Download Chapter 15 The Digestive System

Chapter 15
The Digestive System
DIGESTIVE SYSTEM
(FIGURE 15-1)
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Irregular tube called alimentary canal or
gastrointestinal (GI) tract
Food must first be digested, then
absorbed, and later metabolized
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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—mucous epithelium
Submucosa—connective tissue
Muscularis—two layers of smooth muscle
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 15-4)
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 15-4)
Typical tooth (Figure 15-5)
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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
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Types of teeth—incisors, cuspids, bicuspids, and
tricuspids
Twenty teeth in temporary set; average age for
cutting first tooth about 6 months; set complete at
about 2 years 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 15-6)
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SALIVARY GLANDS
(FIGURE 15-7)
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Parotid glands—largest salivary glands
Submandibular glands—open into mouth
on either side of frenulum
Sublingual glands—open into floor of
mouth
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PHARYNX
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Subdivided into three anatomical
components:
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Nasopharynx
 Oropharynx
 Laryngopharynx
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ESOPHAGUS
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Connects pharynx to stomach
Dynamic passageway for food
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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 gastroesophageal
(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
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Wall—contains smooth muscle fibers that
contract to produce peristalsis
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
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
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, 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 15-2)
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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)
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
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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
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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
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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