First off, i would like to clear up a worldwide rumor; the Heart is not located on the left side of your body! Its actual location is found in the center of your chest, just SLIGHTLY of to the left hand side, and underneath the sternum. For protection, the heart is enveloped in a sac called the pericardium, and is surrounded by the lungs. First thought is, "why does the strongest muscle in the body need so much protection?". It's fragile! Weighing in at about 300 grams, the heart consists of 4 chambers; 2 lower ventricles and 2 upper atria. Valves between the atrium and the ventricle control proper blood flow from one to the other.
The job of pumping blood everywhere in the body is a huge responsibilty. Each and every beat of the heart consists of a sequence of events called the cardiac cycle. "Cardiac" is greek, meaning "heart". There is 3 major steps in the sequence: atrial systole, ventricular systole and complete cardiac diastole. After the blood has completely left the atria, the atrioventricular valves (located between atria and ventricular chambers), close to prevent backflow. This function is what you would recognize as your heartbeat. Next, there is a contraction of the ventricles and flow of blood into the circulatory system. This is known as the ventricular systole. Once again, valves called "pulmonary" and "aortic semilunar" close to prevent backflow. After these 2 steps, the heart takes a quick break called complete cardiac diastole. This allows the refilling of blood and to start the process over.
