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Chapter 14
The Digestive
System and Body
Metabolism
Lecture Presentation by
Patty Bostwick-Taylor
Florence-Darlington Technical College
© 2015 Pearson Education, Inc.
The Digestive System Functions
 Ingestion—taking in food
 Digestion—breaking food into nutrient molecules
 Absorption—movement of nutrients into the
bloodstream
 Defecation—elimination of indigestible waste
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Organs of the Digestive System
 Two main groups of organs
 Alimentary canal (gastrointestinal or GI tract)—
continuous, coiled, hollow tube
 These organs ingest, digest, absorb, defecate
 Accessory digestive organs
 Includes teeth, tongue, and other large digestive
organs
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Figure 14.1 The human digestive system: Alimentary canal and accessory organs.
Mouth (oral cavity)
Tongue
Parotid gland
Sublingual gland
Submandibular
gland
Salivary glands
Pharynx
Esophagus
Stomach
Pancreas
(Spleen)
Liver
Gallbladder
Small
intestine
Duodenum
Jejunum
Ileum
Anus
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Transverse colon
Descending colon
Ascending colon
Cecum
Sigmoid colon
Rectum
Appendix
Anal canal
Large intestine
Organs of the Alimentary Canal
 The alimentary canal is a continuous, coiled, hollow
tube that runs through the ventral cavity from
stomach to anus:
 Mouth
 Pharynx
 Esophagus
 Stomach
 Small intestine
 Large intestine
 Anus
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Mouth (Oral Cavity)
 Anatomy of the mouth
 The mouth (oral cavity)—mucous membrane–lined
cavity
 Lips (labia)—protect the anterior opening
 Cheeks—form the lateral walls
 Hard palate—forms the anterior roof
 Soft palate—forms the posterior roof
 Uvula—fleshy projection of the soft palate
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Mouth (Oral Cavity)
 Anatomy of the mouth (continued)
 Vestibule—space between lips externally and teeth
and gums internally
 Oral cavity proper—area contained by the teeth
 Tongue—attached at hyoid bone and styloid
processes of the skull, and by the lingual frenulum to
the floor of the mouth
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Mouth (Oral Cavity)
 Anatomy of the mouth (continued)
 Tonsils
 Palatine—located at posterior end of oral cavity
 Lingual—located at the base of the tongue
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Figure 14.2a Anatomy of the mouth (oral cavity).
Nasopharynx
Hard
palate
Oral
cavity
Soft palate
Lips (labia)
Palatine tonsil
Vestibule
Lingual tonsil
Lingual
frenulum
Tongue
Hyoid bone
Trachea
(a)
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Uvula
Oropharynx
Epiglottis
Laryngopharynx
Esophagus
Figure 14.2b Anatomy of the mouth (oral cavity).
Upper lip
Gingivae
(gums)
Hard palate
Soft palate
Uvula
Palatine tonsil
Oropharynx
Tongue
(b)
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Mouth
 Functions of the mouth
 Mastication (chewing) of food
 Tongue mixes masticated food with saliva
 Tongue initiates swallowing
 Taste buds on the tongue allow for taste
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Pharynx
 Food passes from the mouth posteriorly into the:
 Oropharynx—posterior to oral cavity
 Laryngopharynx—below the oropharynx and
continuous with the esophagus
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Figure 14.2a Anatomy of the mouth (oral cavity).
Nasopharynx
Hard
palate
Oral
cavity
Soft palate
Lips (labia)
Palatine tonsil
Vestibule
Lingual tonsil
Lingual
frenulum
Tongue
Hyoid bone
Trachea
(a)
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Uvula
Oropharynx
Epiglottis
Laryngopharynx
Esophagus
Pharynx
 The pharynx serves as a passageway for food,
fluids, and air
 Food is propelled to the esophagus by two skeletal
muscle layers in the pharynx
 Longitudinal inner layer
 Circular outer layer
 Alternating contractions of the muscle layers
(peristalsis) propel the food
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Esophagus (Gullet)
 Anatomy
 About 10 inches long
 Runs from pharynx to stomach through the
diaphragm
 Physiology
 Conducts food by peristalsis (slow rhythmic
squeezing) to the stomach
 Passageway for food only (respiratory system
branches off after the pharynx)
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Figure 14.5 Peritoneal attachments of the abdominal organs.
