Mathematical model of avian urine concentrating mechanism
A mathematical model of the avian urine concentrating mechanism was used to investigate how concentrating capability depends on morphological and tubular transport parameters. In the bird, urine is concentrated in the medullary cones, subunits of the kidney that contain countercurrent multiplier systems. The collecting ducts and loops of Henle of a single medullary cone were modeled as interacting flow-tubes; the interstitium and vasculature were represented by a central core (CC). The model included active transport of NaCl from thick ascending and prebend thick descending limbs into the CC and passive diffusion of NaCl from the CC into thin descending limbs. Simulations conducted with parameters based on experimental measurements produced urine-to-plasma osmolality ratios of about 1.7, consistent with experimentally measured osmolalities. Active NaCl transport from the descending limb prebend segment was found to contribute about 35% of this concentrating capability. In addition, simulations indicated that concentrating capability is highly sensitive to loop-of-Henle population as a function of medullary depth.