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Lecture 8
The Digestive System
Part I
Digestive System
• Two groups of organs
– Alimentary Canal
– Assessory organs
• Tuesday (Today)
– Gross and histological anatomy of organs and their
function
• Thursday
– Chemical process, and hormona regulation, of digestion
Alimentary Canal
• Gastrointestinal or GI tract
• Mouth to anus
– Digests food and absorbs nutrients and water
• Includes: Mouth, pharynx, esophagus,
stomach, small intestine, and large intestine
Accessory Organs
•
•
•
•
•
•
Mouth/Teeth/Tongue
Gallbladder
Digestive glands
Salivary glands
Liver
Pancreas
Figure 23.1 Alimentary canal and related accessory digestive organs.
Mouth (oral cavity)
Tongue*
Parotid gland
Sublingual gland
Submandibular gland
Salivary
glands*
Pharynx
Esophagus
Stomach
Pancreas*
(Spleen)
Liver*
Gallbladder*
Transverse colon
Small
intestine
Duodenum
Jejunum
Ileum
Anus
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Descending colon
Ascending colon
Cecum
Sigmoid colon
Rectum
Appendix
Anal canal
Large
intestine
Digestion
• Six essential activities
– Ingestion
– Propulsion
– Mechanical breakdown
– Digestion
– Absorption
– Defecation
Digestive Processes
• Ingestion
– taking food in the mouth
• Mechanical breakdown
– increase surface area for digestion enzymes
– chewing, mixing with saliva
– churning in the stomach
– segmentation – constriction or small intenstine
Digestive Processes
• Propulsion
– swallowing (voluntary) and peristalsis
(involuntary)
– Peristalsis, alternating waves of contraction and
relaxation
Figure 23.3 Peristalsis and segmentation.
From
mouth
Peristalsis: Adjacent segments of alimentary
tract organs alternately contract and relax,
moving food along the tract distally.
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Segmentation: Nonadjacent segments of
alimentary tract organs alternately contract and
relax, moving food forward then backward.
Food mixing and slow food propulsion occur.
Digestive Processes
• Digestion
– enzymatic breakdown of food into their chemical
building blocks
• Absorption
– passage of digest end products into blood or lymph
• Defecation
– elimination
Histology of the Alimentary Canal
• Four basic layers (tunics)
– Mucosa
– Submucosa
– Muscularis externa
– Serosa
Mucosa
• Lines lumen
• Functions
– Secretes mucus, digestive enzymes, and hormones
– Absorbs end products of digestion
– Protects against infectious disease
• Three sublayers:
– epithelium
– lamina propria
– muscularis mucosae
Three Sub layers of Mucosa
• Epithelium
– Simple columnar epithelium and mucus-secreting
cells (most of tract)
– Mucus protects digestive organs from enzymes
and eases food passage
Three Sub layers of Mucosa
• Lamina propria
– Loose connective tissue
– Capillaries for nourishment and absorption
– Lymphoid follicles (part of MALT)
• Muscularis mucosae: smooth muscle for local
movements of mucosa
Figure 23.6 Basic structure of the alimentary canal.
Intrinsic nerve plexuses
• Myenteric nerve plexus
• Submucosal nerve plexus
Glands in
submucosa
Mucosa
• Epithelium
• Lamina propria
• Muscularis
mucosae
Submucosa
Muscularis externa
• Longitudinal muscle
• Circular muscle
Mesentery
Nerve
Artery
Gland in mucosa
Vein
Duct of gland outside
Lymphatic vessel
alimentary canal
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Serosa
• Epithelium
(mesothelium)
• Connective tissue
Lumen
Mucosa-associated
lymphoid tissue
Submucosa
• External to the mucosa
• connective tissue filled with blood and
lymphatic vessels, lymphoid follicles, and
nerve fibers
Muscularis externa
• Responsible for segmentation and peristalsis
• Inner and outer layers
• Inner layer contain sphincter muscles to
control food passage from one organ to next
Figure 23.6 Basic structure of the alimentary canal.
Intrinsic nerve plexuses
• Myenteric nerve plexus
• Submucosal nerve plexus
Glands in
submucosa
Mucosa
• Epithelium
• Lamina propria
• Muscularis
mucosae
Submucosa
Muscularis externa
• Longitudinal muscle
• Circular muscle
Mesentery
Nerve
Artery
Gland in mucosa
Vein
Duct of gland outside
Lymphatic vessel
alimentary canal
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Serosa
• Epithelium
(mesothelium)
• Connective tissue
Lumen
Mucosa-associated
lymphoid tissue
The Serosa
• The outermost layer
– Visceral peritoneum
The Peritoneum
• All ventral body cavities contain slippery
serous membranes
• Peritoneum of abdominal cavity most
extensive
Figure 23.5a The peritoneum and the peritoneal cavity.