Diaphragm
Falciform ligament
Liver
Spleen
Gallbladder
Stomach
Visceral peritoneum
Greater omentum
Lesser
omentum
Pancreas
Duodenum
Transverse
colon
Mesenteries
Parietal peritoneum
Small intestine
Peritoneal
cavity
Uterus
Large intestine
Cecum
Rectum
Anus
Urinary bladder
(a)
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(b)
Alimentary Canal Nerve Plexuses
 Two important nerve plexuses serve the alimentary
canal
 Both are part of the autonomic nervous system
 Submucosal nerve plexus
 Myenteric nerve plexus
 Function is to regulate mobility and secretory
activity of the GI tract organs
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Stomach
 The stomach is a C-shaped organ located on the
left side of the abdominal cavity
 Food enters at the cardioesophageal sphincter from
the esophagus
 Food empties into the small intestine at the pyloric
sphincter (valve)
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Stomach
 Regions of the stomach
 Cardial part (cardia)—near the heart
 Fundus—expanded portion lateral to the cardiac
region
 Body—midportion
 Pylorus—funnel-shaped terminal end
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Stomach
 Stomach can stretch and hold 4 L (1 gallon) of food
when full
 Rugae—internal folds of the mucosa present when
the stomach is empty
 External regions
 Lesser curvature—concave medial surface
 Greater curvature—convex lateral surface
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Figure 14.4a Anatomy of the stomach.
Cardia
Fundus
Esophagus
Muscularis
externa
• Longitudinal layer
• Circular layer
• Oblique layer
Pylorus
Serosa
Body
Lesser
curvature
Rugae
of mucosa
Greater
curvature
Duodenum
(a)
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Pyloric
sphincter
(valve)
Pyloric
antrum
Figure 14.4b Anatomy of the stomach.
Fundus
Body
Rugae
of mucosa
(b)
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Pyloric
sphincter
Pyloric
antrum
Figure 14.5a Peritoneal attachments of the abdominal organs.
Diaphragm
Falciform ligament
Liver
Spleen
Gallbladder
Stomach
Greater omentum
Small intestine
Large intestine
Cecum
(a)
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Figure 14.5b Peritoneal attachments of the abdominal organs.
Diaphragm
Liver
Lesser
omentum
Pancreas
Stomach
Visceral peritoneum
Greater omentum
Duodenum
Transverse
colon
Mesenteries
Parietal peritoneum
Small intestine
Uterus
Peritoneal
cavity
Rectum
Urinary bladder
(b)
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Anus
Stomach
 Functions of the stomach
 Temporary storage tank for food
 Site of food breakdown
 Chemical breakdown of protein begins
 Delivers chyme (processed food) to the small
intestine
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Stomach
 Structure of the stomach mucosa:
 Simple columnar epithelium dotted by gastric pits
that lead to gastric glands
 Mucous cells produce bicarbonate-rich alkaline
mucus
 Gastric glands—situated in gastric pits and secrete
gastric juice, including:
 Intrinsic factor, which is needed for vitamin B12
absorption in the small intestine
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Stomach
 Structure of the stomach mucosa (continued)
 Chief cells—produce protein-digesting enzymes
(pepsinogens)
 Parietal cells—produce hydrochloric acid
 Mucous neck cells—produce thin acidic mucus
(different from the mucus produced by cells of the
mucosa)
 Enteroendocrine cells—produce a hormone called
gastrin
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Figure 14.4c Anatomy of the stomach.
Pyloric
sphincter
Gastric
pit
Gastric pits
Surface
epithelium
Gastric gland
Mucous
neck cells
(c)
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Parietal cells
Gastric
glands
Chief cells
Figure 14.4d Anatomy of the stomach.
Pepsinogen
HCI
Pepsin
Parietal cells
Chief cells
(d)
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Enteroendocrine
cell
Small Intestine
 The body’s major digestive organ
 Longest portion of the alimentary tube (2–4 m or
7–13 feet in a living person)
 Site of nutrient absorption into the blood
 Muscular tube extending from the pyloric sphincter
to the ileocecal valve
 Suspended from the posterior abdominal wall by the
mesentery
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Small Intestine
 Subdivisions
 Duodenum
 Attached to the stomach
 Curves around the head of the pancreas
 Jejunum
 Attaches anteriorly to the duodenum
 Ileum
 Extends from jejunum to large intestine
 Meets the large intestine at the ileocecal valve
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Small Intestine
 Chemical digestion begins in the small intestine
 Enzymes are produced by:
 Intestinal cells
 Pancreas
 Pancreatic ducts carry enzymes to the duodenum
 Bile, formed by the liver, enters the duodenum via
the bile duct
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Figure 14.6 The duodenum of the small intestine and related organs.
