There are several ways to analyze a heart. To understand its functioning and the connection of this function to the EKG trace, we can use two approaches. We can analyze the heart as if it were composed of two pumps, or we can analyze it as if it were comprised of two semi-isolated electrical systems. Let’s look at each method of analysis in turn. The heart comprises four chambers: two atria and two ventricles, with the two atria positioned on top of the ventricles. One of the pumps consists of the right atrium and right ventricle (shown in blue in Fig. 7.1), and the other consists of the left atrium and left ventricle. These pumps are enclosed in a serous membrane. A serous membrane consists of two layers: an inner, or visceral layer, and an outer, or parietal layer. The cells that make up the serous membrane contain glands that secrete a lubricating fluid called the serous fluid, which is present between the two layers. This arrangement enables nearly frictionless movement for organs enclosed by the serous membrane. As these organs are packed tightly in our body, this is a necessity. In the serous membrane that encloses the heart, the inner visceral layer is known as the visceral pericardium, and the outer parietal layer is referred to as the parietal pericardium. The serous fluid between these two layers is called the pericardial fluid. Outside the serous layer is a fibrous layer that protects the pumps and is fused to the parietal pericardium. The serous layer and the fibrous layer together are called the pericardium.