## Fraction Absorbed

The calculation of fraction absorbed involves solving a system of two differential equations. (Oh, et al., 1993)

The first equation determines the decrease in radius (i.e., dissolution) as the particles transit the intestine. This rate is proportional to the dissolution number (the greater Dn, the faster the dissolution) and the difference between the saturation solubility and concentration in solution (given by 1 - C*). The second equation determines the change in concentration down the intestinal tract and is the difference between the amount of drug getting in solution via dissolution and the amount of drug being removed from solution via absorption.

Total fraction absorbed is determined by evaluating these equations at the end of the intestine (mathematically when z* = 1) using the expression o

Conceptually, this can be considered as the initial dose fraction (by definition = 1) minus what has not dissolved and is left as a solid at the end of the intestine (= r*(1)3) minus what has dissolved, but has not been absorbed (= C*(1)/Do).