Pharmacokinetics of Oral Drug Absorption
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Physiological considerations.
The GI tract is that part of the body devoted to the transformation of exogenous substances (food) into substances that can be absorbed into blood and became source of energy for the body or maintain the good function of it (e.g., vitamins). The GI accomplish this task by mechanically breaking down the food in the mouth, by the secretion of enzymes and proteins to accomplish the process known as digestion, and finally to absorb them into blood. All the blood flowing in the GI (except for that flowing in the lower part of the rectum) passes through the liver, via the portal vein, before reaching the systemic circulation. The liver has the purpose to quickly recognize and destroy possible toxic substances. That's why many drugs are extensively metabolized in the liver and only a small percent of the dose administered reaches the systemic circulation.
The total transit time ranges from 0.4 to 5 days...