"What Are the Characteristics of Digestion and Absorption in the Small Intestine?"

The small intestine serves as the primary location for digestion and absorption. Within its confines, food undergoes both chemical digestion by pancreatic juice, bile, and intestinal fluids, as well as mechanical digestion. Nutrients are gradually broken down into simple, absorbable small molecules that are then taken up by the small intestine. For instance, carbohydrates in food are decomposed into monosaccharides, while proteins are broken down into amino acids and oligopeptides before they can be absorbed by the small intestine. Dysfunction in the digestive and absorptive capabilities of the small intestine, coupled with the presence of organic lesions, can predispose individuals to acute abdominal conditions.

The sites of absorption for various components of food within the small intestine vary. Monosaccharides are primarily absorbed in the duodenum and the upper jejunum, with the duodenum exhibiting the fastest absorption rate. Protein and fat absorption predominantly occur in the upper section of the small intestine, where the absorptive cells demonstrate the greatest capacity for triglyceride synthesis. By the proximal ileum, fat absorption is complete. Similarly, the sites of absorption for various food components differ, with monosaccharides being primarily absorbed in the duodenum and upper jejunum, the duodenum being the fastest. Protein and fat absorption are also concentrated in the upper small intestine, where absorptive cells exhibit peak triglyceride synthesis capacity, ensuring complete fat absorption by the proximal ileum.

Water is absorbed passively, driven by the osmotic pressure gradient that arises as solutes are taken up into the intercellular spaces. The upper small intestine is the primary site for liquid absorption, with the rat jejunum absorbing sodium ions and water at a rate twice that of the ileum. The absorption of divalent ions such as iron and calcium occurs primarily in the duodenum and proximal jejunum. In contrast, vitamin B1 is mainly absorbed in the ileum.