CCK is a gastrointestinal hormone that acts to decrease gastric emptying when stimulated by the presence of fat and protein. This hormone also has a direct effect on the brain centers responsible for hunger to reduce further eating. The majority of other gastrointestinal hormones are stimulated by the presence of fat.

The correct pathway for food to travel through the gastrointestinal tract after ingestion is the following: esophagus, stomach, duodenum, jejunum, ileum, cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum.

The small intestine receives stomach contents and continues to digest them by using various enzymes, it also absorbs small nutrient molecules. The large intestine produces waste by removing water from undigested material. The liver produces bile for the gallbladder and also processes and stores nutrients. The gallbladder stores bile and assists in the breakdown of fats in the small intestine.

Substances from the small intestine enter the large intestine via the ileocecal valve; thus the first place chyme enters the large intestine is the cecum, which is on the lower right side of the abdominal cavity. The large intestine "outlines" the abdominal cavity; thus next is the ascending colon up towards the liver (on the right side of the abdominal cavity) where it turns toward the midline via the hepatic flexure. The transverse colon moves substances from right to left across the abdominal cavity until it reaches the area near the spleen (on the left side of the body), where it turns down via the splenic flexure into the descending colon. Then comes the sigmoid colon towards the rectum. The anus is not part of the large intestine.

As macromolecules are absorbed in the small intestine, polar molecules are able to float freely into the bloodstream due to their hydrophilic nature. Nonpolar molecules, such as fats and lipids, require a carrier protein in order to move throughout the body. Fats are carried through the lymphatic system by chylomicrons and are then dumped into the bloodstream through the thoracic duct.

Proteins, nucleic acids, and carbohydrates all contain polar regions that allow them to travel easily within aqueous environments.

The duodenum is responsible for the absorption of iron and calcium. Without it, individuals will be iron deficient (anemia) and hypocalcemia (soft bones and etc.). B9 and B12 are both absorbed in the small intestine.

The small intestine is composed of three sections: the duodenum, the jejunum, and the ileum. The duodenum is the first section, and experiences the majority of chemical digestion in the small intestine. The pancreatic duct empties into the section, carrying enzymes to facilitate digestion of carbohydrates, lipids, and fats as well as bicarbonate to neutralize stomach acid. These enzymes break down macromolecules in the duodenum. The jejunum and ileum are largely responsible for absorbing the nutrients digested in the duodenum. The cecum is a region of the large intestine, which is primarily responsible for water absorption. The pylorus is a region of the stomach, and connects the stomach to the small intestine.

Lactose is composed of a galactose monomer and a glucose monomer. The glycosidic bond is a 1-4 linkage.

Sucrose is composed of a glucose monomer and a fructose monomer. The glycosidic bond is an 1-2 linkage.

Maltose is composed of two glucose monomers joined by an 1-6 linkage.

Bile is necessary to emulsify fats. Proteases break down proteins, not fat. Fiber is broken down by bacteria in the large intestine. The stomach does not release bile. Note that the liver synthesizes bile, but it is stored in and released from the gallbladder.