Abdominopelvic
cavity
Vertebra
Dorsal
mesentery
Parietal
peritoneum
Ventral
mesentery
Visceral
peritoneum
Peritoneal
cavity
Alimentary Liver
canal organ
Two schematic cross sections of abdominal cavity illustrate
the peritoneums and mesenteries.
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Nerve Supply
• In-house nerve supply
– enteric neurons
• Regulates digestive activity by controlling GI
tract motility
– segmentation and peristalsis and other
contractions
Functional Anatomy
• Mouth and tongue
• Salivary glands
– cleanses the mouth
– dissolves food chemicals for taste
– moistens food for a compact bolus
– Contains enzymes that begin chemical breakdown
• Teeth, Pharynx and Esophagus
Saliva
• 97-99% water
– pH = 6.75-7.00
– enzymes: amylase and lipase
– proteins: mucin
– helps in innate immunity
• lysozymes, IgG and defensins
• Produce 1.5 L per day!
Esophagus
• Flat muscular tube from laryngopharynx to
stomach
• Pierces diaphragm at esophageal hiatus
• Joins stomach at cardial orifice
– cardiac sphincter muscle
• Collapsed tube when not in use
• Movement down esophagus is involuntary
Figure 23.12a Microscopic structure of the esophagus.
Mucosa
(stratified
squamous
epithelium)
Submucosa
(areolar
connective
tissue)
Lumen
Muscularis
externa
• Circular layer
• Longitudinal
layer
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Adventitia
(fibrous
connective
tissue)
The Stomach
• In upper left quadrant; temporary storage;
digestion of bolus to chyme
• Four parts
– Cardial part
– Fundus
– Body
– Pyloric part
Four parts of the stomach
• Cardial part (cardia)
– Surrounds cardial orifice
• Fundus
– Dome-shaped region beneath diaphragm
• Body
– Midportion
• Pyloric part
– Antrum and canal
Figure 23.14a Anatomy of the stomach.
Cardia
Fundus
Esophagus
Muscularis
externa
• Longitudinal layer
• Circular layer
• Oblique layer
Serosa
Body
Lumen
Lesser
curvature
Rugae of
mucosa
Greater
curvature
Duodenum
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Pyloric sphincter
(valve) at pylorus
Pyloric
canal
Pyloric
antrum
Stomach to duodenum
• The pyloric canal terminates at the pylorus
• Pylorus is continuous with the duodenum
through the pyloric sphincter
– controls emptying of stomach
Holding everything in place
• Two mesenteries (supporting membranes)
– Lesser omentum
– Greater omentum
Figure 23.30b Mesenteries of the abdominal digestive organs.
Liver
Gallbladder
Lesser omentum
Stomach
Duodenum
Transverse colon
Small intestine
Cecum
Urinary bladder
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Figure 23.30c Mesenteries of the abdominal digestive organs.
Greater omentum
Transverse colon
Transverse
mesocolon
Descending colon
Jejunum
Mesentery
Sigmoid
mesocolon
Sigmoid colon
Ileum
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Stomach Histology
• Contains four tunics (layers)
• Modified for function
• Four types of gland cells
–
–
–
–
Mucous Neck Cells
Parietal Cells
Chief Cells
Enteroendocrine Cells
Figure 23.15a Microscopic anatomy of the stomach.
Surface
epithelium
Mucosa
Lamina
propria
Muscularis
mucosae
Submucosa
(contains
submucosal
Oblique
plexus)
layer
Muscularis
Circular
externa
layer
(contains
Longitudinal
myenteric
layer
plexus)
Stomach wall
Serosa
Layers of the stomach wall
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Figure 23.15b Microscopic anatomy of the stomach.
Gastric pits
Surface epithelium
(mucous cells)
Gastric
pit
Mucous neck cells
Parietal cell
Gastric
gland
Chief cell
Enteroendocrine cell
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Enlarged view of gastric pits and
gastric glands
Gastric Glands
• Glands in fundus and body produce most gastric juice
• Mucous neck cells
– thin, acidic mucus of unknown function
• Parietal cells
– HCl and Intrinsic factor (Vit B12 absorption)
• Chief cells
– Pepsinogen and lipases
• Enteroendocrine cells
– Secrete hormones into lamina propria
Figure 23.15c Microscopic anatomy of the stomach.
Pepsinogen
Pepsin
HCI
Mitochondria
Parietal cell
Chief cell
Enteroendocrine
cell
Location of the HCl-producing parietal cells
and pepsin-secreting chief cells in a gastric
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gland
Stomach Filling
• Stretches to accommodate incoming food
– stomach can stretch without increasing its tension
– stress-relaxation response intrinsic to smooth
muscle
Gastric Motility
• Peristaltic waves move toward pylorus at rate
of 3 per minute
• Basic electrical rhythm (BER) set by enteric
pacemaker cells
– linked by gap junctions throughout muscularis
externa
Figure 23.19 Deglutition (swallowing).
