The Body Fluids and Kidneys Lecture 20 Urine

The Body Fluids and Kidneys Lecture 20 Urine Formation II RENAL FUNCTION TESTS 1 1

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 2

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 3

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

GFR ESTIMATION • Inulin • Creatinine • Radioactive isothalamate 5

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

Inulin Clearance= GFR 7

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 8

Serum creatinine level is inversely proportional to GFR 9

Serum creatinine level is inversely proportional to GFR 10

RENAL PLASMA FLOW • PARA AMINOHIPPURIC ACID (PAH) 11

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 12

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 13

PAH Clearance to measure Renal Plasma flow 14

Renal blood flow RPF 1 - Hematocrit 15

Rates • Excretion rate= US ×V • Reabsorption rate= filtered load – excretion rate (GFR ×PS) –(US ×V) • Secretion rate= excretion rate – filtered load 18

Calculation of tubular Reabsorption or Secretion from Renal Clearance • Excretion rate > filtered load =Secretion • Excretion rate < filtered load =Reabsorption 19

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