Clearance of other Substances


The clearance of other substances that are freely filtered at the glomerulus, depends on whether they are secreted or absorbed at the tubules.

Thus to give some examples:

UREA: The clearance of urea utilising th plasma clearance formula, is calculated to be 75ml/min. This is clearly less than the GFR and thus by inference, there is net reabsorption of urea in the nephron.

PARA-AMINOHIPPURIC ACID (PAH): The clearance of PAH has been calculated to be on the average, 625ml/min. This means that a large proportion of the PAH (around 80%) is secreted at the tubules. In fact plasma is cleared of PAH in a single pass through the kidneys. Thus the clearance of PAH can be utilised to measure the total renal plasma flow and from the hematocrit (the fraction of the blood that is occupied by the eryhtrocytes), one can measure the total blood flow in the kidneys. This means that if the PAH clearance is of 625ml/min and the hematocrit is 45%, then total blood flow is equal to 625 divided by (1-0.45) or 1.1L/min.