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Structure of the walls of the heart
Last reviewed: 20.11.2021
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In the wall of the heart, 3 layers are distinguished: a thin inner layer - endocardium, a thick muscular layer - myocardium and a thin outer layer - epicardium, which is a visceral leaf of the serosa of the heart - a pericardium (pericardial sac).
Endocardium lining the heart cavity from the inside, repeating its complex relief, and covers the papillary muscles with their tendon chords. The atrioventricular valves, the aortic valve and the pulmonary trunk valve, as well as the flaps of the inferior vena cava and the coronary sinus are formed by endocardium duplications, inside which connective tissue fibers are located.
The endocardium is formed by a single layer of flat polygonal endotheliocytes located on a thin basal membrane. In the cytoplasm of endotheliocytes, a large number of micropinocytosis vesicles. Endotheliocytes are connected to each other by intercellular contacts, including nexus. On the border with the myocardium there is a thin layer of loose fibrous connective tissue. The middle layer of the heart wall is myocardium, it is formed by cardiac transverse striated muscle tissue and consists of cardiac myocytes (cardiomyocytes). Cardiomyocytes are interconnected by a large number of bridges (intercalary discs), through which they are connected to the muscular complexes forming a narrow-looped network. This muscular network provides a complete rhythmic contraction of the atria and ventricles. The thickness of the myocardium is the smallest in the atria, and the largest - in the left ventricle.
Muscle bundles of the atria and ventricles begin from fibrous rings completely separating the myocardium of the atria from the ventricular myocardium. These fibrous rings, as well as a number of other connective tissue formations of the heart, are part of its soft skeleton. To the skeleton of the heart are: connected between the right and left fibrous rings (annuli fibrosi dexter et sinister), which surround the right and left atrioventricular orifices. These rings constitute the support of the right and left atrioventricular valves {their projection everywhere corresponds to the coronary groove of the heart). The right and left fibrotic triangles (trigonum fibrosum dextrum and trigonum fibrosum sinistrum) are dense plates that on the right and left adjoin the posterior semicircumference of the aorta and are formed by the fusion of the left fibrous ring with the connective tissue ring of the aortic aperture. The right, most dense, fibrous triangle, which actually connects the left and right fibrous rings and the connective tissue ring of the aorta, is in turn connected to the membranous part of the interventricular septum. In the right fibrous triangle there is a small hole through which the fibers of the atrioventricular bundle of the conducting heart system pass.
The myocardium of the atria is separated by fibrotic rings from the myocardium of the ventricles. Synchronism of contractions of the myocardium is provided by a conduction system of the heart, one for the atria and the ventricles. In the atria, the myocardium consists of two layers: superficial, common for both atria, and deep, separate for each of them. In the surface layer, the muscle bundles are located transversely, in the deep layer - longitudinally. Circular muscle bundles looped around the mouth of the veins, flowing into the atrium, like compressors. Longitudinally lying muscle bundles originate from fibrous rings and in the form of vertical cords protrude into the cavities of the atrial auricles and form cristae muscles.
Myocardium of the ventricles consists of three different muscle layers: outer (superficial), middle and inner (deep). The outer layer is represented by obliquely oriented muscle beams, which, starting from the fibrous rings, continue down to the apex of the heart, where the curl of the heart is formed (vortex cordis). Then they pass into the inner (deep) layer of the myocardium, the bundles of which are arranged longitudinally. This layer forms papillary muscles and fleshy trabeculae. The outer and inner layers of the myocardium are common to both ventricles. Located between them is the middle layer, formed by circular (circular) muscle tufts, separate for each ventricle. The interventricular septum is formed in the greater part (its muscular part) by the myocardium and the endocardium covering it. The basis of the upper part of this septum (its membranous part) is a plate of fibrous tissue.
The outer shell of the heart - epicardium (epicardium), adjacent to the myocardium from the outside, is a visceral leaf of the serous pericardium. An epicardium was constructed according to the type of serous membranes and consists of a thin plate of connective tissue covered with mesothelium. Epicardium covers the heart, the initial sections of the ascending aorta and pulmonary trunk, the terminal sections of the hollow and pulmonary veins. On these vessels the epicardium passes into the parietal plate of the serous pericardium.
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