The gastrointestinal system processes food to provide fuel for the body’s functions. During digestion, the body extracts necessary nutrients and biological fuels and isolates waste to be discarded.
The breakdown of food into its constituent parts begins as soon as it is put into the mouth. Enzymes in saliva (such as salivary amylase) begin breaking down food, particularly carbohydrates, as chewing helps prepare food for swallowing. The chewed food is formed into a bolus that travels down the esophagus, aided by a process called peristalsis, rhythmic contractions that move the partially digested food towards the stomach. Upon reaching the stomach, food encounters a powerful acid (composed mainly of hydrochloric acid), which aids the breakdown of food into its absorbable components.
The burning sensation called heartburn occurs when gastric acid from the stomach travels up the esophagus, often as a result of relaxation of the lower esophageal sphincter. This acid can damage the lining of the esophagus.
The stomach produces a semifluid mass of partially digested food called chyme that passes into the small intestine, where nutrients are absorbed into the bloodstream. This absorption occurs through millions of finger-like projections known as villi that increase the surface area available for the absorption of nutrients.
The small intestine itself has three major segments. Proximal to the stomach is the duodenum, which combines digestive substances from the liver and pancreas; next is the jejunum, the primary site of nutrient absorption; finally, the ileum absorbs remaining nutrients and moves the remaining matter into the large intestine. The large intestine (also called the colon) absorbs water from the waste, which then passes into the rectum and out of the body through the anus.
The digestive system also includes accessory organs that aid in digestion:
Common gastrointestinal disorders include infections, autoimmune disorders, and liver disease.