Digestive System II: Digestive Activities
 Digestive Activities in the Mouth, Swallowing
 Digestive Activities in the Stomach
• Gastric juice
• Hormones that stimulate stomach acid and
enzyme secretion
 Contributions of the Liver in Digestion
 Digestion in the Small Intestine
• Pancreatic juice
• Hormonal stimulation of pancreas
• Methods employed to digest and absorb
carbohydrates, proteins, lipids, and nucleic
acids
 Absorption in the Large Intestine and Feces
Formation
Salivary Glands
 Extrinsic salivary glands (parotid,
submandibular, and sublingual)
 Intrinsic (buccal) salivary glands are
scattered in the oral mucosa
 Secretion (saliva)
•
Cleanses the mouth
•
Moistens and dissolves food chemicals
•
Aids in bolus formation
•
Contains enzymes that begin the
breakdown of starch
 Composition
• 97–99.5% water, slightly acidic solution containing:
• Electrolytes—Na+, K+, Cl–, PO4 2–, HCO3–
• Salivary amylase and lingual lipase; mucin;
• Metabolic wastes—urea and uric acid; lysozyme, IgA, defensins, and a
cyanide compound protect against microorganisms
Control of Digestive Activity and Mouth
 Mostly controlled by reflexes via
the parasympathetic division
 Enteric Nervous System
• (ENS) Intrinsic nerve supply of
the alimentary canal
o Sympathetic impulses inhibit
secretion and motility
o Parasympathetic impulses
stimulate
 Chemical and mechanical receptors
are located in organ walls that
trigger reflexes
Digestive System II: Digestive Activities
 Digestive Activities in the Mouth, Swallowing
 Digestive Activities in the Stomach
• Gastric juice
• Hormones that stimulate stomach acid and
enzyme secretion
 Contributions of the Liver in Digestion
 Digestion in the Small Intestine
• Pancreatic juice
• Hormonal stimulation of pancreas
• Methods employed to digest and absorb
carbohydrates, proteins, lipids, and nucleic
acids
 Absorption in the Large Intestine and Feces
Formation
Food Breakdown in the Stomach
 Gastric juice is regulated by neural and
hormonal factors
 Presence of food or falling pH causes
the release of gastrin
pepsinogen
 Gastrin causes stomach glands (chief
cells) to produce protein-digesting
enzymes (pepsinogen)
 Hydrocholoric acid from parietal
cells makes the stomach contents very
acidic, kills bacteria
 Heartburn from Acid Reflux
Acid reflux animation online
Ulcer formation movie online
Chief cell
Digestion and Absorption in the Stomach
 Protein digestion enzymes
(proteases)
Conversion of pepsinogen to pepsin
 Stomach proteins are exported
into the stomach as zymogens
(extra long, inactive polypeptide
chains)
• Pepsin – an active protein
digesting enzyme
(converted from pepsinogen
through action of HCl and
active pepsin)
• Rennin (chymosin)– works
on digesting milk casein
protein in infants
 The only absorption that occurs
in the stomach is of alcohol and
aspirin
The action of chymosin on casein
Regulation of Stomach Activity
Stimulatory events
Cephalic
phase
Gastric
phase
1 Sight and thought
of food
Cerebral cortex
Conditioned reflex
2 Stimulation of
taste and smell
receptors
Hypothalamus
and medulla
oblongata
1 Stomach
distension
activates
stretch
receptors
Vagovagal
reflexes
1 Presence of low
pH, partially digested
foods, fats, or
hypertonic solution
in duodenum when
stomach begins to
empty
Stimulate
Inhibit
Medulla
Vagus
nerve
Vagus
nerve
Local
reflexes
2 Food chemicals
G cells
(especially peptides and
caffeine) and rising pH
activate chemoreceptors
Intestinal
phase
Inhibitory events
Gastrin
release
to blood
Intestinal
(enteric)
gastrin
release
to blood
Lack of
stimulatory
impulses to
parasympathetic
center
Cerebral
cortex
Gastrin
secretion
declines
G cells
Overrides
parasympathetic
controls
Sympathetic
nervous
system
activation
1 Excessive
acidity
(pH <2)
in stomach
2 Emotional
upset
Stomach
secretory
activity
Enterogastric
reflex
Brief
effect
1 Loss of
appetite,
depression
Local
reflexes
Vagal
nuclei
in medulla
Pyloric
sphincter
1 Distension
of duodenum;
presence of
fatty, acidic,
hypertonic
chyme, and/or
irritants in
the duodenum
2 Distension;
Release of intestinal
presence of
hormones (secretin,
cholecystokinin, vasoactive fatty, acidic,
partially
intestinal peptide)
digested food
in the
duodenum
Figure 23.17
Digestive System II: Digestive Activities
 Digestive Activities in the Mouth, Swallowing
 Digestive Activities in the Stomach
• Gastric juice
• Hormones that stimulate stomach acid and
enzyme secretion
 Contributions of the Liver in Digestion
 Digestion in the Small Intestine
• Pancreatic juice
• Hormonal stimulation of pancreas
• Methods employed to digest and absorb
carbohydrates, proteins, lipids, and nucleic
acids
 Absorption in the Large Intestine and Feces
Formation
Liver
 Largest gland in the body
 Connected to the gall bladder via the
common hepatic duct
 Bile Production in the Liver
• Composition
o
Bile salts, bile pigment
(broken down
hemoglobin,
cholesterol,
phospholipids
 Bile is introduced into the
duodenum in the presence of fatty
food to cause emulsification
 Emulsification solubilizes lipids so
that they can come in contact with
water-soluble lipases and be broken
down into fatty acids and glycerol.
