ABSTRACT
The mid-sections of the gut in humans have evolved active transport systems for optimal extraction of nutrients from a complex and varied diet. However, not all compounds which are presented to the gut wall are desirable for absorption. Ingested food may contain small amounts of components which are harmful, for example, plant alkaloids and complex phenolic compounds which arise during cooking. These compounds are not absorbed by active mechanisms but enter the bloodstream by dissolving in the membrane lipid (transcellular transport) or by opportunistic entry between the enterocyte gaps (paracellular transport). They would accumulate in tissues but for mechanisms which act to reduce concentrations by microsomal oxidation, conjugation and secretion. This is a major function of the liver. In the case of drugs, which obviously possess no nutritional benefit, the offending material is present in a great excess and Fickian diffusive processes usually win against secretory and metabolic activity which attempts to eliminate the substance. The overall effectiveness of this process depends on the concentration of the drug, blood flow and the efficiency of intestinal wall metabolism and hepatic extraction. The rate of presentation of drug to the systemic circulation is a function of the rate of gastric emptying, because the small intestine is, par excellence, the organ of absorption.
