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 this single cell. At the other extreme, we have extremely complex multicellular organisms, of which man is perhaps the cardinal member. Obviously, with increased capacity comes increased capabilities. Complex organisms are capable of manipulating their environments to a greater extent than their simpler cousins. While this has many advantages, it also presents some interesting biological problems. With increasing complexity, multicellular organisms must have systems to deliver nutrients, signaling molecules, and biochemical building blocks to each cell. Additionally, waste products and useful cellular products must be removed from the cell and transported to appropriate locations within the body. This function is accomplished by the circulatory system. Although various organisms have a circulatory system, this varies greatly, 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 thorax. It is suspended by its attachment to large vessels. The heart and its attachments are enclosed in a fibrous sac called the pericardium.FigureThe human heart consists of four chambers, these being the left and right atria and the left and right ventricles. The right side of the heart receives blood into the right atrium from the vena cava which is the last venous collection for return of systemic flow. As blood flows through the right atrioventricular valve into the right ventricle, it is surrounded by paper substances, such as digitalis toxicity, catacholamines, and ischemia. Action potential length is also relevant, as increased action potential length results in increased calcium overload. Early afterdepolarizations occur 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 rate dependent. The classic example is that of a patient with long QT syndrome and bradycardia triggered by torsade de pointes. Reentry is the final 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 regain excitability.