058 Renal Clearance Flashcards
What are the properties of inulin that allow it to be a marker for glomerular filtration?
It is freely filtered
It is not reabsorbed
It is not secreted
What is the formula for gloremular filtration rate
(Urine inulin x urine flow rate)/ plasma inulin
What is the definition of renal clearance?
The volume of plasma that would be required to supply that amount of the substance excreted per unit time. (mL/min)
What is the drawback of the clearance method?
It provides no information about whether a substance is filtered or the precise sites and mechanisms of transport.
The clearance of inulin is 125 ml/min. if substance has lower clearance value, what does this mean?
There is either net absorption or the substance is not freely filtered (e.g. bound to plasma proteins).
The clearance of inulin is 125 ml/min. if substance has higher clearance value, what does this mean?
There is net secretion
What does a clearance of 0 mean?
It is not filtered at all or it is totally reabsorbed
Why would you prefer using creatinine clearance?
Using inuline is invasive as you need to infuse it intravenously and continuous measurements must be taken.
What is one of the downsides of using creatinine?
It overestimates GFR by 10-20%
What is the benefits of using creatinine?
Plasma concentration is stable, it is freely filtered, it is naturally produced
What are normal GFR ranges?
90-140ml/min, depending on age, sex, racial group and body size.
What can be used to measure renal plasma flow?
Para-amino hippurate
When is using plasma creatinine concentration to estimate GFR useful?
When GFR levels are low
What does Transport Maximum (Tm) mean?
The point at which an increase in concentration does not result in an increase in movement of a substance across the membrane.
Discuss properities of para-amino hippurate
It is an organic anion not usually present in the body. It is freely filtered and secreted from peritubular capillaries by a Tm dependent mechanism into proximal tubule lumen.
What occurs to PAH when it enters renal artery with low plasma concentrations?
PAH enters artery and 1/5 is filtered in the glomerulus. The remaining 4/5 is secreted into proximal tubule lumen, so that there is none left in the renal vein.
How does para-amino hippurate measure the renal plasma flow?
Volume of plasma cleared of PAH = volume of plasma entering the kidneys = renal blood flow
What is the formula for Renal blood flow (RBF)
RPF x (1 – haematocrit) = renal blood flow.
What is the formula for renal plasma flow?
[PAH]urine x vol urine / [PAH] arterial
What is filtration fraction? What is the normal value?
Ratio of GFR to RPF; inulin clearance/PAH clearance normally at 20%
What occurs when filtration fraction increases?
There will be a decrease in hydrostatic pressure and an increase in oncotic pressure in peritubular capillaries, allowing increased reabsorption.
What occurs when filtration fraction decreases?
There will be an increase in the hydrostatic pressure and a decrease in oncotic pressure in peritubular capillaries. This will reduce reabsorption.
How do we classify chronic kidney injury?
Stage 1 - stage 5 (CKD stage) RIFLE criteria: Risk – GFR decreases by over 25% Injury – GFR decreases by over 50% Failure – GFR decreases by over 75% Loss – AKI persists for over 4 weeks. ESRD (End Stage Renal Disease) – renal replacement therapy above 3 months.
How do you measure the transport maximum?
Filtered load - excretion rate
You can work out how much was able to be absorbed before they are excreted. If Tm = -ve, it is indication of secretion.
