Passive, one-dimensional countercurrent models do not simulate hypertonic urine formation.

Simulations were performed to test the ability of the countercurrent hypothesis to predict measured concentrations of NaCl and urea in the interstitium of the renal medulla. The simulations included one-dimensional representations of loops of Henle, distal tubules, collecting ducts, and vasa recta, and recent estimates of descending limb, thick ascending limb, and collecting duct transport parameters. The nonlinear two-point boundary value problem was solved numerically via quasi-linearization. The simulations failed to predict measured concentrations or concentration gradients of NaCl in the inner medulla. Including countertransport of urea and NaCl in thin ascending limbs added minimally to the performance of the system. The single most effective change in the model was the inclusion of a coefficient to permit preferential solute exchange among vasa recta. This result suggests that the three-dimensional ordering of blood vessels and tubules is an essential construct in the concentrating mechanism.