The small intestine as a homogeneous cylindrical tube

Case I Cq cs and cout cs (the drug is in solution throughout the transit process)  

Case II cq cs and cout cs (solid drug at inlet concentration reaches solubility at a certain point and diminishes thereafter)  

Case III cq cs and c .ul cs (solid drug exists at both ends of the tube). 

The last two integrals of the previous equation are expressed in dimensionless variables, c = c/cq, z = z/L with normalized limits (0,1), while the symbol An is the absorption number of the drug  

The first fraction of the previous equation shows that An is exclusively determined by the effective permeability Peff of drug since all other variables are species-dependent physiological parameters. In terms of characteristic times, the An of a drug can also be defined as the ratio of the mean small intestinal transit time (Tsi), to its absorption time R/Peff. 

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