Right and left
hepatic ducts
from liver
Cystic duct
Common hepatic duct
Bile duct and sphincter
Accessory pancreatic duct
Pancreas
Gallbladder
Jejunum
Duodenal
papilla
Hepatopancreatic
ampulla and sphincter
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Main pancreatic duct and sphincter
Duodenum
Small Intestine
 Three structural modifications that increase surface
area for food absorption
1. Microvilli—tiny projections of the plasma
membrane (create a brush border appearance)
2. Villi—fingerlike projections formed by the mucosa
 House a capillary bed and lacteal
3. Circular folds (plicae circulares)—deep folds of
mucosa and submucosa
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Figure 14.7a Structural modifications of the small intestine.
Blood vessels
serving the small
intestine
Muscle
layers
Villi
(a) Small intestine
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Lumen
Circular folds
(plicae circulares)
Figure 14.7b Structural modifications of the small intestine.
Absorptive
cells
Lacteal
Villus
Blood
capillaries
Lymphoid
tissue
Intestinal
crypt
Muscularis
mucosae
Venule
Lymphatic vessel
Submucosa
(b) Villi
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Figure 14.7c Structural modifications of the small intestine.
Microvilli
(brush border)
(c) Absorptive
cells
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Large Intestine
 Larger in diameter, but shorter in length at 1.5 m,
than the small intestine
 Extends from the ileocecal valve to the anus
 Subdivisions:
 Cecum
 Appendix
 Colon
 Rectum
 Anal canal
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Large Intestine Anatomy
 Cecum—saclike first part of the large intestine
 Appendix
 Accumulation of lymphoid tissue that sometimes
becomes inflamed (appendicitis)
 Hangs from the cecum
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Large Intestine Anatomy
 Colon
 Ascending—travels up right side of abdomen
 Transverse—travels across the abdominal cavity
 Descending—travels down the left side
 Sigmoid—S-shaped region; enters the pelvis
 Sigmoid colon, rectum, and anal canal are located
in the pelvis
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Large Intestine Anatomy
 Anal canal ends at the anus
 Anus—opening of the large intestine
 External anal sphincter—formed by skeletal muscle
and under voluntary control
 Internal anal sphincter—formed by smooth muscle
and involuntarily controlled
 These sphincters are normally closed except during
defecation
 The large intestine delivers undigestible food
residues to the body’s exterior
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Figure 14.8 The large intestine.
Left colic
(splenic) flexure
Transverse
mesocolon
Right colic
(hepatic) flexure
Transverse
colon
Haustrum
Descending
colon
Ascending colon
Cut edge of
mesentery
Ileum (cut)
Ileocecal valve
Teniae coli
Sigmoid
colon
Cecum
Appendix
Rectum
Anal canal
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External anal sphincter
Large Intestine
 Goblet cells produce alkaline mucus to lubricate the
passage of feces
 Muscularis externa layer is reduced to three bands
of muscle called teniae coli
 These bands of muscle cause the wall to pucker
into haustra (pocketlike sacs)
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Accessory Digestive Organs
 Teeth
 Salivary glands
 Pancreas
 Liver
 Gallbladder
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Teeth
 Teeth masticate (chew) food into smaller fragments
 Humans have two sets of teeth during a lifetime:
1. Deciduous (baby or “milk”) teeth
 A baby has 20 teeth by age 2
 First teeth to appear are the lower central incisors
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Teeth
2. Permanent teeth
 Replace deciduous teeth between the ages of 6 and
12
 A full set is 32 teeth, but some people do not have
wisdom teeth (third molars)
 If they do emerge, the wisdom teeth appear between
ages of 17 and 25
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Classification of Teeth
 Incisors—cutting
 Canines (eyeteeth)—tearing or piercing
 Premolars (bicuspids)—grinding
 Molars—grinding
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Regions of a Tooth
 Two major regions of a tooth
1. Crown
2. Root
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Regions of a Tooth
1. Crown—exposed part of tooth above the gingiva
(gum)
 Enamel—covers the crown
 Dentin—found deep to the enamel and forms the
bulk of the tooth, surrounds the pulp cavity
 Pulp cavity—contains connective tissue, blood
vessels, and nerve fibers (pulp)
 Root canal—where the pulp cavity extends into the
root
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Regions of a Tooth
 Note: The neck is a connector between the crown
and root.
