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Heartstrings are the structures scientifically known as the Chordae Tendinae, tiny tendons that connect the mitral and tricuspid valves to the papillary muscles of the heart.
These are the valves between the atria and ventricles and which open to allow the ventricles to fill during the relaxed phase of the heartbeat known as diastole. During the ejection phase, known as systole, these valves close to prevent the backflow of blood into the atria.
The heartstrings act as a stop, preventing the valves from moving too far in the other direction and allowing the regurgitation of blood back into the atria.