Human Digestion and Absorption Chapter 4

Mouth (Oral Cavity) Chewing increases surface area Mixed with saliva food becomes a bolus Saliva –Lysozyme Breaks down bacteria –Mucus Lubricates and hold bolus together –Amylase Breaks down starch –Enhances perception of flavor

Taste and Smell Taste buds – contain receptors –Salty –Sour –Sweet –Bitter –Umami Olfactory cells in nose (smell) –Stimulated with chewing

Stomach Secretions Parietal Cells –Hydrochloric Acid Inactivates proteins Destroys bacteria and viruses Dissolves minerals to aid in absorption Converts pepsinogen into pepsin –Pepsinogen Protein digesting enzyme Gastrin –Hormone that controls release of HCl and pepsinogen

Other Stomach Secretions Chief cells –Gastric lipase Mucus –Protects the stomach from being digested –Production relies on prostaglandins

Small Intestine Most digestion and absorption occurs here Pyloric sphincter allows chyme into the small intestine –Gastric inhibitory peptide (hormone) slows release of chyme Sections –Duodenum, jejunum and ileum

Small Intestine (2) Circular folds Villi – lined with: –Goblet cells - make mucus –Endocrine cells - produce hormones –Enterocytes - produce digestive enzymes and absorb nutrients Contain a brush border of microvilli covered with glycocalyx

Liver, Gallbladder and Pancreas Liver –Provides bile –Enterohepatic circulation (recycling of bile) Gallbladder –Bile storage Pancreas –Produces sodium bicarbonate, lipases, proteases and pancreatic amylase

Gallstones are the most common cause of pancreatitis in the US and accounts for 35–50% of all cases. Despite aggressive and intensive early management, the mortality rate is approximately 10%. Obstruction of the major papilla by the stone causes reflux of bile into the pancreatic duct.

Hormones of the GI Tract Gastrin –Stomach release of HCl and pepsinogen Cholescystokinin (CCK) –Release of bile Secretin –Release of pancreatic bicarbonate Gastric Inhibitory Peptide –Limits release of gastric juices

Absorption Primarily occurs in small intestine Passive diffusion –Concentration gradient Facilitated diffusion –Concentration gradient + carrier protein Active –Carrier protein + energy (regardless of concentration) Endocytosis –Engulfment of compounds or liquids

Moving Nutrients around the Body Blood –Water-soluble nutrients transported via capillaries in villi to portal vein to liver Lymphatic System –Fat-soluble nutrients transported via lacteals into the lymph vessels to thoracic duct

Large Intestine Ileocecal valve Colon –Cecum, ascending colon, transverse colon, descending colon and sigmoid colon Rectum Anus

Functions of Large Intestine Absorption of water and electrolytes Formation and expulsion of feces Housing of bacteria (microbiota)

Microbiota (bacterial flora) Beneficial bacteria –Control pathogenic bacteria –Synthesize Vitamin K and Biotin –Aid dig and fermentation of fibers Probiotics –Live bacteria in food & supplements –Health benefits Prebiotics –Non digestible carbohydrates in food that promote the growth of bacteria- –E.g. inulin, resistant starch

When Digestive Processes Go Awry Heartburn and Gastroesphogeal reflux disease (GERD) –Foods that increase reflux: Citrus, caffeine, chocolate, fatty foods, spicy foods, onion, garlic and tomato based foods (Peptic) Ulcers –Causes: H. pylori and NSAID medications –Treatment: Medications Avoid foods that increase symptoms

When Digestive Processes Go Awry (2) Food Intolerances Intestinal Gas (Flatulence) Constipation –Fiber, fluid and exercise –Laxative use Diarrhea –Replace fluid and electrolytes Irritable Bowel Syndrome (IBS) –Cause unknown Inflammatory Bowel Disease (IBD) –Ulcerative colitis and Crohn’s disease Hemorrhoids Gallstones

Celiac Disease Intolerance to gluten 1 in 133 people affected –Many undiagnosed Can affect many body systems –GI and others Gluten-free diet for life –Corn, rice, quinoa, and buckwheat OK