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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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objectives
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
renal plasma clearance
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

clearance
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 .
clearance1
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
clearance2

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

creatinine clearance
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
slide8

Steady-state relationship between

GFR and serum creatinine concentration

para amino hippuric acid pah
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
slide10

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

pah clearance

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

pah clearance1
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
renal blood flow and oxygen consumption
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
blood flow through the kidneys
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)
blood flow through the kidneys1

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

reabsorbtion and secretion of substances
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
slide20

Inulin

Glucose

Plasma Clearance For substances handled in different ways by the kidney

Urea

PAH

clearance rate
CLEARANCE RATE

Substance

  • Glucose
  • Na+
  • K+
  • Inulin
  • Creatinine
  • Urea
  • PAH

Clearance Rate (ml/min)

0

0.9

12

125

140

70

600

slide26
Q4. If a substance is filtered, then some of it is reabsorbed, its clearance will be less or more than Inulin clearance?
slide27
Q5. If a substance is filtered, then secreted, its clearance will be less or more than Inulin clearance?
q6 what is the plasma clearance of i glucose ii inulin iii creatinine iv urea v pah
Q6. What is the plasma clearance of i). Glucoseii). Inuliniii). Creatinine iv). Ureav). PAH