Although the glomerular filtration rate (GFR) is very low in patients with end-stage renal disease, the urine output is variable, ranging from oliguria to normal or even above normal levels. These findings are related to the fact that the urine output is determined, not by the GFR alone, but by the difference between the GFR and the rate of tubular reabsorption. If, for example, a patient with advanced acute or chronic renal failure has a GFR of 5 L/day (versus the normal of 140 to 180 L/day), the daily urine output will still be 1.5 L if only 3.5 L of the filtrate is reabsorbed.
It had been thought that tubular damage impaired the ability to reabsorb sodium and water, thereby contributing to the maintenance of an adequate urine output in this setting. However, it seems more likely that volume expansion (due to initial sodium retention) and a urea osmotic diuresis (as the daily urea load is excreted by fewer functioning nephrons), due in part to solute intake, play a more important role in the persistent urine output [1-3].
In comparison, water intake (which usually determines the urine output via changes in the secretion of antidiuretic hormone [ADH]) plays relatively little role in regulating the urine output in advanced renal disease. These patients can neither dilute nor concentrate the urine normally; the range of urine osmolality that can be achieved may vary from a minimum of 200 mosmol/kg to a maximum of 300 mosmol/kg, compared with 50 to 1200 mosmol/kg in normal subjects [3,4]. The net effect of this ADH-resistance is that variations in ADH release in response to changes in water intake have relatively little effect on the urine output .
CLINICAL IMPORTANCE OF RESIDUAL RENAL FUNCTION
The loss of residual renal function in patients with end-stage renal disease has important clinical manifestations. This includes adverse effects upon volume control and patient survival [5-7].
●Although the remaining GFR may only be 4 to 5 mL/min in patients begun on maintenance dialysis, this is sufficient to make a significant contribution to the removal of potential uremic toxins since filtration is continuous, as opposed to the 12 hours per week that the patient is undergoing hemodialysis .