Figure 16.6 • Renal Clearance Principle – ?Clearance? describes the volume of plasma that is cleared of a substance per unit time. The renal clearance of a substance provides information on how the kidney handles that substance. Since inulin is freely filtered, and not reabsorbed or secreted, all of the filtered inulin is excreted in the urine. Thus, C_in is equated with the glomerular filtration rate (GFR), and the net handling of other substances can be determined, depending on whether their clearance is greater than (net secretion), less than (net reabsorption), or equal to C_in.