The Digestive System
Functions of
Digestive System
food
blood
Absorbable
nutrients
wall
lumen
Intestines
wall
digestion
food
blood
Absorbable
nutrients
wall
lumen
Intestines
wall
digestion
food
blood
Absorbable
nutrients
absorption
Forms of Nutrients
In Food
Absorbable by Intestine
Protein
Carbohydrates
Fat
DNA, RNA
Vit B12
Amino acids
Monosaccharides (glucose)
Fatty acids, glycerol
Bases + monosaccharides
B12+intrinsic factor
Other vitamins
Cholesterol
Electrolytes
Water
Original form
Original form
Original form
Original form
digestion
food
blood
Amino acids
Glucose
Fatty acids,
Glycerol
Bases +
monosaccharides
vitamins
Cholesterol
Electrolytes,
Water
absorption
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 storage
- host defense
- minor role in digestion and
absorption of nutrients
- The above functions are
performed via gastric secretion
and motility
Gastric pits
mucous cells:
produce
The bottom part of
mucus
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
- is the inactive precursor of
the active enzyme pepsin.
- activated by HCl or pepsin.
Pepsin: chop proteins into
small fragments
Pepsinogen
Hydrochloric
Acid ( HCl )
- reduces pH of
gastric juice to as low
as pH 0.8.
HCl
Functions of HCl:
(1) activates pepsinogen
into pepsin.
(2) breaks up connective
tissues and plant cell
walls.
(3) converts ferric ions to
ferrous ions.
(4) 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
Intrinsic Factor
- is essential to the absorption
of vitamin B12
- is the only indispensable
function of the stomach.
Intrinsic
Factor
Gastrin
secreted by G cells in
gastric gland not into
gastric juice but blood
-
is a hormone
-
stimulates
1) the secretion of HCl
and pepsinogen
2) motility of the large
intestine
Gastric Motility
- relax during swallowing
- stretch further when
food enters the stomach
(stress-relaxation
response)
- Next, pacemaker cells in
the greater curvature initiate
peristaltic contractions.
- Each peristaltic wave pushs ~ 3 mL of chyme into the
duodenum.
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 and mobility
via:
a) the enterogastric reflex.
b) local hormones
Secretin
Cholecystokinin
gastric inhibitory peptide
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
Recycle of Bile acids and Phospholipids
- reabsorbed in the ileum and returned to the liver
via enterohepatic circulation.
Bile acids
phospholipids
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) inactive digestive enzymes and zymogens which
are activated after secreted into duodenum.
The activated enzymes 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.
- The surface area inside the small intestine is
greatly increased by circular folds, villi, and
microvilli.
villi
Microvilli (brush border)
brush border enzymes
brush border enzymes
- activates zymogens
- complete digestion of
carbohydrates and proteins
Secretion by the small intestines
- 1-2 L of intestinal juice per day.
- The duodenum endocrine cells
secret cholecystokinin (CCK) and
secretin. (Both are hormones.)
Chemical Digestion and Absorption
of Nutrients
Chemical Digestion and Absorption of Nutrients
Carbohydrates
Proteins
Lipids
Nucleic Acids
Vitamins
Minerals
Water
Digestion and Absorption of
Carbohydrate
- Most digestible dietary
carbohydrate is starch.
- The starch digestion begins in
the mouth by salivary amylase.
- But fully digestion of starch
occurs in the small intestines.
- Starch is digested to oligosaccharides (3-8 glucose
residues), disaccharide maltose, and glucose.
pancreatic amylase
Brush border
enzymes
oligosaccharides
starch
Intestinal lumen
glucose
glucose glucose
Intestinal
epithelial
cells
blood
Glucose is absorbed by:
- sodium-dependent glucose transporter
(SGLT).
- solvent drag
Digestion and Absorption of Proteins
- Proteins are digested by proteases and
peptidases.
- Protein digestion starts in the stomach.
Protein digestion continues in the small intestine by
pancreatic enzymes trypsin and chymotrypsin.
Protein digestion is completed in the small intestine
by brush border enzymes carboxypeptidase,
aminopeptidase, and dipeptidase.
Amino acid absorption is similar to that of
monosaccharides, via several sodium-dependent amino
acid cotransporters.
Gastric pepsin &
pancreatic proteases
proteins
Brush border
enzymes
short peptides
Intestinal lumen
amino acids
Intestinal
epithelial
cells
blood
Digestion and Absorption of Lipids
- Lipids are digested by enzymes called lipases.
- Most fat digestion occurs in the small intestine via
several steps.
1) Fats are first broken up into smaller
emulsification droplets by lecithin and bile
salts (acids) in the bile.
2) When lipase digests fats, the products are
two fatty acids (FFAs) and a monoglyceride.
3) Bile salts coat these and other lipids and
form droplets called micelles.
Micelles release
their lipids,
which diffuse
freely across the
plasma
membrane.
Resynthesis of triglycerides.
Coating with protein, forming droplets called
chylomicrons.
Intestinal absorptive cell
Chylomicrons are too large to enter blood capillaries
and must be first transported in the lymphatic lacteal.
Bile
salts
lipase &
bile salts
Fat globule
emulsification
droplets
Intestinal lumen
chylomicrons
micelles
Intestinal
epithelial
cells
lacteal
blood
Digestion and Absorption of Nucleic Acids
- The pancreatic nucleases hydrolyze nucleic
acids to their component nucleotides.
- The brush border nucleosidases and
phosphatases further break them down, and
the products are transported across the
intestinal epithelium by membrane carriers.
Absorption of Vitamins
- Vitamins are absorbed without digestion.
- The fat-soluble vitamins are absorbed with other
lipids.
- Water soluble vitamins are absorbed by simple
diffusion, with the exception of vitamin B12.
- Vitamin B12 is an unusually large molecule that
can only be absorbed with the help of intrinsic
factor.
Absorption of Minerals
- Minerals (electrolytes) are absorbed without
digestion.
- Iron and calcium are unusual in that they are
absorbed in proportion to the body's need.
- Other minerals are absorbed at fairly constant
rates regardless of need.
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