Measurment of GFR and Renal Plasma Flow

Various methods of measuring Glomerular Filtration Rate and Renal Plasma Flow

1. MEASURMENT OF GFR AND RENAL PLASMA FLOW FASLU RAHMAN CK JOHNSON KL

2. GLOMERULAR FILTRATION RATE Rate at which glomerular filtrate formed from plasma at glomerulus per minute per unit area of body surface GFR = K f X net filtration pressure Unit millilitters per minute Normal 120 ml per minute About 170 litter blood is filter through kidney GFR is about 20% of renal plasma flow [RPF]

3. DETERMINATION OF GFR RENAL CLEARANCE – kidney^s ability to remove substance from plasma Features of substance used to measure GFR Freely filtered from glomerulus It should not be reabsorbed It should not be secreated Concentration in plasma = conc. In filtrate It cant be degraded in body

4. DETERMINATION OF GFR INULIN is the substance used commonly Inulin – fructose polysaccharide Its freely filtered,neither secreted nor reabsorbed. Because inulin is not produced in our body ,rate of disappearance from blood after i /v injection is strictly related to glomerular filtration.

5. GFR = C inulin = U inulin X V P inulin Cinulin -vol. of plasma cleared of inulin per unit time U inulin -urine conc. Of inulin V – volume of urine collected by time period of collection P-plasma conc. Of inulin

6. Using endogenous creatinine clearance Creatinine is the endogenous by-product of muscle metabolism handled similar to inulin Most widely used clinically – dose not reqire i /v In some species aprox . 10% is secreated. 24 hour urine collection is reqired . Succesful in dog,cat,sheep&cattle

7. GFR = C cre = U cre P cre In birds creatinine clearance cant used because avian renal tubule can secreat creatinine when plasma level is elevated & can reabsorb when when plasma level is normal In clinical practice serum creatinine level – very small increase in serum creatinine level – large reduction in GFR

8. RENAL PLASMA FLOW (RPF) If a substance is completely cleared from plasma , clearance rate of that substance is equal to total renal plasma flow. Features of substance used to measure RPF Freely filtered at the glomerulus Not reabsorbed from tubular lumen Must be secreated by tubular epithelium.

9. PAH (para amino hippuric acid) is commonly used to determine RPF PAH is freely filtered by glomerular capilleris and is also secreated from peritubular capillary blood into the tubular lumen Total renal plasma flow = clearance of PAH /E PAH Extraction ratio (E PAH ) is calculated as difference between the renal arterial PAH and renal venous PAH concenterations,divided by the renal arterial PAH concenteration .

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