The endocardium is a smooth inner skin that covers the entire inner surface of the heart. All four heart valves are also part of the endocardium. Diseases of the inner lining of the heart and the heart valves often lead to heart failure.
What is the endocardium?
The endocardium is the thin layer of tissue that lines the inner surface of the heart. As the innermost layer of three cardiac layers, it is composed of endothelium and connective tissue. Usually, its thickness is between 0.5 and 1 millimeter. It covers the entire interior of the heart. It also includes the tendon filaments of the papillary muscles. The endocardium is responsible for the efficient functioning of the heart. Another important function of the endocardium is the formation of the four heart valves, which are composed of its infoldings. A particularly fibrous tissue is present in the heart valves. As a valve, the heart valves ensure that blood flows in the right direction. The endocardium has no blood vessels of its own. It is supplied by the subendocardial capillary network.
Anatomy and structure
The endocardium consists of several layers. The outer layer is called the endothelium and forms a continuous, single-layer squamous epithelium. This is followed by the stratum subendotheliale as the second layer. It consists of loose connective tissue with some fibroblasts. Another layer consists of elastic connective tissue and smooth muscle. This layer is called the stratum myoelasticum. The stratum myoelasticum is again divided into four sections. These include the lamina elastica interna, the lamina muscularis, and the lamina fibroelastica externa. The final and outer layer of connective tissue is the tela subendocardialis. It consists of loose connective tissue, which also contains Purkinje fibers (nerve fibers of the excitation conduction system). However, the tela subendocardialis is absent in the endocardium over the tendon filaments of the papillary muscles. This layer is also not always attributed to the endocardium. Rather, it lies between the endocardium and the myocardium. In addition to nerve fibers and parts of the excitation system, it also contains blood and lymphatic vessels. The endocardium refers not only to the inner lining of the heart. The four heart valves are also part of the endocardium. These include the mitral valve, the aortic valve, the tricuspid valve and the pulmonary valve. The heart valves are divided into pocket and leaflet valves. Each half of the heart has a pocket valve and a leaflet valve. Leaflet valves are inlet valves and pocket valves are outlet valves of the heart chambers. The leaflet valves include the mitral valve and the tricuspid valve. The aortic valve and the pulmonary valve are pocket valves. In the different sections of the heart, the endocardium has different thicknesses. Thus, it is thinner in the ventricles than in the atria and thicker in the left side of the heart than in the right side. It is likely that the cause of the different layer thickness of the endocardium is its different strain.
Function and Tasks
The endocardium performs two important functions. As the inner lining of the heart, it provides a smooth surface. Thus, it improves blood circulation in the heart. Its mirror-smooth surface prevents blood from sticking to the inner wall of the heart. This eliminates the possibility of thrombus formation. Blood flows evenly and the heart can work effectively. Even the smallest unevenness has a negative effect on the heart’s performance. When flow resistance occurs, vortices form, which can lead to the formation of blood clots. Unevenness is often the result of inflammation of the inner lining of the heart (endocarditis). The second important function is performed by the endocardium in the form of the heart valves. The heart valves, as mentioned earlier, act as valves within the heart. They ensure that blood can only flow in one direction. They either allow blood to flow into the ventricle as leaflet valves or out of the ventricle as pocket valves. The reverse direction is blocked in healthy heart valves. In this process, the two connective tissue layers of the endocardium, the strata, serve as a shifting layer for the endothelium when the heart muscle contracts (systole). When the heart muscle dilates in the relaxation phase (diastole), the elastic fibers and muscle cells ensure that the endocardium is not overstretched.
Diseases
Diseases associated with the endocardium can occur, often leading to chronic heart failure.Inflammatory processes of the inner lining of the heart or heart valves are referred to as endocarditis. There are infectious and non-infectious forms of this disease. Bacterial endocarditis in humans is often triggered by bacteria that originate from infections that have not healed. These can be, for example, streptococci, staphylococci or enterococci. This disease is noticeable by intermittent fever, general weakness, loss of appetite, heart murmurs and water retention, among other symptoms. Bacterial endocarditis is treated with antibiotics. It can occur as a result of pneumonia, urinary tract infections, bronchitis or even tonsillitis. Patients with a pre-existing heart defect have a higher risk of contracting endocarditis in the event of an infection. The risk is also increased after surgery. Sometimes rheumatic fever occurs as a complication of infectious diseases in children and adolescents, which manifests as endocarditis. However, endocarditis can also have non-infectious causes. For example, certain tumors also trigger endocarditis. Autoimmune diseases affecting the heart also damage the endocardium. Furthermore, there are allergic inflammations of the heart that cause endocarditis. This disease is called Löffler syndrome after the Swiss physician Wilhelm Löffler. Last but not least, arteriosclerosis often affects the heart valves. The majority of heart valve defects are due to bacterial and arteriosclerotic previous diseases of the heart. However, surviving rheumatic inflammation of the endocardium often cannot be ruled out as a cause either.
Typical and common heart diseases
- Heart attack
- Pericarditis
- Heart failure
- Atrial fibrillation
- Heart muscle inflammation
