Initial protein digestion takes place in the stomach. Chief cells are responsible for releasing pepsinogen into the stomach, an inactive enzyme (zymogen) that must be cleaved in order to form pepsin. The activation of this enzyme is accomplished by the release of hydrochloric acid into the stomach by parietal cells.

Goblet cells release mucus into the stomach to protect the epithelium from the acidic environment. G cells release gastrin, which stimulates parietal cells to release acid.

All pancreatic enzymes are released as inactive enzymes, or proenzymes, into the small intestine. Enterokinase is an enzyme that activates trypsin in the small intestine. Once activated, trypsin is able to activate the rest of the pancreatic enzymes. Trypsin activates chymotrypsin to digest proteins and pancreatic amylase to digest carbohydrates. Ribonuclease is used to break down old RNA in the cytoplasm of cells, and is not a pancreatic digestive enzyme.

The large intestine, or colon, has the major function of water and electrolyte reabsorption. If this function is inhibited, by a pathogen for example, the body cannot retain water and becomes dehydrated.

Enzyme secretion occurs in the duodenum of the small intestine, where digestive enzymes from the pancreas are released. Protein digestion primarily occurs in the small intestine, but begins in the stomach. The stomach and mouth are primarily responsible for mechanical digestion.

Blood that is rich in nutrients from digestion is collected in various mesenteric veins and then passes through the portal vein directly into the liver, where the nutrients are metabolized. The portal vein supplies about two thirds of the blood to the liver. The hepatic artery supplies the other third. The cisterna chyli collects lymph, not blood, and the femoral vein is associated with the leg.

Critical to proper digestion, bile and pancreatic juices must enter the intestine rather early so that they have ample time to digest foods passing by. After preliminary protein digestion in the stomach, food passes through the pyloric sphincter into the duodenum. The pyloric sphincter simply separates the stomach from the small intestine. The duodenum serves as the main region of chemical digestion in the body. It is in this region that bile and pancreatic enzymes are introduced. Food undergoing digestion subsequently passes through the jejunum, the ileum and the colon, or large intestine. These regions are mostly used for absorption of nutrients and water, rather than chemical digestion.

In the digestive system, food is broken apart mechanically (by the teeth), then chemically (by the stomach). Food then travels to the intestines, where nutrients are absorbed and water is removed from the gut tube, leaving only the waste. The waste is then eliminated from the body. Production of gametes is the function of the reproductive system, and does not take place in the digestive system.

The small intestine has three parts: the duodenum, jejunum and ileum. Note that the ilium is one of the three bones that makes up the pelvis. The small intestine is the primary site of nutrient absorption. Whatever is not absorbed by the small intestine will then be absorbed my the large intestine (mainly water and vitamins). The stomach produces hydrochloric acid, which, like all acids, decrease the pH of their environment below 7.