 Region in contact with the gum
 Connects crown to root
2. Root
 Cement—covers outer surface and attaches the
tooth to the periodontal membrane (ligament)
 Periodontal membrane holds tooth in place in the
bony jaw
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Salivary Glands
 Three pairs of salivary glands empty secretions into
the mouth
1. Parotid glands
 Found anterior to the ears
2. Submandibular glands
3. Sublingual glands
 Both submandibular and sublingual glands empty
saliva into the floor of the mouth through small ducts
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Salivary Glands
 Saliva
 Mixture of mucus and serous fluids
 Helps to moisten and bind food together into a mass
called a bolus
 Contains salivary amylase to begin starch digestion
 Dissolves chemicals so they can be tasted
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Pancreas
 Found posterior to the parietal peritoneum
 Mostly retroperitoneal
 Extends across the abdomen from spleen to
duodenum
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Pancreas
 Produces a wide spectrum of digestive enzymes
that break down all categories of food
 Secretes enzymes into the duodenum
 Alkaline fluid introduced with enzymes neutralizes
acidic chyme coming from stomach
 Hormones produced by the pancreas
 Insulin
 Glucagon
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Liver
 Largest gland in the body
 Located on the right side of the body under the
diaphragm
 Consists of four lobes suspended from the
diaphragm and abdominal wall by the falciform
ligament
 Connected to the gallbladder via the common
hepatic duct
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Liver
 Bile is produced by cells in the liver
 Bile leaves the liver through the common hepatic
duct and enters duodenum through the bile duct
 Bile is a yellow-green, watery solution containing:
 Bile salts and bile pigments (mostly bilirubin from the
breakdown of hemoglobin)
 Cholesterol, phospholipids, and electrolytes
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Liver
 Function of bile
 Emulsify fats by physically breaking large fat
globules into smaller ones
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Gallbladder
 Sac found in shallow fossa of liver
 When no digestion is occurring, bile backs up the
cystic duct for storage in the gallbladder
 During digestion of fatty food, bile is introduced into
the duodenum from the gallbladder
 Gallstones are crystallized cholesterol, which can
cause blockages
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Functions of the Digestive System
 Major functions of the digestive system are
summarized as:
 Digestion
 Absorption
 We will cover 6 more specific processes next
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Functions of the Digestive System
1. Ingestion—placing of food into the mouth
2. Propulsion—movement of foods from one region
of the digestive system to another
 Peristalsis—alternating waves of contraction and
relaxation that squeezes food along the GI tract
 Segmentation—movement of materials back and
forth to foster mixing in the small intestine
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Figure 14.12 Peristaltic and segmental movements of the digestive tract.
(a)
(b)
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Functions of the Digestive System
3. Food breakdown: mechanical breakdown
 Examples:
 Mixing of food in the mouth by the tongue
 Churning of food in the stomach
 Segmentation in the small intestine
 Mechanical digestion prepares food for further
degradation by enzymes
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Functions of the Digestive System
4. Food breakdown: digestion
 Digestion occurs when enzymes chemically break
down large molecules into their building blocks
 Each major food group uses different enzymes
 Carbohydrates are broken to monosaccharides
(simple sugars)
 Proteins are broken to amino acids
 Fats are broken to fatty acids and glycerol
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Figure 14.13 Flowchart of digestion and absorption of foodstuffs. (1 of 3)
Foodstuff
Enzyme(s) and source
Site of action
Starch and disaccharides
Digestion of
carbohydrates
Salivary amylase
Mouth
Pancreatic amylase
Small intestine
Brush border enzymes
in small intestine
(dextrinase,
glucoamylase,
lactase, maltase,
and sucrase)
Small intestine
Oligosaccharides*
and disaccharides
Lactose
Maltose Sucrose
Galactose Glucose Fructose
Absorption of The monosaccharides glucose, galactose, and
carbohydrates fructose enter the capillary blood in the villi and are
transported to the liver via the hepatic portal vein.
*Oligosaccharides consist of a few linked monosaccharides.
