Renal Clearance of Inulin Measurement of GFR

If a substance is neither reabsorbed nor secreted by the tubules, the amount excreted in the urine per minute will be equal to the amount that is filtered out of the glomeruli per minute. There does not seem to be a single substance produced by the body, however, that is not reabsorbed or secreted to some degree. Plants such as artichokes, dahlias, onions, and garlic, fortunately, do produce such a compound. This compound, a polymer of the monosaccharide fructose, is inulin. Once injected into the blood, inulin is filtered by the glomeruli, and the amount of in-ulin excreted per minute is exactly equal to the amount that was filtered per minute (fig. 17.22).

If the concentration of inulin in urine is measured and the rate of urine formation is determined, the rate of inulin excretion can easily be calculated:


Quantity excreted per minute =


x U

(mg/min) |

f ml N

\ fmg'

V min,

I V ml.

rate of urine formation inulin concentration in urine

The rate at which a substance is filtered by the glomeruli (in milligrams per minute) can be calculated by multiplying the milliliters of plasma filtered per minute (the glomerular

Inulin Clearance

To peritubular capillaries

Renal vein inulin concentration lower than in renal artery

Ureter urine containing all inulin that was filtered

To peritubular capillaries

Renal vein inulin concentration lower than in renal artery

Ureter urine containing all inulin that was filtered

■ Figure 17.22 The renal clearance of inulin. (a) Inulin is present in the blood entering the glomeruli, and (b) some of this blood, together with its dissolved inulin, is filtered. All of this filtered inulin enters the urine, whereas most of the filtered water is returned to the vascular system (is reabsorbed). (c) The blood leaving the kidneys in the renal vein, therefore, contains less inulin than the blood that entered the kidneys in the renal artery. Since inulin is filtered but neither reabsorbed nor secreted, the inulin clearance rate equals the glomerular filtration rate (GFR).

filtration rate, or GFR) by the concentration of that substance in the plasma, as shown in this equation:

Quantity filtered per minute =




f—1 (

mg 1

V min J '

P = inulin concentration in plasma

Since inulin is neither reabsorbed nor secreted, the amount filtered equals the amount excreted:

GFR x P = V x U (amount filtered) (amount excreted)

If the preceding equation is now solved for the glomerular filtration rate,


Suppose, for example, that inulin is infused into a vein and its concentrations in the urine and plasma are found to be 30 mg per ml and 0.5 mg per ml, respectively. If the rate of urine formation is 2 ml per minute, the GFR can be calculated as:

This equation states that 120 ml of plasma must have been filtered each minute in order to excrete the measured amount of inulin that appeared in the urine. The glomerular filtration rate is thus 120 ml per minute in this example.

Measurements of the plasma concentration of creatinine are often used clinically as an index of kidney ^ function. Creatinine, produced as a waste product of muscle creatine, is secreted to a slight degree by the renal tubules so that its excretion rate is a little above that of in-ulin. Since it is released into the blood at a constant rate, and since its excretion is closely matched to the GFR, an abnormal decrease in GFR causes the plasma creatinine concentration to rise. Thus, a simple measurement of blood creatinine concentration can indicate whether the GFR is normal and provide information about the health of the kidneys.

Clinical Investigation Clues

Remember that Emily had mild oliguria, edema, and an elevated plasma creatinine concentration. What does an elevated plasma creatinine concentration suggest? How might this be related to Emily's oliguria and edema?

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    How is inulin measured?
    8 years ago
  • lisa
    How to calculate inulin excretion in urine?
    8 years ago
    Is inulin excreted from the body?
    8 years ago
  • yemane
    What doe the concentration of inulin do in urine?
    8 years ago
    Why is inulin excreted in urine?
    8 years ago
  • esmeralda
    What does it mean if a substance is filtered by not reabsorbed or secreted?
    8 years ago
    Which substance is present at a lower concentration in the renal artery than in the renal vein?
    6 years ago
  • Kirsi
    How do i calculate gfr of a substance and inulin?
    6 years ago
  • scott
    How to calculate gfr qith inulin?
    5 years ago
    How does glomerular filtration rate equal clearance?
    5 years ago
  • aimone
    How to calculate inulin clearance?
    5 years ago
    What is the average clearance of inulin?
    5 years ago
  • Raakel
    Why is the carbohydrate inulin used in the inulin clearance test to measure gfr?
    5 years ago
  • sol
    How to calculate renal clearance of inulin?
    4 years ago
  • tony tabb
    How is inulin neither reabsorbed or secreted?
    4 years ago
  • prudenzio folliero
    Why clearance of inulin equals to gfr?
    4 years ago
  • simonetta
    3 years ago
    How to measure renal inuline clearence?
    3 years ago
  • Ellan Villarreal
    Which compound neither reabsorbed nor secreted by renal tubules?
    3 years ago
  • Riia V
    When is clearance rate equal to inulin?
    3 years ago
  • olga
    Why plasma clearance of inulin is equal to GFR?
    3 years ago
  • uwe
    Why inulin is not used nowadays GFR measurement?
    3 years ago
  • primrose
    Why is inulin clearance constant?
    3 years ago
  • allison
    How to measure gfr from renal clearance?
    2 years ago
  • SVEN
    How can clearance rate calculate GFR?
    2 years ago
  • Uta Fruehauf
    When renal clearance of asubstance equals inulin clearance excretion occyrs by ?
    2 years ago
  • bertha sackville
    Why is creatinine used than inulin?
    2 years ago
  • Karen Bock
    How is renal clearance measured?
    2 years ago
  • gavino trevisani
    How is inulin clearance used to determine GFR?
    2 years ago
  • Venera
    Why is glomerular filtration rate is equal to the clearance rate of inulin?
    2 years ago
    Why does glomerular filtration rate equal to clearancc of insulin?
    2 years ago
  • amanda
    Why creatinine clearance is used not inulin?
    2 years ago
  • stefania russo
    Does creatinine clearance equal gfr?
    2 years ago
  • Giuseppe
    Is inulin good for human?
    2 years ago
  • Aira
    How plasma concentration of inulin is calculated?
    1 year ago
  • roma berhane
    Why don't they use inulin for renal study?
    1 year ago
  • primula
    Is 200 milligrams of inulin a lot of inulin?
    1 year ago
    Why is inulin used in finding gfr?
    1 year ago
    How to find someones clearenceof inulin?
    12 months ago
  • Ennio
    How can renal clearance determine gfr?
    12 months ago
  • Selamawit Habte
    Why is inulin not used in clinical gfr?
    11 months ago
  • Dolly
    Is inulin good for kidneys?
    8 months ago
  • JOHN
    How inulin can be utilized to determine glomerular filtration rate (GFR) in a person?
    8 months ago
  • aziz rezene
    How can inulin be utilized in deteriming glomeular filtration rate in a person?
    8 months ago
  • alfrida
    How do we know inulin measures GFR?
    5 months ago
  • cayden
    Why is inulin not used for egfr?
    5 months ago
  • Kyle
    How to figure inulin clearance for urinary excretion rate of 31.9 mg/min?
    5 months ago
    How renal clearance rate can measure gfr?
    4 months ago
  • ercole
    How to calculate clearance of urine insulin?
    4 months ago
  • maria
    How to calculate gfr with urinary excretion rate and plasma inulin?
    1 month ago
  • ana gordon
    Why is inulin not used for gfr calculation?
    1 month ago
    Does a renal clearance lower than inulin indicate no secretion?
    1 month ago

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