2. The Process of Digestion DIGESTION: The breakdown of food into simpler molecules that can be absorbed by the body. Gastrointestinal tract (GI tract) Includes: mouth, pharynx, esophagus, stomach, small intestine, & large intestine. Glands include: salivary, pancreas, & liver. Three tasks: 1) break down food into a fine pulp 2) chemically act on the food, breaking down molecular structure 3) absorption into blood stream The Mouth: Prepares food for entry into GI tract. Mechanically: teeth, cheeks, tongue (taste buds) Types of teeth: canines, molars, inscisors Fig. 39-15 Teeth: enamel and dentin, no nerve cells, hardest material in body Anchored by roots extend into jaw and are held by periodontal membrane. Tooth ache: Pulp cavity w/ blood vessels and nerve cells is disturbed

4. THE MOUTH Salivary Glands: secretes the first digestive enzymes of the GI tract Three impt. Functions: 1) Dissolves some food and speeds mov’t. 2) Contains enzymes that attack many potentially dangerous microorganisms entering the mouth. 3) Contains the enzyme salivary amylase, breaks down starch molecules into maltose. Bolus: Gathering food in the mouth into a small ball for swallowing. Pharynx: The GI tract and Respiratory tract cross each other here. Epiglottis: Flap of cartilage that covers the respiratory tract when swallowing. THE ESOPHAGUS 25cm in length, connecting the pharynx with the stomach Peristalsis: Contraction of muscles in the esophagus pushing the food Sphincter: Thick ring of muscle where esophagus connects stomach

6. THE STOMACH Stomach: Thick muscular sac below the abdomen to store food Has 3 sets of glands: 1) makes mucus to lubricate & protect 2) secretes HCL to break down food 3) secretes pepsin which works w/HCL to begin protein digestion forming smaller polypeptides Chyme: pasty mixture in stomach after a few hours of churning Pyloric valve: Peristalsis forces mixture past this valve into small intestine. Most proteins, some starch broken down by the time of departure. Sugars and fats are not affected as of yet.

8. THE SMALL INTESTINE Duodenum: First part of small intestine where food is flooded with enzymes Enzymes come from three sources: intestine itself, pancreas, & liver Duodenum: peptidases-proteins; maltase-maltose; lactase-lactose Pancreas: Long organ located behind the stomach consisting of glands pancreatic fluid: digestion of carbos, fats, & proteins amylases and protease breaks down carbos and proteins lipase breaks down fats into glycerol & fatty acids sodium bicarbonate- neutralizes HCL in tummy: pH of 7-8 ideal Liver: large brownish organ above the tummy (1500g) Secretes bile which is stored in the gallbladder the bile salts help lipase emulsify fats Very long (7m); peristalsis pushes it through in several hours After leaving duodenum, food goes through jejunum and illium, where digestion is completed and absorbed into blood stream

10. SMALL INTESTINE Small intestine has folds to increase the surface area. Villi: fingerlike projections in the small intestine w/microvilli folds. Richly supplied with blood vessels to absorb and carry nutrients. Carbos and proteins absorbed into capillaries of villi Undigested fat and some fatty acids enter lymph directly When leaving the food is nutrient free; water, cellulose & ? Behind As it leaves it passes the appendix (vestigial organ) THE LARGE INTESTINE (COLON) Remove water from undigested materials passing through it Result is solid waste (feces) removed by peristalsis to rectum. Valvelike anal sphincter muscles prevent early departure. When problems occur: Not enough water absorbed=diarrhea; Too much water=constipation; Contains bacteria aiding in final digestion and giving vitamin K