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Figure 14.13 Flowchart of digestion and absorption of foodstuffs. (2 of 3)
Foodstuff
Digestion
of proteins
Protein
Enzyme(s) and source
Pepsin
(stomach glands) in
the presence of HCI
Site of action
Stomach
Large polypeptides
Small polypeptides,
small peptides
Amino acids
(some dipeptides
and tripeptides)
Absorption
of proteins
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Pancreatic enzymes
(trypsin, chymotrypsin,
carboxypeptidase)
Small intestine
Brush border enzymes
(aminopeptidase,
carboxypeptidase,
and dipeptidase)
Small intestine
Amino acids enter the capillary blood in the villi and are
transported to the liver via the hepatic portal vein.
Figure 14.13 Flowchart of digestion and absorption of foodstuffs. (3 of 3)
Foodstuff
Digestion
of fats
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Site of action
Emulsified by the
detergent action of
bile salts from the liver
Small intestine
Pancreatic lipase
Small intestine
Unemulsified fats
Monoglycerides
and fatty acids
Absorption
of fats
Enzyme(s) and source
Glycerol and
fatty acids
Fatty acids and monoglycerides enter the lacteals of the villi
and are transported to the systemic circulation via the lymph
in the thoracic duct. (Glycerol and short-chain fatty acids are
absorbed into the capillary blood in the villi and transported
to the liver via the hepatic portal vein.)
Functions of the Digestive System
5. Absorption
 End products of digestion are absorbed in the blood
or lymph
 Food must enter mucosal cells and then into blood or
lymph capillaries
6. Defecation
 Elimination of indigestible substances from the GI
tract in the form of feces
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Figure 14.11 Schematic summary of gastrointestinal tract activities.
Ingestion
Mechanical
breakdown
• Chewing (mouth)
• Churning (stomach)
• Segmentation
(small intestine)
Digestion
Food
Pharynx
Esophagus
Propulsion
• Swallowing
(oropharynx)
• Peristalsis
(esophagus, stomach,
small intestine, large
intestine)
Stomach
Absorption
Lymph vessel
Small intestine
Blood vessel
Large intestine
Mainly H2O
Feces
Defecation
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Anus
Activities Occurring in the Mouth, Pharynx,
and Esophagus
 Food ingestion and breakdown
 Food is placed into the mouth
 Physically broken down by chewing
 Mixed with saliva, which is released in response to
mechanical pressure and psychic stimuli
 Salivary amylase begins starch digestion
 Essentially, no food absorption occurs in the mouth
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Activities Occurring in the Mouth, Pharynx,
and Esophagus
 Food propulsion—swallowing and peristalsis
 Pharynx and esophagus have no digestive function
 Serve as passageways to the stomach
 Pharynx functions in swallowing (deglutition)
 Two phases of swallowing:
1. Buccal phase
2. Pharyngeal-esophgeal phase
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Activities Occurring in the Mouth, Pharynx,
and Esophagus
 Food propulsion—swallowing and peristalsis
(continued)
1. Buccal phase
 Voluntary
 Occurs in the mouth
 Food is formed into a bolus
 The bolus is forced into the pharynx by the tongue
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Activities Occurring in the Mouth, Pharynx,
and Esophagus
 Food propulsion—swallowing and peristalsis
(continued)
2. Pharyngeal-esophageal phase
 Involuntary transport of the bolus by peristalsis
 Nasal and respiratory passageways are blocked
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Activities Occurring in the Mouth, Pharynx,
and Esophagus
 Food propulsion—swallowing and peristalsis
(continued)
2. Pharyngeal-esophogeal phase (continued)
 Peristalsis moves the bolus toward the stomach
 The cardioesophageal sphincter is opened when food
presses against it
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Figure 14.14a Swallowing.
Bolus of food
Tongue
Pharynx
Upper
Epiglottis
esophageal
up
sphincter
Glottis (lumen)
of larynx
Esophagus
Trachea
(a) Upper esophageal
sphincter contracted
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Figure 14.14b Swallowing.
Uvula
Bolus
Epiglottis
down
Larynx up
Esophagus
(b) Upper esophageal
sphincter relaxed
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Figure 14.14c Swallowing.
Bolus
(c) Upper esophageal
sphincter contracted
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Figure 14.14d Swallowing.
