GFR and RC Flashcards
Why measure GFR?
Most renal diseases result in a progressive destruction of functioning nephrons. Since total GFR is equal to the sum of the filtration rates of each of the functioning nephrons, a reduction in their number will result in a fall in total GFR. Thus GFR can be used to document the presence, estimate the severity, and follow the course of kidney disease.
Measurement of GFR is also useful in determining the correct dosage of certain drugs. Many drugs, notably digoxin and certain antibiotics, are excreted from the body primarily by GFR in the kidneys. Therefore, if GFR is reduced, the excretion of the drug will fall and it will accumulate in the body, possibly to toxic levels. To prevent this, the dosage must be reduced appropriately.
What is the use of digoxin?
Used in the treatment of heart conditions.
Normal GFR?
120 mL/min per 1.73 m2
[1.73 m2 is a fairly typical value for the body surface area of an adult]
The normal range, however, is quite wide: 90-150 mL/min per 1.73 m2.
How much does the GFR fall on average after the age of 40?
10 mL/min per decade.
Define GFR
Volume of fluid filtered from the glomeruli into the Bowman’s Capsule per unit time (mL/min). Sum of filtration rate of all functioning nephrons.
Define a ‘freely filtered substance’.
If a molecule is freely filtered and neither reabsorbed nor secreted in the nephron then the amount filtered equals amount excreted. Concentration of substance in capillaries = concentration of that substance in the glomerular filtrate (proportion similar to water).
What equates to GFR if a molecule is freely filtered?
RC
How can we determine GFR?
Direct measurement is impossible. However, an indirect determination can be made using a marker substance with certain properties.
Provided that substance (Y) is freeloads filtered, but you don’t know RC how would you calculate GFR?
Since the substance is freely filtered (and hence its concentration in the glomerular filtrate is the same as in plasma) it follows that:
GFR x Plasma conc. of Y (PY) = urine flow rate (V) x Urine conc. of Y (UY)
and therefore, GFR =V x UY / PY
List 5 properties of inulin that make it an ideal marker for GFR calculation.
Freely filtered Not toxic Measurable in the urine Neither reabsorbed nor secreted Not synthesised or metabolised by kidney.
Define renal clearance.
RC of a substance is equal to the volume of plasma which would be required to supply that amount of the substance excreted by the kidneys per unit time.
(An important point to remember is that clearance will always be expressed as a volume of plasma per unit time.)
How can you calculate RC for any substance?
For any substance Z, the renal clearance, CZ, is:
CZ = V x UZ /PZ
PZ and UZ are the concentrations of Z in plasma and urine, respectively and V is the urine flow rate.
(Only if the substance is freely filtered and neither reabsorbed nor secreted will its clearance equal GFR. If we apply the clearance expression to other substances a different answer will be obtained for each, depending on how that substance is handled by the kidneys.)
What conclusions can you draw about a substance whose RC is higher than that of inulin?
It is secreted MORE THAN it is reabsorbed.
If the RC of a substance X is lower than that of inulin, does this rule out the possibility of X being secreted into the tubular fluid?
No, it cold be filtered at very low levels and then secreted but at very low levels OR it could be filtered and secreted and then partially reabsorbed.
Must be more reabsorption than clearance.
