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The Digestive System Part II Structures, Functions & Enzymes The Digestive System The digestive system is responsible for the breakdown of large, complex organic materials into smaller components that are used by the tissues of the body The 4 Stages of Digestion The 4 stages of digestion 1. Ingestion – taking nutrients into the body 2. Digestion – the breakdown of complex compounds into smaller components by enzymes 3. Absorption – the transport of digested nutrients to the tissues of the body 4. Egestion – the removal of food wastes from the body The Mouth • Digestion begins in the mouth. • Teeth helps by mechanically breaking down food into smaller pieces. • Salivary glands produce saliva which contains salivary amylase – an enzyme that breaks starch down into simpler sugars. The Epiglottis • To swallow food, the soft palate raises up to block off the passageway to the nasal passageway and • The epiglottis flips down to block the entrance to the trachea. The Esophagus • Once food leaves the mouth it is called a bolus. • It travels from the mouth to the stomach along the food tube known as the esophagus. • Peristalsis are wave-like contractions that push the bolus along the entire digestive tract. The Stomach • The stomach is the site of food storage and first protein digestion • The rugae (ridges) in the stomach allow it to expand from about 50 ml to 1.5 L. • The stomach has numerous areas: the cardiac area, the fundus, the body and the pyloric area. The Stomach • The cells in the lining of the stomach secrete gastric juices. • Gastric fluids include: 1. 2. 3. Mucus Hydrochloric acid Pepsinogen The Stomach • Mucus – protects the lining of the stomach from the gastric juices; • Hydrochloric acid – kills harmful bacteria and other harmful substances in food. It also converts pepsinogen into its active form – pepsin, a protein digesting enzyme. • Pepsinogen – converts to pepsin to digest protein. The Stomach • Pepsin - breaks down long amino acid chains in proteins into shorter chains called polypeptides. Activation • Pepsiogen + • HCl • Pepsin Active Enzyme Protein Breakdown • Protein Breakdown into • Polypeptides Sphincters • Constrictor muscles the occur along the digestive tract. • Sphincters control the movement of the food and waste from one area of the digestive system to the next. The Small Intestine & Pancreas • Most digestion and nutrient absorption takes place in the small intestine. • It can be divided into 3 zones: 1. Duodenum 2. Jejunum 3. ileum The Small Intestine & Pancreas • The chyme that enters the duodenum from the stomach is very acidic. It must be neutralized. • When acids enter the small intestine, a chemical called prosecretin is converted into the hormone secretin. • Secretin travels through the bloodstream to the pancreas causing it to release bicarbonate ions. The Small Intestine & Pancreas • Now that the chyme is alkaline, the pepsin becomes inactive. • The pancreas also secretes other enzymes such as trypsinogen. Once it reaches the duodenum the enzyme enterokinase converts the inactive trypsinogen into the active form – trypsin. Trypsin further breaks down proteins into shorter chain peptides.. The Small Intestine & Pancreas • The pancreas also secretes: 1. erepsins which complete the digestion of proteins from short-chain peptides into individual amino acids. 2. Pancreatic amylase – further the digestion of carbohydrates into disaccharides. These are finally broken into monosaccharides by the enzyme disaccharidases. The Small Intestine & Pancreas • The pancreas also secretes: 3. Lipases from the pancreas break down lipids. • Pancreatic lipase breaks fats into fatty acids and glycerol. • Phospholipase breaks down phosphlipids. The Liver and Gall Bladder • The liver produces bile salts that speed up the digestion of fats. • When fats enter the duodenum, a hormone cholecystokinin (CCK) is released and carried to the gall bladder. This triggers the gall bladder to relase the bile. • Bile emulsify and breaks down large fat globules. The Large Intestine • Reabsorbs water • Absorbs some vitamins • Contains the bacteria, E.Coli that uses waste materials to make vitamins B and K. • Fibre helps to hold some water in the waste and adds “bulk” making it easier to remove from the body.