Transcript Chapter 15 The Digestive System

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Irregular tube called alimentary
canal or gastrointestinal (GI) tract
and accessory organs
of digestion
 Food must first be digested, and
absorbed

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Ingestion—complex foods taken into the GI tract
Digestion—group of processes that break complex nutrients into
simpler ones
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Mechanical digestion—breakup of large chunks of food into smaller bits
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Chemical digestion—breaks large molecules into smaller ones
Motility—a number of GI movements resulting from muscular
contraction
Secretion—release of digestive juices and hormones that
facilitate digestion
Absorption—movement of digested nutrients into the internal
environment of the body
Regulation—neural, hormonal and other mechanisms that
regulate digestive activity
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Digestive tract described as tube that extends
from mouth
to anus
 Wall of the digestive tube is formed by four
layers of tissue:
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• Mucosa—type varies depending on GI location (tough and stratified
or delicate and simple epithelium); mucus production
• Submucosa—connective tissue layer
• Muscularis—circular, longitudinal, and oblique (in stomach) layers
of muscle important in GI motility
 Peristalsis—“wavelike” movement pushes food down the tract
(Figure 15-3)
 Segmentation—“back-and-forth” movement (Figure 15-4)
• Serosa—serous membrane that covers the outside of abdominal
organs; it attaches the digestive tract to the wall of the
abdominopelvic cavity by forming folds called mesenteries
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Roof—formed by hard palate (parts of maxillary
and palatine bones) and soft palate, an archshaped muscle separating mouth from pharynx;
uvula, a downward projection of soft palate
(Figure 15-5)
Floor—formed by tongue and its muscles;
papillae, small elevations on mucosa of tongue;
taste buds, found in many papillae; lingual
frenulum, fold of mucous membrane that helps
anchor tongue to floor of mouth (Figure 15-5)
Typical tooth (Figure 15-6)
• Three main parts—crown, neck, and root
• Enamel, which covers the crown, is hardest tissue
in body
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Types of teeth—incisors, cuspids, bicuspids,
and tricuspids
Twenty teeth in deciduous or baby set;
average age for cutting first tooth about 6
months; set complete at about 30 months of
age
Thirty-two teeth in permanent set; 6 years
about average age for starting to cut first
permanent tooth; set complete usually
between ages of 17 and
24 years (Figure 15-5)
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
Saliva—exocrine gland secretion flows into
ducts
• Serous type—watery and contains enzymes
(salivary amylase) but no mucus
 Produced by serous-type secretory cells (Figure 15-7, B)
• Mucus type—thick, slippery and contains mucus
but no enzymes
 Lubricates food during mastication
 Produced by mucus-type secretory cells (Figure 15-7, B)