Pyloric
valve
closed
1 Propulsion: Peristaltic
waves move from the fundus
toward the pylorus.
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Slide 1
Pyloric
valve
closed
2 Grinding: The most
vigorous peristalsis and
mixing action occur close to
the pylorus.
Pyloric
valve
slightly
opened
3 Retropulsion: The pyloric
end of the stomach acts as a
pump that delivers small amounts
of chyme into the duodenum,
simultaneously forcing most of its
contained material backward into
the stomach.
Gastric Emptying
• Stomach usually empties completely within
four hours
– high fat (oily) foods digest more slowly and remain
in stomach longer
• Chyme entering duodenum cause receptors to
activate due to stretch
– signals hormones to inhibit acid and pepsin
secretion in stomach
Small Intestine
• Major organ of digestion and absorption
• 2-4 m long (7-13ft); from pyloric sphincter to
ileocecal valve
• Subdivisions
– Duodenum (retroperitoneal)
– Jejunum (attached posteriorly by mesentery)
– Ileum (attached posteriorly by mesentery)
Duodenum
• Curves around head of pancreas
• Bile duct (from liver) and main pancreatic duct
(from pancreas) enter at this point
– Join at hepatopancreatic ampulla
– Enter duodenum at major duodenal papilla
– Entry controlled by hepatopancreatic sphincter
Figure 23.21 The duodenum of the small intestine, and related organs.
Right and left
hepatic ducts
of liver
Cystic duct
Common hepatic duct
Bile duct and sphincter
Accessory pancreatic duct
Mucosa
with folds
Tail of pancreas
Pancreas
Jejunum
Gallbladder
Major duodenal
papilla
Hepatopancreatic
ampulla and sphincter
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Main pancreatic duct and sphincter
Duodenum
Head of pancreas
Jejunum and Ileum
• Jejunum
– Extends from duodenum to ileum
– About 2.5 m (8ft) long
• Ileum
– Joins large intestine at ileocecal valve
– About 3.6 m (12 ft)long
Histology of Small Intestines
• Increase surface area of proximal part for
nutrient absorption
• Circular folds in mucosa and submucosa layers
• Villi
• Microvilli
Figure 23.22a Structural modifications of the small intestine that increase its surface area for digestion and
absorption.
Vein carrying
blood to
hepatic portal
vessel
Muscle
layers
Circular
folds
Villi
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Lumen
Figure 23.22b Structural modifications of the small intestine that increase its surface area for digestion and
absorption.
Microvilli
(brush border)
Absorptive
cells
Lacteal
Goblet
cell
Blood
capillaries
Mucosaassociated
lymphoid
tissue
Intestinal
crypt
Muscularis
mucosae
Duodenal
gland
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Villus
Enteroendocrine
cells
Venule
Lymphatic vessel
Submucosa
Villi and Microvilli
• Epithelial cells, enterocytes, are columnar cells
with a dense capillary bed (lacteal)
• Microvilli create an appearance of a brush
border that also help in aiding digesting
– brush border enzymes
Figure 23.22c Structural modifications of the small intestine that increase its surface area for digestion and
absorption.
Absorptive cells
Goblet
cells
Villi
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Intestinal crypt
Figure 23.23 Microvilli of the small intestine.
Mucus
granules
Microvilli
forming the
brush border
Absorptive cell
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Intestinal Crypts
• Between villi, the mucosa is studded with
intestinal crypts
– secretory cells intestinal juices (mucous)
• Host of other cell types (discussed 5/9)
– Enteroendocrine cells
– Intraepithelian lymphocytes
– Paneth cells
Peyer’s Patches
• Aggregated lymphoid nodules located in
lamina propria along the intestinal tract
• Increasing abundance toward the distal end
– larger bacterial load
Intestinal Juice
• 1-2 L per day
• pH close to blood (isotonic)
• Enzyme poor
– enzymes are bound to brush border for optimal
digestion and immediate absorption
Liver and Gall bladder
• Accessory organs associated with the small
intestines
• Digestive functions:
– Liver: produce bile that breaks down fat
(900ml/day)
– Gallbladder: storage organ for bile
Liver
• Largest gland in body
• Four lobes—right, left, caudate, and quadrate
• Falciform ligament
– Separates larger right and smaller left lobes
– Suspends liver from diaphragm and anterior
abdominal wall
• Round ligament (ligamentum teres)
– Remnant of fetal umbilical vein along free edge of
falciform ligament
Figure 23.24a Gross anatomy of the human liver.
