Fig. 2.

Schematic diagram of pharmacokinetic model of creatinine. Serum creatinine (sCr) is synthesized from the muscle at a zero-order rate $\left(k_{\text{syn}}\right)$ and cleared by the kidney glomerular filtration $\left(\mathrm{G}\mathrm{F}\mathrm{R}\right(t\left)\right)$ into the renal tubules (rtCr). The creatinine from the renal tubules is excreted into the urine by the urinary flow (UF). Transiently, rtCr is also secreted into the plasma by the back-flow $\left(Q\right(t\left)\right)$