Your heart is a muscular pump roughly the size of your fist, beating 60 to 100 times per minute at rest to push blood through your entire body. It weighs between 230 and 340 grams depending on sex, measures about 12 centimeters long and 8.5 centimeters wide, and begins contracting around six weeks after conception, never stopping until death. Despite its small size, the heart turns over roughly 6 kilograms of cellular fuel every day and extracts more oxygen from blood than any other tissue in your body.
Four Chambers, Four Valves
The heart is divided into four hollow chambers. The two upper chambers, called atria, receive incoming blood. The two lower chambers, called ventricles, pump blood out. A muscular wall separates the left and right sides so oxygen-rich and oxygen-poor blood never mix.
Four one-way valves keep blood flowing in the correct direction. The tricuspid valve sits between the right atrium and right ventricle. The pulmonary valve guards the exit from the right ventricle into the lungs. On the left side, the mitral valve connects the left atrium to the left ventricle, and the aortic valve controls flow from the left ventricle into the aorta, the largest artery in the body. Each valve opens and shuts in response to pressure changes, and the familiar “lub-dub” sound of a heartbeat is the sound of these valves snapping closed.
The Path Blood Takes Through the Heart
Blood follows a precise loop every time it passes through the heart. Oxygen-poor blood returning from your body enters through two large veins: the superior vena cava (draining your upper body) and the inferior vena cava (draining your lower body). Both empty directly into the right atrium.
From there, the tricuspid valve opens and blood flows into the right ventricle. When the right ventricle is full, it squeezes. That contraction closes the tricuspid valve behind the blood and opens the pulmonary valve ahead of it, sending blood through the pulmonary artery to the lungs. In the lungs, blood drops off carbon dioxide and picks up fresh oxygen.
Now oxygen-rich, the blood returns to the heart through the pulmonary veins and enters the left atrium. It passes through the mitral valve into the left ventricle, the strongest chamber. The left ventricle contracts with enough force to send blood through the aortic valve, into the aorta, and out to every organ and tissue in your body.
Two Circulatory Loops
The heart powers two separate circulation systems at the same time. The pulmonary circuit is the short loop between the heart and the lungs. Its sole job is gas exchange: dropping off carbon dioxide waste and loading up on oxygen. Because the lungs are close to the heart, this loop operates at relatively low pressure. The right ventricle only generates about 25 mmHg of peak pressure to push blood through it.
The systemic circuit is the long loop. It carries oxygen and nutrients from the left ventricle to every tissue in the body, from your brain down to your toes, then collects carbon dioxide and metabolic waste on the return trip. This loop requires much more force, which is why the left ventricle generates peak pressures around 120 mmHg and has noticeably thicker muscular walls than the right.
How the Heart Feeds Itself
The heart muscle can’t absorb oxygen from the blood passing through its chambers. Instead, it has its own dedicated blood supply: the coronary arteries. Two main coronary arteries branch off from the base of the aorta, the left and right, and divide into smaller vessels that wrap around the heart’s surface and penetrate the muscle.
Here’s a counterintuitive detail: the coronary arteries receive most of their blood flow between beats, not during them. When the heart muscle contracts, it physically compresses the coronary vessels and reduces flow. When the muscle relaxes, the arteries open up and blood rushes in. This is one reason why a very fast heart rate can be a problem. Less time between beats means less time for the heart muscle to receive its own oxygen supply.
The Heart’s Built-In Electrical System
You don’t have to think about making your heart beat. A small cluster of specialized cells in the upper right atrium, called the SA node, acts as the heart’s natural pacemaker. These cells spontaneously generate electrical impulses that set the rhythm for every heartbeat.
Each impulse spreads across both atria, causing them to contract and push blood into the ventricles. The signal then reaches a second cluster of cells called the AV node, which sits between the atria and ventricles. The AV node introduces a brief delay, about a tenth of a second, giving the ventricles time to fill completely before they contract. After that pause, the electrical signal travels down a network of specialized fibers that branch throughout the ventricle walls, triggering a coordinated, bottom-to-top squeeze that efficiently ejects blood.
This entire electrical sequence happens with each heartbeat. When the system works properly, the atria contract first, the ventricles follow a fraction of a second later, and the cycle repeats. When the electrical system misfires, the result is an arrhythmia, a heart rhythm that’s too fast, too slow, or irregular.
Systole and Diastole: The Two Phases of a Beat
Every heartbeat has two phases. Systole is the contraction phase. The ventricles squeeze, pressure inside them spikes above the pressure in the arteries, and the valves open to let blood out. During systole, blood is forced into the arteries with enough energy to stretch their elastic walls.
Diastole is the relaxation phase. The ventricles stop squeezing, pressure drops, the outflow valves snap shut, and the inflow valves open so blood can pour in from the atria. The stretched artery walls recoil during this phase, pushing blood forward even though the heart itself is resting. This is why you have blood pressure between beats, not just during them. The two numbers in a blood pressure reading reflect these phases: the top number (systolic) captures peak pressure during contraction, and the bottom number (diastolic) captures the baseline pressure during relaxation.
There’s also a brief moment called isovolumic relaxation, right after the outflow valve closes but before the inflow valve opens. Pressure in the ventricle plummets during this fraction of a second, creating a suction-like effect that helps pull blood in once the inflow valve opens.
What Powers the Muscle
Cardiac muscle cells are uniquely designed for nonstop work. They’re packed with mitochondria, the energy-producing structures inside cells, which occupy about a third of each cell’s volume. Over 95% of the heart’s energy comes from burning fuel with oxygen, and under normal conditions, 70 to 90 percent of that fuel comes from fat, with the rest coming from sugar, lactate, and other sources. The heart is metabolically flexible, switching fuel sources depending on what’s available.
Cardiac muscle cells are also physically connected to their neighbors through specialized junctions that allow electrical signals to pass directly from one cell to the next. This is what makes the heart contract as a coordinated unit rather than as millions of individual cells firing randomly. Structures within the cell membrane ensure that each electrical signal reaches deep into the core of every cell, producing complete, synchronized contractions.
How Efficiently the Heart Pumps
One key measure of heart function is ejection fraction: the percentage of blood in the left ventricle that gets pumped out with each beat. In healthy adults, the average is around 63%, meaning roughly two-thirds of the blood in the chamber is ejected with every contraction. The normal range runs from about 52% to 74%, with women averaging slightly higher than men.
A resting heart rate between 60 and 100 beats per minute is considered normal for adults. Highly trained athletes often have resting rates closer to 40 beats per minute because their hearts pump a larger volume of blood with each beat, so fewer beats are needed to deliver the same amount of oxygen. A consistently elevated resting heart rate above 100 can signal that the heart is working harder than it should to meet the body’s demands.
When Things Go Wrong
Heart failure doesn’t mean the heart stops. It means the heart can’t pump efficiently enough to meet the body’s needs. This can happen in two distinct ways. In one type, the left ventricle stretches out and loses its ability to contract forcefully, so it ejects less blood with each beat. In the other type, the heart contracts fine but the muscle has thickened and become stiff, so it can’t relax and fill properly between beats. Both result in fatigue, fluid buildup, and shortness of breath, but they involve different mechanical failures.
Coronary artery disease, where the arteries feeding the heart muscle become narrowed or blocked, remains the most common threat to this system. Because the heart depends on continuous oxygen delivery and has the highest oxygen extraction rate of any organ, even a partial reduction in coronary blood flow can impair the muscle’s ability to contract. A complete blockage starves a section of heart muscle of oxygen within minutes, which is what happens during a heart attack.