Relaxed muscles
Cardioesophageal
sphincter open
(d) Cardioesophageal
sphincter relaxed
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Activities in the Stomach
 Food breakdown
 Gastric juice is regulated by neural and hormonal
factors
 Presence of food or rising pH causes the release of
the hormone gastrin
 Gastrin causes stomach glands to produce:
 Protein-digesting enzymes
 Mucus
 Hydrochloric acid
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Activities in the Stomach
 Food breakdown (continued)
 Hydrochloric acid makes the stomach contents very
acidic
 Acidic pH
 Activates pepsinogen to pepsin for protein digestion
 Provides a hostile environment for microorganisms
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Activities in the Stomach
 Food breakdown (continued)
 Protein digestion enzymes
 Pepsin—an active protein-digesting enzyme
 Rennin—works on digesting milk protein in infants,
not adults
 Alcohol and aspirin are virtually the only items
absorbed in the stomach
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Activities in the Stomach
 Food propulsion
1. Peristalsis: Waves of peristalsis occur from the
fundus to the pylorus, forcing food past the pyloric
sphincter
2. Grinding: The pylorus meters out chyme into the
small intestine (3 ml at a time)
3. Retropulsion: Peristaltic waves close the pyloric
sphincter, forcing content back into the stomach.
The stomach empties in 4–6 hours
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Figure 14.15 Peristaltic waves in the stomach.
Pyloric
valve
closed
Pyloric
valve
slightly
opened
1 Propulsion: Peristaltic
waves move from the
fundus toward the pylorus.
2 Grinding: The most
vigorous peristalsis and
mixing action occur close to
the pylorus. The pyloric end
of the stomach acts as a
pump that delivers small
amounts of chyme into the
duodenum.
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Pyloric
valve
closed
3 Retropulsion: The
peristaltic wave closes the
pyloric valve, forcing most of
the contents of the pylorus
backward into the stomach.
Activities of the Small Intestine
 Food breakdown and absorption
 Intestinal enzymes from the brush border function to:
 Break double sugars into simple sugars
 Complete some protein digestion
 Intestinal enzymes and pancreatic enzymes help to
complete digestion of all food groups
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Activities of the Small Intestine
 Food breakdown and absorption (continued)
 Pancreatic enzymes play the major role in the
digestion of fats, proteins, and carbohydrates
 Alkaline content neutralizes acidic chyme and
provides the proper environment for the pancreatic
enzymes to operate
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Activities of the Small Intestine
 Food breakdown and absorption (continued)
 Release of pancreatic juice from the pancreas into
the duodenum is stimulated by:
 Vagus nerves
 Local hormones that travel via the blood to influence
the release of pancreatic juice (and bile):
 Secretin
 Cholecystokinin (CCK)
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Activities of the Small Intestine
 Food breakdown and absorption (continued)
 Hormones (secretin and CCK) also target the liver
and gallbladder to release bile
 Bile
 Acts as a fat emulsifier
 Needed for fat absorption and absorption of fat-soluble
vitamins (K, D, E, and A)
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Figure 14.16 Regulation of pancreatic juice and bile secretion and release.
1 Chyme entering
duodenum causes
duodenal enteroendocrine cells to
release cholecytokinin
(CCK) and secretin.
2 CCK (red dots) and
secretin (blue dots)
enter the bloodstream.
3 Upon reaching the
pancreas, CCK induces
secretion of enzyme-rich
pancreatic juice; secretin
causes secretion of
bicarbonate-rich
pancreatic juice.
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4 Secretin causes
the liver to secrete
more bile; CCK
stimulates the
gallbladder to
release stored bile
and the hepatopancreatic sphincter
to relax (allows bile
from both sources to
enter the duodenum).
5 Stimulation by
vagal nerve fibers
cause release of
pancreatic juice and
weak contractions of
the gallbladder.
Activities of the Small Intestine
 Food breakdown and absorption (continued)
 A summary table of hormones is presented next
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Table 14.1 Hormones and Hormonelike Products That Act in Digestion (1 of 2).
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Table 14.1 Hormones and Hormonelike Products That Act in Digestion (2 of 2).
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Activities of the Small Intestine
 Food breakdown and absorption (continued)
 Water is absorbed along the length of the small
intestine
 End products of digestion
 Most substances are absorbed by active transport
through cell membranes
 Lipids are absorbed by diffusion
 Substances are transported to the liver by the
hepatic portal vein or lymph
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Activities of the Small Intestine
 Food breakdown and absorption
 Peristalsis is the major means of moving food
 Segmental movements
 Mix chyme with digestive juices
 Aid in propelling food
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Figure 14.12b Peristaltic and segmental movements of the digestive tract.
