Measuring kidney function Flashcards
What is renal clearance?
Volume of plasma completely cleared of a substance by the kidneys per unit time
The higher the renal clearance = more plasma is cleared of the substance
Which substances can be used to measure renal clearance?
Any substance
Value obtained depends on how the substance is handled by the kidney
What is the maximum possible renal clearance?
Plasma flow 650 mL/min
What molecule is often used to measure GFR?
Inulin
How does the kidney handle inulin?
Freely filtered at the glomerulus
Not reabsorbed by the tubule
Not secreted by the tubule
What is GFR?
The rate at which the two kidneys form the filtrate
What is the average value of the GFR?
125 mL/min
Renal clearance of inulin = GFR of inulin TRUE or FALSE
TRUE
What is renal clearance of inulin?
Hypothetical volume of plasma completely cleared of inulin by the kidneys per unit time
If a substance is freely filtered, not reabsorbed or secreted renal clearance = GFR TRUE or FALSE
TRUE
In which cases will clearance of a solute be less than that of inulin?
Net reabsorption
Not feely filtered
In which cases will the clearane of a solute be more than that of inulin
Net secretion
In which cases if the clearance of a solute = 0
If solute is not filtered at all
Completely reabsorbed
Which equation do we use to calculate GFR?
Renal clearance equation
Substance must have GFR = renal clearance
Which two subtances are used clinically to measure GFR?
Inulin Serum creatinine
What are the disadvantages of using inulin to measure GFR?
Not made endogenously - has to be injected
Need 45 minutes before inulin concentration remains stable
Hard to keep inulin concentration stable
Continual blood and urine sampling required every 10-20 minutes
Very invasive and time consuming
Chemical analysis - not very accurate
What are the advantages of using creatinine clearance?
Endogenous by-product of metabolism
Released into the blood at a relatively constant rate - plasma concentration is stable
Only need one blood sample
What are the disadvatages of using creatinine?
Some secretion of creatinine from the tubules
Clearance overestimates GFR by around 10-20%
Ease of using compensates for overestimation
What is the normal range of GFR?
90 - 140 ml/min
What conditions change GFR value between individuals?
Age - GFR decreases every year
Sex - 10% lower in women
Race - higher muscle mass in afro-carribeans
Body size - increase in body size = increase in kidney size
What are symptoms of CKD?
Hyperaemia
Anaemia
Calcification of vasculature
How can we calculate renal plasma flow?
Inject PAH in low concentration
20% is filtered across the glomerular capillaries
The remaining 80% is secreted from the peritubular capilaries
Measure the volume of PAH entering the kidney
Use the clearance equation to measure the plasma flow
Since all of it is secreted the volume of plasma cleared of PAH = renal plasma flow
So renal clearance of PAH = renal plasma flow
Renal clearance of PAH = renal plasma flow TRUE or FALSE
True
Why is PAH injected in low concentration?
Needed to prevent oversaturation of the secretory processes
How do you convert the renal plasma flow into the renal blood flow?
Calculate renal plasma flow
Take a blood sample and centrifuge it to calculate haematocrit
Find the volume percentage of the blood ( 1 - % haematocrit )
Plug in renal plasma flow and haematocrit level into the calculation: RPF = RBF x ( 1 - % haematocrit)
What is the average RBF?
1 L/min
What is the average haematocrit %?
40 %
How does kidney stone affect creatinine levels?
Blockage of ureter leads to a drop in GFR
This decreases urine creatinine excretion
Plasma creatinine concentration increases
Leads to an increase in urine creatinine excretion
A steady state will be reached where the creatinine excretion = production
GFR remains low but creatinine exretion remains the same as before.
Plasma creatinine becomes elevated
