The digestive process in the small intestine is induced by free —mostly pancreatic—enzymes, at least for most nutrients. The final breakdown into the smallest molecules, ready for absorption, is achieved by membrane-bound enzymes at the microvilli (A). The active sites of these enzymes are oriented towards the lumen side, achieving the final complete breakdown of nutrient molecules as they migrate towards the enterocytes.
Once inside the cell, nutrients are transported either passively along a concentration gradient or actively. They undergo metabolic processes, which either transform them into transport forms or prepare them to be used by the enterocytes directly. After their passage into the blood capillaries of the intestinal tube, they are carried off through the mesenteric veins which drain into the portal vein. Through this so-called portal system, the nutrients are brought directly to the liver. There, they are further metabolized, preventing unnecessary burdening of the systemic circulatory system into which many of the metabolites will finally be released.
The mechanisms of lipid absorption differ from those used for carbohydrates and proteins (see p. 90). Inside the enterocytes, most of the lipids are integrated into chylomicrons and pass into the intestine's lymphatic system and the large veins in this form, entering the systemic circulatory system and bypassing the liver.
Active absorption mechanisms are not evenly available throughout the intestine. Individual nutrients have preferential absorption sites (C). Since the intestine is very flexible with regard to its absorptive capabilities, sufficient nutrient uptake is possible even after resection of large segments. The intestine is able to adapt to long-term, high-dosage administration of particular nutrients (e.g., lactose): enzyme patterns, as well as absorptive capacity, adjust to the altered nutrient intake.
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