A low pH in the stomach is necessary in order to activate pepsinogen, the zymogen form of pepsin. Pepsin is necessary for breaking down proteins into smaller polypeptides. In a stomach with a higher pH (less acidic), pepsin production will be decreased, and proteins will not be digested as well.

The lining of the stomach secretes hydrochloric acid, pepsinogen, and intrinsic factor. Intrinsic factor binds to vitamin B and allows for its absorption. Hydrochloric acid cleaves the zymogen pepsinogen to create the active enzyme pepsin, which helps with the preliminary chemical digestion of proteins.

The contraction of the stomach causes the breakdown of food. Only food that has been broken down into small boluses can pass through the pyloric sphincter and enter the small intestine. The stomach is thus responsible for the mechanical modification of food, breaking it into small enough particles that it can pass to the next segment of the digestive tract. The majority of chemical digestion and absorption then occurs in the small intestine.

The patient should be most concerned with nutrient absorption. Microvilli provide additional surface area to the small intestine, allowing for nutrient absorption. If the lining of the intestine were completely smooth, nutrients would pass too quickly through the digestive system. Enzyme secretion is primarily provided by the liver and pancreas.

The small intestine is the location of where the majority of enzymatic digestion occurs because enzymes from the pancreas are incorporated into the digestive process at this point. It is important to begin the enzymatic breakdown of food as early as possible to maximize nutrient absorption through the intestinal system.

The large intestine is responsible for water absorption and waste removal. The mouth begins enzymatic digestion with salivary amylase, and the stomach introduces pepsin, but these do not constitute the majority of enzymatic activity in the digestive tract.

The duodenum is the first part of the small intestine. This is where the acidic chyme from the stomach mixes with pancreatic secretions and bile. The pancreatic secretions and bile contain bicarbonate, which helps to neutralize the acidic chyme. 

The small intestine is before the large intestine. It consists of the duodenum, followed by the jejunum, then ileum. The large intestine comes directly after the small intestine and constists of the cecum, followed by the colon, then rectum.

Microvilli surround villa which line the small intestine. These structures greatly increase the surface area (and therefore nutrient absorption capability) of the small intestine.

Microvilli increase the surface area in the small intestine, and thus allow a greater amount of absorption of nutrients once digestion occurs. They do not actually participate in digestion, nor are they linked to intestinal cancer.

The majority of digestion in the small intestine (and the body in general) occurs in the duodenum, the first section of the small intestine, after passing through the stomach. The digestive enzymes secreted by the pancreas enter the digestive tract in the duodenum.

The next two sites, the jejunum and ileum, are sites of absorption. The cecum is part of the large intestine, not the small intestine.  

Calcium is absorbed in the jejunum of the small intestine. Iron is absorbed in the duodenum, calcium in the jejunum, and vitamin B12 (complexed with intrinsic factor) is absorbed in the ileum.