voluntary: composition (often improvised) for a solo instrument (especially solo organ) and not a regular part of a religious service or musical performance
solo: a musical composition for one voice or instrument (with or without accompaniment)
cardiac cycle: the complete cycle of events in the heart from the beginning of one heart beat to the beginning of the next; an electrical impulse conducted through the heart muscle that constricts the atria which is followed by constriction of the ventricles