The Digestive System
• Mechanism for nourishing the body
• Most nutrients in food require either
degradation or release prior to absorption
• Digestion occurs in GI Tract
• Digestion
– Mechanical
• chewing and peristalsis
– Chemical
• enzymes, HCl
Overview of GI Tract
• Upper GI Tract
– Mouth and pharnyx provide entryway
– Lead to Esophagus
• Esophagus wall consists of same layers found in
remainder of GI tract
– epithelium (mucosa), submucosa, muscularis mucosa,
circular and longitudinal muscles
– Secretory glands found throughout GI except
colon
• Exocrine glands and Endocrine glands
Upper GI Tract Continued
• Sphincters
– circular muscles located throughout digestive
tract
• Lower Esophageal Sphincter
– allows passage of food into stomach
• Pyloric Sphincter
– Controls release of chyme from stomach to duodenum
• Stomach
– J shaped organ
– volume ranges from 50 to 1500 mL (2 -52 oz)
Lower GI tract and Accessory
Organs
• Small Intestine
– duodenum, jejunum and ileum
– main site for nutrient digestion and absorption
– duodenum receives secretions from liver,
gallbladder and pancreas
• Liver
– hepatocytes make bile from cholesterol
– right and left hepatic bile ducts join to form
common hepatic duct
Lower GI tract and Accessory
Organs
• Gallbladder
– capacity to hold 40-50 mL of bile
– functions to concentrate and store bile
• Pancreas
– 2 types of active tissue
• Acini or ducted exocrine tissue
– produces digestive enzymes
• secreted into small ducts within pancreas which leads
to common hepatic bile duct
• empties into duodenum
• Ductless endocrine tissue
– secretes hormones, glucagon and insulin
Structure of SI
• Epithelial surface (mucosa) structured to
maximize surface area
– Large folds of mucosa (folds of Kerckring)
– Villi
• projections lined with 100s of absorptive cells
• contain blood capillaries and central lacteal
– Microvilla
•
•
•
•
extensions of plasma membrane of absorptive cells
possess surface coat of glycocalyx
forms brush border
most digestive enzymes produced by SI found here.
Structure of SI
• Crypts of Lieberkuhn
– pits located between villi
– epithelial cells in these crypt migrate upward
and out of crypts toward tips of villi
– turnover every 3-5 days
– Cells include
• paneth cells secrete proteins with unknown function
• globlet cells secrete mucus
• Enterochromaffin cells with endocrine functions
Structure of SI
• Lymphoid Tissues
– mucosa and submucosa
– Peyer’s Patches
• T-lymphocytes and B-lymphocytes
• provide defense against bacteria and foreign bodies
• Ileocecal valve
– allows unabsorbed materials to leave ileum and
enter cecum
Colon
• Cecum
• Ascending, transverse and descending
sections
• Sigmoid sections
• Haustra or pouches
– created by contraction of 1 or 3 muscular strips
(tenaie coli) along with contraction of circular
muscles
• Absorbs water and lytes
Regulation Of Digestive Process
• Regulatory Peptides (GI hormones and neuropeptides)
– Gastrin (hormone)
• produced mainly by gastric cells
• released upon entry of food into stomach
– stimulates HCl release
– gastric and intestinal motility
– pepsinogen release
– Cholecystokinin (CCK, hormone)
• produced by SI cells
• released upon entry of chyme into stomach
– amino acids, fat
• targets pancreas and gallbladder
Regulatory Peptides
• Secretin
– produced by cells of SI
– secreted in response to chyme (acid) into
duodenum
– targets pancreas
– may inhibit GI motility
• Gastric Inhibitory Polypeptide (GIP)
– produced by SI cells
– Inhibits gastric secretions and motility
– Simulates intestinal and insulin secretions
Regulatory Peptides
• Somatostatin
– Produced by pancreatic and SI cells
– Paracrine
• released by endocrine cells but diffuse through
