Final answer:
The main function of the small intestine is the absorption of nutrients through the intestinal villi, with most absorption of carbohydrates and proteins occurring in the jejunum, and bile salts and vitamin B12 in the ileum.
Step-by-step explanation:
One of the main functions of the small intestine is absorption of nutrients. This intricate process involves breaking down food into molecules small enough to be absorbed by the epithelial cells of the intestinal villi. The small intestine is exceptionally efficient, absorbing almost all ingested food, 80 percent of electrolytes, and 90 percent of water. The majority of carbohydrates and proteins are absorbed in the jejunum, while specific nutrients, like bile salts and vitamin B12, are absorbed in the terminal ileum.