Parotid glands (Figure 15-7)
• Largest salivary glands
• Produces serous type saliva
• Mumps—infection of parotids
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Submandibular glands—(Figure 15-7)
• Mixed gland—produces both serous-type
and mucus-type saliva (Figure 15-7, B)
• Located below mandibular angle
• Ducts open on either side of lingual frenulum
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Sublingual glands—(Figure 15-7)
• Produce only mucus-type saliva
• Multiple ducts open into floor of mouth
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Anatomic components: nasopharynx,
oropharynx, laryngopharynx (see
anatomic description in Chapter 4 and
Figure 14-4)
 Oropharynx most involved segment in
digestive process of swallowing or
deglutition
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• Regulation of deglutition movements via
motor cortex of cerebrum (voluntary) and
“deglutition center” of brainstem
(involuntary)
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Connects pharynx to stomach
 Dynamic passageway for food
 Food enters stomach by passing
through lower esophageal sphincter
(LES) or cardiac sphincter
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Size—expands after large meal; about size of
large sausage when empty
Food enters stomach through (cardiac)
sphincter
Pyloric sphincter muscle closes opening
between pylorus (lower part of stomach) and
duodenum
Wall—many smooth muscle fibers; contractions
produce churning movements (peristalsis)
Lining—mucous membrane; many microscopic
glands that secrete gastric juice and
hydrochloric acid into stomach; mucous
membrane lies in folds (rugae) when stomach
is empty
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Size—about 7 meters (20 feet) long
but only 2 cm or so in diameter
 Divisions
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• Duodenum
• Jejunum
• Ileum
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Wall—contains smooth muscle fibers
that contract to produce peristalsis
and segmentation movements
 Lining—mucous membrane; many
microscopic glands (intestinal glands)
secrete intestinal juice; villi
(microscopic finger-shaped
projections from surface of mucosa
into intestinal cavity) contain blood
and lymph capillaries
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Size and location—liver is largest gland;
fills upper right section of abdominal
cavity and extends over into left side
 Liver secretes bile
 Ducts (Figure 15-10)
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• Hepatic—drains bile from liver
• Cystic—duct by which bile enters and leaves
gallbladder
• Common bile—formed by union of hepatic and
cystic ducts; drains bile from hepatic or cystic
ducts into duodenum
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Gallbladder
• Location—undersurface of the liver
• Function—concentrates and stores bile
produced in the liver
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Exocrine gland that lies behind
stomach
 Functions
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• Pancreatic cells secrete pancreatic juice
(most important digestive juice) into
pancreatic ducts; main duct empties into
duodenum
• Pancreatic islets (of Langerhans)—cells not
connected with pancreatic ducts; secrete
hormones glucagon and insulin into the
blood
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
Divisions
• Cecum
• Colon—ascending, transverse, descending,
and sigmoid Rectum
Food enters through ileocecal valve;
external opening called anus
 Wall—contains smooth muscle fibers
that contract to produce churning,
peristalsis, and defecation
 Lining—mucous membrane
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Blind tube off cecum
 No important digestive functions in
humans
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Definitions—peritoneum, continuous
serous membrane lining abdominal cavity
and covering abdominal organs; parietal
layer of peritoneum lines abdominal cavity;
visceral layer of peritoneum covers
abdominal organs; peritoneal space lies
between parietal and visceral layers
 Extensions—largest are the mesentery and
greater omentum
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• Mesentery is extension of parietal peritoneum, which attaches
most of small intestine to posterior abdominal wall
• Greater omentum, or “lace apron,” hangs down from lower edge
of stomach and transverse colon over intestines
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X-ray studies of the GI tract—radiopaque
contrast medium used to help visualize
structures in study images
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Definition—transforms foods into substances
that can be absorbed and used by cells
• Mechanical digestion—chewing (mastication),
swallowing (deglutition), and peristalsis break food into
tiny particles, mix them well with digestive juices, and
move them along the digestive tract
• Chemical digestion—breaks up large food molecules
into compounds that have smaller molecules; brought
about by digestive enzymes (Figure 15-15)
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Enzymes and chemical digestion
• Enzymes are specialized protein molecules that act as
catalysts
• Breakdown process called hydrolysis
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Carbohydrate digestion—mainly in
small intestine
• Pancreatic amylase—breaks
polysaccharides down to disaccharides
• Intestinal juice enzymes
 Maltase—changes maltose to glucose
 Sucrase—changes sucrose to glucose
 Lactase—changes lactose to glucose
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Protein digestion—starts in stomach;
completed in small intestine
• Gastric juice enzyme pepsin partially digests
proteins
• Pancreatic enzyme, trypsin, continues
digestion of proteins
• Intestinal enzymes, peptidases, complete
digestion of partially digested proteins and
convert them to amino acids
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Fat digestion
• Bile contains no enzymes but emulsifies fats
(breaks fat droplets into very small droplets)
• Pancreatic lipase changes emulsified fats to
fatty acids and glycerol in small intestine
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Definition—process by which digested food
moves from intestine into blood or lymph
Foods and most water minerals and vitamins
are absorbed from small intestine; some
water and vitamin K also absorbed from large
intestine
Surface area absorption
• Structural adaptations increase absorptive surface
area
• Fractal geometry—study of fragmented geometric
irregular shapes such as those in lining of intestine
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