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Chapter 24
The Digestive System
Structures and functions, fig 24.1
 Digestive System Organs
Alimentary Canal
Accessory Digestive Organs
 Digestive Processes
Ingestion
Secretion
Propulsion
Digestion
Absorption
Defecation
Anatomy of the Digestive System
________________
Mouth, Pharynx, and Esophagus
Stomach
Small Intestine
Large Intestine
___________ Digestive Organs
Teeth, Tongue, Salivary Glands
Liver and Gallbladder
Pancreas
Functions
1. _______________ taking food into the mouth
2. _______________ release of water, acid,
buffers, & enzymes into lumen of GI tract
3. ___________ and __________ churning &
propulsion of food thru GI tract
4. ______________ mechanical & chemical
breakdown of food
5. _________________ passage of digested
products from the GI tract into blood & lymph
6. ___________ elimination of feces from GI tract
Mouth, Pharynx, & Esophagus
 Anatomy
Mouth
Lips and Cheeks
Palate
Tongue
Salivary Glands
Teeth
Pharynx
Esophagus
 Digestive Processes
Mastication
Deglutition
Mouth (oral or buccal cavity) fig 24.5
Formed by cheeks, hard & soft palate,
tongue
Where mechanical AND chemical
digestion begin
_____________ - chewing
Bolus- soft, flexible mass
______________ – enzyme, initiates
breakdown of starch
______________ – enzyme in saliva, works in
stomach to breakdown dietary triglycerides
Tongue
 accessory organ
 skeletal muscles covered w/ mucous membrane
 Attached to hyoid, styloid process, mandible
 Extrinsic muscles manuever food -chewing, form bolus, force
back for swallowing
 Intrinsic muscles alter shape and size during speech &
swallowing
 Dorsal & lateral surfaces covered w/_____________ which are projections of lamina propria
 Many contain taste buds - ______________
 Lacking taste buds - touch, increase friction
 ____________ – secrete mucus, & serous fluid that
contains lingual lipase
Teeth
figure 24.7
Accessory digestive organs in _________
__________
Crown, neck, root – major external regions
_______________ - calcifed CT, gives shape
and rigidity, harder than bone, majority of tooth
________________- covers dentin on crown,
hardest substance in body, protects tooth from
wear & tear of chewing, and acids
________________ - CT containing bv,
nerves, lymphatic vessels
Dentitions
figure 24.8
 ______________ teeth = primary, milk, or baby
teeth – begin to erupt @ 6 months
20 in full set
All are lost between age 6 to 12
 ______________ or secondary teeth
32 teeth
 8 incisors – chisel shaped for cutting
 4 canine or cuspid – pointed to tear and shred
 8 premolars or bicuspid – crush and grind
 4 first molars (age 6) – crush and grind
 4 second molars (age 12)- crush and grind
 4 third molars or wisdom teeth (age 17)- crush and grind
Oral mucosa
Non-keratinized stratified squamous
______________ – membrane that lines a
body cavity that opens to the exterior
Of mouth and tongue- contain small salivary
glands that open into oral cavity
Labial, buccal, palatal, and lingual
Make small contribution to saliva
Salivary glands
figure 24.6
Keep mouth & pharynx mucous
membranes moist
Cleanse mouth & teeth
When food enters mouth, secretion 
Lubricates, dissolves, begins chemical
digestion
3 major glands (in pairs)- parotid,
submandibular, sublingual glands
Control of salivation
 ______________ stim= continuous secretion
 ______________ stim= dominates during stress
resulting in dryness of mouth
 If body dehydrated, saliva not secreted  dry
mouth  sensation of thirst
 Feel & taste of food stimulate receptors in taste
buds  nuclei in brain stem  parasymp
impulse via facial & glossopharyngeal nerves to
stimulate salivation
Smell, sight, sound or thought of food may also stim.
