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 Chapter 15 - The Digestive System
 Irregular tube; open at both ends, called
“Alimentary canal” or “Gastrointestinal (GI)
Tract”
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29 feet long (adults) - 9 meters
Food & other substances that enter tube are not
really inside body
Passageway of food: broken down (digested) and
absorbed thru walls < entering body - cells
 Both - Mechanical & Chemical Digestion
Break Down of Food
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Teeth- first physical breakdown
Stomach-churning of food (physical)
Mouth- first chemical breakdown (salvia)
Digestive enzymes throughout GI tract
Digestion - Process where large food particles
reduced to absorbable molecules
 Absorption - Process of small molecules passing
thru digestive system walls into body
Key Organs of the GI Tract
 Know Main & Accessory organs, Table 16-2; page
476
 Small Intestine : Duodenum, Jejunum, & Ileum
 Large Intestines (elimination > feces):
 Cecum, Colon: Ascending, Transverse, Descending,
Sigmoid
 Wall of Digestive Tract 
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Mouth to anus
Four layers of tissue; surrounding the hollow
space within the tube “lumen”
May vary in structure in different organs
Mucosa or mucous membrane - tough in
esophagus, delicate, for absorption or
secretion in rest of tract
Submucosa - connective tissue, blood
vessels & nerves
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Muscularis - 2 layers, responsible for
wavelike, rhythmic contractions
(peristalsis), moves contents, assists in
mixing & mechanical breakdown
Serosa - outermost covering, composed
of visceral peritoneum
Mesentery - double folded peritoneal
tissue, anchors loops of digestive tract to
posterior wall of abdominal cavity
 Mouth 
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Oral cavity - hollow chamber (roof, a floor, &
walls)
Entrance of food; digestion begins immediately
Mucous membranes > mucus, protects against
digestive juices & lubricates food passage
This mucous protects & lubricates
Hard palate - bony structure, front portion
Soft palate - posterior, chiefly muscles
Uvula - cone-shaped process hanging down from
soft palate. W/ help of soft pal., prevents food or
liquid from entering nasal cavity
 Floor of the mouth  Tongue - skeletal muscular structure, covered
w/ mucous membrane
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Anchored to bones in skull > hyoid bone
 Frenulum- thin membrane; attaches tongue to
floor of mouth
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Tongue-tied: too short
 Papillae: small elevations on surface
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Vallate type - largest, inverted V-shaped row of
about 10-12 mushroomlike elevations - tastebuds
 Teeth  Four major types  Incisors - (sharp/cutting)
 Canines - cuspids (pierce/tear)
 Premolars - bicuspids & Molars tricuspids (grinding/crush)
 Mastication > chewing of food
 Forms a bolus > ready for swallowing
 By age 2 - full set 20 teeth (cut 1st - 2 yrs.)
 By age 17 to 24 - 32 permanent teeth (cut
1st - 6 yrs.)
 Typical Tooth  Three main parts 
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Crown - visible, covered w/ enamel (hardest
tissue in body)
Neck - narrow portion surrounds by gum tissue
(gingiva)
Root - fitted into socket in upper or lower jaw,
lined by fibrous, periodontal membrane
Inside Structure - Enamel on outside, Dentin, Pulp
cavity (blood vessels & nerves) moving inward
 Salivary Glands  3 Pairs - ducts drain saliva into oral cavity,
secretes about 1 liter/day
 Parotid - in front of each ear (mumps tender)
 Submandibular - ducts by fernullum
 Sublingual - ducts into floor of mouth
 Saliva contents - salivary amylase (begins
CHO digestion), mucus (moistens food)
 Pharynx  Behind nasal cavity & mouth
 Tubelike structure made of muscles, lined
w/ mucous membrane
 Part of respiratory & digestive systems
 Esophagus  Passage for food to stomach
 Tube-like structure, 10 inches long
 Mucous lined
 GERD - often caused by hiatal hernia
 Stomach  Upper part of abdominal cavity, under
diaphragm
 Pouch for food, hollow, expands (can push
up on diaphragm > discomfort)
 Lower esophageal sphincter (LES) or
cardiac sphincter - rings of muscle tissue at
end of esophagus - keeps food from
reentering the esophagus when the stomach
contracts
Chyme - semi-solid mixture, produced
by contraction of stomach muscles that
mixes food w/ gastric juices
 Stomach contractions  Created by 3 layers of muscles, run
lengthwise, around, obliquely
 Makes stomach one of strongest organs >
peristalsis
