Heart Sounds

	There are normally four sounds produced by the heart, only two of which are ordinarily audible by a stethoscope.  The less intense sounds can be detected with electronic amplification and recorded graphically as a phonocardiogram.
	The first heart sound is initiated at the onset of ventricular systole and consists of a series of vibrations of mixed, unrelated, low frequencies.  It is the loudest and longest of the heart sounds and is heard best over the apical region of the heart.  The first heart sound is mainly caused by oscillation of blood in the ventricular chambers and vibration of the chamber walls.  The vibrations are engendered in part by the abrupt rise of ventricular pressure with acceleration of blood back toward the atria, but primarily by sudden tension and recoil of the AV valves and adjacent structures with deceleration of the blood resulting from closure of the AV valves.  The vibrations of the ventricles and the contained blood are transmitted through surrounding tissues and reach the chest wall, where they may be heard or recorded.  The intensity of this sound is a function of the force of ventricular contraction and of the distance between the valve leaflets, being loudest when the leaflets are farthest apart.
	The second heart sound, which occurs with the closure of the semilunar valves, is composed of higher-frequency vibrations, is of shorter duration and lower intensity, and has a more snapping quality than the first heart sound.  The abrupt closure of the semilunar valves causes oscillations of the columns of blood and the tensed vessel walls by the stretch and recoil of the closed valve.  The second sound caused by the closure of the pulmonic valve is heard best in the second thoracic interspace just to the left of the sternum, whereas the that caused by the closure of the aortic valve is heard best in the same intercostal space but to the right of the sternum.
	The third heart sound, which is sometimes heard in children with thin chest walls or in patients with left ventricular failure, consists of a few low-intensity, low-frequency vibrations heard best in the region of the apex.  It occurs in early diastole and is believed to be the result of vibrations of the ventricular walls caused by abrupt cessation of ventricular distension and deceleration of blood entering the ventricles.
	The fourth, or atrial, sound, consisting of a few low-frequency oscillations, is occasionally heard in normal individuals.  It is caused by oscillation of blood and cardiac chambers created by atrial contraction.

Sample Problems:

Which of the heart sounds is the longest and loudest?
What causes the second heart sound?

The first heart sound.
Closure of the semilunar valves.

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