The heart is a muscular organ roughly the size of your fist that pumps blood through your entire body. It sits slightly left of center in your chest, between the lungs, and beats about 100,000 times every day. In an average adult male, the heart weighs between 280 and 340 grams; in an average adult female, between 230 and 280 grams. Despite its small size, it powers a circulatory system that delivers oxygen and nutrients to every tissue you have.
Four Chambers, Two Circuits
The heart is divided into four hollow chambers: two on top (the right atrium and left atrium) and two on the bottom (the right ventricle and left ventricle). A wall of muscle called the septum separates the left and right sides, preventing oxygen-rich and oxygen-poor blood from mixing.
These four chambers run two separate loops of circulation at the same time. The right side handles the lungs: oxygen-poor blood from your body enters the right atrium through two large veins, gets pumped into the right ventricle, and then travels to the lungs through the pulmonary artery. In the lungs, blood picks up fresh oxygen and releases carbon dioxide. That newly oxygenated blood returns to the heart’s left atrium through the pulmonary veins, drops into the left ventricle, and gets pumped out to the rest of the body through the aorta, the largest artery in the body.
The left ventricle has the thickest walls of any chamber because it generates the high pressure needed to push blood all the way to your fingers, toes, and brain in a single powerful squeeze.
How the Valves Keep Blood Moving Forward
Four valves act as one-way doors inside the heart, opening and closing in rhythm with each heartbeat to prevent blood from flowing backward. Each valve has small flaps called leaflets that swing open under pressure and snap shut once blood has passed through.
- Tricuspid valve: sits between the right atrium and right ventricle
- Pulmonary valve: sits between the right ventricle and the pulmonary artery
- 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 is these valves closing. The first sound comes from the tricuspid and mitral valves shutting as the ventricles contract. The second sound is the pulmonary and aortic valves closing after blood has been pushed out.
The Heart’s Three-Layered Wall
The heart wall is built from three distinct layers. The outer layer, called the epicardium, is a thin membrane surrounded by connective tissue and fat that cushions and protects the heart. The middle layer, the myocardium, makes up the bulk of the wall and is composed of specialized muscle cells that contract with each beat. The inner layer, the endocardium, is a smooth lining that lets blood flow through the chambers without sticking or clotting.
Surrounding all three layers is a tough, fluid-filled sac called the pericardium. The thin film of fluid inside it reduces friction so the heart can beat freely without rubbing against the lungs or chest wall.
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 right atrium, known as the sinoatrial (SA) node, fires an electrical signal that spreads across both atria and triggers them to contract. This is why the SA node is often called the heart’s natural pacemaker.
That signal then reaches a second relay point, the atrioventricular (AV) node, which briefly delays it. The delay is important: it gives the atria time to finish squeezing blood into the ventricles before the ventricles contract. From the AV node, the signal travels down a pathway through the septum and branches out into fine fibers that spread across the ventricle walls, triggering a coordinated contraction that pushes blood out to the lungs and body.
How Your Nervous System Adjusts Heart Rate
While the heart sets its own rhythm, your nervous system constantly fine-tunes the speed. Two opposing branches handle this. The sympathetic branch, your “fight or flight” system, speeds the heart up. It increases heart rate, strengthens contractions, and speeds electrical conduction through the heart. This is what happens when you exercise, feel stressed, or get startled.
The parasympathetic branch, working primarily through the vagus nerve, does the opposite. It slows the heart rate, reduces conduction speed through the AV node, and lowers the force of contractions in the upper chambers. When you’re resting, sleeping, or taking slow deep breaths, parasympathetic activity dominates. The constant push and pull between these two systems keeps your heart rate matched to whatever your body needs in the moment.
Normal Resting Heart Rate
A healthy resting heart rate for adolescents and adults falls between 60 and 100 beats per minute. Well-trained athletes often rest closer to 40 or 50 beats per minute because their hearts pump more blood with each beat.
Children have naturally faster heart rates. Newborns range from 100 to 160 beats per minute, infants from 80 to 140, toddlers from 80 to 130, and school-age children from 70 to 100. These rates gradually decrease as the heart grows larger and more efficient.
With each beat, the average adult heart pushes about 70 milliliters of blood into circulation. Multiply that by roughly 70 beats per minute, and the heart pumps around 5 liters of blood every minute at rest. During intense exercise, that output can increase four or five times over.
The Heart’s Own Blood Supply
The heart muscle itself needs a constant supply of oxygen-rich blood to keep working, and it gets that supply through its own dedicated network of blood vessels called the coronary arteries. Two main arteries branch off the aorta right where it leaves the heart.
The right coronary artery feeds the right atrium, right ventricle, and the bottom surface of the heart. It also supplies blood to the SA node and, in about 90% of people, the AV node. The left main coronary artery splits into two major branches: one runs down the front of the heart and feeds the front wall of the left ventricle and most of the septum, while the other wraps around the left side and supplies the left atrium and the back-left portion of the left ventricle.
When one of these coronary arteries becomes blocked, usually by a buildup of fatty plaque, the muscle it feeds starts to die. This is what a heart attack is: not a problem with the heart’s electrical system or valves, but a loss of blood supply to the heart muscle itself.
Cardiovascular Disease at a Glance
Cardiovascular diseases are the leading cause of death worldwide. In 2022, an estimated 19.8 million people died from them, accounting for roughly 32% of all global deaths. Heart attacks and strokes together caused 85% of those deaths. About 80% of cardiovascular deaths occur in low- and middle-income countries, largely due to limited access to prevention and treatment.
The most common conditions affecting the heart include coronary artery disease (narrowed or blocked coronary arteries), heart failure (when the heart can no longer pump efficiently), arrhythmias (irregular heartbeats caused by problems in the electrical system), and valve disease (when one or more valves don’t open or close properly). Many of these conditions share the same root risk factors: high blood pressure, high cholesterol, smoking, physical inactivity, and excess body weight.