4 basic layers of GI tract
fig 24.2
Variations, but same basic organization in
esophagus, stomach, small & large
intestines
From inner to outermost (starting inside
the __________)
Mucosa
Submucosa
Muscularis
Serosa
Layers of GI tract
 1. ____________ - lines lumen, has 3 layers:
______________ mouth, pharynx, esophagus, & anal canal= non-keratinized
stratified squamous for protective function
 stomach & intestines = simple columnar, w/ tight junctions
for secretion & absorption
 Cells renew every 5-7 days (rapid reproduction)
 Some epithelium = exocrine - mucus secreting cells
 Enteroendocrine cells secrete hormones into bloodstream
________________ –areolar CT, many bv &
lymphatic vessels, nutrients  tissues of body
 Supports epithelium & binds to muscularis mucosa
 Contains most mucosa-associated lymphoid tissue
(MALT) – lymphatic nodules containing immune cells
• Also found in tonsils, s.i., appendix and l.i.
________________- thin layer of smooth muscle
Puts mucous membrane of stomach & s. i. into many
small folds ↑ SA for digestion & absorption
movements insure absorptive cells fully exposed
 2. _________________- areolar CT binds
mucosa to muscularis
Highly vascular
__________________- portion of ENS- regulates
movements of mucosa & bv vasoconstriction
Sensory – act as chemoreceptors & stretch receptors
Interneurons – connect myentric & submucosal
Motor - innervates secretory cells of epithelium
May contain glands and lymphatic tissue
 3. _____________- mouth, pharynx, & superiormid esophagus = skeletal muscle- swallow
Skeletal muscle forms external anal sphincter
Thru rest of tract: smooth muscle- generally found
in 2 sheets (exception- stomach has 3):
Inner = circular layer
outer = longitudinal layer
 Involuntary contractions  breakdown food
 Mix food w/ secretions & propel it along tract
 Between 2 sheets: ________________ (ENS)- mostly
controls GI tract motility- freq & strength of contraction
4. __________- found in portions of GI
tract suspended in abdominopelvic cavity
serous membrane: areolar CT & simple
squamous epithelium
AKA- visceral peritoneum
Esophagus lacks this layer, has adventitia
instead- single layer of areolar CT & no simple
squamous
Peritoneum
figure 24.4
largest serous membrane of body
consists of a layer of simple squamous
(mesothelium) w/ supporting areolar CT
__________________- lines abdominopelvic
cavity wall
__________________- covers some organs in
cavity, is serosa
peritoneal cavity- slim space between these layers
• contains serous fluid
Peritoneum (2)
 ___________________- organs lie on posterior
abdominal wall
 covered by peritoneum on their anterior surface
 kidneys and pancreas
 Bind organs to each other & walls of abdominal cavity
 Contain bv, lv, & nerves -supply organs
 Contains large folds between viscera:
1. ___________________ outward fold of serous coat of small
intestine; binds to posterior ab wall
2. ___________________ binds large intestine to posterior ab
wall; carries bv & lv to intestines
*both hold intestines loosely in place
Peritoneum (3)
3. ________________- attaches liver to anterior
ab wall & diaphragm, (liver = only digestive
organ attached to anterior wall)
4. ________________- 2 folds in serosa,
suspends stomach & duodenum from liver,
contains some lymph nodes
5. _______________- largest part of peritoneum
Hangs loosely like a “fatty apron” over transverse colon &
coils of small intestine
Contains considerable adipose & many lymph nodes,
contributes macrophages & plasma cells to combat
infection & prevent spread of infection
Esophagus
figure 24.10
 Extends from laryngopharynx  esophageal hiatus
(surrounded by diaphragm)  stomach
 Stratified squamous epithelium
 Muscularis: superior 1/3 = skeletal
 Intermediate 1/3 = skeletal & smooth
 Inferior 1/3 = smooth
 At each end is sphincter
 ____________ – skeletal, regulate pharynx  esophagus
 ____________ – smooth, regulate esophagus  stomach
 Adventitia- not serosa (no simple squamous cells)
 Secretes mucus, transport food
 No production of enzymes, no absorption
Deglutition (swallowing) - 3 phases
 Movement of food from mouth to stomach
________________ stage- bolus forced back into
oropharynx by movement of tongue
_______________ stage- bolus stimulates receptors in
oropharynx  deglutition center in medulla & pons
 Soft palate & uvula close off nasal cavity
 Epiglottis closes off opening to larynx
 Bolus moves thru oro & laryngopharynx
________________ stage- bolus enters esophagus
 Peristalsis- progression of coordinated contractions &
relaxations of circular & longitudinal layers of muscularis
 Mucus to lubricate & reduce friction
Stomach
Functions
Anatomy
Surface Epithelium
Gastric Glands
Secretory Cells
Digestive Processes
Regulation of Gastric Secretion
Gastric Motility and Emptying
Stomach functions
figure 24.11
Mixes saliva, food & gastric juices to form
__________ – semifluid mixture of partially
digested food
reservoir for holding before release into s.i.
