This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
If the ear be placed on the chest of another person oyer the heart region, two distinguishable sounds will be heard during each round of the heart's work. They are known respectively as the first and second sounds of the heart. The first is of lower pitch and lasts longer than the second and sharper sound; vocally their character may be tolerably imitated by the syllables lūb, dŭp. The cause of the second sound is the closure, or, as one might say, the " clicking up" of the semilunar valves. The first sound takes place during the ventricular systole, and is probably due to vibrations of the tense ventricular wall at that time. In many forms of heart disease these sounds are modified or cloaked by additional sounds which arise when the cardiac orifices are roughened, or narrowed, or dilated, or the valves inefficient. A physician often gets important information as to the nature of a heart disease by studying these new or altered sounds.