= Papillary muscle
A \b[papillary muscle] is a muscular projection inside a ventricle that anchors <chordae tendineae> attached to the <mitral valve> or <tricuspid valve>. During ventricular <systole>, its contraction helps maintain the supporting geometry and <tension> that restrain the <valve leaflets>. Valve opening is driven by the appropriate <pressure> difference; a <papillary muscle> does not act as a cord-operated opener.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§19.1, Right Ventricle and Heart Valve Structure and Function]{ref}
The anchor's position matters as well as its ability to contract. In a primary simulator study, investigators examined six human and five porcine specimens of the <mitral valve>, measuring six selected <chordae tendineae> with force transducers. Eight tested anchor configurations used 5 mm displacement components relative to a reference position. The resulting <tension> changes depended on which chord was measured.<Effects of papillary muscle position on chordal force distribution: an in-vitro study by Jorge Hernan Jimenez and others>[Abstract, Methods, Results and Conclusion]{ref} Moving an anchor therefore need not distribute extra load equally through the apparatus.
An original mechanical approximation illustrates the distinction. If a taut cord has reference length $L$, cross-sectional area $A$ and effective <Young's modulus> $E$, a small extra extension $\delta L$ gives
$$
\delta T\simeq\frac{EA}{L}\,\delta L.
$$
This is a local <Hooke's law> approximation, not a claim that biological cords remain linear at every <strain>. Anchor displacement changes the cord's extension and direction, while active muscle contraction changes the moving support itself. In the equal-tether example under <chordae tendineae>, changing direction also changes the <tension> needed to balance a fixed leaflet load. The apparatus is consequently a coupled geometrical and muscular system; naming its parts alone does not predict its forces.
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