Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Cardiovascular System
Heart
Structure of the Heart Wall

The walls of The Heart chambers vary significantly in thickness: they are relatively thin in the atria (2–5 mm), whereas in the left ventricle (averaging 15 mm) they are typically 2.5 times thicker than in the right ventricle (about 6 mm). The heart wall consists of three layers: the outer epicardium, the middle myocardium, and the inner endocardium.

The epicardium (epicardium) is the inner layer of the serous pericardial sac, or Pericardium (Atlas, Fig. 51). A slit-like space exists between the epicardium and the pericardium, containing a small amount of fluid that acts as a lubricant, facilitating the smooth gliding of the epicardial and pericardial surfaces against one another during heart contractions. The surfaces of the epicardium and pericardium facing the pericardial cavity are lined with mesothelium. The Connective Tissue forming the framework of these two layers contains a rich network of Collagen and elastic fibers, numerous Blood and Lymphatic capillaries, and nerve endings. The epicardium is tightly fused with the myocardium and transitions into the pericardium at the roots of the large vessels entering and leaving the heart. Significant amounts of adipose tissue are sometimes found in the epicardium within the sulci and near the Blood Vessels.

The myocardium (myocardium) is the thickest and most robust layer, formed by Striated Muscle tissue which, unlike Skeletal Muscle, consists of individual Cells—cardiomyocytes—linked together in chains (fibers). The cells are firmly bound to one another via Intercellular junctions known as desmosomes. Thin layers of connective tissue and a well-developed network of blood and lymphatic capillaries lie between the fibers.

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Fig. 2.4. The "Skeleton" of the heart from above (diagram): fibrous rings of the 1 — pulmonary trunk; 2 — aorta; 3 — left and 4 — right atrioventricular orifices

Contractile and conducting cardiomyocytes are distinguished; their Structure is studied in detail in Histology courses. Contractile cardiomyocytes of the atria and ventricles differ from one another: in the atria they are branched, whereas in the ventricles they are cylindrical in shape. The biochemical composition and set of Organelles in these cells also differ. Atrial cardiomyocytes produce substances that reduce blood clotting and regulate blood pressure. Heart muscle contractions are involuntary.

Embedded within the thickness of the myocardium is the robust connective-tissue "skeleton" of the heart (Fig. 2.4). It is formed primarily by fibrous rings positioned in the plane of the atrioventricular orifices. From these, Cytology/practical/45.html">Dense connective tissue extends into the fibrous rings surrounding the orifices of the aorta and the pulmonary trunk. These rings prevent the dilation of the orifices during heart Muscle contraction. The muscle fibers of both the atria and the ventricles originate from the cardiac skeleton, thereby isolating the atrial myocardium from the ventricular myocardium and enabling their separate contractions. The cardiac skeleton also serves as a structural support for the valve apparatus.

Fig. 2.5. Heart muscle (left):

1 — right atrium; 2 — SUPERIOR VENA CAVA; 3 — right and 4 — left Pulmonary Veins; 5 — left atrium; 6 — left auricle; 7 — circular, 8 — outer longitudinal, and 9 — inner longitudinal muscle layers; 10 — left ventricle; 11 — anterior longitudinal sulcus; 12 — semilunar Valves of the pulmonary trunk and 13 — aorta

the deep layers are common to both ventricles. Fibers of the first layer originate from the fibrous rings and descend obliquely toward the apex of the heart. Here they loop around, transition into the deep longitudinal layer, and ascend toward the Base of the heart. A portion of the shorter fibers forms muscular trabeculae and papillary Muscles. The middle circular layer is independent in each ventricle and serves as a continuation of fibers from both the superficial and deep layers. In the left ventricle, this layer is significantly thicker than in the right, which is why the walls of the left ventricle are more powerful than those of the right. All three muscle layers form the interventricular septum. Its thickness is comparable to that of the left ventricular wall, except in its upper portion, where it is considerably thinner.

Specialized, atypical fibers are distinguishable within the heart muscle; they are poor in myofibrils and stain much less intensely in histological preparations. They belong to the so-called Conduction system of the Heart (Fig. 2.6). Running alongside them are a dense plexus of Unmyelinated nerve fibers and groups of Neurons belonging to the Autonomic Nervous system. In addition, Vagus nerve fibers terminate here. The centers of the conduction system are two nodes: the sinuatrial node and the atrioventricular node.

The atrial musculature comprises two layers: a superficial layer consisting of transverse (circular) fibers common to both atria, and a deep layer composed of vertically oriented fibers unique to each atrium. Some of the vertical bundles extend into the cusps of the mitral and tricuspid valves. Furthermore, circular muscle bundles are located around the orifices of the venae cavae and pulmonary veins, as well as along the margin of the fossa ovalis. Deep muscle bundles also form the pectinate muscles.

Fig. 2.6. Conduction System of the heart:

1 — sinuatrial and 2 — atrioventricular nodes; 3 — bundle of His; 4 — Branches of the bundle of His; 5 — Purkinje fibers

The sinuatrial node (sinus node) is located beneath the epicardium of the right atrium, between the entry point of the superior vena cava and the right auricle. The node is an aggregation of conducting myocytes surrounded by connective tissue penetrated by a capillary network. Numerous nerve fibers belonging to both Divisions of the autonomic nervous system penetrate the node. The Cells of the node are capable of generating impulses at a rate of 70 beats per minute. Cell function is influenced by certain Hormones, as well as by sympathetic and parasympathetic inputs. From the node, excitation spreads via specialized muscle fibers throughout the atrial musculature. Some conducting myocytes form the atrioventricular bundle, which descends along the interatrial septum toward the atrioventricular node.

The atrioventricular node lies in the lower part of the interatrial septum. Like the sinuatrial node, it is formed by highly branched and anastomosing conducting cardiomyocytes. The atrioventricular bundle (bundle of His) departs from it into the substance of the interventricular septum. Within the septum, the bundle divides into two branches. At approximately the middle of the septum, numerous fibers known as Purkinje fibers branch off from them. These fibers ramify within the myocardium of both ventricles, penetrate the papillary muscles, and reach the endocardium. The distribution of these fibers ensures that myocardial contraction at the apex of the heart begins earlier than at the base of the ventricles.

The myocytes forming the conduction system of the heart connect with working cardiomyocytes via Gap Junctions (nexus junctions). This allows excitation to be transmitted to the working myocardium, triggering its contraction. The conduction system of the heart coordinates The activity of the atria and ventricles, whose musculature is otherwise structurally separate; it ensures the autonomy of the heart's function and maintains the cardiac rhythm.