Chapter 3 Part 2

 Digestion is the process of breaking down foods into nutrients to prepare for absorption

 Visual, olfactory, and auditory senses  Taste (smell) ◦ Sweet sour, salty, bitter, umami (savoriness)  Presence of food in the mouth ◦ Activate secretory responses from  Salivary glands  Stomach  Pancreas ◦ Contraction of gall bladder  Bile

 Anatomy – Gastrointestinal (GI) tract  Flexible muscular tube from mouth to anus  Lumen - inner space of the tract.

 Anatomy –GI tract ◦ Mouth - beginning of digestive system and digestion  Mastication (chewing) -stimulates taste buds,  Swallowing - epiglottis closes to prevent food from entering the pharynx.

 Anatomy - GI tract ◦ Esophagus - tube carrying bolus to stomach. ◦ Sphincters – control release, keeps food moving forward  Peristalsis  Upper and lower esophageal sphincters (cardiac sphincter - LES)

 Anatomy - GI tract ◦ Stomach – HCl and enzymes turn food into a fluidy paste  Holds 2 to 4 cups of food (6 L)  Grinds the bolus to a semiliquid mass (chyme)  Holding “tank”  Emptying closely regulated  5 ml/min (teaspoon)  1-4 h to empty  Pyloric sphincter

 Anatomy - GI tract ◦ Stomach  Prevention of autodigestion  Mucus lining  HCO 3 -  Prevents HCl in the stomach from burning through the stomach lining

 Stomach Acid (HCl) ◦ Destroys biological activity of protein ◦ Activates digestive enzymes ◦ Partially digests dietary protein ◦ Assists in vitamin B 12 absorption  B 12 requires gastric acid to be released from food ◦ Improves absorption of minerals

 Gastrin stimulates secretion of gastric hydrochloric acid, performing many functions. ◦ Activates pepsin ◦ Stimulates the release of hydrochloric acid ◦ Kills pathogenic organisms ◦ Improves absorption of iron and calcium ◦ Inactivates hormones of plant and animal origin ◦ Denatures food proteins, making them more vulnerable to enzyme action

 Anatomy - GI tract ◦ Small intestine - digestive juices  Gallbladder (Bile) and pancreas (enzymes that break down CHO, fats and proteins)  Three parts of small intestine  Duodenum  Ileum  Cecum ◦ Large intestine (colon)  Ileocecal valve to rectum and anus.

 Anatomy – GI tract ◦ Approximately 10 feet long  Narrow ◦ Duodenum (10 in)  Where enzymes and bile are added  Regulatory center that senses:  Nutrient content (particularly fat)  pH (HCO 3 - from pancreas raises pH of chyme from stomach) ◦ Jejunum (4 ft) ◦ Ileum (5 ft)

 Anatomy – GI tract ◦ Physical structure – intestinal wall  Folded walls  Villi – made up of absorptive cells  Increase surface area greatly  Rapid turnover absorptive cells (daily)

 The Muscular Action of Digestion – under autonomic control ◦ Peristalsis - pushes the digestive contents along.  Muscles are circular, longitudinal, and diagonal muscles. ◦ Sphincter contractions open and close passageways  Prevents reflux, controls the passage of contents.

Fig. 3-2, p. 75 Longitudinal Circular Diagonal

 Salivary glands  Pancreas  Liver (gall bladder)  Stomach  Small intestine

 Begins in the mouth  Saliva ◦ Electrolytes (Na, Cl) ◦ Solvent ◦ Enzymes (salivary amylase) ◦ Mucus  Saliva moistens food ◦ Begins breakdown of CHO

 Small Intestine ◦ Pancreatic amylase (duodenum)  Smaller CHO molecules ◦ Brush border enzymes  Maltase, sucrase, lactase  Disaccharides to monosaccharides

 Maximum rate of glucose absorption ◦ 50 – 80 g/h (70 kg) ◦ 4 g CHO/min or 240 g/h  Absorbed into circulation ◦ Hepatic portal vein  Liver

 Large intestine ◦ Water reabsorption ◦ Some digestion  Mostly bacterial  Vitamin absorption  Vitamins K, B 12, thiamin and riboflavin  Compacts and prepares feces for defecation

 Mouth ◦ Lingual lipase  Stomach ◦ Gastric lipase  Pancreas ◦ Pancreatic lipase ◦ Bile

 Cholecystokinin (CCK) ◦ Released from wall of duodenum ◦ In response to fat and protein in the chyme ◦ Causes the release of bile and pancreatic enzymes  Secretin ◦ Stimulates bile secretion by liver and release of bicarbonate into duodenum  Micelles ◦ Monoglyceride, FFA + bile salts

 Micelles are formed after bile acts on large fat droplets  Pancreatic and brush border enzymes breakdown micelles to fatty acids

 These fatty acids are then repackaged as chylomicrons, which are largely absorbed into the lymphatic system  They then travel to the heart and enter the circulation

 Pepsinogen - precursor  Pepsin ◦ Released from stomach wall  Gastrin ◦ Release of HCL ◦ Controls pepsin release

 Trypsinogen  Secreted by pancreas  Trypsin  Tripeptides, dipeptides, and single amino acids.  Brush border enzymes  Small intestine into portal vein  Active absorption

 Vitamin absorption  Passive diffusion  In the jejunum and ileum.  Fat-soluble vitamins  With dietary lipids  Transport  Chylomicrons, lipoproteins

 Water-soluble vitamins  Diffuse into the blood  Excess (exceeds renal capacity)  Pass into the urine  Vitamin B 12.  Intrinsic factor  Intestine absorbs by endocytosis.

 Dietary factors  Type of food source impacts absorption  Cellular factors  The body does not absorb minerals very well.  Availability depends on its chemical form  Heme vs nonheme

 Passive process of osmosis in small intestine  9 L of water each day. ◦ 72% absorbed in the proximal small intestine ◦ 20% absorbed from the distal segment of the small intestine ◦ 8% absorbed from the large intestine