ABSTRACT
The gastrointestinal (G.I.) tract comprises the oral cavity, esophagus, stomach, small intestine (duodenum, jejunum, ileum) and large intestine (colon and rectum). Accessory organs such as the pancreas, liver and gallbladder play an important role in the digestive process. The process of digestion begins in the mouth with the mechanical grinding of food by the teeth. Saliva produced by the salivary glands lubricates the food and contains
amylase
enzymes that begin the digestion of starches. The chewed food or
chyme
now passes down the esophagus and enters the stomach. In the stomach, the chyme is further broken down by highly acidic gastric juice produced by the stomach lining. The enzyme
pepsin
is released by cells in the stomach lining to begin digestion of proteins. Muscular contractions of the stomach wall also serve to further grind the chyme into a smooth liquid. The chyme is now driven into the small intestine by contraction of the stomach. In the small intestine, further digestion of proteins, sugars and nucleotides/nucleosides will occur via intestinal and pancreatic enzymes. The small intestine is also where the majority of nutrient absorption occurs. By the time the remaining chyme has reached the large intestine, most of the available nutrients have been absorbed from it. The large intestine functions mainly in the absorption of water as well as the synthesis of vitamin K and certain B vitamins.
