Lecture 3: Measuring GFR & Renal Clearance Flashcards
what is clearance
the volume of plasma from which all of the substance is removed and excreted in a given time
how to calculate clearance
Ux = urine concentration
V- = urine flow rate
Px = plasma concentration
Cx = clearance
why can inulin be used to determine GFR
b/c it is freely filtered and neither secreted nor reabsorbed by the tubules
example of a substance that is freely filtered and also secreted and absorbed
Sodium. secreted, but most (99%) gets reabsorbed
how can a solute have a greater clearance than GFR of 150ml/min
if the solute is secreted
what is creatinine
waste product formed in muscle from creatine phosphate
how much (%) creatinine is filtered at the glomerulus
15-20% of plasma creatine is filtered
the portion that is not filtered is returned to systemic circulation via the renal vein
What is the relationship b/w plasma creatinine concentration and GFR
they are inversely related
why isn’t creatinine a perfect substance for measuring GFR
b/c some of it gets secreted in the proximal tubules
*so creatinine clearance tends to be an over-estimation of GFR
what health conditions cause BUN to rise
- heart failure (↓ RBF, ↓GFR)
- dehydration (↓ RBF, ↓ GFR)
- high protein diet (↑protein metabolism)
what health conditions cause BUN to decrease
- liver disease (↓ protein catabolism)
- malnutrition (↓ protein catabolism)
- fluid overload (↑RBF, ↑GFR, ↑ urine flow)
What is BUN
blood urea nitrogen
- the amount of nitrogen derived from urea
what does the level of urea in blood (BUN) reflect
protein intake, protein metabolism, renal handling of urea
renal urea excretion must match ____ otherwise, plasma levels would become pathological (uremia)
hepatic production
the clearance of a substance the is freely filtered and not reabsorbed/secreted is a useful approach to determine _____ ?
GFR