Chapter 21a
The Digestive
System
About this Chapter
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Digestion function and processes
Anatomy of the digestive system
Motility
Secretion
Regulation of GI function
Digestion and absorption
About this Chapter
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The cephalic phase
The gastric phase
The intestinal phase
Immune functions of the GI tract
Activities of the Digestive System
• Ingestion
• Digestion
• Mechanical
• Chemical / Secretion
• Motility
• Peristaltic
• Segmentation
• Absorption
• Immunity
• Elimination
Note:
1. Which of these activities occur
at each region of the GI tract.
2. Explain how these activities
occur, control (hormones or
neural), enzymes and structural
mechanisms, no need to name
the transporters.
Digestive Function and Processes
• The volume of fluid entering the GI tract must
equal the volume leaving 9 liters per day
Fluid input into
digestive system
Ingestion
2.0 L food and
drink
Secretion
1.5 L saliva
(salivary
glands)
0.5 L bile
(liver)
2.0 L gastric
secretions
Fluid removed
from digestive
system
Absorption
7.5 L from small
intestine
1.5 L pancreatic
secretions
1.5 L intestinal
secretions
1.4 L from large
intestine
Excretion
0.1 L in feces
9.0 L Total input
into lumen
9.0 L removed
from lumen
Figure 21-1
Four Basic Processes of the Digestive System
Food
SECRETION
DIGESTION
ABSORPTION
MOTILITY
Lumen of digestive tract
Wall
Interstitial
fluid
Blood
Figure 21-2
Digestive System Anatomy
• Trace through digestive
system, not specific regions
in addition to major parts.
Also note accessory
structures.
• Most simply:
• Oral cavity  pharynx 
esophagus  stomach 
small intestine  large
intestine  rectum
• Accessory
• Salivary glands
• Pancreas
• Liver and gall bladder
ANATOMY SUMMARY
THE DIGESTIVE SYSTEM
Oral cavity
Salivary
glands
Esophagus
Gallbladder
Pancreas
Small
intestine
Rectum
Liver
Stomach
Large
intestine
(a)
Figure 21-3a
Digestive System Anatomy
• Stomach
• Fundus  body  antrum (pylorus)
• Pyloric valve
• Small intestine
• Duodenum  jejunum  ileum
• Accessory organs: pancreas and liver
• Large intestine: colon (ascending, transverse,
descending and sigmoid colons) and rectum
• Anus
Digestive System Anatomy
• A closer look at the structure of the stomach
and small intestine
STRUCTURE OF THE STOMACH AND INTESTINE
Esophagus Fundus
ANATOMY SUMMARY
Diaphragm
THE DIGESTIVE SYSTEM
Body
Antrum
Oral cavity
Salivary
glands
Esophagus
Pylorus
Rugae: Surface folding
increases area
Mesentery
Mucosa
Submucosa
(b) The stomach
Gallbladder
Pancreas
Small
intestine
Rectum
(a)
Liver
Stomach
Large
intestine
Plica
Circular
muscle
Longitudinal
muscle
Submucosal
Villi
glands
(d) Structure of the small intestine
Serosa
Figure 21-3a–b, d
Digestive System Anatomy
• Layers: same throughout, but modified for
different functions
• Mucosa
• Created from
• Epithelial cells
• Lamina propria
• Muscularis mucosae
• Modifications increase surface area
•  Rugae / Plica and villi / crypts
• Submucosa
• Muscularis externa
• Serosa
Digestive System Anatomy
SECTIONAL VIEW OF THE STOMACH
Opening to
gastric gland
Epithelium
Mucosa
Lymph vessel
Lamina propria
Muscularis mucosae
Submucosa
Oblique muscle
Muscularis
externa
Artery
and vein
Circular muscle
Longitudinal
muscle
Serosa
Myenteric
plexus
(c)
Figure 21-3c
Digestive System Anatomy
SECTIONAL VIEW OF THE SMALL INTESTINE
Villi
Crypt
Mucosa
Lymph
vessel
Submucosal
plexus
Muscularis
mucosae
Submucosa
Muscularis
externa
Circular
muscle
Myenteric
plexus
Longitudinal
muscle
Serosa
Submucosal
artery and vein
Peyer’s patch
(e)
Figure 21-3e
Digestive System Anatomy
PLAY
Interactive Physiology® Animation: Digestive System:
Anatomy Review: Overall Function of the GI System
Motility
• Tonic contractions
• Sustained
• Occur in smooth muscle sphincters and
stomach
• Keep bolus from moving backwards
• Phasic contractions
• Last a few seconds
• Peristalsis moves bolus forward
• Segmentation mixes
Contractions in the GI Tract
• Peristalsis promotes forward movement
Figure 21-5a
Motility
• Segemental contractions promote mixing
PLAY
Interactive Physiology® Animation: Digestive System:
Motility
Figure 21-5b
Secretion
• 9 liters / day 7 of which from secretions
• Parietal cells secrete hydrochloric acid into the lumen
of the stomach. Other sources include: duodenum /
pancreas / salivary glands
Interstitial
fluid
H2O
H+
Capillary
Lumen of
stomach
H+ + OH–
ATP
CA
K+
K+
HCO3–
HCO3–
CO2
Cl–
Cl–
Cl–
Cl–
Parietal cell
CA = Carbonic anhydrase
Figure 21-6
Secretion
• Anatomy of the exocrine and endocrine
pancreas acini and islets
Duct cells
secrete NaHCO3
Pancreatic islet cells
Acinar cells
Pancreatic
acini
Figure 21-7
Secretion
• Bicarbonate secretion in the pancreas and
duodenum
Pancreatic duct cell
or duodenal cell
Interstitial
fluid
H2O + CO2
CO2
Capillary
Lumen of
pancreas or
intestine
CA
HCO3–
HCO3– + H+
Na+
Cl–
Na+
Cl–
CFTR
channel
ATP
K+
Na+
2 Cl–
K+
K+
H2O, Na+
Figure 21-8
Secretion
• Cl– secretion by intestinal and colonic crypt
cells
Lumen
Interstitial
fluid
2 Cl– enters lumen through
CFTR channel.
K+
K+ 1
2 Cl–
Na+
2
Cl–
Cl–
1 Na+, K+ and Cl– enter by
cotransport.
3 Na+ is reabsorbed.
CFTR channel
Na+
ATP
3
K+
4
Na+,
H2O
4 Negative Cl– in lumen attracts
Na+ by paracellular pathway.
Water follows.
Na+,
H2O
Figure 21-9
Secretion
• Digestive enzymes secreted into mouth,
stomach and intestine
• Mucous cells in stomach and goblet cells in
intestine
• Saliva is an exocrine secretion
• Liver secretes bile
PLAY
Interactive Physiology® Animation: Digestive System:
Secretion