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The heart is an organ which is composed mainly of one tissue called cardiac muscle, but also has an endothelium and connective tissue. Because it is constantly beating i.e. the muscle is constantly contracting and relaxing (remember that muscle cannot "push") it requires a constant supply of energy. Because the heart wall is so thick, and the blood passing through it flows so quickly, heart mucles cannot get nutrients and oxygen from the blood passing through the heart. Therefore the heart is supplied by a network of its own vessels. When some of these coronary vessels are blocked, it can lead to death of parts of the heart wall and a subsequent myocardial infarction or MI. This is often treated by "bypass" surgery where veins from the legs are used to replace the blocked coronary vessels. Double and triple bypass, for example, mean that two or three veins have been used to replace the blocked vessels. The veins are normally taken from the legs of the patient. This does not cause problems as there is a network of veins in the legs and other veins open up more to compensate for those removed.
Histological structure of the heart:
The heart is composed of three concentric layers: endocardium, myocardium and Epicardium. Endocardium consists of a basal endothelium located in a membrane that is continuous with the endothelial layer, composed of collagen fibers, reticulin fibers, elastic fibers, rare cells conjuctive and many sensitive nerve endings.
Myocardium consists of cardiac muscle bundles,circular oriented atria wall, and oblique fiber-coil in ventricles,in the heart wall. In addition to myocardial cells, there are specialized cells in the generation and conduction of the impulses.
Epicardium is a thin membrane covering the cardiac surface and is the visceral foil ofthe pericardial sac. Between pericardial foils,it is found pericardial cavity, with a thin blade of fluid, which promotes slipping during heart activity.
Properties of heart muscle
Myocardium which is structurally a striated muscle has common property as striated muscles , but also a number of characteristic properties.
-Heart-rhythm is property to contract sequentially as a result of contractions generated by pulse of sinoatrial node. These pulses are the result of metabolic changes that occur in excito conductor system.
-Conductivity is the property of Myocardium, in particular of node tissue, to drive shrinkage waves generated by the sinoatrial node in the whole heart.
-Excitability is the property of myocardium to respond by a contraction to appropriate stimulus.
-Myocardial contractility is property-to contract when stimulated appropriately. Cardiac contractions are called systolics and reliefs, diastole.
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