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PLASMA CLEARANCE AND RENAL BLOOD FLOW

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

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PLASMA CLEARANCE AND RENAL BLOOD FLOW

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  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. Uin x V Cinulin= Pin CLEARANCE 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. UPAH x V CPAH= PPAH 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

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

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

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

  22. Clearance Discussion

  23. Q1. What is Plasma Clearance?

  24. Q2. Which substance is used to measure i) GFRii). 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). Glucoseii). Inuliniii). Creatinine iv). Ureav). PAH

  29. Thank you

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