Calculating the Glomerular Filtration Rate

unit 4 week 12 lesson 6

equation of excretion

Excretion = Filtration – Reabsorption + Secretion

how can GFR be measured

something in the kidney that gets filtered and excreted but not reabsorbed or excreted (waste products like creatinine)

what do u need to measure in order to measure gfr

1. measure how much creatintine is in someones blood [creatinine]_plasma =1mg/L

2. measure how much creatintine is in someones urine [creatine]_urine = 90mg/L

3. measure total urine volume urine/day = 2L

4. formula: ([creatinine]_urine x urine/day) / [creatinine]_plasma =180 L/day = GFR

what are other methods to measure gfr 

Insulin: 

• must be injected thru the vein and therefore be present in the blood of an individual 

• insulin is not naturally produced in humans therefore it will be filtered by the kidneys

• and since its foreign, it isnt metabolized, the epithelial cells of the tubule dont recognize it so it isnt reabsorbed or secreted (100% excerted)

Blood Urea Nitrogen (BUN):

• urea is another waste product excreted by the kidneys, but it is partially reabsorbed by the tubules

• therefore excreted urea does not reflect all of filtration that occurred

• Instead, the blood plasma levels of urea can be measured to help determine kidney function

• The concentrations of urea in the blood can be used to determine if less urea is being filtered from the blood.

• If kidney function declines, less urea can be filtered, and therefore, more urea accumulates in the blood.

• The test for measuring urea is a measure of the nitrogen, found within the urea molecule (why urea levels in the blood can increase include a high-protein diet and strenuous exercise.

Serum Creatinine:

• A quick way to estimate GFR is to measure the blood levels of creatinine, and not measure the urine excretion at all

• If serum creatinine levels increase in the blood, this can indicate that the kidneys are no longer filtering as much total fluid

•  But, a flaw of this estimate is what values are considered normal, and how normal concentrations of plasma creatinine can be different for different individuals

how to convert L/day to mL/min (GFR unit)

180L/day x 1 day/1440min (how many mins in a day 24 hrs) x 1000ml/L

= 125mL/min

what should gfr be

180L/day or 125mL/min

As we age, there is a natural decline in renal function, so it is expected that the measured GFR would be a little lower than this value (125 mL/min).

The lower the GFR value, the more significant the kidney disease. Kidneys that are filtering less than 15 mL/min are classified as being in kidney failure.

chronic kidney disease

• progressive disease that can result in complete kidney failure if not treated early

• even with early intervention, damage to kidneys cant be reversed so treatment can slow down/stop the progression of kidney damage

• once nephrons are damaged, they dont heal

• if a critical number of nephrons are damaged, symptoms of ckd become more apparent

stages of CKD

• Stage 1-2 There is a mild decrease in GFR that can be measured, however, a patient may not be experiencing any symptoms at this time (early-stage kidney disease)

• Stage 3 - GFR has decreased further and a patient may notice symptoms like swelling in the hands and feet, due to less blood being filtered by the damaged kidneys (kidney disease)

• Stage 4 - This is the last stage before kidney failure, symptoms are more severe (kidney disease).

• Stage 5 - When a kidney is in failure, this means it is no longer functioning in a way to support the body. In kidney failure, the treatment options are dialysis or a kidney transplant.