Gastrointestinal System Review

The Digestive System Ingestion, Digestion, Absorption Digestive Structure

Mucosa or the mucous membrane layer is the most innermost of the wall. It lines the lumen of the tract. In certain regions the mucosa secretes mucus, digestive enzymes and hormones. Submucosa- is a thick layer of loose connective tissue that surrounds the mucosa. This layer also contains blood and lymphatic vessels, nerves and some glands. Muscularis- consists of two layers of smooth muscle and is responsible for peristalsis in the lower portion of the esophagus Serosa- a squamous (dense connective tissue) epithelial layer that is the outermost covering of the digestive tube that secretes a serous fluid for lubrication so that the abdominal organs move smoothly against each other to prevent friction I. Ingestion

The oral cavity is the first part of the digestive tract. It is adapted to receive food by ingestion, which be broken down into small particles and mixed with saliva to forma bolus. The oral cavity has salivary glands which secrete several things which help in digestion: 1. Amylase: this functions to start the digestion of carbohydrates in the mouth 2. Saline: moisten pharyngel tissue 3. Mucous: provide lubrication for food, helps form the bolus II. Digestion After the bolus has been formed, it travels down the esophagus, via peristalsis, into the stomach. The stomach consists of four different sections: 1. Cardium 2. Fundus 3. Body 4. Pyloric Each section contains a gastric pit which contains parietal cells, chief cells, and gastric glands which secrete things which are needed for digestion. Parietal cells secrete HCl and intrinsic factor, which is needed to absorb vitamin B12. Chief cells secrete pepsinogen, which will then be transformed into pepsin, when pepsinogen combines with HCl. When the food mixes with the HCl/pepsin, chyme is formed. The chyme is moved into the small intestine. There, secretions from the pancreas, liver, and gall bladder enter the small intestine via the pancreatic duct. The pancreas is an accessory organ which secretes alpha (glucagon) and beta (insulin) cells. The pancreas is responsible for controlling blood glucose levels; when levels are high, the pancreas releases insulin, which takes glucose and converts it into glycogen, however when levels are low, the pancreas releases glucagon which converts glucagon back into glucose which the body can then use. The pancreas also releases bicarbonate, to neutralize the HCl, trypsin, and chymotrypsin. The liver and bile ducts release bile salts, which act like emulsifying agents that attach to fats and make absorbing them easier. The small intestine is made up of three parts; The beginning or duodenum, the middle, jejunum, and the end portion, ileum. Most digestion takes place in the duodenum, and absorption occurs throughout the rest of the intestine. Specialized cells called villi are present, which increase the surface area for absorption. Carbohydrates are broken down into monosaccharides by amylase and through facilitated transport are absorbed. Proteins are broken down into amino acids by trypsin and chymotrypsin and through active transport are absorbed. Fats are broken down into monoglycerides with the help of the bile salts and enter the lymphatic duct to be absorbed.

The large intestine is the final part of the digestive system in which water and some minerals are absorbed. The first part of the colon, the cecum contains the largest number of bacteria and therefore is responsible for the production of vitamins, such as B12 and K. Control of Digestion

Essential Nutrients Vitamins: A (retinol)- used in making visual pigments; green vegetables, milk B1- coenzyme during cell respiration B2 (riboflavin)- part of coenzyme which play metabolic roles B3 (niacin)- part of coenzymes NAD, NADP B5 (pantothenic acid) B6- metabolism B12-metabolism Biotin-metabolism Folic acid- metabolism C- collagen, important antioxidant D- increases absorption of calcium E- protects cell membranes from oxidation, antioxidant K- essential to blood clotting