Drug absorption physiological factors

Orally administered dosage forms are absorbed into the systemic circulation following dissolution in the GI tract. Because substances must be in solution for the absorption from the GI lumen, the absorption rate of poorly water-soluble drugs is limited by their rate of dissolution. The dissolution rate is affected by the unique physicochemical properties of the drug and by physiological factors the pH, composition, and hydrodynamics of the GI medium. 

In situ techniques suffer the disadvantage that the animal is anaesthetised for the duration of the experiment. Surgery and anaesthesia change several physiological factors that can alter drug absorption characteristics. Anderson et al. [117] found the ABL thickness in laparotomised rats to be higher than in conscious non-laparotomised rats. Yuasa et al. [118] reported that... 

In relation to systemic absorption of drugs, absorption in the lung can be described as the passage of a series of barriers by the drug in order to enter the systemic circulation. It is important to realize that physiological conditions in the lung differ widely from site to site. Major physiological factors that affect pulmonary absorption are [10] ... 

Absorption from the Gl tract is governed by many factors. Broadly, it can be categorized into three classes physicochemical properties, biopharmaceutical factors, and physiological and pathophysiological factors (Mojaverian et al., 1985,1988 Nomeiretal., 1996). Since this chapter focuses on the pharmacokinetic perspectives, the main factors that could affect drug absorption are merely listed below, and will be discussed in detail in other chapters. 

Macheras, P., C. Reppas, and J.B. Dressman. 1995. Physiological factors related to drug absorption. In Biopharmaceutics of orally administered drugs. London Ellis Horwood. 

Many physiological aspects affect drug absorption from the rectum (Table 7.1). Influential factors include the pH of the rectal contents, state of the mucus layer, volume and viscosity of rectal fluid, luminal pressure from the rectal wall on the dosage form, enzymatic and microbacterial degradation by rectal epithelium, presence of stools, and venous drainage differences within the rectosigmoid regions. 

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