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Digestive System
 Function and processes of the digestive system
 Anatomy of the digestive system
 Motility
 Secretion
 Regulation of GI function
 Digestion and absorption
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Digestive System
 The gastrointestinal tract is a long tube passing
through the body that opens to the outside world on
both ends. Thus, the contents can be considered to be
part of the external environment.
 The primary function is to move nutrients, water, and
electrolytes from the external environment into the
body’s internal environment
 The food we eat is in the form of macromolecules
that must be broken down. The digestive system
secretes enzymes to break down food.
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
5 Basic Processes of the Digestive System
1. Digestion: Mechanical and chemical breakdown of
foods into units that can be taken across the intestinal
epithelium into the body.
2. Absorption: Active or passive transfer of substances
from the lumen of the GI tract to the extracellular fluid.
3. Motility: Movement of material in the GI tract as a result
of muscle contraction.
4. Secretion: Both the transepithelial transfer of water and
ions from the ECF to the digestive tract lumen and the
release of substances synthesized by the GI epithelial
cells.
5. Immune: Repelling foreign invaders via gut-associated
lymphoid tissue (GALT). 80% of lymphocytes found in
small intestive.
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Basic Processes of the Digestive System
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 21-1
Digestive System Anatomy
Oral cavity  esophagus  stomach  small intestine
 large intestine  rectum
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Figure 21-2a
Digestive System Anatomy

Mouth and pharynx
 Mechanical and chemical digestion (secretion of saliva by salivary glands)


Esophagus
Stomach
 Fundus (top)  body  antrum
 Mechanical and chemical digestion: Mixing food and acid and enzymes to create
chyme.

Pyloric valve
 Lets out only small amount of chyme at a time, regulated by integrated signals and
feedback loops between the intestines and stomach.

Small intestine
 Duodenum (25cm)  jejunum  ileum
 Most digestion and absorption takes place here via intestinal enzymes and exocrine
secretions from the liver and pancreas (which enter the duodenum via ducts)


Accessory organs: pancreas and liver
Large intestine: colon and rectum
 About 1.5L/day of the 7L/day of chyme generated ends up entering the colon.
 Water and electrolytes are absorbed out of the chyme and into the ECF to make
feces.

Anus
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Digestive System Anatomy: 4 Layers of GI
Tract Wall
1. Mucosa (Inner Layer)




Epithelial cells
Lamina propria (connective tissue and immune cells)
Muscularis mucosae (thin layer of smooth muscle)
Modifications increase surface area
2. Submucosa (connective tissue, nerves, blood and
lymph vessels)
3. Muscularis externa


Smooth muscle in layers
Nerves
4. Serosa (Outer Layer)

Connective Tissue
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Digestive System Anatomy: Folds of GI Tract
 Entire wall folds
 Rugae in the stomach
 Plicae in the small intestine
 Mucosal folds
 Intestinal Villi project into the lumen
 Tubular Invaginations of the Surface
 Gastric Glands in the stomach
 Crypts in the intestine
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Digestive System Anatomy
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 21-2c
Digestive System Anatomy
PLAY
Animation: Digestive System: Anatomy Review:
Overall Function of the GI System
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 21-2e
Epithelial Cells


The type changes from section to section.
4 Epithelial Cell Types
1. Transport Cells:
 Move water and ions into and out of the lumen
and absorb nutrients from lumen.
2. Endocrine
 Release hormones
3. Secretory Cells
 Release enzymes, mucus, and paracrine
molecules into the lumen
4. Stem
 Create new cells
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Motility: For Mixing and Movement of Food
 Tonic contractions
 Sustained
 Smooth muscle sphincters and stomach
 Phasic contractions
 Last a few seconds
 Peristalsis moves bolus forward
 Segmentation mixes
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Contractions in the GI Tract
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Figure 21-4a
Motility
PLAY Animation: Digestive System: Motility
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Figure 21-4b
Secretion
Daily mass fluid balance in digestive system
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Figure 21-5
Acid Secretion by Parietal Cells
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Figure 21-6
Neutralizing Bicarbonate Secretion in Duodenum:
From Pancreas and Duodenal Cells
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Figure 21-8
Secretion
Cl– secretion by intestinal colonic crypt cells
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Figure 21-9
Secretion
 Digestive enzymes secreted into mouth, stomach and
intestine
 Mucous cells in stomach and goblet cells in intestine:
Protective of the lining
 Saliva is an exocrine secretion with digestive
enzymes.
 Liver secretes bile. Gallbladder stores the bile until
needed for digestion:
 Bile salts act as detergents to solubilize fats,
facilitating enzymatic fat digestion
PLAY Animation: Digestive System: Secretion
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Regulation of GI Function
 Long reflexes integrated in CNS
 Short reflexes integrated in Enteric Nervous System
(ENS)
 Intrinsic neurons of the two nerve plexuses of the
digestive tract lie completely w/in the wall of the gut.
They carry out reflexes that are independent of control
from the CNS
 Reflexes involving GI peptides (Hormones,
Neuropeptides and Cytokines)
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Regulation of GI Function
Integration of long and short reflexes in the digestive
system
External
stimulus
KEY
Stimulus
(sight, smell,
etc.)
Receptor
Sensory
receptors
Integrating
center
Efferent pathway
The
cephalic
brain
Local
stimulus
(pH, stretch,
osmolarity,
products of
digestion)
Sensory
receptors
and
neurons
Interneurons
Tissue response
Short reflexes
Sympathetic and
parasympathetic
neurons
Neurons
of
myenteric
and
submucosal
plexuses
Enteric nervous system
“The little brain”
Long reflexes
Smooth
muscles
or
secretory
cells
Effectors
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Muscle contraction/relaxation
or
Exocrine secretion (enzymes,
mucus, acid, bicarbonate),
Paracrines
or
Endocrine secretions
Digestive system
responses
Figure 21-11
REGULATION OF GI FUNCTION
 Cephalic Reflexes: long reflexes that originate
completely outside the digestive system. The smooth
muscle and glands of the GI tract are under
autonomic control. In general, parasympathetic
neurons to the GI tract are excitatory and sympathetic
are inhibitory.
 Feedforward Reflexes
 Begin with stimuli such as the sight, smell, sound,
or thought of food.
 Prepare the digestive system for food (mouth
watering, movement of the stomach)
 Emotional Reflexes
 Psychologically induced diarrhea
 Butterflies in your stomach
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Regulation of GI Function
Reflexes involving GI Peptides
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 21-12