Download Chapter 15 The Digestive System

Chapter 15
The Digestive System
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OVERVIEW OF THE DIGESTIVE SYSTEM
(Figure 15-1 and Table 15-1)
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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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PRIMARY MECHANISMS OF THE
DIGESTIVE SYSTEM (Table 15-2)
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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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WALL OF THE DIGESTIVE TRACT
(Figure 15-2)
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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)
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• 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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MOUTH
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Roof—formed by hard palate (parts of maxillary and
palatine bones) and soft palate, an arch-shaped
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)
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Three main parts—crown, neck, and root
 Enamel, which covers the crown, is hardest tissue
in body
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MOUTH (cont.)
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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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SALIVARY GLANDS
(Figure 15-7)
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Saliva—exocrine gland secretion flows into ducts
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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
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• 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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SALIVARY GLANDS
(Figure 15-7) (cont.)
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Submandibular glands—(Figure 15-7)
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Mixed gland—produces both serous-type and mucustype 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)
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Produce only mucus-type saliva
 Multiple ducts open into floor of mouth
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PHARYNX
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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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ESOPHAGUS
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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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STOMACH (Figure 15-8)
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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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SMALL INTESTINE (Figure 15-9)
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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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SMALL INTESTINE (cont.)
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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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LIVER AND GALLBLADDER
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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
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Location—undersurface of the liver
Function—concentrates and stores bile produced in
the liver
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PANCREAS
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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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LARGE INTESTINE (Figure 15-12)
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Divisions
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Cecum
 Colon—ascending, transverse, descending, and
sigmoid Rectum
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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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APPENDIX
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Blind tube off cecum
No important digestive functions in humans
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PERITONEUM (Figure 15-14)
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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
X-ray studies of the GI tract—radiopaque contrast
medium used to help visualize structures in study
images
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DIGESTION (Table 15-3)
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Definition—transforms foods into substances that can
be absorbed and used by cells
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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
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Enzymes are specialized protein molecules that act as
catalysts
 Breakdown process called hydrolysis
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DIGESTION (cont.)
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Carbohydrate digestion—mainly in small intestine
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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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DIGESTION (cont.)
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Protein digestion—starts in stomach; completed in
small intestine
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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
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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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ABSORPTION
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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
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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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