How are the small intestines in mammals adapted to their functions?

Small intestines consists of the duodenum and the ileum, - most digestion of food occurs in the duodenum. Bile from the gall bladder of the liver is secreted through the bile ducts and it is used to emulsify fats/break fat particles into tiny droplets to increase the surface area for enzyme action.

The pancreas secretes pancreatic juice to the duodenum, the juice contains pancreatic amylase that helps to breakdown the remaining starch into maltose, trypsin (that is secreted in its inactive form, trypsinogen, and activated by enterokinase enzyme) hydrolyses proteins into shorter peptides, pancreatic lipase converts lipids into fatty acids and glycerol.

Sodium hydrogen carbonate is also produced to neutralize the acidic chyme from the stomach and provide a suitable alkaline medium for pancreatic and other intestinal enzymes. The ileum is long and narrow to increase the surface area for complete digestion of food and maximum absorption of digested food - highly-coiled to reduce speed of food flow for maximum digestion and absorption.

There is the presence of villi and microvilli to increase surface area for maximum absorption, dense network of capillaries to transport blood for efficient transport of absorbed food.

Also there is the presence of lacteals in the villi for absorption of fatty acids and glycerol molecules, presence of enzymes: Lipase for digestion of lipids into fatty acids and glycerol, maltase for digestion of maltose to glucose molecules peptidase for breakdown of peptides into amino acids, sucrase for digestion of sucrose into glucose and fructose, lactase for digestion of lactose into glucose and galactose.

Goblet cells produce mucus to lubricate the walls of the ileum for smooth flow of food and coats the walls of ileum to prevent digestion by peptidase enzyme.