The endocardium is the thin, smooth membrane that lines the inside of the heart’s chambers and covers the surfaces of its valves. As the innermost layer of the heart wall, it is in direct contact with the blood flowing through the organ, distinguishing it from the myocardium and the epicardium. This creates a continuous lining, ensuring a seamless interface between the circulating blood and the heart muscle.
Anatomy of the Endocardium
The endocardium consists of two primary layers. The superficial layer, which directly interfaces with the blood, is a single sheet of specialized epithelial cells called the endothelium. These cells are flat and fit together tightly, creating a smooth surface that minimizes friction as blood is pumped through the atria and ventricles. This endothelial layer is continuous with the cellular lining found throughout the entire circulatory system.
Beneath the endothelium is a layer of loose connective tissue. This subendothelial layer provides structural support and helps bind the endocardium to the myocardium. It contains collagen and elastin fibers, which give it strength and flexibility to withstand the pressures of the cardiac cycle. Within this tissue are small blood vessels and components of the heart’s electrical conduction system.
Core Functions of the Endocardium
A primary function of the endocardium is to facilitate smooth blood flow. Its smooth surface prevents turbulence that could lead to the formation of blood clots within the heart chambers. The endothelial cells maintain this non-thrombogenic environment by producing substances that inhibit the aggregation of platelets and regulate clotting factors. This ensures that blood remains fluid and can be efficiently pumped out to the body.
The endocardium is also involved in the mechanics of the heart valves. It covers the aortic, pulmonary, mitral, and tricuspid valves, giving them a frictionless surface. This allows the valve leaflets to open and close cleanly with each heartbeat, preventing wear and ensuring a tight seal when closed. Without this smooth layer, valve surfaces could become rough, leading to improper function like stenosis (narrowing) or regurgitation (leaking).
Endocarditis and Other Disorders
The most well-known disorder affecting this tissue is endocarditis, an inflammation of the endocardium. It most commonly arises from an infection, a condition known as infective endocarditis. This process begins when bacteria or other microorganisms enter the bloodstream and travel to the heart, where they can adhere to damaged areas of the endocardium, particularly on the heart valves.
Once attached, these microorganisms multiply and form clusters of bacteria and cell fragments called vegetations. These growths can interfere with normal valve function and may break off, traveling through the bloodstream to lodge in other parts of the body. Common symptoms of endocarditis include:
- Persistent fever
- Fatigue
- Shortness of breath
- A new or altered heart murmur
Diagnosis is confirmed through blood cultures to identify the microorganism and an echocardiogram to visualize the vegetations on the heart valves.
Treatment for infective endocarditis involves a long course of intravenous antibiotics, often administered over several weeks. If the infection has severely damaged a heart valve, surgery may be required to repair or replace it. A less common condition is nonbacterial thrombotic endocarditis, where sterile clots form on the heart valves, often in association with other medical conditions.