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Glomerular Filtration Rate and the Measurement of Kidney Function Quiz
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Which of the following best describes the normal total volume of glomerular filtrate produced by the kidneys in a healthy adult per day?
- A Approximately 10 to 20 litres per day You picked ✕
- B Approximately 1 to 2 litres per day You picked ✕
- C Approximately 300 to 400 litres per day You picked ✕
- D Approximately 50 to 70 litres per day You picked ✕
- E Approximately 140 to 180 litres per day You picked ✓
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The answer is E: Approximately 140 to 180 litres per day
Explanation
Why E is correct
See Glomerular Filtration. Option E is correct: healthy kidneys filter roughly 140 to 180 litres of plasma every day, which is the glomerular filtrate. The glomeruli (the tufts of capillaries that begin each nephron) are very leaky to water and small solutes and receive about a fifth of the plasma flowing through the kidneys, so filtration is fast. The normal GFR (glomerular filtration rate, the volume of plasma filtered each minute) is 90 to 120 mL per minute for each 1.73 square metres of body surface. Multiply by the 1,440 minutes in a day and you get about 130 to 173 litres, and a textbook figure of about 180 litres a day is quoted for men. Almost all of this is reclaimed by the tubules (the tubes of the nephron that reabsorb water and solutes back into the blood) so only about 1 to 2 litres leave as urine. The exam trap is mixing up the filtrate with the urine. Memory aid: about 180 litres filtered, about 1.5 litres passed, so more than 99 per cent is reabsorbed.
- Normal filtrate: the kidneys filter about 140 to 180 litres of plasma a day.
- Urine volume: only about 1 to 2 litres a day leave as urine.
- Reabsorption: the tubules reclaim more than 99 per cent of the filtrate.
Why the other options are incorrect
- A. Approximately 10 to 20 litres per day: Ten to twenty litres a day would correspond to a GFR of under 15 mL per minute, which is established kidney failure. It is far below the normal filtration rate. Your answer
- B. Approximately 1 to 2 litres per day: One to two litres a day is the typical volume of urine passed, not the volume of filtrate. More than 99 per cent of the filtrate is reabsorbed before it leaves the kidney. Your answer
- C. Approximately 300 to 400 litres per day: Three to four hundred litres a day would be a GFR of roughly twice the normal range. No healthy adult filters at this rate. Your answer
- D. Approximately 50 to 70 litres per day: Fifty to seventy litres a day would mean a GFR of only about 35 to 50 mL per minute, which is a markedly reduced filtration rate. It is not normal for a healthy adult. Your answer
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1
Which set of properties best describes an ideal substance for accurately measuring the glomerular filtration rate (GFR)?
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2
A 22-year-old woman presents to her GP with dysuria and increased urinary frequency for two days. A urine dipstick test is performed at the bedside. Which combination of findings on urinalysis most strongly supports a diagnosis of an uncomplicated urinary tract infection?
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