secretes ___________ (2-3L/day), contains:
HCl - kills bacteria & denatures proteins
Pepsin begins the digestion of proteins
Intrinsic factor- aids absorption of vitamin B12
Gastric lipase - aids digestion of triglycerides
Secretes ___________ into blood when
stomach is distended or pH is too high
Stomach anatomy
figure 24.11
 4 main regions:
Cardia
Fundus
Body
Pylorus
 Pyloric sphincter
 Lesser curvature
 Greater curvature
 ____________ – large folds of mucosa when
stomach is empty
Histology of stomach
fig 24.12, 13
 Mucosa – simple columnar = surface mucous
cells that extend into lamina propria forming
columns of ____________ secretory cells –
gastric glands which line gastric pits
 Mucous neck cells- secrete mucus
 Parietal cells- secrete intrinsic factor & HCl
 Chief cells- secrete pepsinogen & gastric lipase
____________________ cells also part of gastric gland
 G cells- secretes gastrin, which stimulates gastric acitivity,
located mainly at pyloric antrum
 Muscularis – 3 layers: outer-longitudinal, midcircular, inner-oblique
Chemical digestion in stomach
 food may remain in fundus for hr, salivary amylase works
 chyme mixes w/gastric juices & lingual lipase activated
 HCl denatures proteins & stimulates secretion of
hormones that promote flow of bile and pancreatic juice
 Also, acidity kills microbes
 Enzymatic digestion of proteins begins w/pepsin
 Gastric lipase breaks down short chain triglycerides
(BUT, optimum pH is 5-6)
 Pancreatic lipase is more important
 Small amt of nutrients absorbed in stomach
 Mucous cells absorb some water, ions, some f.a., drugs
(such as aspirin) & alcohol
What prevents auto-digestion?
_____________ secreted in inactive form:
pepsinogen  cannot digest proteins
inside chief cells
Pepsinogen  pepsin in presence of HCl or
other active pepsin
Stomach epithelium protected by 1-3 mm
of _______________ secreted by surface
mucous cells & mucous neck cells
3 Phases of digeston, p 937
 _________________ sight, smell, thought or
initial taste activates neural centers in cerebral
cortex, hypothalamus, & brain stem
Facial and glossopharygeal nerves stimulate saliva
secretion
Vagus nerve stimulates gastric juice secretion
(all to prep mouth & stomach for food to be eaten)
Phases (2)
figure 24.24
 _________________ food in stomach, digestion begins
 Neural regulation- when either of following occur, sets off
negative feedback loop  submucosal plexus activates
parasym & enteric neurons  peristalsis & gastric secretion
 Stretch receptors monitor distention
 Chemoreceptors monitor  pH
 Hormonal regulation- gastrin regulates gastric secretions,
released in response to distension, partially digested proteins,
 pH (food present), caffeine, or Ach (parasymp)
 Glands to secrete large amt. gastric juice
 Strengthens contraction of LES
  stomach motility
 Relaxes pyloric sphincter
Phases (3)
 _________________ – food enters small intestine, inhibitory effects
to slow exit of chyme from stomach (contrary to cephalic & gastric)
 Neural regulation: Enterogastric reflex- duodenum distention signals
stretch receptors  medulla oblongata  inhibit parasym & activate
symp. contraction of pyloric sphincter   gastric emptying
 Hormonal regulation: (see also table 24.8)
 Secretin – acidic chyme stimulates release
• stimulates secretion of pancreatic juice & bile, rich in HCO3 • Inhibits release of gastric juice
• Enhances effects of CCK
 Cholescystokinin (CCK)- secreted in response to a.a. & f.a.
