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Cardiac cycle

Your Heart and the Cardiovascular System

Your Heart is just one of the components of your body’s cardiovascular system. This system can be broken down into several components which includes: blood, blood vessels, coronary circulation, and the heart.

During a single cardiac cycle, the atria and ventricles do not beat simultaneously; the atrial contraction occurs prior to ventricular contraction. This timing delay allows for proper filling of all four chambers of the heart. Recall that the left and right heart pumps function in parallel. The diastolic phase of the cardiac cycle begins with the opening of the tricuspid and mitral valves (atrioventricular valves). The atrioventricular valves open when the pressures in the ventricles fall below those in the atria. This can be observed in here for the left heart, in which the mitral valve opens when the left ventricular pressure falls below the left atrial pressure. At this moment, passive filling of the ventricle begins. In other words, blood that has accumulated in the atria behind the closed atrioventricular valves passes rapidly into the ventricles, and this causes an initial drop in the atrial pressures. Later, pressures in all four chambers rise together as the atria and ventricles continue to passively fill in unison with blood returning to the heart through the veins (pulmonary veins to the left atrium, and the superior and inferior vena cava to the right atrium).

Contractions of the atria are initiated near the end of ventricular diastole, which is initiated by depolarization of the atrial myocardial cells (sinoatrial node). Atrial depolarization is elicited at the P wave of the electrocardiogram (ECG lead II trace). The excitation and subsequent development of tension and shortening of atrial cells cause atrial pressures to rise. Active atrial contraction forces additional volumes of blood into the ventricles (often referred to as "atrial kick"). The atrial kick contributes a significant volume of blood toward ventricular preload (approximately 20%). At normal heart rates, the atrial contractions are considered essential for adequate ventricular filling. As heart rates increase, atrial filling becomes increasingly important for ventricular filling because the time interval between contractions for passive filling becomes progressively shorter. Atrial fibrillation and/or asynchronized atrial-ventricular contractions can result in minimal contribution to preload, via atrial contraction. Throughout diastole, atrial and ventricular pressures are nearly identical due to the open atrioventricular values which offer little or no resistance to blood flow. It should also be noted that contraction and movement of blood out of the atrial appendage (auricle) can be an additional source for increased blood volume.

Ventricular systole begins when the excitation passes from the right atrium through the atrioventricular node, and through the remainder of the conduction system (His bundle and left and right bundle branches) to cause ventricular myocardial activation. This depolarization of ventricular cells underlies the QRS complex within the ECG. As the ventricular cells contract, intraventricular pressures increase above those in the atria, and the atrioventricular valves abruptly close. Closure of the atrioventricular valves results in the first heart sound, S1. As pressures in the ventricles continue to rise together in a normally functioning heart, they eventually reach a critical threshold pressure at which the semilunar valves (pulmonary valve and aortic valve) open.