The esophagus is a muscular tube about 25 cm (10 inches) long. It is the passageway between the pharynx and the stomach. By means of waves of muscular contractions (peristalsis), food is pushed along this tube to the stomach. When peristalsis is reversed, vomiting occurs.
The stomach is a saccular enlargement of the gastrointestinal tube, connecting the lower end of the esophagus and the first portion of the small intestine (duodenum). It lies in the left upper quadrant of the abdomen. Muscular rings, or sphincters, at each end of the stomach form valves to close off the stomach and to prevent its contents from escaping in either direction while they are being mixed by peristaltic muscular contractions of the stomach wall. The sphincter at the esophageal end is the cardiac sphincter; at the duodenal end it is the pyloric sphincter.
The stomach acts as an initial storehouse for swallowed material and helps in the chemical breakdown of food substances. Small glands in the wall of the stomach secrete gastric juice, the principal components of which are hydrochloric acid and pepsinogen. Hydrochloric acid activates pepsin from pepsinogen, kills bacteria that enter the stomach, inhibits the digestive action of ptyalin, and helps regulate the opening and closing of the pyloric sphincter. The action of pepsin is confined to protein, which it splits. The stomach is half-empty within 1 hour of a normal meal and completely empty in 6 hours.
Most food absorption takes place in the small intestine. There is little food absorption in the stomach. One exception is alcohol, which is absorbed directly through the stomach wall. For this reason intoxication happens quickly when alcohol is taken on an empty stomach.
The stomach and intestines are enclosed in the abdominal cavity, the space between the diaphragm and the pelvis. This cavity is lined with serous membrane, the PERITONEUM. The peritoneum covers the intestines and the organs and, by secreting a serous fluid, prevents friction between adjacent organs. The MESENTERY (double folds of peritoneum) extends from the cavity walls to the organs of the abdominal cavity, suspending them in position and carrying blood vessels to the organs.
The small intestine is a muscular, convoluted (coiled) tube, about 7 meters (23 feet) long and attached to the posterior abdominal wall by its mesentery. The mesentery is gathered together like a folding fan, permitting coiling of the intestine, allowing this long organ to be contained in a relatively small space.
The small intestine is divided into three continuous parts: the duodenum, jejunum, and ileum. It receives digestive juices from three accessory organs of digestion: the pancreas, liver, and gallbladder.
The DUODENUM is about 25 cm (10 inches) long and forms a C-shaped curve around the head of the pancreas, posterior to the liver. It is lined with a mucous membrane that contains small glands. These glands secrete intestinal juices containing the enzymes carbohydrate, peptidase, and lipase.
The JEJUNUM is the middle part of the small intestine and is about 2.5 meters (7.5 feet) long. Its enzymes continue the digestive process.
The ILEUM is the last and longest part of the small intestine. Most of the absorption of food occurs in the ileum where fingerlike projections (villi) provide a large absorption surface. After ingestion it takes 20 minutes to 2 hours for the first portion of the food to pass through the small intestine to the beginning of the large intestine.
The large intestine is so called because it is larger in diameter than the small intestine. It is considerably shorter, however, being about 1.5 meters (5 feet) long. It is divided into three distinct parts: the cecum, colon, and rectum.
The unabsorbed food or waste material passes through the CECUM into the COLON. The cecum is a pouch at the beginning of the large intestine, located in the lower right portion of the abdominal cavity. Twelve hours after the meal, most of the waste material passes through the colon slowly, building in mass and reaching the rectum 24 hours after the food is ingested.
The APPENDIX, a long narrow tube with a blind end, is an outpouching of the cecum located near the junction of the ileum and the cecum. It has no known function but frequently becomes infected and an inflammation known as appendicitis develops.
The RECTUM is 12.5 cm (5 inches) long and follows the contour of the sacrum and coccyx until