Cardiac endocardium: structure, functions, common pathologies

The heart is one of the most important organs of the complex system that is commonly called the human body. It is its engine, supplying blood to the most remote corners so that all organs receive sufficient nutrition and can function smoothly. Despite the apparent simplicity of the organ from the outside, its internal structure seems quite interesting. Take at least its walls, which in fact consist of not one, but three different layers, the tissues of which have their own characteristics: endocardium, myocardium, epicardium. Each of these layers has its own structure and functions, the disruption of which causes certain malfunctions in the heart. In this article, we will talk about the inner lining of the main circulatory organ, called the endocardium.

Histology of the epicardium

For a reader who is not versed in the issues of medicine and biology, the meaning of the word "histology" may seem unclear. We are talking about a section of biology that studies the structure, features of vital activity and functioning of various tissues of any living organism, including humans. This means that we will now talk about the structure of the epicardium, its development and the functions it performs.

The human heart can otherwise be called the largest blood vessel, which acts as a pump, ensuring the smooth movement of blood in the body. The pumping function is one of the main functions of the heart, which is provided by the contraction of the central muscular layer of the organ - the myocardium.

It would seem that if the myocardium is capable of ensuring the heart's performance, i.e. pumping blood, then why is the endocardium needed? To understand this, we need to take a closer look at the structure of the endocardium, which is the inner lining of the heart and is tightly adjacent to the myocardium, lining the area of the left and right ventricles and atria.

The endocardium is a continuous membrane that fills any irregularities in the structure of the myocardium, covers the cardiac chambers and valves, the posteromedial and anterolateral papillary muscles, and tendon threads. In the area where large vessels join the heart, the endocardium smoothly transitions into the internal vascular membrane, which is similar in structure and function.

Both the heart wall as a whole and the epicardium itself have a layered structure. It has 4 layers:

• The outer layer, consisting of connective tissue cells and directly adjacent to the myocardium. It has a loose structure and contains thick elastic, collagen and reticular fibers that extend deep into the muscular layer, where they smoothly transition into the connective layers (stroma) of the myocardium.
• The muscular-elastic layer, consisting of smooth elongated myocytes and elastin fibers and resembling in its structure the middle layer of blood vessels. Due to this layer, the endocardium moves after the myocardium during the contractile movements of the latter.
• Subendothelial layer. It, like the outer layer, consists of loose connective tissue.
• Endothelial layer.

Perfectly smooth endothelial cells (endotheliocytes) are attached to a cell-free structure called the basement membrane. The endothelial layer can be considered a type of flat epithelium, because its cells have a slight convexity only in the region of the nucleus, while the cytoplasm evenly fills the free space (externally, when viewed from above, endothelial cells resemble a blot or scrambled eggs). Endothelial cells are microscopic in size and fit tightly together, leaving no space between them.

The endothelium surface is very smooth, and this is not accidental, because it is with it that blood cells directly contact. And one of the important functions of the endocardium is considered to be the ability of blood cells to pass through the cavity of the heart and adjacent vessels (both large and small) without hindrance, without damage. By the way, damage to endothelial cells entails a violation of blood clotting.

In addition to the fact that the endocardium lines the inner surface of the heart, it is also capable of forming peculiar folded structures inside the organ. Endocardial folds are usually called heart valve flaps, the atrial side of which is lined with endothelium and has a smooth surface, and the ventricular side is attached to the tendon threads with irregularities. Thanks to the heart valves, the blood flow through the heart is regulated.

The heart is a vital organ, the formation of which occurs at the very beginning of the embryonic period. The development of the endocardium begins already in the second week of the embryo's life, when groups of cells appear in the germ layer, which in the future will form blood vessels, including the heart. The doubled fold of the mesoderm is transformed into primary endocardial tubes, which subsequently unite into one two-layer structure, called the primary cardiac tube. The endocardium is formed from the inner layer of this tube, and its outer layer gives rise to the myocardium and epicardium.

The peculiarity of the endocardium is that only its outer layer, connected to the myocardium, contains blood vessels. The main part of the endocardium receives nutrients from the blood by diffusion.

