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Next is the ''isovolumic relaxation'', during which pressure within the ventricles begin to fall significantly, and thereafter the atria begin refilling as blood returns to flow into the [[Atrium (heart)|right atrium]] (from the [[vena cavae]]) and into the [[Atrium (heart)|left atrium]] (from the [[pulmonary vein]]s).<ref name=pollock/> As the ventricles begin to relax, the mitral and tricuspid valves open again, and the completed cycle returns to ventricular diastole and a new "Start" of the cardiac cycle.<ref name=pollock/><ref name=wiggers/>
Throughout the cardiac cycle, [[blood pressure]] increases and decreases. The movements of [[cardiac muscle]] are coordinated by a series of electrical impulses produced by specialized [[cardiac pacemaker|pacemaker cells]] found within the [[sinoatrial node]] and the [[atrioventricular node]]. Cardiac muscle is composed of [[
In an [[electrocardiogram]], electrical systole initiates the atrial systole at the P wave deflection of a steady signal; and it starts contractions (systole).<ref name=wiggers/>
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===Physiology===
[[File:Cardiac-Cycle-Animated.gif|thumb|[[
The [[heart]] is a four-chambered [[organ (anatomy)|organ]] consisting of right and left halves, called the [[right heart]] and the [[left heart]]. The upper two chambers, the left and right [[Atrium (heart)|atria]], are entry points ''into'' the heart for blood-flow returning from the [[circulatory system]], while the two lower chambers, the left and right [[Ventricle (heart)|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.<ref name=pollock/><ref name=wiggers/>
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