The " lub " sound is caused by the closure of the AV valves at the beginning of atrial systole.
The first sound is the simultaneous closure of the A/V valves, which is at the start of the ventricular systole.
A high velocity jet from the feeding artery enters the aneurysm sac during systole.
The systole is not altered in length, but the diastole is very much prolonged, and since this is the period not only of cardiac rest but also of cardiac "feeding" - the coronary vessels being compressed and occluded during systole - the result is greatly to benefit the nutrition of the cardiac muscle.
The individual muscle-fibres contract and expand more perfectly, and thus the diastole and systole of the heart are rendered more complete, the pulse is slowed, and the blood-pressure is raised.