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