The human heart is a fist-sized muscle containing four hollow chambers, four one-way valves, a built-in electrical system, and a network of blood vessels that keep it fed with oxygen. In an average adult, it weighs roughly 230 to 340 grams and measures about 12 centimeters long and 8.5 centimeters wide. Despite its compact size, every structure inside has a specific job in moving blood through your body.
Four Chambers and What Each One Does
The heart is divided into four compartments. The two upper chambers are called atria, and the two lower chambers are called ventricles. A muscular wall called the septum runs down the middle, separating the left side from the right so oxygen-rich blood never mixes with oxygen-poor blood.
The right atrium collects blood returning from your body after it has delivered its oxygen. That blood drops into the right ventricle, which pumps it to your lungs to pick up a fresh supply. Once oxygenated, the blood flows into the left atrium, then down into the left ventricle. The left ventricle is the strongest chamber. It pumps oxygen-rich blood out through the aorta, the largest artery in the body, to reach every organ and tissue. This entire loop happens with each heartbeat.
Valves That Keep Blood Moving Forward
Four valves sit between the chambers and at the exits of the ventricles. They’re made of thin but strong flaps of tissue called leaflets, and they open and close with each heartbeat to prevent blood from flowing backward.
- Tricuspid valve: sits between the right atrium and right ventricle, with three leaflets.
- Pulmonary valve: guards the exit from the right ventricle into the arteries leading to the lungs.
- Mitral valve: sits between the left atrium and left ventricle, with two leaflets.
- Aortic valve: guards the exit from the left ventricle into the aorta.
The tricuspid and mitral valves do not simply flap open and shut on their own. They are anchored by tiny cord-like structures called chordae tendineae, which attach the valve leaflets to small columns of muscle on the ventricle walls called papillary muscles. When the ventricles squeeze, the papillary muscles pull the cords taut, preventing the valve leaflets from being pushed backward into the atria. Without these cords, the valves would buckle under pressure, a condition known as valve prolapse.
The Heart’s Built-In Electrical System
Your heart doesn’t need your brain to tell it when to beat. It has its own electrical wiring. A small cluster of cells in the upper right atrium, called the SA node, fires an electrical signal that triggers each heartbeat. This is your natural pacemaker. The signal spreads across both atria, causing them to contract and push blood into the ventricles.
The signal then reaches a second relay station called the AV node, which pauses briefly so the ventricles have time to fill. From there, the impulse travels down a pathway called the bundle of His, splits into left and right branches, and fans out through a web of fibers that cause the ventricles to contract almost simultaneously. This coordinated sequence is what produces the rhythmic “lub-dub” you hear through a stethoscope.
Three Layers of the Heart Wall
The wall of the heart itself is built from three distinct layers. The innermost layer, the endocardium, is a smooth lining that reduces friction as blood flows through the chambers. The middle layer, the myocardium, is thick cardiac muscle that does the actual work of contracting and pumping. The outer layer, the epicardium, is a thin protective membrane that also houses the coronary blood vessels running along the heart’s surface.
The myocardium in the left ventricle is noticeably thicker than in the right, because the left side has to generate enough force to push blood throughout the entire body, while the right side only needs to reach the nearby lungs.
Ridges and Strands Inside the Ventricles
If you could look inside the ventricles, the walls wouldn’t be smooth. They’re covered with irregular ridges and small finger-like strands of muscle tissue called trabeculae carneae. These tiny structures, seldom more than 3 millimeters long, increase the surface area of the inner walls and help the ventricles grip blood during contraction. They also have an interesting oxygen advantage: unlike the rest of the heart muscle, which relies entirely on coronary arteries for oxygen, trabeculae can absorb oxygen directly from the blood filling the ventricle chambers around them. Very thin trabeculae may have no capillaries at all, surviving purely on this direct oxygen supply.
Coronary Arteries and the Heart’s Own Blood Supply
The heart muscle works nonstop and needs its own dedicated blood supply. Two main coronary arteries branch off the aorta and wrap around the outside of the heart. The left main coronary artery feeds the left side, splitting into a branch that supplies the front of the heart and another that curves around to the back. The right coronary artery feeds the right side and also delivers blood to the SA and AV nodes, the electrical centers that keep the heart beating on rhythm.
When one of these arteries becomes blocked, the muscle tissue it feeds starts to die. That is what happens during a heart attack.
Major Blood Vessels Connected to the Heart
Five great vessels connect the heart to the rest of the circulatory system. Two large veins, the superior vena cava and inferior vena cava, deliver oxygen-depleted blood from the upper and lower body into the right atrium. The pulmonary trunk exits the right ventricle and carries that blood to the lungs. Four pulmonary veins return freshly oxygenated blood from the lungs into the left atrium. Finally, the aorta exits the left ventricle and channels high-pressure, oxygen-rich blood to every part of the body.
The Protective Sac Around It All
The entire heart sits inside a double-layered sac called the pericardium. The outer layer is tough fibrous tissue that anchors the heart in place within the chest. The inner layer is a thinner membrane that hugs the heart’s surface. Between these two layers sits a small amount of fluid, typically 20 to 60 milliliters in a healthy adult. This pericardial fluid acts as a lubricant, allowing the heart to expand and contract with minimal friction against surrounding structures. If excess fluid accumulates in this space, it can compress the heart and interfere with its ability to pump.