GFR and Renal clearance Flashcards
(19 cards)
What is the meaning of freely filtered?
When something is filtered and as a result the concentration of it in plasma and filtrate is the same
What is the conditions when GFR equals renal clearance of a substance? What is the name of this ideal molecule
Freely filtered but either reabsorbed nor secreted
inulin
What is the formulae for renal clearance
(Concentration of substance in urine X volume of urine per minute)/ conc of substance in plasma
What properties of inulin make it an ideal molecule for getting an accurate GFR
Freely filtered
Not rebsorbed
Not secreted
not toxic
Measurable in urine and plasma
What happens if the renal clearance of substance is higher or lower than that on inulin
Lower- NET reabsorption; like Na in which 99% is reabsorbed
Higher - Net secretion
What substance can be used for measuring renal plasma flow ? Explain why
PAH- para-amino hippuric acid
PAH is secreted into tubules; hence all PAH arriving at arterioles should be in tubules and none leaves in renal vein. Provided PAH plasma conc isn’t too high
How do you calculate the rate at which PAH enters kidney per minute
[Plasma PAH] x RPF (Renal plasma flow rate)
N.B RPF is equal to it’s renal clearance
How do you calculate the rate at which PAH is excreted in urine
[Urine PAH] X Volume of urine excreted per minute
How do you calculate what proportion of blood entering glomeruli is filtered (filtration fraction)
Inulin clearance / PAH clearance
Why is PAH clearance rarely performed clinically when renal disease is suspected?
GFR measurement is sufficient to diagnose renal disease, hence no need to calculated filtration fraction (using PAH). Also PAH will need to be infused which will inconvenience the patient..
What are the problems what using inulin for estimating GFR
Not given orally as enzymes may break it down an hence lose it’s special properties
What are the 3 main methods for estimating GFR
Inulin clearance
Creaitnine clearance
57Cr EDTA clearance
What are the features for creaitnine clearance?
It’s endogenous
requires:
- 24 hr urine collection
- a single plasma sample taken at some time during clearance period
Bladder catheterisation is unnecessary as % volume error incurred by incomplete bladder emptying at the start and end of 24h urine collection is insignificant
What is the problem of using creatinine for GFR? How can it be resolved?
Some (tiny amount) is secreted into proximal tubule; hence GFR may be an overestimate
however there’s error in the colorimetric calculation in plasma creaitnine concentration; it measures the concentration of non-creatinine chromogens
Hence the 2 cancel out
Explain the shape of this curve
Some EDTA injected enters ECF (then try to equilibrate), some enter urinary system. Hence initially, theres a large decrease in EDTA activity in plasma. Amount leaving blood is equal to amount entering urinary system + amount entering ECF - amount entering blood via ECF
over time after equilibrium has been reached between blood and ECF, theres a steady decrease . Only EDTA leaving blood is though urinary system
Is there any need to collect urine using EDTA
No; as EDTA emits gamma radiation and hence can be measured
How will the graph of EDTA acitve against time change of theres a renal disease
Renal disease means that there’s a lower rate of EDTA leaving the blood via the urinary system . The rate of decline will be shallower
How can clearance be calculated from the EDTA activity vs time graph
what are the advantages of using EDTA
A second log graph is plotted and then slope calculated. The great the gradient, the higher the clearance.
Advantages are:
- No need to collect urine samples
- Injections of a single dose of EDTA
- colleciton of 2-3 plasma samples - to map out straight line of the graph.
How does the concentration of inulin in efferent arteriole and renal vein compare
explain why
Efferent arteriole inulin concentration is same as afferent arteriole
renal vein is lower due to reabsorption of water (99.9%)
