Your heart is made primarily of muscle, but it also contains connective tissue, a network of blood vessels, specialized electrical cells, and an outer protective sac. The muscular layer, called the myocardium, is the thickest component and does the mechanical work of pumping blood. But the heart’s full structure involves several distinct layers and materials, each with a specific job.
Three Layers of the Heart Wall
The heart wall is built from three concentric layers, each made of different tissue. The outermost layer, the epicardium, is a thin sheet of elastic connective tissue and fat. It cushions the heart against friction and trauma, and it houses the coronary blood vessels that feed oxygen to the heart itself.
Beneath that sits the myocardium, the thick middle layer of cardiac muscle that generates the force behind every heartbeat. This is what most people picture when they think of “what the heart is made of.” Cardiac muscle cells, called cardiomyocytes, contract rhythmically and continuously from before birth until death. Because they never stop working, they need a constant supply of oxygen and nutrients delivered by the coronary arteries. Under conditions of peak demand, the coronary circulation can increase blood flow more than fivefold above its resting level.
The innermost layer, the endocardium, is a smooth lining of endothelial cells. It creates a slick, non-stick surface so blood can flow through the chambers without clotting or sticking. The endocardium also helps regulate the chemical environment around the muscle cells, which in turn affects how forcefully the heart contracts.
What the Muscle Cells Contain
Inside each cardiomyocyte, the actual machinery of contraction comes from proteins arranged in repeating units called sarcomeres. Two proteins do the heavy lifting: actin (thin filaments) and myosin (thick filaments) slide past each other to shorten the cell, producing a contraction. A third giant protein called titin runs the length of each sarcomere and acts as a molecular spring. Titin keeps the thick and thin filaments precisely aligned, gives the heart its elasticity, and generates the recoil force that helps the chambers refill with blood between beats.
This spring-like property matters more than it might sound. When the heart stretches as it fills with blood, titin’s elastic tension increases, which contributes to a stronger contraction on the next beat. This is one of the mechanisms behind a fundamental principle of heart function: the more blood flows in, the more forcefully the heart pumps it out.
More Than Just Muscle Cells
Cardiomyocytes get most of the attention, but they actually make up only about 31% of the cells in the human heart by number. The majority, roughly 54%, are endothelial cells lining the vast network of tiny blood vessels woven through the heart tissue. A small fraction, around 3%, are immune cells that patrol for damage or infection. The rest include fibroblasts and other supporting cells embedded in the connective tissue between muscle fibers.
Cardiomyocytes are far larger than these other cell types, so by volume they still dominate the organ. But the sheer number of endothelial cells reflects just how blood-vessel-dense the heart is. Every muscle fiber sits close to a capillary, ensuring oxygen is never more than a short distance away.
The Fibrous Skeleton
Running through the center of the heart is a framework of dense connective tissue made mainly of collagen. This internal scaffold, called the cardiac skeleton, consists of four fibrous rings that encircle the base of the heart’s major openings: the aorta, the pulmonary trunk, and the two large valves between the upper and lower chambers.
The skeleton serves two purposes. Structurally, it anchors the heart valves in place and gives them a firm attachment point so they can open and close under pressure without distorting. Electrically, it acts as an insulator, blocking electrical signals from passing directly between the upper chambers (atria) and the lower chambers (ventricles). This forced separation is what allows the heart to beat in a coordinated top-to-bottom rhythm. Electrical impulses must travel through a single controlled gateway, giving the upper chambers time to finish contracting and push blood downward before the lower chambers fire.
What the Valves Are Made Of
The heart’s four valves are not muscle. They are thin, flexible flaps made of extracellular matrix, a mesh of collagen, elastin, and other structural proteins. In a healthy aortic valve, about 70% of the collagen is type I (the same tough variety found in bone and tendons), with another 25% being type III (a more flexible form common in blood vessels and skin). Scattered through this matrix are specialized cells called valve interstitial cells, which maintain and repair the tissue over a lifetime.
The Pacemaker System
Embedded in the heart’s muscle are clusters of specialized cells that generate and conduct electrical signals. The most important of these sits in the upper right chamber: a small patch of tissue called the sinoatrial node, or the heart’s natural pacemaker. These cells are structurally different from regular cardiomyocytes. They produce very low levels of the gap-junction proteins that ordinary heart muscle cells use for fast electrical communication, and they carry a unique ion channel that lets them spontaneously generate rhythmic electrical impulses without any input from the brain.
From the sinoatrial node, the signal travels through a defined pathway of conduction cells to the lower chambers. This wiring system is what keeps the heart beating in an organized sequence rather than quivering randomly.
Ions That Power Every Beat
The heart’s electrical and contractile activity depends on the movement of charged particles (ions) in and out of cells. Sodium ions rushing inward trigger each electrical impulse. Calcium ions then flood into the cell and signal the release of even more calcium from internal storage compartments called the sarcoplasmic reticulum. This calcium surge is what actually causes the muscle proteins to slide and the cell to contract. Potassium ions flowing outward then reset the cell, preparing it for the next beat.
This cycle of sodium in, calcium in, potassium out happens with every single heartbeat, roughly 100,000 times a day. Disruptions to any of these ion flows can cause arrhythmias or weaken the heart’s pumping ability.
The Pericardial Sac
Surrounding the entire heart is a protective enclosure called the pericardium. Its outermost layer is a tough, fibrous shell of connective tissue that prevents the heart from overstretching if it fills with too much blood. Inside this shell is a thinner, two-layered membrane called the serous pericardium. The inner surface of this membrane produces a small amount of pericardial fluid, a lubricant that allows the heart to beat with minimal friction against the surrounding tissues of the chest.