Physiology of the Digestive System
Chapter 26
Anatomy & Physiology
Mechanical Digestion
• Consists of all motility that brings
about:
– Change in physical state of food from large
to small
– Churning
– Propelling food forward
Mastication (chewing)
• Begins in mouth
• Tongue, cheeks, lips keep food between
the surfaces of teeth
• Reduce particle size & mix with saliva
Deglutition
•Oral stage: mouth to
oropharynx (voluntary
control); under control of
cerebral cortex
•Pharyngeal stage:
oropharynx to esophagus
(involuntary); deglutition
center in medulla
•Esophageal stage:
esophagus to stomach
(involuntary); deglutition
center in medulla
Oral stage
• Bolus in the middle of tongue pressed
against the palate & moved back into
the oropharynx
• Soft palate & uvula prevent food from
entering nasopharynx
Pharyngeal stage
• Involuntary reflexes push bolus toward
esophagus
• Upward movement of larynx &
downward movement of food closes
epiglottis
Motility of smooth muscle in GI tract
• Peristalsis
• Segmentation
Peristalsis
•Wavelike ripple;
always in a forward
direction
•Bolus stretching GI
wall triggers a reflex
contraction of circular
muscle
Segmentation
•Mixing
movement;
forward &
backward
movement
Gastric motility
• Takes 2-6 hours to empty stomach
• Food churned with gastric juices:
chyme
• Hormonal control: gastric inhibitory
peptide secreted by intestinal mucosa
• Nervous mechanism: triggered by
presence of acid & distentionenterogastric reflex
Intestinal motility
• Both segmentation & peristalsis
• Segmentation in duodenum & jejunum
mixes chyme w/digestive juices
• Takes 5 hours in small intestine
• Peristalsis also stimulated by
cholecystokinin-pancreozymin (CCK)
secreted by intestinal mucosa in
presence of chyme
Digestive enzymes
• Extracellular, protein catalysts
• Enzyme principles
– Specific in action
– Function best a specific pH
– Catalyze reaction in both directions
– Continually being destroyed or eliminated
so continually being synthesized
– Most digestive enzymes are inactive
proenzymes
Carbohydrate digestion
•Polysaccharides
hydrolyzed by
amylases (in saliva
& pancreatic juices)
to disaccharides
•Sucrase, lactase,
maltase found in
cell membrane of
villi cells hydrolyze
into
monosaccharides,
mostly glucose
Protein digestion
•Proteases catalyze
hydrolysis into
intermediate
compounds then
finally into amino acids
•Main proteases
–Pepsin-stomach
–Trypsin-pancreas
–Chymotrypsin- “
–Peptidase-intestines
Fat digestion
•Lecithin & bile acids
in bile emulsify
(become small
droplets soluble in
water) fats
•Main fat digesting
enzyme: pancreatic
lipases
Residues of digestion
• Cellulose: dietary fiber
• Undigested connective tissue from
meat
Saliva
• Secretion of salivary glands
– Water (mostly)
– Mucus: lubricates food
– Salivary amylase: begins digestion of
starches
– Sodium bicarbonate: increases pH for
optimal amylase activity
Gastric juice
• Secreted by gastric glands
– Pepsin (by chief cells as pepsinogen)begins digestion of proteins
– HCl (by parietal cells) decreases pH for
activation & function of pepsin
– Intrinsic factor (by parietal cells) protect
vitamin B12
– Mucus & water
Pancreatic juice
• Secreted by acinar cells & duct cells
– Proteases (trypsin & chymotrypsin)
– Lipases
– Nucleases: digest nucleic acids
– Amylase
– Sodium bicarbonate: increase pH
• All pancreatic enzymes are secreted as
inactive proenzymes
Bile
• Secreted by liver, stored &
concentrated in gall bladder
– Lecithin & bile salts
– Sodium bicarbonate for optimum pH
– Cholesterol, detoxification products, bile
pigments all eliminated in feces
Intestinal juice
• Secreted by cells of intestinal exocrine
cells
– Mucus & water: lubricate & aid in mixing
– Sodium bicarbonate
Control of salivary secretions
• Only by reflex mechanisms
• Chemical, mechanical, olfactory, visual
stimuli
Control of gastric
secretion
•Three phases:
–Cephalic phase
–Gastric phase
–Intestinal phase
Cephalic phase
• Activated by mental factors
• Vagus nerve stimulates production of
gastrin which stimulates gastric
secretions
Gastric phase
• Products of protein digestion stimulate
release of gastrin
• Distension of stomach also stimulates
release of gastrin
Intestinal phase
• Fats, carbohydrates & acid in chyme
stimulate release of gastric inhibitory
peptide (GIP), CCK, secretin which
decrease gastric secretion
Control of pancreatic secretion
• Stimulated by hormones secreted by
intestinal mucosa
– Secretin: production of pancreatic fluid low
in enzyme but high in bicarbonate
– CCK
• Increased exocrine secretion by pancreas
• Opposes gastrin, thus inhibiting gastric
secretion
• Stimulates contraction of gallbladder
Absorption
• Passage of substances through the
intestinal mucosa into blood
• Most occurs in small intestines
• Water: osmosis
• Secondary active transport: sodium
• Sodium cotransport: glucose
• With the aid of bile salts in lacteals
Elimination
• Expulsion of the residues of digestion
• Act of expelling feces is defecation
• Defecation results as a reflex
stimulated by receptors in rectal
mucosa when rectum is distended