(b)
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Activities of the Large Intestine
 Food breakdown and absorption
 No digestive enzymes are produced
 Resident bacteria digest remaining nutrients
 Produce some vitamin K and B
 Release gases
 Water and vitamins K and B are absorbed
 Remaining materials are eliminated via feces
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Activities of the Large Intestine
 Food breakdown and absorption (continued)
 Feces contains:
 Undigested food residues
 Mucus
 Bacteria
 Water
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Activities of the Large Intestine
 Propulsion of the residue and defecation
 Sluggish peristalsis begins when food residue arrives
 Haustral contractions are most seen in the large
intestine
 Mass movements are slow, powerful movements that
occur 3 to 4 times per day
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Activities of the Large Intestine
 Propulsion of the residue and defecation
(continued)
 Presence of feces in the rectum causes a defecation
reflex
 Internal anal sphincter is relaxed
 Defecation occurs with relaxation of the voluntary
(external) anal sphincter
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Nutrition and Metabolism
 Most foods are used as metabolic fuel
 Foods are oxidized and transformed into adenosine
triphosphate (ATP)
 ATP is chemical energy that drives cellular activities
 Energy value of food is measured in kilocalories
(kcal) or Calories (C)
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Nutrition
 Nutrient—substance used by the body for growth,
maintenance, and repair
 Major nutrients
 Carbohydrates
 Lipids
 Proteins
 Water
 Minor nutrients
 Vitamins
 Minerals
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Table 14.2 Five Basic Food Groups and Some of Their Major Nutrients (1 of 2).
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Table 14.2 Five Basic Food Groups and Some of Their Major Nutrients (2 of 2).
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Dietary Sources of Major Nutrients
 Carbohydrates
 Dietary carbohydrates are sugars and starches
 Most are derived from plants such as fruits and
vegetables
 Exceptions: lactose from milk and small amounts of
glycogens from meats
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Dietary Sources of Major Nutrients
 Lipids
 Saturated fats from animal products (meats)
 Unsaturated fats from nuts, seeds, and vegetable
oils
 Cholesterol from egg yolk, meats, and milk products
(dairy products)
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Dietary Sources of Major Nutrients
 Proteins
 Complete proteins—contain all essential amino acids
 Most are from animal products (eggs, milk, meat,
poultry, and fish)
 Essential amino acids: those that the body cannot
make and must be obtained through diet
 Legumes and beans also have proteins, but the
proteins are incomplete
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Dietary Sources of Major Nutrients
 Vitamins
 Most vitamins are used as coenzymes
 Found mainly in fruits and vegetables
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Dietary Sources of Major Nutrients
 Minerals
 Mainly important for enzyme activity
 Foods richest in minerals: vegetables, legumes, milk,
and some meats
 Iron is important for making hemoglobin
 Calcium is important for building bone, blood clotting,
and secretory activities
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Metabolism
 Metabolism is all of the chemical reactions
necessary to maintain life
 Catabolism—substances are broken down to simpler
substances; energy is released
 Anabolism—larger molecules are built from smaller
ones
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Carbohydrate Metabolism
 Carbohydrates are the body’s preferred source to
produce cellular energy (ATP)
 Glucose (blood sugar)
 Major breakdown product of carbohydrate digestion
 Fuel used to make ATP
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Carbohydrate Metabolism
 Hyperglycemia—excessively high levels of glucose
in the blood
 Excess glucose is stored in body cells as glycogen
or converted to fat
 Hypoglycemia—low levels of glucose in the blood
 Glycogenolysis, gluconeogenesis, and fat
breakdown occur to restore normal blood glucose
levels
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Fat Metabolism
 Fats
 Insulate the body
 Protect organs
 Build some cell structures (membranes and myelin
sheaths)
 Provide reserve energy
 Excess dietary fat is stored in subcutaneous tissue
and other fat depots
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Fat Metabolism
 When carbohydrates are in limited supply, more fats
are oxidized to produce ATP
 Excessive fat breakdown causes blood to become
acidic (acidosis or ketoacidosis)
 Breath has a fruity odor
 Common with:
 “No carbohydrate” diets
 Uncontrolled diabetes mellitus
 Starvation
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Protein Metabolism
 Proteins form the bulk of cell structure and most
functional molecules
 Proteins are carefully conserved by body cells
 Amino acids are actively taken up from blood by
body cells
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Protein Metabolism
 Amino acids are oxidized to form ATP mainly when
