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The organization of tubules and blood vessels in the quail
medullary cone is highly structured.
This structural organization may result in
preferential interactions among tubules and vessels,
interactions that may enhance urine concentrating capability.
In this study, we formulate a model framework for the
urine concentrating mechanism of the quail kidney.
The model simulates preferential interactions among renal tubules by
representing two concentric cores and by specifying the fractions of tubules
assigned to each of the concentric cores.
The model equations are based on standard expressions for transmural
transport and on solute and water conservation.
Model results suggest that the preferential interactions among tubules
enhance the urine concentration capacity of short medullary cones
by reducing the diluting effect of the descending limbs on the region of
the interstitium where the collecting ducts are located;
however, the effects on longer cones are unclear.
In a separate study, we assess how the avian kidney's
morphological and transepithelial transport properties affect the urine
concentrating mechanism (UCM).
To this end, an inverse problem is solved for a mathematical model of the
quail UCM.
A measure of concentrating mechanism efficiency---the ratio of the
free-water absorption rate to the total NaCl active
transport rate (TAT)---is optimized by varying a set of parameters within
bounds that are suggested by physiological experiments.
The parameters varied include transport properties of renal tubules,
length of the prebend enlargement of the descending limb (DL),
DL and collecting duct (CD) inflows, plasma concentration,
cortical length, solute diffusivity, and
population distribution of loops of Henle and of CDs along the medullary cone.
By selecting parameter values that increase urine flow rate (while maintaining
a sufficiently high urine osmolality) and that reduce TAT,
the optimization algorithm identified a set of parameter values that increased
efficiency by ~60% above base-case efficiency.
The algorithm also identified two sets of parameters that reduced efficiency
by ~70%, via the production of either a urine of high osmolality
(approximately twice of base-case value) at high TAT,
or of a urine of near-plasma osmolality at near-base-case TAT.
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