The Digestive System
Processes of the Digestive System: Overview
• Ingestion
• Digestion:
enzymatic
• Motility
• Secretion
• Absorption
• Elimination
• (Self protection)
Digestive Secretions/Abosrptions
•
•
•
•
•
•
•
•
Salivary glands
Pancreas
Water
Enzymes
Mucus
Ions: H+, K+, Na+
HCO3-, ClMass Balance (H2O)
Figure 21-5: Daily mass balance in the digestive system
The Organs of the Digestive System
The Mouth
The Salivary Glands
The Esophagus
The Stomach
The Liver
The Gallbladder
The Pancreas
The Small intestines
The Large intestines
The Mouth
has the following functions :
1) breaking food
2) sense of taste
3) swallowing
4) speech
Saliva and the Salivary Glands
1) moisten food and the
mouth
2) clean the teeth, inhibit
bacterial growth,
3) dissolve molecules so
they can stimulate taste
buds
4) digest a small amount of
starch and fat
Esophagus
Esophagus
The Stomach
The Stomach
Functions
- food breakdown by gastric juices
- host defense
- minor role in absorption of nutrients
Gastric pits
mucous cells:
produce
The bottom
mucus
part of
the pits is the
gastric
parietal
cells:gland.
produce HCl
and intrinsic
factor
chief cells:
produce
pepsinogen
G cells:
produce
hormones
that regulate
digestion
The gastric glands produce
2-3 L of gastric juice daily,
containing:
pepsinogen
HCl
intrinsic factor
Pepsinogen
Contains the enzyme
pepsin. Chop proteins into
small fragments (amino
acids)
Pepsinogen
Hydrochloric Acid
Breaks down food and
reduces pH of gastric
juice to as low as 0.8.
HCl
Functions of HCl:
(1) Helps activate
pepsinogen.
(2) breaks up connective
tissues and plant cell
walls.
(3) destroys ingested
pathogens.
HCl
Self protection of the stomach
from HCl and pepsin by
1) a highly alkaline mucous
coat.
2) rapid replacement of
epithelial cells (3-6 d)
3) tight junctions between
epithelial cells
HCl
Regulation of Gastric Function
Gastric secretion and motility is divided
into three stages:
1) Cephalic
2) Gastric
3) Intestinal phases
1) The Cephalic Phase
The cephalic phase is
stimulated by the sight,
smell, taste, or mere
thought of food.
2) The Gastric Phase
- is stimulated by food in the
stomach
- accounts for two-thirds of
gastric secretion.
3) The Intestinal Phase
After entering small intestines,
chyme inhibit gastric secretion
via:
a) the enterogastric reflex.
b) local hormones
The small intestine receives not
only chyme from the stomach but
also secretions from the liver and
pancreas.
The Liver
has tremendous
variety of functions,
including the
secretion of bile
for digestive
purposes.
Hepatocytes secret bile into the bile canaliculi and
bile ductules.
bile ductule
common
Bile ductules  hepatic ducts  hepatic
duct
cystic
duct
common hepatic duct
common bile duct
gallbladder
hepatopancreatic
sphincter
pancreas
duodenum
When no chyme is in the small intestine, the
hepatopancreatic sphincter is closed. Bile flows
into gallbladder.
cystic
duct
common hepatic duct
common bile duct
gallbladder
hepatopancreatic
sphincter
pancreas
duodenum
Function of Gallbladder:
Absorbs water and stores the bile for later
use.
gallbladder
hepatopancreatic
sphincter
duodenum
Bile
- is constantly produced by the liver (500-1,000
mL of bile per day).
- is a yellow-green fluid containing :
minerals
bile pigments
bile acids
phospholipids
cholesterol
neutral fats
facilitate fat
digestion and
absorption
The Pancreas
The Pancreas
Exocrine:
- secretes pancreatic
juice into the lumen of the
pancreatic duct
Endocrine:
- secretes insulin, glucagon, somatostatin into
the blood.
Exocrine
secretes 1,200-1,500 mL of pancreatic juice per
day into the main pancreatic duct.
It empties into
duodenum
when hepatopancreatic
sphincter
opens.
Pancreatic juice is an
alkaline mixture
containing:
1) sodium bicarbonate:
neutralize HCl
2) Digestive enzymes which are secreted into the
duodenum and digest carbohydrates, lipids, RNA,
DNA, and proteins.
