Transcript Chapter 17 The Digestive System

The Digestive System
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Digestion
• Definition—process that transforms food into a
form that can be absorbed and used by cells
– Mechanical digestion—chewing, swallowing, 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 having smaller molecules; brought
about by digestive enzymes
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The Digestive System
• Alimentary canal or GI tract
– From mouth to anus—9 m (29 feet)
– Involved in digestion, absorption and metabolism
of nutrients
• System includes main and accessory organs
– Main organs: mouth, pharynx, esophagus,
stomach, small intestine, large intestine, rectum,
and anal canal
– Accessory organs: teeth and tongue, salivary
glands, liver, gallbladder, pancreas, and vermiform
appendix
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Digestion
• Enzymes and chemical digestion
– Enzymes—protein molecules that act as
catalysts, speeding up chemical reactions
– Chemical digestion—specific enzymes
speed up breakdown of specific molecules
and no others
– Hydrolysis—enzymes speed up reactions
that add water to break large molecules
into smaller molecules
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Digestion
• Carbohydrate digestion—mainly in
small intestine
– Pancreatic amylase—changes starches to
maltose
– Intestinal juice enzymes
• Maltase—changes maltose to glucose
• Sucrase—changes sucrose to glucose
• Lactase—changes lactose to glucose
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Digestion
• Protein digestion—starts in stomach;
completed in small intestine
– Gastric juice enzymes, rennin and pepsin,
partially digest proteins
– Pancreatic enzyme, trypsin, completes
digestion of proteins to amino acids
– Intestinal enzymes, peptidases, complete
digestion of partially digested proteins to
amino acids
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Digestion
• 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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Absorption
• Digested food moves from intestine into
blood or lymph
• Absorption site—foods and most water
are absorbed from small intestine; some
water also absorbed from large intestine
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Mouth
• Also known as oral cavity—hollow
chamber with a roof, floor, and walls
• Roof—formed by hard palate (parts of
maxillary and palatine bones) and soft
palate (an arch-shaped muscle
separating mouth from pharynx)
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Mouth
• Uvula—a downward projection of the
soft palate
– Uvula and soft palate prevent food and
liquid from entering nasal cavities
– Assists in speech and swallowing
(deglutition)
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Mouth
• Floor—formed by tongue
• Lingual frenulum — helps anchor the
tongue to the floor of the mouth
– Taste buds—found in many papillae
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Teeth
• Types of teeth—incisors, cuspids,
bicuspids, and tricuspids
– Deciduous (also known as baby or
primary) teeth—full set equals 20 teeth
• First tooth erupts at about 6 months
• Complete set in place at about 2 years of age
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Teeth
• Permanent teeth—full set equals 32 in
most; 28 teeth is a normal variation in
others
– First permanent tooth erupts at about 6
years of age
– Set complete between ages 17 and 24
years
• Structures of a typical tooth—crown,
neck, and root
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Disorders of the
Mouth and Teeth
• Infections, cancer, congenital defects,
and other disorders can cause serious
complications including malnutrition
• Leukoplakia—precancerous mouth tissue
– Snuff dipper’s pouch—from use of chewing tobacco
– Squamous cell carcinoma—most common form of
mouth cancer
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Disorders of the
Mouth and Teeth
• Dental caries “cavity”
– Food + bacteria = acid
• Gingivitis—gum inflammation or infection
– Most cases result from poor oral hygiene
– Can be a complication of diabetes, vitamin deficiency, or
pregnancy
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Disorders of the
Mouth and Teeth
• Thrush, or oral candidiasis—caused by yeastlike
fungal organism
– Patches of “cheesy”-looking exudate form over an
inflamed tongue and oral mucosa
• Periodontitis—inflammation of periodontal
membrane
– Often a complication of advanced or untreated gingivitis
– Leading cause of tooth loss among adults
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Disorders of the
Mouth and Teeth
• Cleft lip and cleft palate are most common
types
– May occur alone or together
– Caused by failure of mouth structures to fuse during
embryonic development
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Salivary Glands