 Breaks food into tiny particles
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Mucous membranes line stomach contains gastric glands > secrete gastric
juice & hydrochloric acid
 When empty, wrinkled folds - rugae
 Three divisions of stomach  Fundus, body, pylorus
 Pyloric sphincter - holds food in stomach,
empties contents slowly into small
intestine
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Ulcer - carterlike wound or sore in
membrane of stomach
• 1 in 10 persons suffer in USA
• Helicobacter pylori bacterium (H. pylori)
 Small Intestines 
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Portion of digestion tract that extends from the
pylorus to the ileocecal valve
12- 20 feet in length, coiled, convoluted, and
occupies most of the abdominal cavity
Intestinal glands - secrete digestive juices
Smooth muscle wall - contracts > peristalsis
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Plicea - circular folds covered w/ villi,
increases surface area > absorption
In or on the villi Blood capillaries - absorb CHO & protein end
products (glucose & amino acids)
Lacteal - lymphatic vessel - absorb lipids
Microvilli - brushlike border, > surface more
Chemical digestion - most occurs in 1st
subdivision of duodenum
Minor & major duodenal papillae - ducts
where pancreatic enzymes & bile enter small
intestine
 Liver 
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Large organ, fills R upper abdominal cavity
Exocrine gland - secretes bile into ducts
Hepatic - means liver
Bile - essential for breaking up or emulsification
of fats
CCK (cholecystokinin) - hormone secretion
triggered by lipids in chyme > makes gallbladder
contract & release bile
Drains from common bile duct into duodenum
 Gallbladder - concentrates & stores bile
 Pancreas  C-shaped, exocrine gland that lies behind the
stomach & duodenum
 Pancreatic juice - most important digestive
juice - contains enzymes for all 3 food groups
 Sodium bicarbonate (alkaline substance) neutralizes hydrochloric acid
 Enters small intestine thru same duct as bile
 Islets of Langerhans - hormones produced
 Pancreatitis - inflammation (blockage, CF)
 Large Intestine  Begins with the ending of the ileum at the
ileocecal valve - called the cecum
 Approximately 5 feet in length, much larger
in diameter than small intestine
 Contents - not called chyme
 Function - reabsorb water & salts
 Material acted on by bacteria > more nutrients
from cellulose & other fibers
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Synthesis Vit. K needed for blood clotting,
Production of some B-complex vit.
 Not as well suited for absorption as small
intestine - no villi
 Normal passage of material thru large intestine
- 3 to 5 days
 Subdivisions - flow in GI Tract one-way
 Cecum - pouchlike area
 Ascending colon - right side of body
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Bends at hepatic or right colic flexure
 Transverse colon - extends across front
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Bends at splenic or left colic flexure
 Descending colon - left side abdomen
 Sigmond colon - S-shaped segment,
terminates in rectum
 Anal canal - terminal end of rectum, ends at
external opening - anus
 Inner anal sphincter - involuntary, smooth
muscle, keeps anus closed except during
defecation
 Outer anal sphincter - striated, voluntary
muscle
 Appendix 
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Vermiform appendix - “worm-shaped”, tubular
structure, blind tube
No important digestive fnc. - digest cellulose
Appendicitis - inflammation
 Peritoneum 
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Large, moist, slippery sheet of serous membrane
Peritoneal space - small space between parietal &
visceral layers - surfaces slide freely
Retroperitoneal - organs outside peritoneum
Extensions of peritoneum-mesentary, greater
omentum - both assist in anchoring abd. contents
 Digestion -Chemical & mechanical breakdown
 CHO - amylase in mouth, slight effect
 amylase from pancreas - into small intestine
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Absorption of simple sugars (glucose)
 Proteins - stomach (HCL/pepsinogen> pepsin)
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Finished in small intestine by pancreatic (trypsin)
& peptidases in intestinal juice
Amino acids - basic protein units
 Fats - in small intestine
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Bile emulsification of fats > pancreatic
lipase > fatty acids & glycerol
 Key digestive juices & enzymes
 * page 410 Table 15-2
 Absorption - taking food, breaking it
down in form for utilization of body
 Just as important as digestion