Page 12, Cardiac Cycle, Dr. D. Penney
Heart Sounds:
As mentioned earlier, there are "timing" differences between the right and left hearts. Right atrial contraction precedes left atrial contraction, primarily because the SA node is located in the right atrium. Left ventricle contraction begins before right ventricle contraction, because the left bundle branch 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 S2. In both cases, the differences in opening and closing of the semi-lunar valves in the right and left heart are due to the greater afterload (i.e. arterial blood pressure) on the left ventricle than on the right ventricle. See Misconceptions number 5 and 6
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