<P> The heart is a four - chambered organ consisting of right and left halves, called the right heart and the left heart . The upper two chambers, the left and right atria, are entry points into the heart for blood - flow returning from the circulatory system, while the two lower chambers, the left and right ventricles, perform the contractions that eject the blood from the heart to flow through the circulatory system . Circulation is split into pulmonary circulation--during which the right ventricle pumps oxygen - depleted blood to the lungs through the pulmonary trunk and arteries; or the systemic circulation--in which the left ventricle pumps / ejects newly oxygenated blood throughout the body via the aorta and all other arteries . </P> <P> In a healthy heart all activities and rests during each individual cardiac cycle, or heartbeat, are initiated and orchestrated by signals of the heart's electrical conduction system, which is the "wiring" of the heart that carries electrical impulses throughout the body of cardiomyocytes, the specialized muscle cells of the heart . These impulses ultimately stimulate heart muscle to contract and thereby to eject blood from the ventricles into the arteries and the cardiac circulatory system; and they provide a system of intricately - timed and persistent signaling that controls the rhythmic beating of the heart muscle cells, especially the complex impulse - generation and muscle contractions in the atrial chambers . The rhythmic sequence (or sinus rhythm) of this signaling across the heart is coordinated by two groups of specialized cells, the sinoatrial (SA) node, which is situated in the upper wall of the right atrium, and the atrioventricular (AV) node located in the lower wall of the right heart between the atrium and ventricle . The sinoatrial node, often known as the cardiac pacemaker, is the point of origin for producing a wave of electrical impulses that stimulates atrial contraction by creating an action potential across myocardium cells . </P> <P> Impulses of the wave are delayed upon reaching the AV node, which acts as a gate to slow and to coordinate the electrical current before it is conducted below the atria and through the circuits known as the bundle of His and the Purkinje fibers--all which stimulate contractions of both ventricles . The programmed delay at the AV node also provides time for blood volume to flow through the atria and fill the ventricular chambers--just before the return of the systole (contractions), ejecting the new blood volume and completing the cardiac cycle . (See Wiggers diagram: "Ventricular volume" tracing (red), at "Systole" panel .) </P> <P> Cardiac diastole is the period of the cardiac cycle when, after contraction, the heart relaxes and expands while refilling with blood returning from the circulatory system . Both atrioventricular (AV) valves open to facilitate the' unpressurized' flow of blood directly through the atria into both ventricles, where it is collected for the next contraction . This period is best viewed at the middle of the Wiggers diagram--see the panel labeled "Diastole". Here it shows pressure levels in both atria and ventricles as near - zero during most of the diastole . (See gray and light - blue tracings labeled "Atrial pressure" and "Ventricular pressure"--Wiggers diagram .) Here also may be seen the red - line tracing of "Ventricular volume", showing increase in blood - volume from the low plateau of the "Isovolumic relaxation" stage to the maximum volume occurring in the "Atrial systole" sub-stage . </P>

Outline the events of the cardiac cycle that are occuring during this qrs interval