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.
In order that the contracting ventricles may not force blood back into the auricles, it is essential that the flaps of the mitral and tricuspid valves be held together across the openings which they close, and not pushed back into the auricles. If they were like swinging doors and opened both ways they would be useless; they must so far resemble an ordinary door as only to open in one direction, namely, from the auricle to the ventricle. At the commencement of the ventricular systole this is provided for by the chordae tendineae, which are of such a length and so arranged as to keep the valve flaps shut across the opening, and to maintain their edges in contact. But, as the contracting ventricles shorten, the chordae tendineae would be slackened and the valve-flaps pushed up into the auricle. The little papillary muscles prevent this. Shortening as the ventricular systole proceeds, they keep the chord® taut and the valves closed.
What then happens? How long does the ventricle continue to contract? What then follows? How are the semilunar valves closed?
What is essential In order that blood may not be forced back from ventricle to auricle? Illustrate. How is the pushing back of the valve-flaps between auricle and ventricle prevented at the beginning of a ventricular systole? When would the chordae? tendineae be slackened? What would result? How is the slackening prevented?