Chapter 4: Human Digestion and Absorption

cells that cover surfaces inside and outside the body

support and protect the body by holding structure together

contract and relax to permit movement

transmit nerve impulses

• mouth
• esophagus
• stomach
• small intestine
• large intestine

mechanical/chemical processes of breaking down food into smaller components

uptake of nutrients from the GI tract into the blood or lymph

waves of contraction that propels material along the GI tract

back and forth action that breaks food apart

peristaltic wave that contracts over a large area of the large intestine to help eliminate waves

• peristalsis
• segmentation
• mass movement
• elimination

• long hollow muscular canal from mouth to anus
• nearly 15 ft

• mucosa - epithelia cells and glands, innermost
• submucosa - CT and glands, contains blood vessels carrying nutrients to GI tract
• smooth muscle - moves food forward, longitudinal and circular
• serosa - outside layer that protects the tract, secretes lubricating fluid to keep it cushioned

• ring link muscles that control the flow of contents in the GI tract
• open and close valves to control flow of contents

• lower esophageal sphincter
• pyloric sphincter
• hepatopancreatic sphincter
• ileocecal valve
• anal sphincter

• prevents back glow of stomach contents into the esophagus (reflux)
• aka cardiac sphincter

• control the flow of stomach contents into the small intestine
• only allows 1 tsp of chyme at a time into SI to neutralize the acid and digest nutrients

• control the flow of bile and pancreatic juice from the common bile and pancreatic ducts into the small intestine
• aka sphincter of oddi

prevent the contents of the large intestine from reentering the small intestine

prevent defecation until the person desires to do so

• saliva
• mucus
• digestive enzymes
• hydrochloric acid
• bile
• bicarbonate ions
• hormones

• dissolves taste forming compounds
• aids in swallowing and digestion and protection of teeth

• protects GI tract cells
• lubricates digesting food

break down carbs, fats, and proteins small enough for absorption

• promotes digestion of protein
• destroys microorganisms
• increases solubility of minerals

aids in fat digestion (emulsifies fat)

neutralizes stomach acid when it reaches the small intestine

regular food intake, digestion, and absorption

bolus

• lysozyme - breaks down bacteria
• mucus - lubricates and holds bolus together
• amylase - breaks down starch, only 5%
• lingual lipase - initiates fat digestion

• salty - from metal ions
• sour - from acids
• sweet - from organic compounds like sugar
• bitter - from many groups, caffeine, plants
• umami - savory tastes from amino acids and MSG

moves bolus from mouth to esophagus

prevents food from lodging in the trachea by covering larynx

• circular muscles
• longitudinal muscles
• relax and contract to move food along

chyme

• Hcl
• intrinsic factor (IF)

• destroys bacteria and viruses
• inactivates proteins (such as plant and animal hormones to prevent them from affecting human functions)
• inhibits salivary amylase
• converts pepsinogen into active form of pepsin

binds to B12 and aids in its absorption

• pepsinogen (the inactive enzyme)
• gastric lipase

• turns into the enzyme pepsin by Hcl, which breaks down proteins into AA chains
• first location of protein digestion

enzyme that breaks down lipids

mucus

protects stomach from being digested

presence of prostaglandins

inhibits prostaglandin production -> can damage the stomach

gastrin

• stimulated by food, thoughts of food, and action potentials of autonomic nervous system
• stimulates secretion of pepsinogen and HCl
• stimulates contraction of lower esophageal sphincters
• slows gastric emptying

slows down the release of chyme into the SI

• duodenum (10 in)- primary site of digestion
• jejunum (4ft) - upper part of digestion, lower for absorption
• ileum (5ft)

enzymes produced in the small intestine

• secretin
• cholecystokinin (CCK)
• gastric inhibitory peptide (GIP)

• stimulated by presence of acidic chyme and presence of peptones in the duodenum
• stimulates secretion of pancreatic bicarbonate which neutralizes the HCl

• stimulate by food, presence of fat and protein in duodenum
• stimulates contraction of gallbladder and release of bile
• stimulates release of enzyme rich pancreatic fluids

• stimulated by fats and protein
• inhibits secretion of stomach acid and enzymes
• slows gastric emptying/motility

• recycling of bile
• bile is stored in the gallbladder until needed
• it is release into the duodenum and then reabsorbed in the ileum to be returned to liver

• produces bile to aid in digestion and absorption of fat
• emulsifies fat

bile storage

produces pancreatic juice to help break down large molecules

• sodium bicarbonate - neutralizes acidic chyme coming into SI to protect the SI from damage by the acid
• lipase - digest fat, break down large molecules
• proteases - digest protein, break large AA chains into smaller AA chains
• pancreatic amylase - digest starch and carbs

speed digestion by catalyzing chemical reactions

• passive diffusion
• facilitate diffusion - carrier proteins allows substances through membrane into cell
• active transport
• endocytosis - phagocytosis and pinocytosis

• water soluble nutrients (proteins, carbs, medium chain fatty acids, B vitamins, vitamin C)
• portal vein delivers nutrient rich blood to liver before going throughout the body
• it gives the liver a chance to process and store nutrients before being used (liver is main processing location in the body)

• fat soluble nutrients (long chain fatty acids, fat soluble vitamins, large particles
• nutrients enter circulation at thoracic duct near the heart, far from the liver

• colon
• rectum
• anas

• cecum
• ascending colon
• transverse colon
• descending colon
• sigmoid colon

• houses gut microbiota to keep GI tract healthy
• absorbs water and electrolytes
• forms and expels feces

ileocecal valve

• protect against infection by pathogens (crowd them out, produce antimicrobial substances)
• synthesize vitamins to be absorbed in colon (especially vitamin K and biotin)
• digest and metabolize complex carbs/fibers/starches + form short fatty chain acids
• modulate information with metabolic products

• imbalance between beneficial and pathogenic bacteria
• can cause GI infections

• live bacteria that will colonize the LI
• found in fermented foods

found in non digestible carbohydrates that promote growth of bacteria

• water
• sodium
• potassium
• short chain FA
• vitamins produced by bacteria in LI

• very small erosion of the top layer of cells in the stomach or duodenum
• aka peptic ulcer

• hub pylori bacteria (acid resistant) - weakness mucus coating that protects stomach + duodenum -> allows HCl + acids to erode the cells
• heavy use of aspirin, NSAIDs, alcohol, or smoking

• antibiotics
• antacid
• medicines that reduce acid (pepsin)

• smaller meals
• less fatty meals
• stop smoking

long term use decreases stomach acidity = Ca and Mg less well dissolved from food = decreased absorption of them = decreased bone mineralization process = increased risk of osteoporosis

• liver cells store excess fat instead of metabolizing it -> swelling, inflammation, and scarring
• gradual weight loss and daily exercise can reverse the effects

develop in gallbladder when cholesterol in bile forms crystal-like particles

• causes - low fiber diet (bc fiber helps stimulate peristalsis and forms soft feces). Antacids, calcium, iron supplements.
• treatment - plenty of dietary fiber and fluids, laxatives

insertion of fluid into rectum -> stimulates the bowel -> liquid and feces are expelled

swollen veins of the rectum and anus

inflammation + ulceration extend through all layers of GI tract