FIGURE 13.5 • Chambers of the heart and the impulse-conducting system. The heart has been cut open in the coronal plane to expose its interior and the main parts of its impulse-conducting system (indicated in yellow). Impulses are generated in the sinuatrial (SA) node, transmitted through the atrial wall to the atrioventricular (AV) node and then sent along the AV bundle to the Purkinje fibers.
impulses that cause the heart to contract and supply blood to the body. Sudden cardiac arrest is a medical emergency; first-aid treatment such as cardiopulmonary resuscitation (CPR) and defibrillation (delivering a therapeutic dose of electrical energy to the heart) can improve the chances of survival. If not treated, cardiac arrest leads to sudden cardiac death. Heart rhythm pathologies associated with cardiac arrest include tachycardia (accelerated heart rhythm), fibrillation (rapid, irregular, and ineffective contractions), bradycardia (decelerated heart rhythm), and asystole (total absence of heart rhythm).
 A coronary vasculature that consists of two coronary arteries and cardiac veins. The right and left coronary arteries provide the arterial blood supply to the heart. They originate from the initial part of the ascending aorta near the aortic valves and circle the base of the heart, with branches converging toward the apex of the heart. Venous drainage of the heart occurs via several cardiac veins, most of which drain into the coronary sinus located on the posterior surface of the heart. The coronary sinus drains into right atrium.
The wall of the heart is composed of three layers: epicardium, myocardium, and endocardium.
The structural organization of the wall of the heart is continuous within the atria and ventricles. The wall of the heart is composed of three layers. From the outside to the inside, they are as follow.
 The epicardium, also known as the visceral layer of serous pericardium, adheres to the outer surface of the heart (Fig 13.6). It consists of a single layer of mesothelial cells and underlying connective and adipose tissue. The blood vessels and nerves that supply the heart lie in the epicardium and are surrounded by adipose tissue that cushions the heart in the pericardial cavity. The epicardium is reflected back at the great vessels entering and leaving the heart as the parietal layer of serous pericardium, which lines the inner surface of the pericardium that surrounds the heart and roots of great vessels. Thus, there is a potential space containing a minimal amount (15–50 ml) of serous (pericardial) fluid between the visceral and parietal layers of the serous pericardium. This space is known as the pericardial cavity; its lining consists of mesothelial cells (see Fig. 13.6). The condition in which excess ﬂuid (blood or pericardial effusion) rapidly accumulates in the pericardial cavity is called cardiac tamponade. It is commonly caused by both blunt and penetrating chest injuries and by myocardial rupture or pericarditis (inﬂammation of pericardium). This is a life-threatening condition in which accumulating ﬂuid compresses the heart, preventing the heart’s chambers from filling properly with the blood. Relieving the pressure is usually accomplished with pericardiocentesis (a procedure to drain the ﬂuid from the pericardial cavity).