Definition and Function of Endocardium

The endocardium is a thin, smooth tissue that makes up the lining of the chambers and valves of the heart. The innermost layer of the heart’s walls, it serves as a barrier between cardiac muscles and the bloodstream and contains necessary blood vessels. It also houses the heart’s conduction system, which regulates the activity of cardiac muscles.

Given this vital role—of both directing blood through the heart and regulating heartbeat—problems in the endocardium can have drastic health effects. Most notable among these is endocarditis, an infection and inflammation of these tissues that affect the valves particularly.

Let’s take a quick look at the anatomy and function of endocardium, as well as its association with health problems.

Definition and Function

The heart, tasked with pumping blood throughout the body, is composed of four chambers, and these are lined with endocardium. As the innermost layer of the heart’s walls, it serves two important functions:

  • Anatomic function: A tissue covering the inside of the heart, the endocardium keeps the blood flowing through the heart separate from the myocardium, or cardiac muscles. It also lines the valves, which open and close to regulate blood flow through the chambers of the heart.   
  • Conduction system: Heart activity and rhythm are regulated by electrical signals, which travel through the nerves embedded in the endocardium. These nerves are connected to the myocardium, causing the muscle to contract and relax, pumping blood through the body.


Alongside what it does, it’s also important to get a sense of the location and structure of the endocardium.  


As the inner lining of the heart, the endocardium is found along the walls of the four chambers of the heart (the left and right ventricles as well as the left and right atria). In addition, this tissue represents the outer layer of the tricuspid, pulmonary, mitral, and aortic valves, which serve as gateways between the chambers.

The endocardium is one of three layers that make up the heart’s walls. As the innermost of these, it’s connected to the myocardium, which is the thickest of the layers and consists of the heart muscles. Surrounding the myocardium is the epicardium, a tissue that contains the primary nerves, vessels, and arteries that serve the heart itself.


The endocardium is composed of three sub-layers, which define its function. These are:

  • The endothelium is the innermost layer, which controls the exchange of any materials between bloodstream and the heart muscles. It consists of specialized endothelial cells, which are the same type found lining the arteries and veins.  
  • The elastic tissue layer consists of smooth muscle, which applies pressure on veins running through the layer, and connective tissue.
  • Subendocardial layer is the outermost sub-layer of endocardium that serves as a connecting tissue to the cardiac muscle. Alongside nerves and vessels, it contains fibrous collagen cells, which provide structure and stability, and Purkinje fibers, which deliver electrical signals to the myocardium. 

Associated Conditions

Given its central role in the heart, conditions of the endocardium can have serious implications.


Endocarditis is an infection and inflammation of the endocardium. It is the most significant and most common condition of the endocardium. As an overview:

  • Symptoms: Endocarditis leads to a range of symptoms, including fever, chills, fatigue, chest pains, swelling in the extremities or abdomen, night sweats, as well as muscle and joint aches. If unchecked, broader health effects are seen.
  • Causes: This disease is usually caused by a bacterial infection (called infective endocarditis), though it can arise from other causes.
  • Diagnosis: If suspected, doctors diagnose this condition by performing blood tests, alongside heart imaging, such as echocardiogram, magnetic resonance imaging (MRI), and cardiac angiography, among others.
  • Treatment: Antibiotics delivered intravenously (via IV) are the first-line of treatment, though surgery may be employed if these don’t yield results. The latter involves carefully removing infected endocardial tissue, draining any swollen areas, and, as necessary, repairing and/or replacing the affected tissue. 
  • Prognosis: A condition that usually arises when patients have had other heart problems or have artificial valves, the prognosis for endocarditis is relatively poor, with 10 to 26% of patients experiencing in-hospital mortality, and with 60 to 70% estimated to survive at five years. This condition is severely impacted by the age of the patient, as well as previous medical history.

Mitral Valve Prolapse (MVP)

This congenital condition is characterized by an abnormally thick tissue (including the endocardium) on the mitral valve (between the left ventricle and atrium). MVP causes the valve to “flop” back into the left atrium, leading to a backflow of blood.

Usually asymptomatic, it can be characterized by bursts of rapid heartbeat, fatigue, and chest discomfort. It is detected using imaging techniques, such as echocardiogram (echo). Treatment isn’t usually required; however, if advanced to a point where there’s valve leakage, intervention is necessary to prevent stroke or heart attack.    

Carcinoid Heart Disease

Sometimes referred to as “Hedinger syndrome,” is an advancement of carcinoid syndrome, a spectrum of disorders arising from excess hormone secretion. As such, symptoms include facial flushing, chronic diarrhea, hypotension (low blood pressure), and others.

Eventually, this can lead to right heart failure, causing difficulty breathing, fatigue, swelling in the legs, rapid and irregular heartbeat, and rapid weight fluctuations. It is diagnosed using standard cardiac imaging techniques, such as electrocardiogram (EKG), computer tomography (CT) scan, and others. It can be treated with medication, or by surgically removing problematic tissues.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Mark Gurarie
Mark Gurarie is a freelance writer, editor, and adjunct lecturer of writing composition at George Washington University.