<prompt> What is the clearance of a substance when its concentration in the plasma is 10 mgdL, its concentration in urine is 100 mgdL, and urine flow is 2 mLmin?2 mLmin10 mLmin20 mLmin200 mLmin  <response>: Ans. c 20 mlmin Ref. Ganong physiology, 21st ed., 806MEASURING GFR The glomerular filtration rate GFR can be measured in intact experimental animals and humans by measuring the excretion and plasma level of a substance that is freely filtered through the glomeruli and neither secreted nor reabsorbed by the tubules. The amount of such a substance in the urine per unit of time must have been provided by filtering exactly the number of milliliters of plasma that contained this amount. Therefore, if the substance is designated by the letter X, the GFR is equal to the concentration of X in urine UX times the urine flow per unit of time V. divided by the arterial plasma level of X PX, or UXVPX. This value is called the clearance of X CX.In our MCQ, the clearance will be Cx100 x 2 10 20 mlmin. PX is, of course, the same in all parts of the arterial circulation, and if X is not metabolized to any extent in the tissues, the level of X in peripheral venous plasma can be substituted for the arterial plasma level.SUBSTANCES USED TO MEASURE GFR In addition to the requirement that it be freely filtered and neither reabsorbed nor secreted in the tubules, a substance suitable for measuring the GFR should be nontoxic and not metabolized by the body. Inulin, a polymer of fructose with a molecular weight of 5200 that is found in dahlia tubers, meets these criteria in humans and most animals and is extensively used to measure GFR.UIN 35 mgmLV 0.9 mLminPIN 0.25 mgmLCIN UINV---PIN35 x 0.9---0.25CIN 126 mLmin Endogenous creatinine clearance is easy to measure and is a worthwhile index of renal function. Normal GFR- The GFR in an average-sized normal man is approximately 125 mLmin.- Its magnitude correlates fairly well with surface area, but values in women are 10 lower than those in men even after correction for surface area.- A rate of 125 mLmin is 7.5 Lh, or 180 Ld, whereas the normal urine volume is about 1 Ld. Thus, 99 or more of the filtrate is normally reabsorbed.- At the rate of 125 mLmin, the kidneys filter in 1 day an amount of fluid equal to 4 times the total body water, 15 times the ECF volume, and 60 times the plasma volume. Control of GFR- The factors governing filtration across the glomerular capillaries are the same as those governing filtration across all other capillaries, i.e., The size of the capillary bed, The permeability of the capillaries, and The hydrostatic and osmotic pressure gradients across the capillary wall. For each nephron:- GFR Kf PGC-PT-nGC-nT Kf, the glomerular ultrafiltration coefficient, is the product of the glomerular capillary wall hydraulic conductivity ie, its permeability and the effective filtration surface area. PGC is the mean hydrostatic pressure in the glomerular capillaries, PT the mean hydrostatic pressure in the tubule, pGC osmotic pressure of the plasma in the glomerular capillaries, and pT the osmotic pressure of the filtrate in the tubule. 