other fuel sources are not available
 Ammonia, released as amino acids are catabolized,
is detoxified by liver cells that combine it with
carbon dioxide to form urea
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The Central Role of the Liver in Metabolism
 Liver is the body’s key metabolic organ
 Roles in digestion:
 Manufactures bile
 Detoxifies drugs and alcohol
 Degrades hormones
 Produces cholesterol, blood proteins (albumin and
clotting proteins)
 Plays a central role in metabolism
 Liver can regenerate if part of it is damaged or
removed
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Body Energy Balance
 Energy intake = Total energy output
(heat + work + energy storage)
 Energy intake is the energy liberated during food
oxidation
 Energy produced during glycolysis, Krebs cycle, and
the electron transport chain
 Energy output
 Energy we lose as heat (60%)
 Energy stored as fat or glycogen
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Body Energy Balance
 Interference with the body’s energy balance leads
to:
 Obesity
 Malnutrition (leading to body wasting)
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Regulation of Food Intake
 Body weight is usually relatively stable
 Energy intake and output remain about equal
 Mechanisms that may regulate food intake
 Levels of nutrients in the blood
 Hormones
 Body temperature
 Psychological factors
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Metabolic Rate and Body Heat Production
 Nutrients yield different amounts of energy
 Energy value is measured in kilocalorie (kcal)
 Carbohydrates and proteins yield 4 kcal/gram
 Fats yield 9 kcal/gram
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Basal Metabolic Rate
 Basic metabolic rate (BMR)—amount of heat
produced by the body per unit of time at rest
 Average BMR is about 60 to 72 kcal/hour for an
average 70-kg (154-lb) adult
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Basal Metabolic Rate
 Factors that influence BMR
 Surface area—a small body usually has a higher
BMR
 Gender—males tend to have higher BMRs
 Age—children and adolescents have higher BMRs
 The amount of thyroxine produced is the most
important control factor
 More thyroxine means a higher metabolic rate
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Total Metabolic Rate (TMR)
 TMR—total amount of kilocalories the body must
consume to fuel ongoing activities
 TMR increases dramatically with an increase in
muscle activity
 TMR must equal calories consumed to maintain
homeostasis and maintain a constant weight
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Body Temperature Regulation
 When foods are oxidized, more than 60% of energy
escapes as heat, warming the body
 The body has a narrow range of homeostatic
temperature
 Must remain between 35.6°C and 37.8°C
(96°F and 100°F)
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Body Temperature Regulation
 The body’s thermostat is in the hypothalamus
 Hypothalamus initiates mechanisms to maintain
body temperature
 Heat-loss mechanisms involve radiation of heat from
skin and evaporation of sweat
 Heat-promoting mechanisms involve
vasoconstriction of skin blood vessels and shivering
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Body Temperature Regulation
 Fever—controlled hyperthermia
 Results from infection, cancer, allergic reactions,
CNS injuries
 If the body thermostat is set too high, body proteins
may be denatured, and permanent brain damage
may occur
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Developmental Aspects of the Digestive
System and Metabolism
 The alimentary canal is a continuous, hollow tube
present by the fifth week of development
 Digestive glands bud from the mucosa of the
alimentary tube
 The developing fetus receives all nutrients through
the placenta
 In newborns, feeding must be frequent, peristalsis is
inefficient, and vomiting is common
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Developmental Aspects of the Digestive
System and Metabolism
 Common congenital defects that interfere with
normal nutrition:
 Cleft palate
 Cleft lip
 Tracheoesophageal fistula
 Common inborn errors of metabolism:
 Phenylketonuria (PKU)
 Cystic fibrosis (CF)
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Developmental Aspects of the Digestive
System and Metabolism
 Newborn reflexes
 Rooting reflex helps the infant find the nipple
 Sucking reflex helps the infant hold on to the nipple
and swallow
 Teething begins around age 6 months
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Developmental Aspects of the Digestive
System and Metabolism
 Problems of the digestive system:
 Gastroenteritis—inflammation of the gastrointestinal
tract; can occur at any time
 Appendicitis—inflammation of the appendix;
common in adolescents
 Metabolism decreases with old age
 Middle-age digestive problems
 Ulcers
 Gallbladder problems
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Developmental Aspects of the Digestive
System and Metabolism
 Later middle-age problems
 Obesity
 Diabetes mellitus
 Activity of the digestive tract in old age
 Fewer digestive juices
 Peristalsis slows
 Diverticulosis and gastrointestinal cancers are more
common
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