Regulation of Bile and Pancreatic
Secretion
- secreted in response to
similar stimuli.
-Cephalic and Gastric
phases
The vagus nerves
stimulate pancreatic
secretion.
The Intestinal Phase
- Chyme with acid and fat,
stimulate the duodenal
mucosa to secrete
cholecystokinin (CCK).
1) relaxation of the hepatopancreatic sphincter,
2) the contraction of the gallbladder
3) secretion of pancreatic juice and enzymes.
The Intestinal Phase
- Acidic chyme also
stimulates the duodenum
to release secretin.
secretion of bicarbonate
by both the hepatic and pancreatic ducts
The Small Intestine
Overview
- Nearly all chemical
digestion and nutrient
absorption occur in the
small intestines.
1) The duodenum
- ~ 25 cm
2) The jejunum
- comprises the next 2.5 m.
3) The ileum
- forms the last 3.6 m.
Small Intestine
• Extends from stomach
(pyloric sphincter)
• Regions
– Duodenum
– Jejenum
– Ileum
Chemical Digestion and Absorption of Nutrients
Carbohydrates
Proteins
Lipids
Nucleic Acids
Vitamins
Minerals
Water
Small Intestine
• Secretes digestive enzymes
– Peptidases
• Amino• Di• Tri-
–
–
–
–
Sucrases
Maltase
Lactase
Saccharidases
• Di• Tri-
– Lipase
– Nucleases
Small Intestine
• Absorbs
–
–
–
–
–
80% ingested water
Electrolytes
Vitamins
Minerals
Carbonates
• Active/facilitated transport
• Monosaccharides
– Proteins
• Di-/tripeptides
• Amino acids
– Lipids
• Monoglycerides
• Fatty acids
The surface area inside the small intestine is
greatly increased by circular folds called villi and
microvilli.
villi
Figure 21-2e: ANATOMY SUMMARY: The Digestive System
Structure of the Villi in the Small
Intestine
Absorption of Water
- The digestive tract receives about 9 L of water per
day
0.7 L in food
1.6 L in drink
6.7 L in gastrointestinal secretions
- ~ 8 L absorbed by the small intestine via osmosis;
Intestinal Motility
serve three functions:
1) to mix chyme with intestinal
juice, bile, and pancreatic juice
(segmentation)
2) to churn chyme and bring it in
contact with the brush border for
digestion and absorption;
(segmentation)
3) to move residue toward the
large intestine. (peristalsis)
- Segmentation is the most
common type of movement of
the small intestine.
- When nutrients have been
absorbed, segmentation slows
and peristalsis begins.
- The intensity of the
contractions is modified by
nervous and hormonal
influences.
The Large Intestine
- No further chemical
digestion
- Water (~ 1L) is further
absorbed.
- The feces consists of:
75% water
25% solid matter, of which
30% is bacteria, and 30%
undigested fiber.
Bacterial Flora
- refer to several species of
useful bacteria in large intestine.
- ferment cellulose and other
undigested carbohydrates
- synthesize B vitamins and
vitamin K, which are absorbed by
the colon.
Strong contractions
called mass
movements occur
one to three times a
day, last about 15
minutes each, and
occur especially an
hour after breakfast.
Neural Control of Defecation
1. Filling of the rectum
2. Reflex contraction of
rectum & relaxation of
internal anal sphincter
3. Voluntary relaxation of
external sphincter
Diarrhea
too little water absorbed
Constipation
too much water absorbed, causing
difficulty in defecation
In the absence of bile, a fat-rich
diet causes diarrhea.
Accumulation of fat molecules
osmolarity of intestinal content
retain water
diarrhea
People lacking lactase have
diarrhea after drinking milk.
Lactose (a dimer)
lactase
glucose
galactose
People lacking lactase have
diarrhea after drinking milk.
Accumulation of Lactose
osmolarity of intestinal content
retain water
diarrhea
SUMMARY
The Organs
Digestion and
Absorption of Nutrients
The Stomach
Carbohydrates
The Liver
Proteins
The Gallbladder
Lipids
The Pancreas
Nucleic Acids
The Small intestine
The Large intestine
Vitamins
Minerals
Water