How the Heart Works: Anatomy, Electrical System, and Blood Flow

Overview of the Human Heart

The human heart is a muscular organ roughly the size of a closed fist, located slightly left of center in the chest between the lungs. It beats approximately 100,000 times per day, pumping about 2,000 gallons of blood through a network of blood vessels that stretches over 60,000 miles. This relentless pumping delivers oxygen and nutrients to every cell in the body while removing carbon dioxide and metabolic waste.

The heart is made primarily of a specialized muscle tissue called cardiac muscle (myocardium), which is unique in the body because it contracts rhythmically and involuntarily without ever tiring in the way skeletal muscle does. The myocardium is surrounded by a protective double-layered sac called the pericardium, which contains a small amount of fluid that reduces friction as the heart beats.

Despite its modest size, the heart is an engineering marvel. It functions simultaneously as two pumps working in parallel: the right side sends blood to the lungs for oxygenation, while the left side sends oxygenated blood to the rest of the body.

The Four Chambers

The heart is divided into four chambers: two upper chambers called atria (singular: atrium) and two lower chambers called ventricles. A muscular wall called the septum separates the left and right sides, preventing oxygen-rich and oxygen-poor blood from mixing.

The right atrium receives deoxygenated blood returning from the body through two large veins: the superior vena cava (from the upper body) and the inferior vena cava (from the lower body). It passes this blood to the right ventricle, which pumps it to the lungs through the pulmonary arteries.

The left atrium receives freshly oxygenated blood from the lungs via the pulmonary veins. It delivers this blood to the left ventricle, the most powerful chamber, which pumps oxygenated blood out through the aorta to the entire body. The left ventricle's walls are significantly thicker than the right ventricle's because it must generate enough pressure to push blood throughout the systemic circulation.

Heart Valves and Their Function

Four heart valves act as one-way gates that ensure blood flows in the correct direction and does not leak backward. Each valve opens and closes in response to pressure changes during the cardiac cycle.

The tricuspid valve sits between the right atrium and right ventricle, while the mitral valve (also called the bicuspid valve) separates the left atrium and left ventricle. These two are collectively known as the atrioventricular (AV) valves because they separate the atria from the ventricles.

The pulmonary valve guards the exit from the right ventricle into the pulmonary artery, and the aortic valve guards the exit from the left ventricle into the aorta. These are called semilunar valves because of their half-moon shaped cusps. The characteristic "lub-dub" sound of the heartbeat is produced by the closing of these valves: the first sound (lub) comes from the AV valves closing, and the second sound (dub) comes from the semilunar valves closing.

The Electrical Conduction System

The heart generates its own electrical impulses through a specialized conduction system that coordinates the contraction of all four chambers in precise sequence. This system operates independently of the brain, which is why a heart can continue beating even when removed from the body, as long as it receives oxygenated blood.

The process begins at the sinoatrial (SA) node, a small cluster of cells in the upper wall of the right atrium. The SA node is the heart's natural pacemaker, firing electrical impulses at a rate of 60 to 100 beats per minute at rest. This signal spreads across both atria, causing them to contract and push blood into the ventricles.

The impulse then reaches the atrioventricular (AV) node, located at the junction between the atria and ventricles. The AV node briefly delays the signal (about 0.1 seconds), allowing the ventricles to fill completely before they contract. From the AV node, the signal travels down the bundle of His, splits into left and right bundle branches, and spreads through the Purkinje fibers that trigger simultaneous contraction of both ventricles.

The Cardiac Cycle: Systole and Diastole

Each heartbeat consists of two phases: systole (contraction) and diastole (relaxation). During atrial systole, the atria contract and push the final 20 to 30 percent of blood into the ventricles, topping off the volume that already flowed in passively during diastole. This is sometimes called the atrial kick.

During ventricular systole, the ventricles contract forcefully. The AV valves snap shut to prevent backflow into the atria, and the semilunar valves open as ventricular pressure exceeds arterial pressure, ejecting blood into the pulmonary artery and aorta. A healthy heart ejects about 55 to 70 percent of the blood in the ventricle with each beat, a measurement called the ejection fraction.

During diastole, the heart muscle relaxes, the semilunar valves close to prevent backflow from the arteries, and the AV valves open as the ventricles fill with blood from the atria. The coronary arteries, which supply blood to the heart muscle itself, receive most of their flow during diastole when the myocardium is relaxed and not compressing the vessels.

Pulmonary and Systemic Circulation

Blood flow through the body follows two distinct circuits. Pulmonary circulation carries deoxygenated blood from the right ventricle to the lungs, where carbon dioxide is exchanged for oxygen across the thin walls of the alveoli. The freshly oxygenated blood returns to the left atrium through the pulmonary veins. Notably, the pulmonary arteries are the only arteries in the body that carry deoxygenated blood, and the pulmonary veins are the only veins that carry oxygenated blood.

Systemic circulation begins when the left ventricle pumps oxygenated blood into the aorta, the largest artery in the body. The aorta branches into progressively smaller arteries and arterioles that deliver blood to every organ and tissue. At the capillary level, oxygen and nutrients diffuse into cells while carbon dioxide and waste products diffuse into the blood.

The now-deoxygenated blood collects in venules that merge into veins, eventually draining into the superior and inferior vena cava and returning to the right atrium. The entire circuit takes approximately 20 seconds at rest, meaning a single red blood cell completes a full loop through both circuits about three times per minute.

Keeping Your Heart Healthy

Cardiovascular disease remains the leading cause of death worldwide, but many risk factors are modifiable through lifestyle choices. Regular aerobic exercise -- at least 150 minutes of moderate-intensity activity per week -- strengthens the heart muscle, improves ejection fraction, lowers resting heart rate, and reduces blood pressure.

A heart-healthy diet emphasizes fruits, vegetables, whole grains, lean proteins, and healthy fats while limiting sodium, added sugars, and saturated fats. The Mediterranean and DASH diets have the strongest evidence for cardiovascular benefit. Maintaining a healthy weight, avoiding tobacco, managing stress, and getting adequate sleep (7 to 9 hours per night) further reduce risk.

Regular health screenings for blood pressure, cholesterol, and blood sugar help catch problems early when they are most treatable. Understanding how your heart works is the first step toward appreciating the importance of these preventive measures and recognizing symptoms that warrant medical attention, such as chest pain, shortness of breath, irregular heartbeat, or unexplained fatigue.