Frequency of 1st hea sound: (A) 10-15 Hz, (B) 20-25 Hz, (C) 25-45 Hz, (D) 50 Hz
Answer is C. The first sound has a duration of about 0.15 s and a frequency of 25-45 Hz. It is soft when the hea rate is low because the ventricles are well filled with blood and the leaflets of the AV valves float together before systole.Ref: Ganong&;s Review of medical physiology 25th edition. Page: 542