The mouth, stomach, and small intestine all contain enzymes that help chemically digest food. The mouth contains salivary amylase to digest carbohydrates. The stomach contains pepsin to digest proteins. The small intestine contains numerous enzymes from pancreatic secretions that digest carbohydrates, fats, and proteins.

The esophagus serves to transport food from the mouth to the stomach via smooth muscle contractions. No chemical digestion takes place in the esophagus, and no digestive enzymes are secreted into this region.

Peristalsis is the rhythmic contraction of smooth muscle to propel digested contents through the digestive tract. It is most commonly examined in the esophagus, as the means by which a bolus is transported from the mouth to the stomach.

Segmentation contractions occur in the small intestine as a means of mixing chyme with digestive enzymes, but do not result in propulsion. Churning movements are most common in the stomach, where food particulates are mechanically digested and exposed to hydrochloric acid. Mastication refers to the mechanical digestion of food via crushing and tearing, primarily achieved when food is chewed in the mouth.

Cholecystokinin causes the gall bladder to contract so that it can releae bile into the small intestine.

Secretin is released in response to acid in the small intestine, and causes a release of pancreatic bicarbonate to neutralize the acid. Pepsin, trypsin, and carboxypeptidase are all involved in chemical digestion of proteins.

15% of protein digestion occurs in the stomach and pepsin is the enzyme responsible. Remember, pepsin has an optimum pH of about 2.0, which is the pH of the stomach. Trypsin, another protease, has an optimum pH of 7.5-8.5, and therefore would not function in the stomach. Rather, it is produced by the pancreas and carries out its enzymatic reaction in the small intestine. Amylase breaks down carbohydrates. Trypsinogen is the inactive form of trypsin, thus it does not directly break down proteins.

The pancreas is responsible for secreting digestive enzymes into the small intestine. These enzymes are responsible for digesting lipids (lipase), proteins (trypsin), nucleic acids (nucleases), and starches (amylase). In addition, the pancreas secretes bicarbonate to help neutralize the acids coming from the stomach. Pepsin's zymogen, pepsinogen is released by chief cells in the stomach, and gets activated by the acid that is also produced by the stomach (by parietal cells).

Amylase is secreted in active form. Pepsin and trypsin are first secreted as zymogens, inactive precursors that must be activated in order to function. Zymogens are produced with peptides blocking their active sites that must first be cleaved to function. Angiotensin is also secreted as a zymogen, angiotensinogen. Zymogens are useful in allowing the body to quickly activate an enzyme/hormone rather than synthesizing it from scratch.

The body compartmentalizes each hormone to help ensure it is it its optimum pH. Try to remember where each enzyme functions to help remember each enzyme's optimum pH. Pancreatic amylase is secreted by the pancreas into the small intestine. Amylase also occurs in the mouth, which is neutral. It's optimum is around 7. Pancreatic lipase is secreted by the pancreas into the small intestine, which is slightly basic because of the release of bicarbonate. Pancreatic lipase functions best at a pH of about 8. Trypsin is also secreted by the pancreas into the small intestine, and has an optimal pH of about 8. Pepsin functions in the stomach, which is extremely acidic. Pepsin's optimum pH is about 2.

Just after eating, the body is busily converting carbon-based life forms to glucose. If the body is about to have an influx of glucose, the the body must produce more insulin to allow for the passage of glucose into its cells. Glycogen, the stored form of glucose, would naturally increase since the person just replenished lost glucose or energy with food and will store it for later use. Glucagon would decrease because this breaks down glycogen. If the body is receiving fresh glucose, it doesn't need to break down its stores of glycogen into glucose. And finally, if the body is breaking down food into glucose, the liver doesn't need to release more freshly broken down (glucose that was previously stored as glycogen) glucose.

Salivary amylase is produced by the salivary glands and functions to hydrolyze starch to maltose. Pepsin is produced by the stomach to hydrolyze peptide bonds. Lipase is produced by the pancreas to hydrolyze lipids. Lactase is produced in intestinal glands to hydrolyze lactose to glucose and galactose. 

Vitamin B12 absorption requires intrinsic factor (IF), which is a protein secreted by the parietal cells of the stomach. When vitamin B12 binds to IF, it is able to be absorbed by the body in the terminal ileum. Therefore, removal of the ileum would likely result in a vitamin B12 deficiency, since the body cannot absorb the B12-IF complex.