The large intestine is responsible for the absorption of water and electrolytes. Most digestion and nutrient absorption occurs in the small intestine, leaving water and insoluble wastes for the large intestine. Once absorption takes place, the wastes become more solid and the body is able to retain water. Loss of this large intestine function can lead to severe dehydration.

Fat digestion is primarily a function of lipase and bile. Lipase helps cleave lipid molecules and bile helps emulsify particles for transport into the lymphatic system via lacteals.

The digestive system is responsible for the breakdown of food as it moves from the mouth, down through the stomach and the intestines. The food is mechanically broken down into small pieces in the mouth, and then further digested by the gastric secretions and contractions of the stomach. The small intestine is the site of chemical digestion due to the presence of pancreatic secretions. Once digested, the nutrients are absorbed in the jejunum and ileum of the small intestine. The large intestine is responsible for absorption of water and electrolytes, after which the remaining products are carried to the rectum for elimination.

If the water is not absorbed due to an infection or anatomical abnormality the feces will retain the water content, resulting in diarrhea.

The kidneys are responsible for the excretion of fluid wastes from the blood and help to maintain water balance and blood pressure. They are not involved in the digestive tract.

Amino acids, sugars, and fats are absorbed in the small intestine, but water is absorbed in the large intestine.

All enzymes released into the lumen of the small intestine by the pancreas are zymogens. Enterokinase is an enzyme that activates trypsin in the lumen. Once trypsin is activated, it is capable of activating every other enzyme found in the lumen of the small intestine, even other trypsin enzymes.

Bile is produced by the liver and released from the gall bladder into the small intestine. It has the function of emulsifying fat, increasing its surface area by forming micelles. This allows the protein lipase to break down and digest the fat in the small intestine much more quickly.

The pancreas secretes bicarbonate to neutralize the acidity of chyme and pancreatic amylase to aid in the digestion of carbohydrates.

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).