A simplified explanation of filtration, water balance and changes in creatinine. Not a clinically validated simulation, an eGFR estimate or a treatment recommendation. Height, age, sex, diseases and medications are not individually accounted for.
Direct GFR
You set the absolute GFR for both kidneys. Cardiac output = 5 L/min, renal blood flow = 1,100 mL/min and haematocrit = 45% remain constant. Renal plasma flow is therefore 605 mL/min. Filtration fraction equals GFR / plasma flow.
Urine formation
Urine flow = GFR × (1 − water reabsorption). At 125 mL/min and 99.2% reabsorption, 180 litres of filtrate and 1.44 litres of final urine are produced per day. Reabsorption is freely adjustable; ADH, RAAS, osmolality, sodium transport and tubular secretion are not calculated separately. Clinically, a low GFR does not automatically mean a proportionally low urine output.
Circulation model
Stroke volume is fixed at 71.4 mL. Renal flow depends on cardiac output, a simplified autoregulation factor and the resistance of both arterioles. Glomerular pressure follows a schematic resistance divider. GFR = Kf × positive net filtration pressure; Kf starts at 12.5 mL/min/mmHg. Capsular pressure is 15 mmHg. A simplified opposing oncotic pressure increases with filtration fraction. Coefficients are calibrated to the baseline for teaching, not derived from patient data. Filtration capacity scales Kf; nephron hyperfiltration and long-term compensation are not included.
Creatinine over time
A single compartment with a distribution volume of 42 litres and constant creatinine production of 1.125 mg/min. dC/dt = (production − GFR × C / 100) / 420. At steady state, C = 112.5 / GFR. Tubular creatinine secretion and extrarenal elimination are not included. At GFR 0, this model has no finite steady-state value. Creatinine therefore rises over time rather than changing instantly.
The clock, urine collection and creatinine curve use the same model time. The particle animation is illustrative: it is neither to scale nor a measurement of an individual nephron. Collection stops when the 500 mL container is full. Model time pauses in an inactive browser tab.
