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The Digestive System …cont’d
Wednesday, January 20th, 2010
Today’s Targets!
• Functions of anaerobic bacteria in the
colon
• Review the actions of various
enzymes
• Understand how release of enzymes
controlled by hormones
• Understand the regulation of
hormones
The Large Intestine (or Colon)
• Size: ~1.5 m long, 6.5 cm in
diameter
• LI joins SI in lower right corner of
abdomen - near top of pelvic bone
 junction is at right angles but not
quite at the end - there is a blind
end to the LI called the cecum
(projecting from the cecum is the
appendix)
The Large Intestine
Major parts:
• cecum
• ascending colon rises up
right side of abdomen,
• transverse colon crosses top
of abdomen
• descending colon goes
down left side becoming…
• sigmoid colon, which curves
to join rectum
The Large Intestine
1. Mechanical Movement - peristalsis moves feces along
2. Absorption - water and some salts absorbed from feces
Figure 41.25
3. Bacteria (E. coli ) work on undigested food from the SI and produce gases flatulence- about 1.5 L/day (mainly nitrogen gas, and carbon dioxide,
with small amounts of hydrogen, methane and hydrogen suphide),
amino acids and some vitamins
 amino acids and vitamins (K) produced absorbed through intestinal
lining (LI does not have villi like SI)
4. Defecation - reflex contraction of muscles lining the filled rectum force the
sphincter muscles open and expel the feces
5. Feces contain bile pigments, heavy metals and billions of E. Coli
Animation: Reflexes in the Colon
• http://highered.mcgrawhill.com/classware/ala.do?isbn=0072956208&
alaid=ala_996422&showSelfStudyTree=true
Physical Digestion:
-teeth, tongue, mouth
Chemical Digestion:
-Salivary amylase
(carbohydrates  disaccarides)
Liver  SI (Physical):
-bile (emulsify fats)
Pancreas  SI:
Amylase, trypsin,
peptidase, lipase,
nuclease, HCO3SI: maltase,
lactase, sucrase,
peptidase
Saliva: water,
mucus, HCO3-,
salivary amylase
Physical Digestion:
-peristalsis
Physical Digestion:
-peristalsis
Gastric Juice: mucus
HCL (kills pathogens,
lowers pH, stimulates
pepsin production),
pepsinogen  pepsin
(protein  peptides)
Control of Digestive Gland
Secretions
• NERVOUS STIMULATION – sight and smell receptors send
messages to brain and cause release of saliva, gastric and
pancreatic juices
• PRESENCE OF FOOD in digestive system triggers exocrine
glands to secrete enzymes and bicarbonate
• HOW?
HORMONES released from endocrine glands into blood 
travels to glands to trigger release of enzymes
Control of Digestive Gland
Secretions
• As with all body systems, the nervous system and
endocrine system tightly control digestion
• The amount of saliva produced, the amount of
stomach acid produced, the timing and amount of
pancreatic and gall bladder secretions and the rate of
peristalsis all controlled
Gastrin (Hormonal) Regulation
• When food eaten, sensory cells in stomach detect
presence of peptides
• Other sensory receptors detect stomach distending
(i.e. stretching)
• This causes specialized stomach cells in the lower
stomach to release GASTRIN into the blood. Gastrin
travels through the blood to the upper stomach
which causes the release of gastric juices, (HCl and
pepsinogen)
Gastrin (Hormonal) Regulation
STRETCHING
HCl
PEPSINOGEN
FOOD
BLOOD
GASTRIN (hormone)
CCK (Hormonal) Regulation
• Presence of amino acids or fatty acids in
duodenum triggers release of
CHOLECYSTOKININ (CCK)
• CCK stimulates the release of digestive
enzymes by the pancreas and bile by the
gallbladder
Secretin (Hormonal) Regulation
RECALL:
- Most digestion occurs in duodenum
- Chyme is acidic when it leaves the stomach
• SECRETIN produced by small intestine
SI  blood  pancreas (stimulates release of
SODIUM BICARBONATE)
Also secretin will decrease motor activity in the
stomach
GIP (Hormonal) Regulation
• ENTEROGASTRONE (or Gastric Inhibitory
Peptide, or GIP) is released by small intestine
when chyme is rich in fats
• Slows digestion by INHIBITING stomach
peristalsis and acid secretion (allowing more
time to digest)
• Hormones help coordinate the secretion of
digestive juices into the alimentary canal
When acid chyme rich in
fats enters duodenum,
enterogastrone
secreted by duodenum to
inhibit peristalsis and acid
secretion by stomach
Liver
Enterogastrone
Gallbladder
Gastrin
CCK
Stomach
Amino acids or fatty acids
in duodenum trigger
release of
cholecystokinin (CCK),
which stimulates release of
digestive enzymes from
pancreas and bile from
gallbladder.
Pancreas
Secretin
Duodenum
CCK
Key
Stimulation
Figure 41.22
Gastrin from stomach
recirculates via blood
back to stomach, where it
stimulates production
of gastric juices
Inhibition
Secreted by duodenum,
secretin stimulates
pancreas to release
sodium bicarbonate to
neutralize acid chyme
from stomach
Three Phases of Gastric Secretion
• http://highered.mcgrawhill.com/classware/ala.do?isbn=0072956208&
alaid=ala_996427&showSelfStudyTree=true
Digestion Summary
Animation:
• http://www.linkstudio.info/portfolio/animatio
n08.htm
Digestion
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• Posted on
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