The heart is a very important organ in the body. It is the size of a hand closed into a fist and is located in the chest, between the lungs. The heart's main purpose is to pump blood around the body (The Structure of the Heart). The heart is made up of layers of epicardium, myocardium and endocardium that protect its walls. The epicardium is the membrane of the outer surface of the heart. The endocardium lines the inside of the heart chambers. Finally, between these two layers and the thicker one, the myocardium is made up of cardiac muscles and carries out the work of the heart. The collagenous and elastic fibers between the walls and chambers of the heart are called the fibrous skeleton (Anatomy and Physiology pages 717-718). It functions as a structural support and provides electrical insulation between the atria and ventricles. This is important for the timing and coordination of electrical and contractile activity. Inside the heart there are four important chambers that play an important role in blood circulation: right/left atrium and right/left ventricles. The upper half of the heart, both atria, are thin-walled receiving chambers for blood returning to the heart. Both ventricles, the lower half, expel blood into the arteries and keep it flowing throughout the body. It is essential that blood flows in the correct direction through the heart, so numerous valves are needed. The tricuspid valve separates the right atrium from the right ventricle, and the pulmonary valve divides the right ventricle from the pulmonary artery. The bicuspid valve separates the left atrium from the left ventricle, and the aortic valve separates the left ventricle from the ascending aorta (The S...... middle of paper ...... begins with depolarization of the atria, which is represented in the ECG by the P wave. After the P wave, the QRS complex wave represents ventricular depolarization, but also shows repolarization of the atria, but cannot be seen due to the strength of the ventricular depolarization wave QRS wave, T wave represents ventricular repolarization In addition to the waves of the ECG, there are also intervals and segments that play a role. An interval is part of the ECG that has at least one wave and a straight line. The PR interval is the time it takes for the impulse sent from the SA node to travel to the ventricles Segments refer to a period of time from the end of one wave to the beginning of the next wave AV node to delay and send signals to the ventricles (Lesson). No. 5: Physiology of the heart).