Heart Tube Looping: Buckling & Chirality

The very first organ-level left-right asymmetry in a vertebrate body appears when the straight embryonic heart tube bends into a C-shape and then an S-shape — cardiac looping. This simulator models the accepted mechanical explanation: the tube is anchored at its arterial and venous poles inside a pericardial cavity that grows more slowly than the tube itself, so the excess length is absorbed by buckling into a circular arc whose bend angle follows directly from the length-to-cavity ratio. A left-right bias slider stands in for the Nodal/Lefty/Pitx2c signaling cascade that normally makes the loop bend rightward (the D-loop); drive that bias toward zero and the loop direction becomes a coin flip, mirroring how loss of that signaling produces randomized organ situs (heterotaxy) in real embryos. Scrub or auto-play the developmental-stage timeline to watch the C-loop deepen and roll into an S-loop, with live readouts for the bend angle, the length/cavity ratio, and an approximate Carnegie stage and embryonic day.