1. Lecture 20 Urine Formation II RENAL FUNCTION TESTS
Kidney lecture 18'19
The Body Fluids and Kidneys
Lecture 20
Urine Formation II
RENAL FUNCTION TESTS
Dr. Rashid Mahmood

2. RENAL FUNCTION TESTS Quantify the excretory functions of the kidney.
Quantify the rate at which the blood flows through the kidney.
Basic functions of the kidney
1.GFR
2. Tubular reabsorption.
3. Tubular Secretion

3. Use of Clearance Methods to Quantify Kidney Function
Renal Clearance of a Substance:
It is the volume of Plasma that is
Completely Cleared of the Substance
by kidney per unit time

4. Cs × Ps = Us × V S = Substance Cs = Clearance Rate
Ps = Plasma Concentration
Us = Urine Concentration
V = Urine Flow Rate

5. GFR ESTIMATION
Inulin
Creatinine
Radioactive isothalamate

6. Inulin Clearance= GFR
Rate of excretion in urine=filtration rate of that substance
Us × V = GFR × Ps
For GFR
Freely filtered
Not reabsorbed
Not secreted
Not Metabolized
Harmless
GFR = Cs = Us × V Ps

7. Inulin Clearance= GFR

8. Creatinine Clearance to estimate GFR
Creatinine is by-product of muscle metabolism
Cleared almost entirely by glomerular filtration
Intravenous injection not required therefore easy to use clinically
Not a perfect marker
Small amount secreted
Amount excreted is slightly > amount filtered
Palma Creatinine measurement clinically Overestimates
Two errors cancel each other

9. Serum creatinine level is inversely proportional to GFR

10. Serum creatinine level is inversely proportional to GFR

11. RENAL PLASMA FLOW
PARA AMINOHIPPURIC ACID (PAH)

12. PAH Clearance to measure Renal Plasma flow
For Renal Plasma Flow
Completely Cleared( Filtration+ Secretion )
Amount of the substance delivered to kidneys=amount excreted in urine
Renal plasma flow × Ps= Us × V
RPF=Cs= Us × V / Ps

13. PAH Clearance to measure Renal Plasma flow
Extraction Ratio = 90 %
Total renal plasma flow= Clearance of PAH/ Extraction ratio of PAH
Extraction ratio=EPAH=PPAH-VPAH/PPAH
VPAH= Venous concentration of PAH

14. PAH Clearance to measure Renal Plasma flow

15. Renal blood flow
RPF
1- Hematocrit

16. Rates Excretion rate= US ×V
Reabsorption rate= filtered load – excretion rate
(GFR ×PS) –(US ×V)
Secretion rate= excretion rate – filtered load

17. Calculation of tubular Reabsorption or Secretion from Renal Clearance
Excretion rate > filtered load
=Secretion
Excretion rate < filtered load
=Reabsorption