•
•
•
•
stimulates secretion of pancreatic juice rich in digestive enzymes
causes ejection of bile from gallbladder
Causes opening of hepatopancreatic ampulla
induces satiety
Gastric motility and emptying
 Several minutes after food enters, peristaltic waves
body pylorus every 15-25 sec, create chyme, & forces
@ 3mL into duodenum = ________________
 During gastric phase, emptying is proportional to volume
ingested due to distension & stretching of smooth mus.
 Extremely large amts of food  motility
 2-4 hr after meal stomach usually empties
  carbs take least amt of time, followed by protein-rich meal
 Lipid rich meals cause slowest emptying
  f.a. & a.a. in duodenal chyme slows stomach motility
 Gastrin  motility, secretin and CCK  emptying
Small Intestine & Accessory Organs
Small Intestine
Duodenum, jejunum, ileum, ileocecal sphincter
Villi, Microvilli, plicae circulares
Liver
Hepatic portal vein & hepatic artery
Hepatic vein
Bile duct
Gallbladder
Pancreas
Functions of small intestine
 _________________ mixes chyme w/ digestive
juices & brings food into contact w/mucosa for
absorption
 _______________ propels chyme thru s.i.
 Completes digestion of carbs, proteins & lipids
 Begins & completes digestion of nucleic acids
 ____________ about 90% of nutrients and
water
Other 10% in stomach & large intestine
Peristalsis and Segmentation
Anatomy of small intestine
fig 24.17
3 regions:
Duodenum- shortest (10 in), retroperitoneal,
pyloric sphincter  jejunum
Jejunum- 3 ft, extends to the:
Ileum- 6 ft, joins w/ l.i. at ileocecal sphincter
Differences seen in microanatomy:
_______________ in duodenum
_______________ in ileum
Histology of small intestine fig 24.18, 19
 Epithelium– simple columnar, many types:
 ________________ cells- digest & absorb nutrients in chyme
 ________________cells- secretes mucus
 Additional glandular epithelial cells found in intestinal
glands- crypts of Lieberkuhn – in mucosa of all s.i.
 Paneth cells- secrete lysozyme (bacteriocide), are also
phagocytic
 Enteroendocrine cells:
 S cells- secretes secretin (stim secretion p.j & bile)
 CCK cells- secretes CCK (stim secretion p.e., release of bile)
 K cells- secretes (GIP) glucose-dependent insulinotropic peptide
Histology (2)
 In the lamina propria:
MALT
Solitary lymphatic nodules in ileum
Aggregated lymphatic follicles= Peyer’s patches in ileum
 Submucosa:
_____________ or _____________- secrete alkaline
mucus to neutralize gastric acid in chyme
Lymphatic tissue may extend into
 Serosa absent in duodenum
3 specializations that  absorption in s. i.
 _________________ (circular folds) – folds in mucosa &
submucosa, permanent ridges
 In duodenum to mid- ileum
 Enhance absorption by  SA & cause chyme to spiral thru
 ____________- fingerlike projection of mucosa,  SA
 Lacteal- lymph capillary inside villus
 nutrients absorbed thru epithelial cells pass thru wall of lacteal 
lymph or blood capillaries  blood
 ___________ - projections of apical surface of
absorptive epithelium
 greatly  surface area of plasma membrane
 form fuzzy line = brush border- contains several digest. enzymes
Digestion of carbohydrates
 ______________- destroyed by  pH
 ________________- in pancreatic juice, acts in
small intestine
Acts on glycogen and starch but not cellulose
 -dextrinase- brush border enzyme works on dextrins after amylases work on starch
 3 brush border enzymes- ________ _________
________ work on their respective
disaccharides  monosaccharides  absorbed
Digestion of proteins
 Starts in stomach
 __________ fragments proteins  peptides
 Pancreatic juice enzymes continue protein 
peptide, each breaking peptide bonds between
different a.a.
