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Heart wall structure
Last reviewed: 04.07.2025
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The wall of the heart is divided into 3 layers: a thin inner layer - the endocardium, a thick muscular layer - the myocardium and a thin outer layer - the epicardium, which is the visceral layer of the serous membrane of the heart - the pericardium (pericardial sac).
The endocardium lines the cavity of the heart from the inside, repeating its complex relief, and covers the papillary muscles with their tendinous chords. The atrioventricular valves, the aortic valve and the pulmonary valve, as well as the valves of the inferior vena cava and coronary sinus are formed by duplications of the endocardium, inside which connective tissue fibers are located.
The endocardium is formed by a single layer of flat polygonal endotheliocytes located on a thin basement membrane. In the cytoplasm of the endotheliocytes there is a large number of micropinocytotic vesicles. The endotheliocytes are connected to each other by intercellular contacts, including nexuses. At the border with the myocardium there is a thin layer of loose fibrous connective tissue. The middle layer of the heart wall - the myocardium - is formed by cardiac striated muscle tissue and consists of cardiac myocytes (cardiomyocytes). Cardiomyocytes are connected to each other by a large number of bridges (intercalated discs), with the help of which they are connected into muscle complexes that form a narrow-mesh network. This muscle network ensures complete rhythmic contraction of the atria and ventricles. The thickness of the myocardium is the smallest in the atria, and the greatest in the left ventricle.
The muscular bundles of the atria and ventricles originate from the fibrous rings, which completely separate the atrial myocardium from the ventricular myocardium. These fibrous rings, like a number of other connective tissue formations of the heart, are part of its soft skeleton. The skeleton of the heart includes: the interconnected right and left fibrous rings (annuli fibrosi dexter et sinister), which surround the right and left atrioventricular openings. These rings form the support of the right and left atrioventricular valves (their projection everywhere corresponds to the coronary groove of the heart). The right and left fibrous triangles (trigonum fibrosum dextrum et trigonum fibrosum sinistrum) are dense plates, which are adjacent to the posterior semicircle of the aorta on the right and left and are formed as a result of the fusion of the left fibrous ring with the connective tissue ring of the aortic opening. The right, most dense, fibrous triangle, which actually connects the left and right fibrous rings and the connective tissue ring of the aorta, in turn is connected to the membranous part of the interventricular septum. In the right fibrous triangle there is a small opening through which the fibers of the atrioventricular bundle of the cardiac conduction system pass.
Atrial myocardiumseparated from the ventricular myocardium by fibrous rings. The synchronicity of myocardial contractions is ensured by the cardiac conduction system, which is common to both atria and ventricles. In the atria, the myocardium consists of two layers: a superficial layer, common to both atria, and a deep layer, separate for each of them. In the superficial layer, the muscle bundles are located transversely, in the deep layer - longitudinally. Circular muscle bundles loop-like embrace the mouths of the veins flowing into the atria, like constrictors. Longitudinal muscle bundles originate from the fibrous rings and protrude into the cavities of the atrial auricles in the form of vertical strands and form pectineal muscles.
Myocardium of the ventriclesconsists of three different muscle layers: the outer (superficial), middle and inner (deep). The outer layer is represented by obliquely oriented muscle bundles, which, starting from the fibrous rings, continue downwards to the apex of the heart, where they form the cardiac curl (vortex cordis). Then they pass into the inner (deep) layer of the myocardium, the bundles of which are located longitudinally. Due to this layer, papillary muscles and fleshy trabeculae are formed. The outer and inner layers of the myocardium are common to both ventricles. The middle layer located between them, formed by circular muscle bundles, is separate for each ventricle. The interventricular septum is formed in its greater part (its muscular part) by the myocardium and the endocardium covering it. The basis of the upper section of this septum (its membranous part) is a plate of fibrous tissue.
The outer membrane of the heart - the epicardium, adjacent to the myocardium from the outside, is the visceral layer of the serous pericardium. The epicardium is constructed like serous membranes and consists of a thin plate of connective tissue covered with mesothelium. The epicardium covers the heart, the initial sections of the ascending aorta and pulmonary trunk, the final sections of the vena cava and pulmonary veins. Through these vessels, the epicardium passes into the parietal plate of the serous pericardium.
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