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:

where

Quantity excreted per minute =

V

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 =

GFR x

P

(mg/min)

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,

GFR,

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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Responses

  • EVELIINA
    How is inulin measured?
    7 years ago
  • lisa
    How to calculate inulin excretion in urine?
    7 years ago
  • PIPPIN
    Is inulin excreted from the body?
    7 years ago
  • yemane
    What doe the concentration of inulin do in urine?
    7 years ago
  • HILDIFONS
    Why is inulin excreted in urine?
    7 years ago
  • esmeralda
    What does it mean if a substance is filtered by not reabsorbed or secreted?
    7 years ago
  • MAISIE MCDONALD
    Which substance is present at a lower concentration in the renal artery than in the renal vein?
    5 years ago
  • Kirsi
    How do i calculate gfr of a substance and inulin?
    4 years ago
  • scott
    How to calculate gfr qith inulin?
    4 years ago
  • BURTUKA
    How does glomerular filtration rate equal clearance?
    4 years ago
  • aimone
    How to calculate inulin clearance?
    4 years ago
  • ERMENEGILDO PIAZZA
    What is the average clearance of inulin?
    3 years ago
  • Raakel
    Why is the carbohydrate inulin used in the inulin clearance test to measure gfr?
    3 years ago
  • sol
    How to calculate renal clearance of inulin?
    3 years ago
  • tony tabb
    How is inulin neither reabsorbed or secreted?
    3 years ago
  • prudenzio folliero
    Why clearance of inulin equals to gfr?
    3 years ago
  • simonetta
    WHY IS INULIN CONCENTRATION LOWER IN RENAL VEIN THAN IN RENAL ARTERY?
    2 years ago
  • ASMARA
    How to measure renal inuline clearence?
    2 years ago
  • Ellan Villarreal
    Which compound neither reabsorbed nor secreted by renal tubules?
    2 years ago
  • Riia V
    When is clearance rate equal to inulin?
    2 years ago
  • olga
    Why plasma clearance of inulin is equal to GFR?
    1 year ago
  • uwe
    Why inulin is not used nowadays GFR measurement?
    1 year ago
  • primrose
    Why is inulin clearance constant?
    1 year ago
  • allison
    How to measure gfr from renal clearance?
    1 year ago
  • SVEN
    How can clearance rate calculate GFR?
    1 year ago
  • Uta Fruehauf
    When renal clearance of asubstance equals inulin clearance excretion occyrs by ?
    12 months ago
  • bertha sackville
    Why is creatinine used than inulin?
    10 months ago
  • Karen Bock
    How is renal clearance measured?
    8 months ago
  • gavino trevisani
    How is inulin clearance used to determine GFR?
    7 months ago
  • Venera
    Why is glomerular filtration rate is equal to the clearance rate of inulin?
    7 months ago
  • MELBA
    Why does glomerular filtration rate equal to clearancc of insulin?
    7 months ago
  • amanda
    Why creatinine clearance is used not inulin?
    7 months ago
  • stefania russo
    Does creatinine clearance equal gfr?
    5 months ago
  • Giuseppe
    Is inulin good for human?
    2 months ago

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