Small intestine Digestion in the small and large intestines 9.5 The structure of the small intestine • Most digestion and absorption of nutrients takes place in the small intestine. • Only about 2.5 cm in diameter.7m in length. • Large intestine: 7.6cm in diameter but 1.5 m in length. • Small intestine is made up three sections: • duodenum., the jejunum, and the ileum. • carbohydrates, proteins and lipids are digested in the small intestine with the help of hormones and enzymes. • Takes about 3h to 5h to digest a meal in the small intestine. The structure of the small intestine • Inner surface of the small intestine gives the maximum surface area for efficient nutrient absorption. • Villi - finger like projections of the inner layer folded into ridges • Microvillus - microscopic projection of the cell membrane; increases the surface area of the cell • Lacteal - a lymphatic vessel within a villus, through which digested fats enter the circulatory system • Duodenum - where most enzymes are added; start of digestion • Jejunum - digestion continues and some nutrients are absorbed • Ileum - most nutrients are absorbed; the last section of small intestine. Chemical digestion in the small intestine The role of Pancreas in Digestion • The pyloric sphincter controls the passage of food from the stomach into the small intestine. • When the food in the stomach is mixed properly with gastric juice and the proteins are partially digested, sphincter will relax and chyme will enter duodenum. • This process is slow and steady -> prevents overloading and allows time for thorough digestion. • Pancreas is in between the stomach and the duodenum. • Pancreas has two roles: 1. secrete enzymes for digestion, 2. secretes hormones that regulate the absorption and storage of glucose form the blood. • The enzyme amylase (in saliva) is also secreted by the pancreas. • Digestion of carbohydrates form the mouth continues to duodenum. Digestion in Duodenum Proteins to amino acids • Acidic chyme enters the small intestine, a chemical called prosecretin is converted to its active form, secretin - secreted by the duodenum that stimulates pancreatic and bile secretions (enzymes for digestion) • Primary function: release bile to neutralize the acidic chyme and raise the pH. Secretin protects the small intestine from stomach acids.