Chordae tendineae

The chordae tendineae (tendinous chords) of the heart (also known as heart strings), are cord-like tendons that connect the papillary muscles of the ventricles to the atrioventricular (tricuspid and mitral) valve leaflets. 

Chordae tendineae are approximately 80% collagen, while the remaining 20% is made up of elastin and endothelial cells.