extracellular space to target tissue
– Enters gastric juice
– Inhibits gastrin secretion, as well as GIP and secretin
– Inhibits gastric acid release, gastric motility,
pancreatic exocrine secretions and gall bladder
secretions
Neural Regulation
• Enteric Nervous System
–
–
–
–
located in wall of GI tract
begins in esophagus and ends at anus
controls peristaltic activity
affected by parasympathetic and sympathetic
nervous systems
• Acetylcholine increase GI motility
• Norepinephrine and epinephrine inhibit GI activity
– Also influences GI secretions
Neurocrines (Regulatory Peptides)
• Neurocrines
– Peptides originating from nerves of gut
• Vasoactive Intestinal Peptide (VIP)
– stimulates intestinal secretions
– relaxes GI sphincters
– inhibits gastric acid secretion
– stimulates bicarbonate release
• Gastrin Releasing Peptide (GRP)
– Stimulates release of HCl, gastrin and CCK
• Neurotensin (Bombesin)
– Mediates gastric emptying, gastric acid secretion and SI
motility
Process of Digestion
• Oral Cavity and Salivary Glands
– mastication of food
– mixed with salivary gland secretions
• parotid glands
– water, lytes and enzymes
• submandibular and sublingual glands
– water, lytes, enzymes and mucus
– Enzymes
• alpha amylase and lingual lipase
Process of Digestion
• Esophagus
– Entry of food results in peristalsis
• acetylcholine
– LES
• LES pressure decreases upon swallowing
• Neural and hormonal regulation
• functions to prevent gastric reflux
Process of Digestion
• Stomach
– Body extends from LES to angular notch
• includes fundus
• serves as reservoir as well as producer of gastric
juice
– Antrum (Pyloric Portion)
• extends from angular notch to duodenum
• grinds and mixes food with digestive juice (chyme)
• provides strong peristalsis
Stomach
• Gastric Juices
– produced by 3 different gastric glands
• Cardiac glands (fundus)
– mucus cells (HCO3 and mucus) and endocrine cells
– no parietal (oxyntic cells)
• Parietal glands (body)
– mucus cells, oxyntic cells (secrete HCl and IF), chief cells
secrete pepsinogen upon stimulation by acetylcholine
• Pyloric glands (antrum)
– both mucus and oxyntic cells
– endocrine G cells that produce gastrin
– Acetylcholine stimulates chief, oxyntic and
other gastric cells
Gastric Juices
• HCl
– activates inactive zymogen pepsinogen to
active pepsin
– denatures proteins
– serves as bactericide
• Mucus
– lubricates and protects gastric lining
• Intrinsic Factor (IF)
– necessary for vitamin B12 absorption
HCl
• Release from oxyntic cells stimulated by
– acetylcholine, gastrin, CCK, histamine
– Histamine
• secreted from mast cells
• paracrine that binds to H2 receptors on parietal cells
to stimulate HCl release
• mechanism used in drug therapy to lower acid
– drugs prevent histamine from binding to H2 receptors
– decreases acid release
Gastric Emptying
• GI hormones and neuropeptides affect a
pacemaker
– determines frequency and rate of contractions
– 1-5 mL chyme allowed to enter duodenum
every 30 seconds
– Fat appears to slow gastric emptying
• Inhibitors
– Secretin, GIP, Somatostatin, CCK
Small Intestine
• Chyme has low pH
– SI protected by pancreatic secretions and
secretions from Brunner’s glands
– Secretin and CCK
• Digestive juices
– Glands within crypts of Lieberkuhn
– Pancreas
• Responsible for enzymes that digest 50% CHO,
50% protein, 90% lipids
Digestive Juice
• Pancreatic Proteases
– trypsinogen, chymotrypsinogen,
procarboxypeptidases, proelastase, collagenase
– secreted in vesicles
– must be activated
• trypinsogen converted to trypsin by
enteropeptidase (enterokinase) and by free trypsin
– hydrolyze peptide bonds either internally or from
ends
– mono, di and some tri can be absorbed