• Three pairs of salivary glands
– Secrete about 1 L of saliva/day
– Located outside of GI tract
– Convey secretions via ducts
– Parotid glands—largest of salivary glands
– Located in front of ear at angle of jaw
– Ducts open into mouth opposite second molars
– Inflamed in mumps
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Salivary Glands
• Submandibular glands—ducts open on
either side of lingual frenulum
• Sublingual glands—ducts open into
floor of mouth
• Saliva contains salivary amylase—
begins digestion of carbohydrates
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Pharynx
• Muscular tube (throat) lined with
mucous membrane
• Functions as part of both respiratory
and digestive systems
• Subdivided into three anatomical
segments
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Wall of the Digestive Tract
• Lumen—hollow space within the “tube” of
the digestive tract
• Tissue layers of the wall of the digestive
tube from inside to outside
– Mucosa—mucous epithelium
– Muscularis—two layers of smooth muscle
that move food through the tube by rhythmic
muscular waves known as peristalsis
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Wall of the Digestive Tract
• Tissue layers (cont’d)
– Serosa—serous membrane that covers the
outside of abdominal organs
• Composed of visceral peritoneum in abdominal
cavity
• It attaches the digestive tract to the wall of the
abdominopelvic cavity by forming folds called
mesenteries
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Esophagus
• Muscular, mucus-lined tube about 25 cm (10
inches) long
• Connects pharynx with stomach
• Muscular walls help push food toward stomach
• Sphincters at each end of esophagus help keep
ingested material moving in one direction down
the tube
– Upper esophageal sphincter (UES)
– Lower esophageal sphincter (LES)
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Esophagus
• GERD—gastroesophageal reflux disease
– Backflow of acidic stomach contents into
esophagus causes symptoms of heartburn
and indigestion
– Mild symptoms treated by nonsurgical
measures include dietary changes, weight
loss, acid-blocking or buffering medications,
and drugs that strengthen LES
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Esophagus
• GERD
– Severe and frequent episodes of GERD can
trigger asthma attacks, cause severe chest pain,
bleeding, or narrowing and chronic irritation of
esophagus (Figure 17-12)
– Untreated GERD may result in a precancerous
condition called Barrett esophagus
– Common symptom of hiatal hernia
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Stomach
• Pouch for food that lies in upper part of
abdominal cavity just under diaphragm
– The size of a large sausage when empty
– Expands considerably after a large meal
• Contraction of muscular walls of
stomach mixes food with gastric juice
and breaks it down into chyme
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Stomach
• Mucous membrane lines the stomach
– Membrane lies in folds (rugae) when stomach is
empty
– Many microscopic glands secrete gastric juice and
hydrochloric acid into stomach
• Divisions of stomach—fundus, body, and
pylorus
• Pyloric sphincter muscle closes opening of
pylorus (lower part of stomach) to retain food
to facilitate partial digestion
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Disorders of the Stomach
• Gastroenterology—study of stomach
and intestines and their diseases
– Stomach is site of numerous diseases and
conditions
– Gastric diseases often exhibit the following
signs or symptoms: gastritis
(inflammation), anorexia (appetite loss),
nausea (upset stomach), and emesis
(vomiting)
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Disorders of the Stomach
• Pylorospasm—abnormal spasms of the pyloric
sphincter
– Common in infants
– Pyloric stenosis is similar abnormality—obstructive
narrowing of the pyloric opening
• Ulcers—open wounds caused by acid in gastric
juice
– Often occurs in duodenum or stomach
– Associated with infection by the bacterium
Helicobacter pylori and use of NSAIDs
– Current treatment involves triple therapy
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Disorders of the Stomach
• Stomach cancer
– Associated with consumption of alcohol or
preserved food and use of chewing
tobacco
– No practical way to screen for early stages
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Small Intestine
• About 7 m (20 feet) long but only 2 cm
or so in diameter
• Divisions
– Duodenum
– Jejunum
– Ileum
• Wall—contains smooth muscle fibers
that contract to produce peristalsis
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Small Intestine
• 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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Disorders of the Small Intestine
• Enteritis—intestinal inflammation
• Gastroenteritis—inflammation of
stomach and intestines
• Malabsorption syndrome—group of
