Digestive system

Abstract

In essence, the digestive system is a four-layered tube that extends from mouth to anus. Its main purpose is the enzymatic digestion of food to produce smaller molecules that can then be absorbed into the body as nutrients. To achieve this the gut is regionally specialized to enable the serial processing of food and the absorption of food, water, and electrolytes as materials pass along the bowel. The four layers of the bowel are: 1) A mucosa surrounding the lumen, made up of a specialized epithelium, a lamina propria of connective tissue, and a layer of smooth muscle—the muscularis mucosae. 2) A submucosa, a layer of connective tissue oft en containing glands. 3) A muscularis externa with, usually, an inner layer of circular smooth muscle and an outer layer of longitudinal smooth muscle, responsible for peristalsis. 4) An outermost layer of epithelia and connective tissue called the adventitia, or serosa if the bowel is enfolded by peritoneum or mesentery. Despite this common arrangement along the whole bowel, the four layers show characteristic differences in each region, reflecting the specialization of function found in the oesophagus, stomach, small intestine, and large intestine. Indeed, differences can also be seen between the subdivisions of these regions. Associated with the gut are two major organs, the liver and the pancreas. The liver processes the newly absorbed nutrients passed to it from the bowel by the hepatic portal vein. It also produces bile, which is eventually secreted into the bowel. Bile, stored and modified between meals in the gallbladder, is a vehicle for the removal from the body of conjugated bile pigments from the breakdown of haemoglobin. Bile also delivers to the small intestine the bile salts essential for the proper digestion of fats. The pancreas is divided into an exocrine pancreas, whose secretions of pro-enzymes, bicarbonate, and water pass to the small intestine to neutralize gastric acid and support digestion, and islets of endocrine tissue that produce insulin, glucagon and somatostatin — hormones concerned, in part, with glucose regulation. Control of bowel function is complex.