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

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

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

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

GFR ESTIMATION Inulin Creatinine Radioactive isothalamate

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

Inulin Clearance= GFR

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

Serum creatinine level is inversely proportional to GFR

Serum creatinine level is inversely proportional to GFR

RENAL PLASMA FLOW PARA AMINOHIPPURIC ACID (PAH)

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

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

PAH Clearance to measure Renal Plasma flow

Renal blood flow RPF 1- Hematocrit

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

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