Oral bioavailability physiological factors

M. Rowland, Effect of some physiologic factors on bioavailability of oral dosage forms, in Dosage Form Design and Bioavailability (J. Swarbrick, Ed.), Lea Febiger, Philadelphia, 1973, pp. 181-222. 

Hurst, S., Loi, C. M., Brodfuehrer, J., and El-Kattan, A. (2007). Impact of physiological, physicochemical and biopharmaceutical factors in absorption and metabolism mechanisms on the drug oral bioavailability of rats and humans. Expert Opin. Drug Metab. Toxicol. 3 469-489. 

Physiological factors which affect oral bioavailability include ... 

Physiological factors affecting oral transmucosal bioavailability 172... 

Absorption from the rectum depends on various physiological factors such as surface area, blood supply, pH, fluid volume, and possible metabolism by microorganisms in the rectum. The rectum is perfused by the inferior and middle rectal arteries, whereas the superior, the middle, and the inferior rectal veins drain the rectum. The latter two are directly connected to the systemic circulation the superior rectal vein drains into the portal system. Drugs absorbed from the lower rectum are carried directly into the systemic circulation, whereas drugs absorbed from the upper rectum are subjected to hepatic first-pass effect. Therefore, a high-clearance drug should be more bioavailable after rectal than oral administration. The volume of fluid in the rectum, the pH of that fluid, and the presence of stool in the rectal vault may affect drug absorption. Because the fluid volume is... 

Table 4.8 Physicochemical and physiological factors that influence drug bioavailability after oral administration (from Ungell 1997, with permission). ...

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