One patient has a GFR of 5 ml/min and one patient has a GFR of 120 ml/min. Both have a daily urine output of 1.8 L per day.
Please explain how they both can have the same daily urine output despite such large differences in GFR.
Imagine a person with a glomerular filtration rate of 120 ml/min which is equivalent to almost 180 L/day. This means that about 180 litres per day of water (along with solute) is filtered through all the millions of glomeruli in this person’s kidneys. Almost all of this water is reabsorbed along the nephron.
Imagine if this didn’t happen – they would put out about 7.5 L of water in just 1 hour, leading to hemodynamic collapse quite quickly!
Normally, the kidney reabsorbs large amounts of filtered NaCl and water. If a normal person with a GFR of 120 ml/min reabsorbs 99% of all filtered water, the urine output would be:
0.01 * 180 L = 1.8 L per day.
In the patient with 5 ml/min of GFR (advanced chronic kidney disease), this is equivalent to about 7.2 litres per day of water being filtered through all the glomeruli. If only 70% of water is reabsorbed, 30% or:
0.3 * 7.2L = 1.8L will pass as urine.
So, it becomes quite clear that is it possible for urine volume to be preserved even in advanced chronic kidney disease.
In a patient with a GFR of 5 ml/min, can diuretics increase urine output?
In advanced CKD, diuretics can certainly increase urine volume. Since diuretics act along the renal tubules to diminish salt and water reabsorption, if the fraction of reabsorbed water can be decreased, the total urine excretion will increase. This additional water will contain Na and K which can assist in managing the hypertension and hyperkalemia of chronic renal failure.
Do diuretics have any impact on clearance of uremic toxins?
Since diuretics act AFTER filtration has occurred at the glomerulus, they have no impact on clearance of uremic toxins. The total clearance of uremic toxins is determined by the how much water + solute passes through all the glomeruli combined. If we increase urine volume by using diuretics, the same total amount of toxins are excreted, just in a large amount of water.
Do diuretics have any impact on clearance of potassium?
As mentioned above, diuretics can increase urinary potassium excretion. This is not by increasing the amount of K filtered at the glomerulus, but rather by increasing excretion in the distal nephron.