Final answer:
Blood turbulence causes the abnormal swishing or whooshing sound when blood regurgitates back into an atrium from its associated ventricle.
Step-by-step explanation:
The abnormal swishing or whooshing sound that is heard as blood regurgitates back into an atrium from its associated ventricle is caused by blood turbulence. When the ventricle relaxes, the pressure differential causes blood to flow back into the ventricle from the pulmonary trunk or aorta. This flow of blood fills the pocket-like flaps of the pulmonary or aortic valve, causing the valve to close and producing the audible sound.