Diseases of the endocardium

As we can see, the endocardium is a very important structural part of the heart wall, the health of which determines the intensity of blood flow and even the quality of blood, which supplies oxygen and nutrients to various tissues of the body. Despite the fact that the tissues of the endocardium itself have an insignificant number of blood vessels, it is it, together with the myocardium, that provides the regulatory function of blood circulation (both as an atraumatic surface along which blood flows freely through the main blood vessel, and as heart valves that ensure the correct direction of blood flow).

But, like any human organ, the endocardium is not immune to diseases. These can be either congenital pathologies (heart defects associated with underdevelopment of the valve system, due to which the organ cannot function normally) or acquired ones, most often associated with an inflammatory process in the endocardium.

Generally speaking, inflammation of the endocardium can be considered one of the most common pathologies of the inner layer of the heart, although according to statistics, endocarditis is considered a rather rare disease (1 person out of 25,000). It would seem, how can the inner lining of our "motor" become inflamed, where access from the outside is limited for all environments except blood? But it is necessary to take into account the fact that the most common factor in the development of inflammation is an infection, which can easily spread throughout the body along with the blood, and therefore, can get inside the heart.

It turns out that any bacterial infection present in the body can provoke inflammation of the endocardium? Yes, scientists have concluded that although the most common pathogens of the disease are the well-known streptococci and staphylococci, the development of the disease can also be caused by the presence of gram-negative microorganisms, chlamydia, rickettsia, some fungi and viruses in the body.

However, there is no need to be too scared, because in order for an infectious factor to provoke inflammation, certain prerequisites are needed, namely: congenital and acquired defects of the heart and its valves, plus weakened immunity. The most dangerous in terms of the development of endocarditis are such congenital pathologies as aortic valve stenosis, ventricular septal defect, common arterial trunk, transposition of the great vessels, mitral valve prolapse, etc. Acquired pathologies include: rheumatic heart valve disease, aortic and mitral insufficiency, narrowing of the aorta, etc.

In principle, the development of inflammation on an intact endocardium is rather an exception to the rule, indicating low immunity. Most often, the inflammatory process develops against the background of an existing cardiac pathology.

Congenital and acquired heart defects are associated with hemodynamic disturbances (the formation of turbulent blood flow and high blood pressure on the vessel walls), which can cause damage to the inner lining of the heart. Damage to the endocardium, in turn, entails disturbances in the blood coagulation system and the formation of blood clots, on which pathogenic microorganisms subsequently settle. Thrombotic elements themselves do not cause inflammation, although when torn off, they can move with the blood flow to the brain and other organs, causing vascular occlusion (in the case of the brain, this can threaten a stroke). Bacteria that settle on thrombotic formations contribute to the further destruction of the inner layer of the heart, which further disrupts hemodynamics and the functioning of the heart as a whole.

Inflammation of the endocardial layer is most often observed in the area of the heart valve flaps, which are more susceptible to damage by the blood flow. It is in the area of the heart valves that infection most often settles, causing an inflammatory process and proliferation of connective tissue, leading to thickening of the endocardium. In addition, detachment of the upper layer of the endocardium, the formation of blood clots and threads of a special protein fibrin, covering tissue defects, and again leading to their thickening, can be observed.

In order for infectious (aka bacterial, fungal, viral, septic, etc.) endocarditis to develop, there must be a source of infection in the body, which can be venereal pathologies, bacterial lesions of the gastrointestinal tract, caries, stomatitis and even respiratory infections. By the way, this pathology is often diagnosed in children aged 8-13 precisely against the background of inadequate treatment of infectious respiratory pathologies, which significantly reduce the body's defenses.

In addition, the bacterial factor can enter the blood during medical procedures: colonoscopy, bronchoscopy, catheterization, implantation, biopsy, dental treatment, etc. For example, the prevalence of pathology among drug addicts is due to the use of non-sterile needles and syringes. But heart patients can get an infection during the implantation of prostheses and shunts.

The main symptoms of endocarditis are: fever against the background of relative health, the appearance of heart murmurs and hemorrhages on the skin and whites of the eyes, myalgia, pain in the chest and head, cough, shortness of breath, hyperhidrosis at night, edema syndrome, weight loss, etc.

