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Updated: Apr 17, 2026

Physiology Lab Demonstration: Glomerular Filtration Rate in a Rat
Published on: July 26, 2015
Parameters of phosphorus homeostasis at normal and reduced GFR: empiric observations
Aims:
Influx (IP) determines urinary excretion of phosphorus (EP). Contributions of IP and reabsorption (TRP) to serum phosphorus ([P]s) can be depicted by normalization to creatinine clearance (EP/Ccr and TRP/Ccr) or by calculation of fractional excretion and reabsorption (FEP and FTRP). We analyzed these parameters at normal and reduced GFR.
Methods:
We studied 29 patients with chronic kidney disease (CKD) and 28 controls. From [cr] and [P] in serumand urine we calculated [P]u/[cr]u, EP/Ccr, TRP/Ccr, (TRP/Ccr)/(EP/Ccr), FEP, and FTRP. We compared means between groups and examined pertinent linear regressions.
Results:
[P]s was not different in CKD and controls. [Cr]s, EP/Ccr, and FEP were higher and TRP/Ccr, (TRP/Ccr)/(EP/Ccr), and FTRP were lower in CKD. [P]u/[cr]u, a surrogate for IP, was similar in both groups. In CKD, [P]s correlated with EP/Ccr and TRP/Ccr; EP/Ccr with [P]u/[cr]u and [cr]s; and FEP with EP/Ccr, TRP/Ccr, [P]u/[cr]u, and [cr]s. In controls, [P]s correlated with TRP/Ccr; EP/Ccr with [P]u/[cr]u; and FEP with EP/Ccr and [P]u/[cr]u. In both groups, FEP was a precise inverse function of (TRP/Ccr)/(EP/Ccr). Despite wide variation in TRP/Ccr, FEP was < 20% in 26/28 controls and > 20% in 27/29 patients with CKD.
Conclusions:
GFR affected determinants of [P]s, EP/Ccr, and FEP. FTRP was often dissociated from TRP/Ccr at normal or reduced GFR.
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