Tubular Transport . Note that we can calculate only the net reabsorption or secretion, which may be the result of a combination of secretion and reabsorption in different segments of the nephron. If the clearance of a solute is less than the clearance of inulin, the substance must be reabsorbed. Fractional Excretion .
Paracellular transport is by passive diffusion or by solvent drag.
Transcellular transport may be mediated, passive, or a combination of the two Transcellular transport involves luminal and basolateral steps. They also exhibit a threshold , i.e. a plasma level after which the solute involved starts to appear in the urine. Other solutes, such as phosphate, have low threshold, i.e. they appear in urine at plasma levels only slightly above normal. Only the plasma levels of those solute with low threshold are regulated by the kidney. Low capacity, high affinity transporters are usually located more distally.