Trypsin
Chymotrypsin
Carboxypeptidase
Elastase
 __________ of brush border complete digestion
Aminopeptidase- cleaves a.a. at amino end
Dipeptidase- splits dipeptides
Digestion of lipids
 Lipase- splits triglycerides & phospholipids
Lingual- works in stomach
Gastric- works in stomach
Pancreatic- works in s.i., where most lipid digestion
occurs
 Triglycerides  f.a. and monoglyceride
 In order to be digested, lipid globules must be
___________ - globule is broken down to
several globules, done by _____________
Bile salts- amphipathic molecules that interact with lipid
globule and watery intestinal chyme
 surface area of small globule allows for area on which
pancreatic lipase works
Absorption
figure 24.20
Passage of digested nutrients from
___________ to_______ or _________
Diffusion
Facilitated diffusion
Osmosis
Active transport
___% in _________, 10% in stomach & l.i.
Undigested & unabsorbed passes thru l.i.
Monosaccharides
Form in which all carbs are absorbed
Lumen  apical surface via facilitated
diffusion (fructose) or active transport
(glucose & galactose)
All 3 monos  thru basolateral membrane
to capillaries  liver via hepatic portal
system  general circulation (if not
removed by hepatocytes)
Amino acids, dipeptides, tripeptides
Most proteins absorbed via active
transport in duodenum & jejunum
½ proteins come from food
½ digestive juices and dead cells
________% of protein in s.i. is absorbed
Different transporters for different a.a.
a.a.  basolateral membrane to
capillaries via diffusion liver (like monos)
Absorption of lipids
 Via simple diffusion
 Bile salts surround f.a. & monoglycerides in chyme
making more soluble, form tiny spheres= __________
 Micelles can also solubilize vitamins A, E, D, K & cholesterol
 Micelles ferry f.a. & monoglycerides to brushed border 
diffuse micelle  absorptive cells
 Inside cell recombine form triglycerides
 Aggregate w/ phospholipids & cholesterol coated w/protein =
_____________ exocytosed  lacteals  liver & adipose tissue
 Lipoprotein lipase breaks down triglycerides in chylomicron & f.a.
& glycerol diffuse  hepatocytes or adipocytes
 Bile salts reabsorbed in ileum  liver for recycling
Lipoprotein
figure 25.13
Nutrients to blood and lymph
Liver functions
 produces___- lipid emulsifier *main digestive fnc
 filters blood- destroying worn out RBC, WBC,
bacteria & other foreign material in venous blood
draining GI tract
 metabolizes carbs, lipids, protein
 processes drugs & hormones
 Excrete _______- byproduct hemoglobin break
 storage of glycogen, some vitamins & minerals
 Phagocytosis – ____________
 Activation of vitamin D
Liver anatomy & histology fig 24.15
Right and left lobe, falciform ligament
Lobule = functional unit of liver
6 sided, consists of hepatocytes arranged
around a central vein. Blood flows from:
_________ at each corner  _________ c.v.
Bile duct- collect bile flowing thru canaliculi from
hepatocytes
Hepatic portal vein- blood from GI tract
Hepatic artery- oxygenated blood
Kupffer cells in sinusoids (large capillaries)
Hepatic blood flow
figure 24.16
Gallbladder
Bile secreted by hepatocytes stored here
Mucosa– simple columnar, rugae
Smooth muscle causes bile _________
Along w/storage, functions to concentrate
bile
water & ions absorbed in mucosa
Flow of bile
figure 24.14
Bile production & release from g.b.
