The Digestive System
Chapter 23
The Digestive System
Structures of the Digestive
Tract:
• Alimentary canal - The
continuous tube from mouth to
anus whose organs perform
various digestive functions
• Accessory structures - the
additional organs and glands that
play a role in digestion
Figure 23.1
The Digestive System
Functions of the Digestive System:
• Ingestion - taking food into the
alimentary canal at the mouth
• Propulsion - the movement of food
through the alimentary canal
• Peristalsis – waves of muscle
contraction that propel food
• Mechanical digestion - physical
breakdown of food into smaller pieces
• Segmentation – contractions of
the alimentary canal that mixes
food
• Chemical digestion - food
molecules are broken down via
enzymes
• Absorption - transport of digested
materials from lumen through an
epithelium into the blood or lymph
• Defecation – removal of indigestible
materials through the anus
Figure 23.2
The Peritoneum
The Peritoneum - the serous membrane lining the abdominal/pelvic body cavity
Visceral layer - lines the surfaces of the organs in the cavity
Parietal layer – lines the inside of the body wall
Peritoneal cavity - serous fluid-filled space between the visceral and
parietal layers
Figure 23.5
The Peritoneum
Mesentery - a double layer of serous membrane that suspends organs in the
abdominal body cavity
Intraperitoneal – refers to organs that hang into the peritoneal cavity, suspended
from mesenteries
Retroperitoneal – refers to structures behind parietal peritoneum
Figure 23.5
The Alimentary Canal
Layers of the Alimentary
Canal:
• Mucosa - Innermost layer
composed of epithelium,
specialized for secretion and
absorption
• Submucosa - Connective
tissue with glands, blood
vessels, lymphatics, and nerves
• Muscularis - Layers of smooth
muscle
• Circular layer – inner
layer controlling lumen size
• Longitudinal layer –
outer layer controlling canal
length
• Serosa – serous membrane,
also called the visceral
peritoneum
Figure 23.6
The Oral Cavity
Oral cavity - start of the alimentary
canal where ingestion occurs. Begins
mechanical digestion via chewing
(mastication) and chemical digestion
via enzymes
• Teeth - bony structures used in
mastication
• Tongue – skeletal muscle used to
manipulate food and form a bolus
• Palate – roof of the oral cavity, hard
and soft regions
• Uvula – extension of the soft palate,
blocks the nasopharynx during
swallowing
Figure 23.7a
Salivary Glands
Salivary glands – paired glands that secrete saliva into the oral
cavity to moisten food and cleanse the mouth
Saliva – secretion containing salivary amylase which begins
chemical digestion of starches. Also contains IgA antibodies and
lysozymes to clear potential pathogens
Figure 23.9
The Pharynx
Pharynx - Begins propulsion,
transporting food from the oral
cavity to the esophagus,
performing the act of deglutition
(swallowing)
• Only the oropharynx and
laryngopharynx transport food
Figure 23.7a
The Esophagus
Esophagus - Transports food from the pharynx to the stomach
Esophageal Hiatus – Opening in the diaphragm, allowing passage
of esophagus from thorax to abdomen
Figure 23.12
The Esophagus
Deglutition – when swallowing, the elevation of the larynx and uvula
block the respiratory passages ensuring flood enters the esophagus
Figure 23.13a
The Esophagus
Gastroesophageal
sphincter (cardiac
sphincter) - circular
muscle allowing
passage into stomach
and prevents
regurgitation
Heartburn – burning
when acidic gastric
juices regurgitate into
the esophagus
Figure 23.13d, e
The Stomach
Stomach – stores food, performing mechanical digestion via churning and
chemical digestion via enzymes
Chyme – the acidic, paste-like substance passed to the small intestine after about
4 hours
Regions: The cardia is nearest to the esophagus, the fundus is the superior
dome, the body is the central region, and the pylorus is nearest the small intestine
Figure 23.14
The Stomach
Curvatures: the lateral surface of the stomach is the convex greater curvature,
while the medial surface is the concave lesser curvature
Rugae – large folds of the mucosa allowing expansion
Muscle layers – only organ in the alimentary canal with 3 layers, outer
longitudinal, middle circular, and inner oblique
Pyloric sphincter – circular muscle regulating chyme movement into the small
intestine
Figure 23.14
The Omenta
Omenta – the 2 mesenteries that
attach to the surfaces of the stomach
• Lesser omentum – mesentery
from liver to the lesser curvature of
the stomach
• Greater omentum – largest
mesentery draping from the greater
curvature of the stomach
Figure 23.30d
The Stomach
Gastric pits/glands – invaginations
of the stomach mucosa containing
cells that produce the gastric juices
• Chief cells – secrete pepsinogen
that quickly becomes pepsin, a
protein enzyme that works best in
acidic pH
• Parietal cells – secrete
hydrochloric acid (HCl), to produce
acidic environment
• Mucous neck cells – produce
mucus that with the mucous of goblet
cells protects the mucosa
• Enteroendocrine cells – secrete
local hormones to regulate stomach
activity
Figure 23.15
The Stomach
Gastic Juice Secretion:
1. Cephalic phase –
smell or thought of food
begins limited gastric
secretions
2. Gastric phase –
stretch and chemicals
in stomach trigger
largest secretions
3. Intestinal phase – a
balance of excitatory
and inhibitory actions
• Excitatory –
movement of
chyme into
intestine triggers
continued secretion
Figure 23.16
The Stomach
Gastic Juice Secretion:
3. Intestinal phase – a
balance of excitatory
and inhibitory actions
• Excitatory –
movement of
chyme into
intestine triggers
continued gastric
secretion
• Inhibitory – stretch
of duodenum
inhibits gastric
secretion and
motility (prevents
overload of
duodenum)
Figure 23.16
The Small Intestine
Small intestine – coiled digestive
organ transporting food from stomach
to large intestine. Completes digestion
and performs all nutrient absorption.
