The small intestine is the primary site for which processes?

• A. Mechanical digestion only
• B. Enzymatic digestion and nutrient absorption
• C. Water reabsorption only
• D. Protein storage

Answer: (B)

The small intestine is where enzymatic digestion is completed and nutrients are absorbed. After chyme leaves the stomach, pancreatic enzymes and bile enter the duodenum to chemically digest carbohydrates, proteins, and fats, with brush-border enzymes on the intestinal lining finishing the job. The resulting nutrients—monosaccharides, amino acids, fatty acids, and monoglycerides—are absorbed across the highly folded lining equipped with villi and microvilli, which greatly increases the surface area for absorption. Fat absorption occurs with the help of bile to emulsify fats and the formation of chylomicrons that enter the lymphatic system. While there is some mixing and segmentation in the small intestine, mechanical digestion is primarily a function of earlier steps, and water reabsorption is mostly a large-intestine activity. There is no storage of proteins in the small intestine.