythm problems are so dangerous. Each one disrupts a different part of the system: the pump, the fuel supply, or the wiring. When any one of those fails, the entire circuit is compromised.