Gastrointestinal tract absorptive function

Gastrointestinal Tract Absorption. The structure and function of this tract is varied and complex. The structure of the pesticide may be altered within the G.I. tract due to changes in pH in the stomach and intestine, or due to enzymatic action within the gut before it is absorbed into the lacteals and eventually into the hepatic portal system or lymphatic system. 

As patients lose exocrine function of the pancreas, they have decreased ability to absorb lipids and protein ingested with normal dietary intake. Weight loss from nutritional malabsorption is a common symptom of chronic pancreatitis not often seen in acute pancreatitis. Fatty- or protein-containing stools are also common carbohydrate absorption is usually unaffected. Even though patients with chronic pancreatitis have decreased ability to absorb lipid from the gastrointestinal tract, there does not appear to be an increased incidence of fat-soluble vitamin deficiency in these patients.34... 

The gastrointestinal tract (GIT) is a highly specialized region of the body whose primary functions involve the processes of secretion, digestion, and absorption. Since all nutrients needed by the body, with the exception of oxygen, must first be ingested orally, processed by the GIT, and then made available for absorption into the bloodstream, the GIT represents a significant barrier and interface with the environment. The primary defense mechanisms employed by the gut... 

The important stages in delivering a drug to its desired target after an oral dose can be summarized as shown in Fig. 6.2. Initially the formulation has to be swallowed and survive the transition to the site of absorption - the gastrointestinal tract (GIT). The time required for this to happen will depend on the stomach emptying time, which in turn will be a function of the fed/fasted state of the subject or animal that is being studied (see for example Ref. [7]). This kind of information can only be obtained from in vivo studies. 

Fig. 17. Biological model recommended for describing the uptake and retention of cerium by humans after inhalation or ingestion. Numbers in parentheses give the fractions of the material in the originating compartments which are cleared to the indicated sites of deposition. Clearance from the pulmonary region results from competition between mechanical clearances to the lymph nodes and gastrointestinal tract and absorption of soluble material into the systemic circulation. The fractions included in parentheses by the pulmonary compartment indicate the distribution of material subject to the two clearance rates however, these amounts will not be cleared in this manner if the material is previously absorbed into blood. Transfer rate constants or functions, S(t), are given in fractions per unit time. Dashed lines indicate clearance pathways which exist but occur at such slow rates as to be considered insignificant compared to radioactive decay of the cerium isotopes.

The pharmacokinetics of nalidixic and hydroxy-nalidixic acids have been studied by several different groups. Takasugi ejt al studied in-situ and in-vitro absorption of nalidixic acid from the gastrointestinal tracts of rats as a function of pH. They reported that the absorption of non-ionized nalidixic acid was faster than the ionized form, with the maximum absorption rate constant found when the drug was administered from a pH=3 buffer solution. The absorption in-sltu was found to be ten times the rate in-vitro, but this was dependent on several factors.(13)... 

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