1. PLASMA CLEARANCE AND RENAL BLOOD FLOW
Lecture-6
Dr. Zahoor

2. Objectives Describe the concept of renal plasma clearance
Use the formula for measuring renal clearance
Use clearance principles for inulin, creatinine etc. for determination of GFR
Use PAH clearance for measuring renal plasma flow
Calculate Renal Blood Flow

3. RENAL PLASMA CLEARANCE
It is the volume of plasma cleared of a substance per unit time
Plasma clearance is calculated by the formula:
Cs = Us x V Ps
Cs = clearance of a substance
Us = urine concentration of a substance
V = volume of urine ml/min
Ps = plasma concentration of a substance

4. CLEARANCE
Inulin clearance is used to measure GFR {Glomerular Filtration Rate}
WHY ?
Because Inulin is filtered but not reabsorbed or secreted
If a substance X [Inulin] is filtered from 125ml/min, then it is excreted but not reabsorbed, not secreted, therefore, 125ml of filtrate is cleared of substance X .

5. CLEARANCE Inulin is used to measure GFR
Inulin is a carbohydrate, polysaccharide, not produced in the body but found in certain plants
Inulin is infused I/V continuously to maintain a constant plasma level

6. CLEARANCE Cinulin = Pin Uin x V GFR = If Uin = 30mg/ml of urine,
V = 1.25ml/min,
Pin = 0.3mg/ml in plasma then
C Inulin = (30 × 1.25)/ 0.3
GFR = 125ml/min

7. CREATININE CLEARANCE
In clinical practice endogenous substance Creatinine is often used to find GFR
Creatinine is end product of muscle metabolism
Creatinine is filtered, not reabsorbed ,but slightly secreted, therefore, creatinine clearance is not so accurate for measurement of GFR as Inulin
But creatinine clearance provides close approximate value and easy to use as it does not require I/V injection

8. Steady-state relationship between
GFR and serum creatinine concentration

9. PARA AMINO HIPPURIC ACID (PAH)
PAH clearance is used to measure renal plasma flow
WHY ?
Because PAH is filtered, not reabsorbed but secreted from all the plasma that flows through the kidneys
Renal plasma flow is 650ml/min

10. Use of Clearance to Estimate Renal Plasma Flow
Theoretically, if a substance is completely cleared from
the plasma, its clearance rate would equal renal plasma flow.
Cx = renal plasma flow

11. PAH clearance example

12. PAH CLEARANCE
PAH is 90% cleared from the plasma called extraction ratio
Renal plasma flow (RPF) can be calculated by PAH CLEARNCE
U = 5.85mg/ml
V = 1 ml/min
P = 0.01mg/ml
Therefore, C = (5.85 × 1)/0.01
RPF = 585 ml/min
CPAH =
UPAH x V
PPAH

13. PAH CLEARANCE
Actual Renal Plasma Flow can be calculated by dividing 585/0.9 = 650ml/min
We are dividing by 0.9 because PAH removed from the plasma i.e extraction ratio is about 90% in normal kidneys

14. RENAL BLOOD FLOW

15. RENAL BLOOD SUPPLY

16. Renal blood flow and Oxygen consumption
Oxygen consumption per 100 g of renal tissue is 5 ml/min. which is next highest to the heart (8ml/min).
Total renal blood flow is ≈ 1200/min.
90 % of RBF goes to the renal cortex, 9% goes to outer medulla and 1% to the inner medulla

17. BLOOD FLOW THROUGH THE KIDNEYS
Total blood flow through the kidneys can be calculated from renal plasma flow and Hematocrit (percentage of RBC in the blood)

18. BLOOD FLOW THROUGH THE KIDNEYS
We apply formula
If renal plasma flow = 650ml/min
HCT = 0.45
Therefore,
RBF =
RPF
1 - Hct
RBF =
650
RBF =
650
0.55
= 1182ml/min

19. REABSORBTION AND SECRETION OF SUBSTANCES
If a substance has clearance rate equal to Inulin then substance is only filtered but not reabsorbed, not secreted
If a substance clearance rate is less than Inulin clearance, then substance has been reabsorbed by nephron tubule E.g. Urea
If clearance rate of a substance is greater than that of Inulin then substance must be secreted by nephron tubule e.g. creatinine

20. Inulin
Glucose
Plasma Clearance For substances handled in different ways by the kidney
Urea
PAH

21. Clearance Rate (ml/min) 0 0.9 12 125 140 70 600
Substance
Glucose
Na+ K+
Inulin
Creatinine
Urea
PAH
Clearance Rate (ml/min)

22. Clearance Discussion

23. Q1. What is Plasma Clearance?

24. Q2. Which substance is used to measure i) GFR ii). Renal Plasma Flow

25. Q3. What is the formula for the clearance?

26. Q4. If a substance is filtered, then some of it is reabsorbed, its clearance will be less or more than Inulin clearance?

27. Q5. If a substance is filtered, then secreted, its clearance will be less or more than Inulin clearance?

28. Q6. What is the plasma clearance of i). Glucose ii). Inulin iii)
Q6. What is the plasma clearance of i). Glucose ii). Inulin iii). Creatinine iv). Urea v). PAH

29. Thank you