Sternum
Nipple
Bare area
Liver
Falciform
ligament
Left lobe of
liver
Right lobe of liver
Gallbladder
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Round ligament
(ligamentum
teres)
Figure 23.24b Gross anatomy of the human liver.
Lesser
omentum
(in fissure)
Left lobe
of liver
Porta hepatis
containing
hepatic
artery (left)
and hepatic
portal vein
(right)
Quadrate
lobe of liver
Ligamentum
teres
Bare area
Caudate lobe
of liver
Sulcus for
inferior
vena cava
Hepatic vein
(cut)
Bile duct
(cut)
Right lobe
of liver
Gallbladder
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Histology of Liver
• The liver is composed of sesame seed-sized
structural and functional units called liver
lobes
– liver cells: hepatocytes
• Liver’s main function is to filter and process
nutrient, rich blood
– central vein running through each lobe
Figure 23.25a–b Microscopic anatomy of the liver.
Lobule
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Central Connective
vein tissue septum
Figure 23.25c Microscopic anatomy of the liver.
Interlobular veins
(to hepatic vein)
Central vein
Sinusoids
Bile canaliculi
Plates of
hepatocytes
Bile duct (receives
bile from bile
canaliculi)
Fenestrated
lining (endothelial
cells) of sinusoids
Stellate macrophages
in sinusoid walls
Portal vein
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Bile duct
Portal venule
Portal arteriole
Portal triad
Gallbladder
• About the size of a kiwi-fruit, on the inferior
surface of the liver
• Stores and concentrates bile not immediately
needed
• Muscular wall contracts to expel bile when
stimulated
Pancreas
• Endocrine function
– Pancreatic islets secrete insulin and glucagon
• Exocrine function
– Acini (clusters of secretory cells) secrete
pancreatic juice
– To duodenum via main pancreatic duct
Figure 23.27 Activation of pancreatic proteases in the small intestine.
Stomach
Pancreas
Epithelial
cells
Membrane-bound
enteropeptidase
Trypsinogen
(inactive)
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Trypsin
Chymotrypsinogen
(inactive)
Chymotrypsin
Procarboxypeptidase
(inactive)
Carboxypeptidase
Figure 23.26a Structure of the enzyme-producing tissue of the pancreas.
Small
duct
Acinar cell
Basement
membrane
Zymogen
granules
Rough
endoplasmic
reticulum
Duct cell
One acinus
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Pancreatic Juice
•
•
•
•
1200 – 1500 ml/day
pH 8 to neutralize chyme
Electrolytes (primarily HCO3–)
Enzymes
– Amylase, lipases, nucleases and proteases
The Large Intestines
• Major digestive function Is to absorb remaining
water
• Unique anatomy
– Teniae coli
• Three bands of longitudinal smooth muscle in muscularis
– Haustra
• Pocketlike sacs caused by tone of teniae coli
– Epiploic appendages
• Fat-filled pouches of visceral peritoneum (unknown
function)
Regions of Large Intestines
• Five Regions
– Cecum
– Appendix (MALT)
– Colon
– Rectum
– Anal canal
Figure 23.29a Gross anatomy of the large intestine.
Left colic
(splenic) flexure
Right colic
(hepatic) flexure
Transverse
mesocolon
Transverse colon
Epiploic
appendages
Superior
mesenteric artery
Descending colon
Haustrum
Ascending colon
IIeum
Cut edge of
mesentery
IIeocecal valve
Tenia coli
Sigmoid colon
Cecum
Appendix
Rectum
Anal canal
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External anal sphincter
Figure 23.30c Mesenteries of the abdominal digestive organs.
Greater omentum
Transverse colon
Transverse
mesocolon
Descending colon
Jejunum
Mesentery
Sigmoid
mesocolon
Sigmoid colon
Ileum
© 2013 Pearson Education, Inc.
Histology
• Mucosa layer is simple columnar epithelium
– thicker and deeper for increased number of goblet
cells for mucous secretion
• Mucosa layer of anal canal is stratified
squamous epithelium
– increase protection against abrasion
The Anal Canal
• Last segment of large intestine
• Muscle Sphincters
– Internal anal sphincter— smooth muscle
• Involuntary
– External anal sphincter— skeletal muscle
• Voluntary
Motility in Colon
• Most contractions of colon; Haustral
contractions
– Slow segmenting movements for 1 min, every 30
minutes
• Occur in ascending and transverse colon
– distension of muscle wall regulate contractions
Gastrocolic reflex
• Activated by presence of food in stomach
• Mass movements – long, slow moving and
powerful contractile waves the move over
entire colon
– force contents to rectum
Lab Exercise 38
• Do not worry about mouth, tongue or teeth