The areas of the stomach, from superior to inferior, are the fundus, the body, and the pyloric antrum. The rugae are folds in the stomach that increase its surface area and expandability. 

The ligament of Treitz suspends the duodenum, while the splenorenal ligament goes between the left kidney and the spleen. Ligamentum flava are ligaments present in the spinal cord.

Amylase enzymes, produced by the salivary gland and the pancreas, break starched down into simple sugars. Peptidase and protease enzymes break down proteins, while lipase enzymes break down lipids.

Bile acids, produced in the liver and stored in the gallbladder, help emulsify fats during digestion. The process of emulsification makes fats collect into small droplets, increasing the surface area of ingested lipids and increasing the ability of pancreatic lipase to break fats down into fatty acids and glycerol. Hydrochloric acid aids in the breakdown of proteins in the stomach, and cholecystokinin is a hormone with a wide variety of functions in the digestive process, including the stimulation of the release of bile (though it does itself not play a role in emulsification of fats).

Normal gastric pH ranges from 1.5 to 3.5. This highly acidic environment aids the stomach in denaturing proteins, a fundamental part of digestion. 

While a small amount of fat digestion may occur in the mouth via the action of lingual lipase, the majority of lipid digestions begins in the duodenum. The presence of fat in the small intestine triggers release of pancreatic lipase, which enters the duodenum via the pancreatic duct. Fat digestion does not occur in the stomach.

The pancreatic duct and common bile duct come together at the ampulla of vater, allowing for the combination of pancreatic enzymes and bile. This combination is released into the duodenum via the relaxation of the sphincter of Oddi, a muscular ring at the end of the ampulla. The sphincter of Oddi also prevents any back-flow of duodenal contents into the ampulla.

The pyloric sphincter controls flow of chyme between the stomach and the duodenum, while the ileocecal valve separates the small and large intestine. 

Lipids are transported out of the digestive system via lacteals, lymphatic capillaries within the vilii of the small intestine that absorb fatty acids in the form of chylomicrons. They are then transported through the lymphatic system and re-enter the blood stream via the thoracic duct.

The hepatic system produces bile, a vital part of lipid digestion, but it does not absorb lipids. Nor does the excretory system, which is not really a single system but rather a collection of organ systems including the kidney, skin, liver, and colon.

Gastrin is produced by the G cells of the stomach. Parietal cells of the stomach secrete hydrochloric acid, while chief cells secrete pepsinogen and chymosin. Beta-islet cells of the pancreas produce glucagon. 

The hydrolysis of oligosaccharides into disaccharides occurs in the duodenum, after the addition of pancreatic amylase from the pancreatic duct.