The three layers of the heart wall are the epicardium (outer layer), the myocardium (middle layer), and the endocardium (inner layer). Each layer has a distinct structure and role, and together they form a wall that protects the heart, generates the force to pump blood, and provides a smooth interior surface for blood to flow through.
Epicardium: The Outer Layer
The epicardium is a thin layer of elastic connective tissue and fat that wraps around the outside of the heart. It serves as a protective cushion, shielding the heart from friction as it beats inside the chest. If you’ve heard the term “visceral pericardium,” that’s the same structure. The epicardium is technically the innermost layer of the pericardium, the sac that surrounds the heart.
Just beneath the epicardium sits a region called the subepicardial layer, a zone of loose connective tissue that houses the coronary arteries and veins, along with nerves that help regulate heart rhythm. This is significant because it means the heart’s own blood supply runs through the space between the outer and middle layers before branching inward to feed the muscle itself.
Myocardium: The Muscle Layer
The myocardium is the thickest of the three layers and the one responsible for the heart’s pumping action. It’s composed of cardiac muscle cells, which are specialized striated cells roughly 25 micrometers wide and 100 micrometers long. Unlike the muscle in your biceps, cardiac muscle cells form a branching network connected by structures called intercalated discs, which allow electrical signals to pass rapidly from cell to cell so the heart contracts in a coordinated wave.
Inside each muscle cell are repeating units called sarcomeres, the basic engines of contraction. Proteins within these sarcomeres slide past one another to shorten the cell, and when millions of cells shorten together, the chamber squeezes blood out. The heart also has a built-in way to adjust its power: when a chamber fills with more blood and stretches the muscle, the sarcomeres lengthen slightly, which increases the force of the next contraction. This is why your heart can pump harder during exercise without you consciously telling it to.
The myocardium isn’t the same thickness everywhere. The left ventricle, which pumps blood to the entire body, has the thickest walls. In healthy adults, the left ventricular wall measures roughly 5 to 8 mm depending on the location and the person’s sex, with men averaging slightly thicker walls than women. The right ventricle, which only needs to push blood to the nearby lungs, is considerably thinner.
Endocardium: The Inner Lining
The endocardium is a single-cell-thick layer of specialized cells that lines the inside of all four heart chambers and covers the heart valves. Its surface is smooth, elastic, and resistant to blood clotting, which is critical because blood is constantly flowing across it. Without this slick lining, blood cells and clotting proteins would stick to the inner walls and form dangerous clots.
Between the endocardium and the myocardium is a thin zone of loose connective tissue called the subendocardial layer. This region contains the heart’s internal electrical wiring, including specialized fibers that carry electrical impulses rapidly across the ventricles. These fibers ensure that the lower chambers contract from the bottom up, efficiently pushing blood out through the valves.
The endocardium also plays a role during heart development before birth. Endocardial cells act as a source for several different cell types, contributing to the formation of heart valves, connective tissue cells, and even parts of the coronary blood vessels.
How the Layers Work Together
Think of the three layers as a team with different jobs. The endocardium keeps blood flowing smoothly and carries the electrical signals that coordinate each beat. The myocardium does the heavy mechanical work of contraction. The epicardium protects the outside of the heart and holds the coronary vessels that supply the myocardium with oxygen and nutrients. No single layer can function well without the others.
What Happens When a Layer Gets Inflamed
Each layer can become inflamed independently, and the names follow the same pattern: endocarditis (inner layer), myocarditis (muscle layer), and pericarditis (outer layer and surrounding sac). Because symptoms vary by type and overlap with other conditions, heart inflammation can be difficult to diagnose.
Pericarditis often produces a distinctive grating sound called a friction rub that a doctor can hear with a stethoscope, along with sharp chest pain that worsens when lying down. Endocarditis may cause a new heart murmur, fever, skin changes, and an enlarged spleen. Myocarditis can lead to abnormal heart rhythms, fatigue, and signs of heart failure like leg swelling, since the muscle layer itself is weakened. All three conditions typically require imaging, blood tests, and sometimes additional procedures to confirm.