id (a material resembling cartilage). The base of each valve is supported by a fibrocollagenous ring. Each valve also has a dense fibrocollagenous central plate that is covered by simple squamous epithelium. Chordae tendonae connect with the valves at the edge of each cusp as well as underneath each cusp at one end and they attach to papillary muscles in the ventricles at the other end. Endocardial endothelium completely covers the papillary muscles, valves, and the chordae tendonae. The junctions between the cusps of each valve are known as commissures. The conducting system of the heart consists of four main components; the sinuatrial node (SA), the atrioventricular node (AV), the bundle of his, and the Purkinje fibers/cells. All the parts of this conducting system are composed of modified cardiac muscle cells. The SA node is located in the right atrium, at the point where the superior vena cava enters. The small muscle fibers of the SA node contain a central nodal artery and desmosomes. The muscle fibers do not contain intercalated discs. The AV node is located in the medial wall, in front of the opening of the coronary sinus and above the tricuspid ring. Its small muscle fibers are more regularly arranged than those of the SA node. The AV node contains a rich nerve and blood supply. The bundle of his has a right (single bundle) and a left (branched bundle) bundle branch located underneath the endocardium. It is histologically similar to the other components of the conducting system. The Purkinje fibers/cells can be found in clusters of about six cells which are located under the endocardium in the ventricles. The cytoplasm of Purkinje fibers appears pale under the microscope and contains many glycogen granules.7 Physiology of the Heart: The principle function of the heart and circulatory system is to provide oxygen and nutrients and to remove metabolic waste products from tissues and organs of the body. ...
