It is possible, of course, to calculate the clearance of any substance that is brought to the kidney in the blood and excreted in the urine by simply measuring the rate of excretion of the material as well as its concentration in the blood plasma. However, the clearance of a substance other than inulin will be greater than the simultaneous clearance of inulin if it undergoes net secretion by the renal tubules or less than the simultaneous clearance of inulin if it undergoes net reabsorption by the renal tubules. The glomerular filtrate is modified as it moves along the renal tubules by the processes of tubular reabsorption and secretion. Tubular reabsorption means the removal of a substance present in the filtrate by the tubular epithelium. In most cases, this involves transport of the substance from the tubule lumen into the peritubular fluid.
Tubular secretion means the addition of a substance to the filtrate in the tubule lumen.
In most cases, this involves transport of the substance from the peritubular fluid into the tubule lumen. No energy is expended directly in such movement, although energy is expended indirectly in establishing the chemical or potential gradient down which the substance moves. Determination of rate of net reabsorption for the whole kidney. If it is not freely filtered, then the degree to which it is filtered must be taken into account. Maximum rate of glucose reabsorption.