26. Heart

The heart is a muscular organ, composed primarily of cardiac muscle tissue, which contracts rhythmically to pump blood throughout the body. Structure of the heart wall: the walls of the heart are constructed in layers that are similar to those of the major blood vessels.

Endocardium is the innermost layer of the heart and is lined with endothelium. Veins, nerves, and components of the impulse conduc—ting system are present in the suben—docardial connective tissue layer.

Myocardium is composed of branching, anastomotic cardiac myocytes attached to one another by intercalated disks. Most of these cells are involved in the pumping function of the heart; others are specialized for the control of rhythmicity (impulse conducting system) or secretion (myocardial endocrine cells).

Epicardium is a serous membrane that forms the visceral lining of the pericardium. Its external mesothelium is supported by a loose connective tissue subepicardial layer.

Cardiac skeleton is composed mainly of dense connective tissue and consists of the annuli fibrosi, the trigonum fibrosum, and the septum membranaceum.

Cardiac valves are composed of dense fibrous tissue covered by endothelium. Unidirectional flow is maintained from the.

Right atrium to the right ventricle (tricuspid valve).

Right ventricle to the pulmonary artery (pulmonic semilunar valve). Left atrium to the left ventricle (mitral/bicuspid valve).

Left ventricle to the aorta (aortic semilunar valve).

Tricuspid and mitral valves are attached to papillary muscles by cords of fibrous connective tissue (chordae tendineae) and prevent reflux of blood into the atria during 26б ventricular con traction (systole). Semilunar valves (aortic and pulmonic) prevent reflux of blood back into the ventricles during ventricular relaxation (diastole).

Impulse conducting system of the heart consists of specialized cardiac myocytes that are characterized by auto—maticity and rhythmicity (i. e., they are independent of nervous stimulation and possess the ability to initiate heart beats). These specialized cells are located in the sino—atrial (SA) node (pacemaker), intern—odal tracts, atrioven—tricular (AV) node, AV bundle (of His), left and right bundle branches, and numerous smaller branches to the left and right ventricular walls. Impulse conduct ing myocytes are in electrical contact with each other and with normal contractile myocytes via communicating (gap) junctions. Specialized wide—diameter impulse conducting cells (Pur—kinje myocytes), with greatly reduced myofilament components, are well—adapted to increase conduction velocity. They rapidly deliver the wave of depolarization to ventricular myocytes.

New words

heart – сердце

muscular – мышечный

cardiac – сердечный

to pump – качать

endocardium – эндокардиум

innermost – самый внутренний

conducting system – проведение системы

subendocardial – внутрисердечный

impulse – импульс

fibrosi – фиброзные кольца