symptoms resulting from failure to
absorb nutrients properly (anorexia,
abdominal bloating, cramps, anemia,
and fatigue)
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Liver and Gallbladder
• Liver
– Size and location
• Liver is largest gland
• Fills upper right section of abdominal cavity and
extends over into left side
– Classified as exocrine gland
• Secretes bile
• Has a variety of metabolic functions
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Liver and Gallbladder
• Liver
– Ducts
• Hepatic—drains bile from liver
• Cystic—duct by which bile enters and leaves gallbladder
• Common bile—formed by union of hepatic and cystic
ducts and drains bile from hepatic or cystic ducts into
duodenum
• Gallbladder
– Location—undersurface of the liver
– Function—concentrates and stores bile produced
in the liver
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Disorders of the Liver and
Gallbladder
• Gallstones—calculi (stones) made of
crystallized bile pigments and calcium
salts
– Cholelithiasis—condition of having
gallstones
– Cholecystitis—inflammation of the
gallbladder; may accompany cholelithiasis
– Stones can obstruct bile canals, causing
jaundice
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Disorders of the Liver and
Gallbladder
• Hepatitis—liver inflammation
– Characterized by liver enlargement,
jaundice, anorexia, discomfort, gray-white
feces, and dark urine
– Caused by a variety of factors—toxins,
bacteria, viruses, and parasites
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Disorders of the Liver and
Gallbladder
• Cirrhosis—degeneration of liver tissue
involving replacement of normal (but
damaged) tissue with fibrous and fatty
tissue
• Portal hypertension—high blood pressure
in the hepatic portal veins caused by
obstruction of blood flow in a diseased
liver; may cause varicosities of
surrounding systemic veins
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Pancreas
• Location—behind stomach
• Functions
– Pancreatic cells secrete pancreatic juice
into pancreatic ducts; main duct empties
into duodenum
– Pancreatic islets (of Langerhans)—cells
not connected with pancreatic ducts;
secrete hormones glucagons and insulin
into the blood
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Pancreas
• Pancreatic disorders
– Pancreatitis—inflammation of pancreas
• Acute pancreatitis results from blocked ducts
that force pancreatic juice to backflow
• Pancreatic enzymes digest the gland
– Cystic fibrosis—thick secretions block flow
of pancreatic juice
– Pancreatic cancer is very serious—fatal in
the majority of cases
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Large Intestine
• Size and location—1.5 m long; forms lower,
or terminal, portion of digestive tract
• Divisions
– Cecum
– Colon—ascending, transverse, descending, and
sigmoid
– Rectum
– Anal canal
• Opening to exterior—anus
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Disorders of the Large Intestine
• Disorders of the large intestine often relate to
abnormal motility (rate of movement of
contents)
– Diarrhea—results from abnormally increased
intestinal motility; may result in dehydration or
convulsions
– Constipation—results from decreased intestinal
motility
– Diverticulitis (inflammation of abnormal
outpouchings called diverticula)—may cause
constipation
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Disorders of the Large Intestine
– Colitis—general name for any inflammatory
condition of the large intestine
– Colorectal cancer—a common malignancy
of the colon and rectum associated with
colonic polyps; advanced age; low-fiber,
high-fat diets; and genetic predisposition
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Appendix and Appendicitis
• Vermiform appendix is blind tube attached
directly to cecum; no important digestive
function in humans
• Appendicitis—inflammation or infection of
appendix
– If appendix ruptures, infectious material may
spread to other organs
– Most common acute abdominal condition requiring
surgery
– Affects 7% to 12% of population younger than 30
years
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Peritoneum
• Description—large sheet of serous
membrane
– Parietal layer of peritoneum lines
abdominal cavity
– Visceral layer of peritoneum covers
abdominal organs
– Peritoneal space lies between parietal and
visceral layers
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Peritoneum
• Extensions of peritoneum—largest are
the mesentery and greater omentum
– Mesentery—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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Peritoneum
• Peritonitis—inflammation of peritoneum
resulting from infection or other irritant;
often a complication of ruptured
appendix
• Ascites—abnormal accumulation of fluid
in peritoneal space, often causes
bloating of abdomen
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