Digestion in the mouth involves the mechanical chewing of food and the action of salivary amylase, which breaks down starch. Proteins do not start getting digested until they are in the stomach, where pepsin breaks them down. Fats and carbohydrates are broken down in the small intestne by lipases and other enzymes like lactase.

A layer of mucous forms between the epithelial cells of the stomach and the acid within the stomach. This mucous is secreted by mucous cells lining the stomach. When the mucous layer is broken down, certain complications can take place (e.g. stomach ulcers).

D cells in the antrum of the stomach are neuroendocrine cells that secrete somatostatin, a neuropeptide that increases stomach pH by decreasing gastric acid secretion from fundal parietal cells. In a negative feedback mechanism, when the stomach pH falls too low, hydrogen ions (H⁺) will stimulate D cells to secrete somatostatin. This somatostatin blocks histamine release from enterochromaffin-like (ECL) cells in the stomach, preventing the stimulation of parietal cells to release hydrochloric acid.

Histamine is released by ECL cells, stimulating acid secretion from parietal cells. Gastrin is released by G cells, also stimulating acid secretion from parietal cells. Ghrelin is a neurohormone that stimulates hunger.

Iron, usually ingested in the Fe³⁺ form (ferric iron), must be reduced to Fe²⁺ (ferrous form) in order to be absorbed in the duodenum of the small intestine. The iron undergoes an oxidation-reduction reaction in the stomach, facilitated by the low pH of the stomach lumen.

The pyloric sphincter, between the antrum of the stomach and the duodenum of the small intestine, regulates the passage of chyme from the stomach to the small intestine. The antrum has rhythmic contractions that force chyme against the pyloric sphincter, allowing approximately five milliliters to flow through every contraction; thus, the pyloric sphincter allows for the gradual but continual digestion of gastric contents.

The upper and lower esophageal sphincters are involved in the process of swallowing, while the ileocolic sphincter joins the small intestine and large intestine.

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.