SODIUM SIEVING — the single most load-bearing constant in the engine.

WHY THESE LEVELS. In the three-pore picture, ultrafiltrate leaves through small pores
(carrying sodium at nearly plasma concentration) AND through aquaporins (carrying NO
sodium at all). The apparent overall sodium sieving coefficient is therefore roughly
(1 - fFWT), where fFWT is the free-water fraction of ultrafiltration. Helman et al. 2024
(PMC10914194) state that fFWT is a fixed 40-50% of UF in patients and use 50% in their
model. That implies an apparent S_Na near 0.50-0.60. The engine ships 0.72, which implies
fFWT of only about 28% — below the measured range.

  S_Na (implied fFWT)         cumUF L   gluAbs g/d   serum Na        BUN
  0.72  (shipped, fFWT~28%)       40.3          184      141.8         23
  0.60  (fFWT 40%)               51.4          198      142.4         23
  0.55  (literature centre)       57.0          205      142.7         23
  0.50  (fFWT 50%, Helman)       63.2          212      143.1         23

  P4 anuric 72 kg, 30 days. cumUF is the headline ultrafiltration figure in every
  published document. This is a SENSITIVITY ARM, not a change: nothing was altered on disk.