• Brush border aminopeptidases to hydrolyze
further
Digestive Juice
• CHO
– Pancreatic alpha amylase
• hydrolyzes alpha 1-4 bonds
– Alpha dextrinase to hydrolyze 1-6 branches
– Brush border enzymes (maltase, sucrase,
lactase)
• Lipids
– Pancreatic lipase hydrolyzes TG to yield MG,
FFA and glycerol
Bile Synthesis
• Made in liver cells from cholesterol
– cholesterol oxidized
• chenodeoxycholic acid and cholic acid
– conjugated to glycine or taurine
• glycocholic acid and taurocholic acid
– conjugation of bile acids with amino acids
improves its ability to form micelles
– cholesterol and PLs secreted into bile
• bile acid dependent frx
– Water, Lytes and bilirubin secreted in bile
Bile Storage
• Gallbladder
– Concentrates bile by removing 90% of water
– Stores bile
– Stimulated to release bile by CCK
• CCK secreted in response to amino acids and lipids
– Bile is secreted into duodenum
Function of Bile
• Digestion
– Bile acids are amphipathic
– Decrease surface tension of fat
– Permits emulsification of fat
• Increases exposure of area of lipids
– Allow digestive enzymes (lipase) to get close to
fat
• Absorption
– Micelle Formation
Micelles
• Bile acid monomers from simple micelles
– Hydrophobic centers, hydrophilic periphery
• Fatty acids and MAGs enter micelles
– Mixed Micelles
• Micelles travel to brush border
– MAGs and fatty acids diffuse through unstirred
water layer and into enterocytes
– Bile acids released for reuse
Enterohepatic Circulation
• > 90% of bile acids secreted in duodenum are
reabsorbed into ileum
• Enters portal vein for transport via blood back to
liver
• Reabsorbed bile acids are secreted in bile along
with newly made bile acids
• New bile mixed with recirculated bile is sent via
cystic duct for storage in gallbladder
• Pool of bile (2-4 g) may recycle 1-2 times/meal
Secondary Bile Acids
• Bile acids not reabsorbed in ileum may be
deconjugated by bacteria in colon
– deconjugated bile acids form secondary bile
acids
• cholic acid is converted to deoxycholic acid which
may be reabsorbed
• chenodeoxycholic acid is converted to lithocholic
acid which is excreted in feces
– ~ 0.5 g of bile salts are lost in feces daily
Decreasing Blood Cholesterol
• Only route of cholesterol excretion is in
feces
• By increasing excretion of bile in feces, one
can lower blood cholesterol levels
– necessitates use of body cholesterol for
synthesis of new bile acids
– drugs (powdered resins) bind bile and enhance
excretion from body
– soluble fiber behaves like resins
Role of Intestinal Brush Border
in Digestion
• CHO Digestion
– Isomaltase
• 1-6 bonds in oligosaccharides and dextrins
– Sucrase
• alpha 1-4 bonds in sucrose and maltose
– Glucoamylase, glucosidase, maltase
• alpha 1-4 bonds in oligosaccharides, maltotriose and
maltose
– Maltase, Lactase and Sucrase
Role of Intestinal Brush Border
in Digestion
• Protein Digestion
– Aminopeptidases hydrolyze N terminal amino
acids from oligopeptides, tripeptides and
dipeptides
– Tripeptidases
– Dipeptidases
Absorption
• Begins in duodenum
• Continues throughout jejunum and ileum
• May be accomplished by
– simple diffusion
– facilitated diffusion
– active transport
– pinocytosis or endocytosis
– paracellular absorption
• Mechanism depends on
– nutrient solubility; electrical gradient; size
Colon
• Greater absorption of sodium, chloride and water
• Secretion of bicarbonate into colon
– neutralize acids produced by colonic anaerobic
bacteria via action on CHO
– also synthesize vitamin K, biotin and folate
• Acids produced in colon include short chain fatty
acids
– acetate, butyrate, propionate
– may be absorbed by colonocyte
• Progressive dehydration of unabsorbed materials
– 1 L of chyme reduced to < 200 mL of defecated