Receives secretions form liver, gall
bladder, and pancreas.
Regions – divided into duodenum,
jejunum, and ileum
Length: 6-7 meters (about 20 ft) in a
living person (add 10ft in a cadaver)
Diameter: 2.5 - 4cm
Figure 23.1
The Small Intestine
Hepatopancreatic sphincter (of Oddi) – regulates the entrance of secretions
from the liver, gall bladder and pancreas into the duodenum
Figure 23.20
The Small Intestine
Specializations for
absorption:
• Circular folds – large folds of
the mucosa and submucosa
that help slow food movement
Figure 23.21a
The Small Intestine
Specializations for
absorption:
• Villi– finger-like projections of
the mucosa that increase
surface area of the epithelium.
Each villus contains a capillary
bed and a lacteal for nutrient
absorption.
• Intestinal crypts –
invaginations producing
intestinal juices, a mucous
secretion
Figure 23.21b
The Small Intestine
Specializations for absorption:
• Microvilli – finger-like extensions of the epithelial cell membranes with
attached “brush border enzymes,” further increasing surface area
Figure 23.21b, c
The Liver
Liver – a 4-lobed gland whose digestive function is the production of bile
Hepatitis – inflammation of the liver, usually resulting from viral infection
Cirrhosis – chronic inflammation usually associated with alcohol abuse
Bile – a green, alkaline fluid containing bilirubin pigment and bile salts that
perform emulsification of fats. Drains from the liver via the hepatic ducts
Figure 23.23a
The Liver
Emulsification – the physical
breakdown of large fat globules into
smaller fat droplets, thus increasing
surface area for enzymatic digestion
• bile salts are partially polar and
partially nonpolar, thereby breaking
up fat globules like a detergent
Figure 23.35
The Gall Bladder
Gall bladder - muscular sac on the posterior surface of the liver that stores
and concentrates bile. The gall bladder is drained by the cystic duct
Gall stones - crystallization of bile that accumulate in the gall
bladder, may be passed naturally or removed via ultrasound, drugs,
or surgery
Figure 23.23b
The Pancreas
The Pancreas – gland whose acinar cells produces pancreatic juices containing
digestive enzymes such as proteases, lipases, nucleases, and amylases.
Figure 23.26
The Pancreas
Pancreatic duct - drains pancreatic juices into the duodenum through the
sphincter of Oddi
Accessory duct – alternate pathway that drains directly
Figure 23.20
Intestinal Hormones
Intestinal hormones - chemicals
released by enteroendocrine cells
when chyme enters the duodenum
• Cholecystokinin (CCK) - triggers
secretions of bile from the gall
bladder and relaxation of
hepatopancreatic sphincter
• Secretin – triggers liver to
increase bile production
Figure 23.25
Intestinal Hormones
• Cholecystokinin (CCK) triggers release of pancreatic
juices rich in enzymes
• Secretin – triggers release of
pancreatic juices rich in
bicarbonate
Figure 23.28
The Large Intestine
Large Intestine (Colon) transports food from the
small intestine to the anus
while absorbing water and
forming feces
Ileocecal valve –
regulates movement of
food from the small to the
large intestine
Haustra – individual
pouches of the large
intestine wall
Teniae coli – thin
longitudinal muscle layer
forming the haustra
Figure 23.29a
The Large Intestine
Regions:
• Cecum - pouch-like region
with the hanging vermiform
appendix
• Ascending colon –
transports food up the right
side of the abdomen, turning
at the hepatic flexure
•Transverse colon –
transports food across the
abdomen, turning at the
splenic flexure
• Descending colon –
transports food down the left
side of the abdomen
• Sigmoid colon – s-shaped
region
Figure 23.29a
The Rectum and Anus
Rectum – most distal region
of the colon, where feces is
stored until defecation
Rectal valves – projections
that allow gases to pass
around feces
Anus – end of the alimentary
canal from which defecation
occurs
Internal anal sphincter –
smooth muscle regulating
anal opening
External anal sphincter –
skeletal muscle regulating
anal opening
Figure 23.29b
The Defecation Reflex
Defecation reflex – series of impulses
that trigger release of feces
1. Sensory fibers sense stretch of
rectum wall due to filling
2. Parasympathetic fibers trigger
contraction of rectum wall and
relaxation of internal anal sphincter
3. External anal sphincter can allow
defecation, or voluntarily contract
to temporarily stop the reflex
Figure 23.32
Chemical Digestion Overview
Figure 23.33a
Chemical Digestion Overview
Figure 23.33b