HistologyTissue
The endocardium is a flat, unicellular layer that separates the chamber muscles from the blood. It corresponds functionally to the inner lining of the blood vessels (endothelium). Its function, preventing the formation of a blood clot (thrombus), is ensured by its special smooth surface and by the production of anticoagulants (nitrogen monoxide (NO), prostacyclin).
The myocardium (heart muscle) is the driving force for blood flow (convection) throughout the body. The muscle cells are a kind of mix of smooth and striated muscles. They have the same mobile protein complexes (sarcomeres of actin, myosin and titin) as the muscles of the musculoskeletal system (striated muscles) and therefore the same mechanism to control the contraction of the protein complexes.
This mechanism consists of other proteins (troponins), which can assume different structures and which, depending on their condition, can allow or prevent the individual components of the protein complex from contracting together. What distinguishes the heart muscle cells from the skeletal muscle cells is the arrangement of the individual cells in all directions of the three-dimensional space and their centrally located cell nucleus – both characteristics of smooth muscles (visceral muscles). The muscle cells are connected to each other by fixed cell-cell connections (desmosomes).
In addition, there is another type of cell-cell connection (gap junction), which fulfils an electrical function by connecting the individual cells with each other in an electrically conductive manner.This is why we also speak of a functional syncytium (cell group without cell boundaries). The muscle layer is not the same thickness in the whole heart. The thickness of the muscle layer ranges from 2-3 mm in the right atrium to 12 mm in the left chamber.
These differences are thus an expression of the different pressures that prevail in the individual heart cavities. In the wall of the right atrium there are other specialized cells, so-called myoendocrine cells. They are muscle cells from their origin, but they produce the hormones ANP (atrial natriuretic peptide) and BNP (brain natriuretic peptide).
They are formed when excessive blood is measured in the atrium. Their effect lies in increased fluid excretion (diuresis) by the kidneys to prevent excess blood. Epicardium and pericardium are the two leaves of the classical serous organ coating.
The leaf close to the organ (visceral) is the epicardium, the parietal (distal) leaf is the pericardium. At the border between the two leaves they are very smooth and separated by a very narrow, fluid-filled cavity. They thus allow the heart to move almost without friction.
Furthermore, the outer (parietal) leaf (pericardium) with its taut connective tissue gives the heart mechanical stability. The heart is supplied with oxygen by its own vascular system (coronary arteries). The vessels are located inside the pericardium.
The two arteries of the heart (arteria coronaria dextra and sinistra) both originate directly from the initial part of the aorta, a few millimeters behind the aortic valve. The left coronary artery (LCA= Left coronary artery) runs anteriorly at the level of the atrial-ventricular junction and then divides into a descending branch (ramus interventricularis anterior (LAD= Left anterior descending) and a branch that runs further horizontally (RCX= Ramus circumflexus)). The right coronary artery (RCA) is the smaller of the two coronary arteries and runs backwards, also at the level of the atrial-ventricular junction.
It supplies the sinus and AV node to the two decisive stations of excitation formation. Of all these arteries named here, smaller branches extend into the musculature to be supplied in the direction of the heart cavities. Only the innermost layers of the myocardium are supplied directly by diffusion (absorption of blood components due to concentration differences) from the heart cavities.
Due to the high pressure (>120 mmHg) that is generated during systole, especially in the left ventricle, the vessels in the systole are pressed shut. As a result, the supplying blood stream only advances in diastole. The problem that results from the diastolic blood flow: With increased heart frequency the Diastole is disproportionately shortened – the time for an oxygen supply thereby likewise.
However, the increased cardiac output increases the demand for oxygen. This is a contradiction that can become dangerous for the preexisting heart. There are basically two paths for venous return: The main path collects the blood in a cardiac vein (sinus coronarius) and flows into the right atrium, as does the rest of the body’s used blood.
A by-path for the venous blood are tiny veins that open directly into all four heart cavities. Here it must be added that the high pressure during a heart contraction literally squeezes out the veins – the return flow works without any problems in almost all hearts.
