Digestive System

The human gastrointestinal tract is the stomach and intestine, sometimes including all the structures from the mouth to the anus. (The "digestive system" is a broader term that includes other structures, including the accessory organs of digestion). In an adult male human, the gastrointestinal (GI) tract is 5 meters (20 ft) long in a live subject, or up to 9 meters (30 ft) without the effect of muscle tone, and consists of the upper and lower GI tracts. The tract may also be divided into foregut, midgut, and hindgut, reflecting the embryological origin of each segment of the tract. The GI tract always releases hormones to help regulate the digestion process. These hormones, including gastrin, secretin, cholecystokinin, and grehlin, are mediated through either intracrine or autocrine mechanisms, indicating that the cells releasing these hormones are conserved structures throughout evolution. Upper gastrointestinal tract: The upper gastrointestinal tract consists of the esophagus, stomach, and duodenum. The exact demarcation between "upper" and "lower" can vary. Upon gross dissection, the duodenum may appear to be a unified organ, but it is often divided into two parts based upon function, arterial supply, or embryology. Lower gastrointestinal tract: The lower gastrointestinal tract includes most of the small intestine and all of the large intestine. According to some sources, it also includes the anus. Bowel or Intestine: *Small Intestine: Has three parts: -Duodenum: Here the digestive juices from the pancreas (digestive enzymes) and the gallbladder (bile) mix. The digestive enzymes break down proteins and bile and emulsify fats into micelles. The duodenum contains Brunner's glands which produce bicarbonate. In combination with bicarbonate from pancreatic juice, this neutralizes HCl of the stomach. -Jejunum: This is the midsection of the intestine, connecting the duodenum to the ileum. It contains the plicae circulares, and villi to increase the surface area of that part of the GI Tract. Products of digestion (sugars, amino acids, fatty acids) are absorbed into the bloodstream. -Ileum: It has villi and absorbs mainly Vitamin B12 and bile acids, as well as any other remaining nutrients. *Large Intestine: Has three parts: -Cecum: The Vermiform appendix is attached to the Cecum. -Colon: Includes the Ascending Colon, Transverse Colon, Descending Colon and Sigmoid Flexure: The main function of the Colon is to absorb water, but it also contains bacteria that produce beneficial vitamins like Vitamin K. -Rectum *Anus