IntroductionIn our world, organisms occupy a sliding scale of complexity. On the one hand we have unicellular organisms, where all the functions necessary for their life are carried out within that single cell. At the other extreme we have extremely complex multicellular organisms, of which man is perhaps the cardinal member. Of course, with greater capacity comes greater skill. Complex organisms are able to manipulate their environment to a greater degree than their simpler cousins. While this has many advantages, it also presents interesting biological problems. With increasing complexity, multicellular organisms must have systems to deliver nutrients, signaling molecules, and biochemicals to each cell. Furthermore, wastes and useful cellular products must be removed from the cell and taken to appropriate places within the organism. This function is carried out by the circulatory system. Although a variety of organisms have a circulatory system, they range substantially from primitive organisms to more complex mammals. In this case we are particularly interested in the human circulatory system. At the center of this system is the heart. The heart is a muscular organ located in the center of the chest. It is suspended by its attachment to the large vessels. The heart and attachments are enclosed in a fibrous sac called the pericardium. Figure The human heart consists of four chambers, which are the left and right atria along with the left and right ventricles. The right side of the heart receives blood into the right atrium from the vena cava which is the final venous collection for the return of systemic flow. Flowing through the right atrioventricular valve into the right ventricle, blood is the focus of substances such as digitalis toxicity, catacholamines, and ischemia. The length of the action potential is also relevant since a longer length of the action potential leads to an increase in calcium overload. The first successive depolarizations take place during the plateau (phase 2) and repolarization (phase 3) phases of the cardiac action potential. Like delayed afterdepolarization, early afterdepolarization relies on a prolonged action potential that triggers additional activity. The amplitude of early afterdepolarization is strongly velocity dependent. The classic example is a patient with long QT syndrome and bradycardia that triggers torsades de pointes. Reentry is the last category of arrhythmias. Normally, a cardiac action potential ends when all cells have been stimulated and are refractory. However, if a group of cells somehow manages to recover excitability.