The human body contains a monogastric digestive system. In a monogastric digestive system, the stomach only contains one chamber in which it digests food. Several other animal species also have monogastric digestive systems, including pigs, horses, dogs, and birds. This chapter, however, focuses on the human digestive system.

Saliva and Diagnostics

Saliva is a watery substance secreted by the salivary glands into the mouth. Human saliva contains 99.5% water with electrolytes, mucus, white blood cells, epithelial cells, enzymes, and antimicrobial agents. The enzymes found in saliva are essential in beginning the process of digestion. They also play a role in breaking down food particles trapped around the teeth, protecting them from decay.

Saliva is obtained easily, inexpensively, and non-invasively from patients which spurs research interest. Ongoing research identified novel ways of using saliva in molecular diagnostics. DNA, RNA, and proteins found in saliva serve as useful sources of diagnostic information in the early detection of various cancers including oral, pancreatic, and gastric cancer.

Gastric Acid

The primary component of gastric acid is hydrochloric acid. Hydrogen and chloride ions released by parietal cells lining the stomach react in the stomach cavity to form hydrochloric acid. Parietal cells are coupled to feedback systems that increase and decrease acid production as needed. To prevent the acidic environment from damaging the cells lining the stomach, other cells in the stomach produce mucus, which lubricates the stomach lining and contains bicarbonate that neutralizes the acid.

The Gut Microbiome and Fecal Microbiota Transplant

Immunocompromised patients are at an elevated risk for developing Clostridium difficile infection (CDI). CDI, alarmingly, has increased to epidemic proportions in North America over the past decade. There has been increasing interest in understanding the role of the human gut microbiome in order to counter this epidemic. One potential therapy is fecal microbiota transplant (FMT), or stool transplant. FMT is the process in which fecal bacteria from an individual with a healthy gut microbiome are transferred to a recipient to increase their microbial diversity. Some studies have shown that FMT is an effective treatment for patients with CDI. FMT has also been used experimentally in the treatment of other gastrointestinal diseases such as inflammatory bowel disease.