Flashcards in Exam #4: Renal Clearance Deck (19):
What is "clearance?" What is renal clearance?
Clearance= the rate at which a substance is removed from the plasma
- Thus, renal clearance is the rate at which a substance is removed from the plasma by the kidneys
- Specifically, clearance is the Volume of substance cleared/ time (C=mL/min)
Write the equation for renal clearance.
N/A (p. 249)
What is the renal clearance the ratio of?
Urinary excretion/plasma concentration
- High clearance= High excretion, low plasma concentration
- Low clearance= Low excretion, high plasma concentration
What is the renal clearance of albumin?
~0--it is not filtered across the glomerular capillary; thus, it is not cleared
What is the renal clearance of glucose?
~0--it is filtered but then completely reabsorbed
Why is the renal clearance of inulin a measure of GFR?
GFR= Glomerular Filtration Rate (rate of filtration across the glomerular capillaries)
- Inulin is freely filtered across the glomerular capillaries
- However, it is neither reabsorbed or secreted afterward i.e. ALL the inulin that is filtered appears in the urine
Thus, Inulin clearance= GFR i.e. inulin filtered is exactly equal to inulin excreted
What is creatinine?
Creatinine is the byproduct of muscle catabolism of creatine; it is released into systemic circulation at a fairly constant rate
Why is clearance of creatinine a reasonable estimate of GFR?
- Creatinine is freely filtered & not reabsorbed
- However it is slightly secreted
*Thus, creatinine results in a slightly elevated measure of GFR
Why is measuring the clearance of creatinine easier than measuring the clearance of inulin?
Determining renal clearance by inulin is cumbersome & requires:
1) Extended office visit with IV inulin infusion
2) Catheterization of bladder
3) Collection of urine for 2 hours
Creatinine is much more convenient:
- No IV infusion
- No catheterization of the bladder
Why does creatinine clearance as a measure of GFR become less accurate in patient with renal disease and low GFR? In such patient does the creatinine clearance over estimate or underestimate the GFR?
- Normally, the secreted fraction of creatinine is 10-20% of what is excreted; thus, clearance overestimates GFR
- In patients with v. low GFR; this fraction is considerably HIGHER
- In patients with LOW GFR, creatinine clearance severely OVERESTIMATES GFR
Assuming creatinine production remains constant, how do changes in GFR affect serum creatinine level?
High GFR= low serum creatinine concentration
Low GFR= high serum creatinine concentration
Note, 125mL/min is a normal GFR, which corresponds to plasma creatinine between 0.7 & 1.5 mg/100mL
If serum creatinine is above normal, is GFR above or below normal?
GFR is below normal
What variables are used to estimate GFR using the Cockcroft-Gault equation?
1) Serum creatinine
3) Lean body weight
How could you use the estimated GFR to help in determining dosages of drugs that are excreted in the urine?
What are the variables used to estimate GFR using the Modification of Diet in Renal Disease (MDRD) study?
1) Serum creatinine
What is the the effect of age on the number of functional nephrons, and GFR?
In most people there is a 10% decrease in the number of functional nephrons each 10 years after 40
- Progressive decrease in GFR after 40
How do hypertension & DM affect renal function changes with age?
These pathologies accelerate the loss of nephrons
- GFR will decline at a faster rate in individuals with these conditions compared to those without
The clearance of substance x is lower than the clearance of inulin. What does this mean?
- Substance is NOT filtered at all, or
- Substance is reabsorbed