The stomach is a pear-shaped, muscular sac. Its main function is to store food. The stomach walls are made of three layers of muscle, each arranged on a different angle. As the food enters the stomach, muscle contractions begin to twist, turn, and churn the food, which is part of mechanical digestion. The stomach lining contains a great many glands, which produce a clear, yellow liquid that mixes with the food called gastric juice. Gastric juices are secreted into the stomach to help liquefy food and break it into simpler forms. Gastric juices contain water, enzymes, and acid. This acid softens fibers in the food and kills many of the bacteria. The juice also contains an enzyme that acts on proteins. Muscles in the stomach wall continue to contract and relax, and this mixes the food with the gastric juices. Gastric juices begin the digestion of proteins and fats and turn them into amino acids. Food will usually remain in the stomach for around four hours and while it is there, it becomes partially digested food that changes into a thick liquid called chyme. The sphincter at the end of the stomach allows this chyme to move into the small intestine a little bit at a time.
As food passes through the specialized compartments, it is gradually made ready for absorption into the blood. Most of this absorption occurs in the small intestine with the exception of drugs and alcohol, which are absorbed chiefly in the stomach. The small intestine is a long, coiled organ about one inch in diameter and about two centimeters across and seven meters long. The digesting food takes several hours to pass through it. The stomach connects with the duodenum, the first part of the small intestine. The duodenum is about a foot long and is shaped like the letter C. The major digestion of food occurs here.
While the food is in the small intestine, there are three juices that act upon it. One is the intestinal juice produced by the intestine lining; another is pancreatic juice that flows through a duct from an important gland lying near the stomach. This gland is the pancreas, which has a duct that enters the small intestine near its upper end. The pancreas is a long, soft gland lying behind the stomach, and it secretes digestive enzymes into the duodenum. The intestinal glands are tiny organs found in the lining of the small intestines. These tiny organs release digestive enzymes and mucus. There is also a juice called bile, which also enters the intestine at this same place and is produced by the liver.
In the small intestine are villi, very tiny fingerlike projections that line the walls. These many projections of villi increase the surface area of the small intestine allowing the small intestine to absorb most of the nutrients that enter the body. A fluid called lymph is located inside the villi blood vessels. Fat soluble vitamins and fatty acids are absorbed into the lymph system. These blood vessels also absorb glucose, amino acids, and water-soluble vitamins and minerals; then the blood and lymph carry the completely digested food throughout the body.