 Hepatocytes secrete 800-1000mL/day
Continuously release bile
 _____ – neutral pH, water, bile salts, cholesterol,
lecithin, bile pigments (biliruben), & ions
  production & secretion if portal blood contains
more bile acids
  as digestion & absorption continue in s.i. bile
release increases
 When absorption , bile  gallbladder
_____________________ closed
Glucose, glycogen & gluconeogenesis
 Liver helps maintain normal blood glucose
Blood glucose 
 breakdown glycogen & release glucose
 Convert a.a. & lactic acidglucose=gluconeogenesis
 Convert galactose & fructose to glucose
Blood glucose 
 Convert glucose to glycogen & triglycerides
 _______________ – glucose formation from
non-carbohydrate source
When glycogen is depleted, lipids & proteins catabolized
Glycogenesis & glycogenolysis
25.11
Gluconeogenesis
figure 25.12
Lipid metabolism
figure 25.14
Catabolism:
__________ - triglyercides  glycerol & f.a
Done by lipases
Necessary for liver, muscle, adipose: f.a.  ATP
Anabolism:
_________ - liver & adipose: glu & a.a.lipids
Occurs when consume too many calories
Triglyercides can be:
• Stored in adipose tissue
• More rxns  lipoprotein, phospholipids, & cholesterol
Plasma proteins & urea production
____________ (removal of NH2 or amine
group from a.a.) occurs in hepatocytes
a.a. used for:
ATP production
Converted to carbohydrates and fats
Ammonia (NH3) is toxic & converted to urea
Hepatocytes synthesize most ________
_________:
Globulins & albumin (transport)
Prothrombin & fibrinogen (clotting)
Pancreas
figure 24.15
 99% acini- ___________ function
Secrete fluid & enzyme mixture = pancreatic juice
 Pancreatic amylase- carbohydrate-digesting enzyme
 Trypsin, chymotrypsin, carboxypeptidase, & elastase- protein digesting
enz
 Pancreatic lipase- triglyceride digesting enzymes
 Ribonuclease & deoxyribonuclease- digest n.a.
 1% Islets of Langerhans – __________ function
Glucagon
Insulin
Somatostatin
Pancreatic polypeptide
Pancreas (2)
 2 passageways secretions  small intestine:
pancreatic duct- joins common bile duct
duodenum thru h.p. amp
accessory duct = duct of Santorini
 Pancreatic juice secretion stimulated by:
_______________
_________ & ________ (in s.i) CCK also
opens hepatopancreatic ampulla
Large Intestine
Anatomy
Digestive Processes
Water
Electrolytes
Vitamins
Defecation
Functions of the large intestine
Haustral churning, peristalsis, &
_____________ drive contents of
colonrectum
Bacteria in large intestine convert proteins
to a.a., break down a.a., produce some B
vitamins & ____________
Absorbs some water, ions & vitamins
Forming feces
______________ emptying the rectum
Anatomy of large intestine
fig 24.22
 cecum ascending transverse
descending sigmoid colon rectum  anal
canal
 Epithelium mostly simple columnar – absorptive
& goblet cells, and are found lining intestinal
glands– highly mucus prod.
 ___________ – 3 longitudinal bands of muscle
extending along most l.i.,
Tonic contractions create pouches - ____________
Digestion in large intestine
 _______________ – after meal, ileal peristalsis to force
all chyme from ileum  cecum
 Gastrin also relaxes ileocecal sphincter
 Haustral churning- remain relaxed but after distended
will contract
 Peristalsis- slower than other areas of tract
 Mass peristalsis- strong wave from mid-transverse colon
quickly sending contents to rectum
 Is called a gastrocolic reflex- initiated by food in stomach
 Usually takes place 3-4 times/ day (immediately after meal)
 ____________ secreted, just mucus
 ____________ prepares chyme for elimination
Absorption, feces, & defecation
 Chyme in l.i. 3-10 hr  solid or semi-solid due to water
absorption = feces
 Water, inorganic salts, GI mucosal cells, bacteria, products of
bacterial decomp, unabsorbed digested & undigested material
 Water absorption here is crucial
 0.5 -1.0 L enters, 100 - 200mL leaves
 Absorbs ions (Na+, Cl-) & some vitamins
 Mass peristalsis _____________ – distention 
impulse  parasym motor neuron causes contraction,
also external anal sphincter- voluntary
 Bowel movements depend on diet, health, stress
 Amt varies: 2-3/day to 3-4/week
Daily fluids