1) Digestive System Digestion is the process of breaking down foods and nutrients that you eat into molecules that the body can actually use. Digestion begins at the mouth and ends at the anus.The it of the digestive system is the gastrointestinal tract, where digestion takes place. Digestion is usually divided into mechanical digestion and chemical digestion. In the mechanical phase, the body physically breaks down chunks of food into smaller particles. Mechanical digestion increases the surface area on which enzymes can act. The mechanical phase of digestion begins when you take a bite of food. After you ground up your food into a bolus, chemical digestion begins. Chemical digestion involves a change in the chemical nature of the nutrients. Salivary glands produce saliva, which is a mixture of water, mucus, and the digestive enzyme salivary amylase. This salivary amylase begins the chemical digestion of carbohydrates by breaking down some starch into the disaccharide maltose. Instead of entering the trachea, the epiglottis forces the bolus from the pharynx to the esophagus. The extremely effective process of peristalsis pushes the bolus through the esophagus and into the stomach. The stomach is located just below the diaphragm in the upper left side of the abdominal cavity and is present in both mechanical and chemical digestion. When food is present, the three layers/types of muscle that form the stomach work together to churn the contents of the stomach. This churning helps the stomach carry out mechanical digestion. The inner mucous membrane of the stomach is scattered with gastric pits, which are the open ends of gastric glands that release secretions into the stomach. Some of the cells in gastric glands secrete mucus, some secrete digestive enzymes, and others secrete hydrochloric acid. This mixture of these secretions forms the acidic digestive fluid. Gastric fluid carries out chemical digestion of proteins that starts in the stomach when pepsin splits complex protein molecules into shorter chains of amino acids called peptides. Hydrochloric acid kills bacteria and dissolves minerals that enter the stomach with food. Food usually remains in the stomach for three to four hours. During this time, muscle contractions in the stomach churn the contents, breaking up food particles and mixing them with gastric fluid. This process forms a mixture called chyme. Between the stomach and the small intestine is the pyloric sphincter, which regulates the amount of chyme that peristalsis forces out of the stomach and into the small intestine. When chyme is present in the small intestine, the gallbladder releases bile made in the liver through the common bile duct in the small intestine. By neutralizing stomach acid, the pancreas protects the interior of the small intestine. The chemical digestion of chyme continues as secretions from the liver and pancreas enter the duodenum. This triggers intestinal mucous glands to release large quantities of mucus, which protects the intestinal wall from protein - digesting enzymes and the acidic chyme. Glands in the lining of the small intestine release enzymes that complete digestion by breaking down peptides into amino acids, disaccharides into monosaccharides, and fats into glycerol and fatty acids. During absorption, the end products of digestion are transferred into the circulatory system through blood and lymph vessels in the lining of the small intestine. Nutrients are absorbed through this surface through diffusion and active transport. Peristalsis then moves the remaining material to the large intestine after absorption is completed. The large intestine (colon) is the final organ of digestion. Slow contractions move material in the large intestine toward the rectum. As the remnants move through the large intestine, the absorption of water from the small intestine solidifies the mass into feces. As the fecal matter solidifies, cells lubricate the intestinal wall, making the