Final answer:
The S4 heart sound occurs when the atria contract to enhance ventricular filling, indicating blood being pushed into a stiff or hypertrophic ventricle, often a sign of left ventricular failure.
Step-by-step explanation:
The sound that occurs when the atria contract to enhance ventricular filling is the S4 heart sound. This sound is associated with the atria contracting and pushing blood into a stiff or hypertrophic ventricle, which can be an indication of the failure of the left ventricle.
S4 occurs before S1, and together with S1 and S2, some cardiologists refer to this rhythm as the "Tennessee gallop" due to its rhythmic similarity to a galloping horse with a specific gait.
In contrast, S1 and S2 heart sounds are the normal sounds heard in a healthy heart, corresponding to the closing of the atrioventricular valves and the semilunar valves, respectively.
S3 may be heard in specific situations like in young individuals or athletes but may signal a problem such as congestive heart failure if heard later in life.