"Timing" differences exist between the right and left hearts (Figure 1.03). Right atrial contraction precedes left atrial contraction, primarily because the SA node is located in the right atrium and thus receives the depolarization signal earlier.
Left ventricle contraction begins before right ventricle contraction, because the left bundle branch of the septal conduction system is larger and conduction is faster there. Right ventricle ejection begins before left ventricle ejection and left ventricle ejection ends before right ventricle ejection. The latter phenomenon is responsible for the splitting of the second heart sound, S2. In both cases, the differences in opening and closing of the semilunar valves in the right and left heart (later opening, earlier closing) are due to the greater afterload (i.e. arterial blood pressure) on the left ventricle than on the right ventricle.
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