Treatment of infective endocarditis is primarily the introduction of effective antimicrobial drugs - antibiotics - into the body. A quarter of patients undergo surgical intervention due to the high probability of developing various complications, most often irreversible.

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Consequences of endocarditis

Endocardial inflammation quite often causes other equally dangerous pathologies of the inner lining of the heart. For example, such a pathology of newborns as endocardial fibroelastosis. The disease consists of thickening of the heart wall, as a result of which the heart chambers are smaller. This state of affairs causes the development of a severe form of congestive heart failure, which in turn can lead to death, which is often observed among babies with this diagnosis.

Intensive treatment in some cases allows the disease to become chronic and prolong the period of remission, and in rare cases even cure the disease. It is important that the child's body actively responds to drug therapy.

The etiology of endocardial fibroelastosis (a rather rare pathology) has not yet been fully studied, however, there are all the prerequisites to suspect intrauterine infection of the fetus as the main predisposing factor, which causes an inflammatory process with subsequent tissue thickening. Other causes of the pathology can be considered: subendocardial ischemia (impaired blood supply to the subendocardial layer of the myocardium), reduced lymphatic drainage of cardiac tissue, general carnitine deficiency.

Secondary endocardial fibroelastosis can develop against the background of congenital and acquired heart defects (aortic stenosis, genetic mutation in the form of the absence of physiologically determined openings in the heart, myocardial damage, etc.).

Treatment of the disease involves lifelong administration of cardiac glycosides, anticoagulants, and glucocorticosteroids.

Another rare disease with damage to the endocardium of the heart can be called endocardial fibrosis. Here some clarification is required: it is more correct to call the pathology endomyocardial fibrosis, since it affects not only the endocardium, but also the middle membrane of the heart (myocardium), and is manifested by inflammation and thickening of the endocardial and myocardial layers of the heart. Most often, changes are diagnosed in the apices of the ventricles of the heart, but sometimes they can be detected on the atrioventricular valves, consisting of endocardium.

Scientists believe that the main causes of this pathology, common in the tropics and subtropics, are the inflammatory process, the presence of infection in the body, poor nutrition (malnutrition, deficiency of vitamins and minerals, intoxication with serotonin, which is contained in plantain, which is actively consumed by local residents).

The main symptom of the disease is progressive heart failure, which causes death in most patients within 1-2 years after the onset of the disease.

Effective drug therapy in this case has not yet been developed, since the etiology of the disease has been studied very little. In some cases, surgical intervention helps, consisting of endocardiectomy, which is carried out together with plastic surgery of the atrioventricular valves located between the atria and ventricles of the heart.

Inflammatory pathologies of the cardiac membranes can lead to metabolic disorders, such as calcium, in the organ tissues even in the absence of endocrine diseases. Calcium, along with many other elements of the periodic table (sodium, potassium, zinc, magnesium, etc.), is a substance necessary for our body to carry out vital functions, but its excess can cause calcification (calcinosis) of various tissues and organs, including the endocardium. The whole point is that calcinosis can develop against the background of various inflammatory pathologies, accompanied by the proliferation of fibrous tissues.

Calcification is most often diagnosed in the area of the aortic valve, as a result of which calcareous growths form on its walls, disrupting hemodynamics (normal blood flow) and provoking the development of organic lesions of various heart tissues.

Among the most common causes of myocardial calcification, one can also consider rheumatic lesions of body tissues, provoking degenerative changes in them. Rheumatism is considered an infectious-allergic disease with a wave-like course, which mainly affects the heart and blood vessels. Its causative agent is streptococcus, the immune response to the substances secreted by it provokes the appearance of symptoms of the disease.

Rheumatism manifests itself as mucoid edema of the heart tissue, softening and necrosis of collagen fibers and penetration of fibrin threads into them, inflammatory reactions occurring at the cellular level with the formation of specific rheumatic granulomas in the endocardium (connective tissue that forms the inner lining and heart valves) and other heart tissues.

In principle, endocarditis can be considered one of the most striking manifestations of rheumatism. And at the same time, inflammation of the endocardium caused by a bacterial infection can itself provoke the development of rheumatism. Thus, endocarditis can be considered both as a cause and as a consequence of rheumatism of the heart and blood vessels. In this case, the disease becomes chronic and is difficult to treat.

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