hereby the kidney is isolated in a double cup, thus decreasing any movements due to the animals respiration. All the above experiments used the free-flow micropuncture technique. A micropipette is inserted through the tubule wall and fluid enters the pipette spontaneously due to intratubular hydrostatic pressure. One problem with this technique is that if too much fluid is withdrawn from the tubule, either by negative pressure or simply by capillarity, tubular flow will change affecting tubular pressure, decreasing transit time and increasing the glomerular filtration rate. Analytical data from such samples is then of little use. To avoid this firstly, fluid should be allowed to enter only very slowly (therefore a little positive counter pressure should be applied to the pipette). Secondly tubular diameter should be closely monitored to avoid collapse. Thirdly the sample size should not be greater than 10-20
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