Role of the Liver in Metabolism
•
•
•
•
•
Produces blood proteins (albumin, clotting proteins) and lipoproteins
• Degrades hormones
•
The Liver Processes/Detoxifies Absorbed Food in Lobules
(a)
Lobule
(b)
Figure 23.25a, b
Central vein
Connective
tissue septum
Digestive System II: Digestive Activities
 Digestive Activities in the Mouth, Swallowing
 Digestive Activities in the Stomach
• Gastric juice
• Hormones that stimulate stomach acid and
enzyme secretion
 Contributions of the Liver in Digestion
 Digestion in the Small Intestine
• Pancreatic juice
• Hormonal stimulation of pancreas
• Methods employed to digest and absorb
carbohydrates, proteins, lipids, and nucleic
acids
 Absorption in the Large Intestine and Feces
Formation
Digestion in the Small Intestine
 Enzymes from the brush border
• Break double sugars into simple
sugars (carbohydrases)
• Complete some protein
digestion (proteases)
 Pancreatic enzymes play the major
digestive function; usually zymogens
• Help complete digestion of
starch (pancreatic amylase)
• Carry out about half of all
protein digestion (trypsin, etc.)
• Responsible for fat digestion
(lipase)
• Digest nucleic acids (nucleases)
• Bicarbonate ion neutralizes
acidic chyme, raises pH to 8
Chemical Digestion : Pancreas
Produces a wide spectrum of digestive enzymes that break down all categories of food
Acinar cells produce enzymes that are secreted into the duodenum
Alkaline fluid introduced with enzymes neutralizes acidic chyme
Figure 14.6
Stimulation of the Release of Pancreatic Juice
 Vagus nerve:  secretion of enzymes,
weak release of bile
 Local hormones from duodenal mucosa
•
Secretin (enzyme release,  bile
production, gastrin &  stomach
activities)
•
Liver +
Cholecystokinin (enzyme and
bicarbonate release,  bile release)
Digestive System II: Digestive Activities
 Digestive Activities in the Mouth, Swallowing
 Digestive Activities in the Stomach
• Gastric juice
• Hormones that stimulate stomach acid and
enzyme secretion
 Contributions of the Liver in Digestion
 Digestion in the Small Intestine
• Pancreatic juice
• Hormonal stimulation of pancreas
• Methods employed to digest and absorb
carbohydrates, proteins, lipids, and nucleic
acids
 Absorption in the Large Intestine and Feces
Formation
Other Activities in the Small Intestine
 Water absorption
 Nutrient absorption by
mucosal cells via active or
passive transport
 Lipids are absorbed by
diffusion into lacteals
 Substances are transported
to the liver by the hepatic
portal vein or lymph
(lacteal)
Summary of Carbohydrate Digestion
Carbohydrate digestion
Foodstuff
Enzyme(s)
and source
Site of
action
Starch and disaccharides
Oligosaccharides
and disaccharides
Lactose Maltose Sucrose
Galactose Glucose Fructose
(monosaccharides)
Salivary
amylase
Pancreatic
amylase
Brush border
enzymes in
small intestine
(dextrinase, glucoamylase, lactase,
maltase, and sucrase)
Mouth
Small
intestine
Small
intestine
Path of absorption
• Glucose and galactose
are absorbed via
cotransport with
sodium ions.
• Fructose passes via
facilitated diffusion.
• All monosaccharides
leave the epithelial
cells via facilitated
diffusion, enter the
capillary blood in the
villi, and are
transported to the liver
via the hepatic portal
vein.
Figure 23.32 (1 of 4)
Summary of Protein Digestion
Protein digestion
Foodstuff
Protein
Large polypeptides
Small polypeptides,
small peptides
Amino acids
(some dipeptides
and tripeptides)
Enzyme(s)
and source
Pepsin
(stomach glands)
in presence
of HCl
Pancreatic
enzymes
(trypsin, chymotrypsin,
carboxypeptidase)
Brush border
enzymes
(aminopeptidase,
carboxypeptidase,
and dipeptidase)
Site of
action
Path of absorption
• Amino acids are absorbed
by cotransport with
Stomach
sodium ions.
• Some dipeptides and
tripeptides are absorbed
via cotransport with H++
Small
and hydrolyzed to amino
intestine
acids within the cells.
• Amino acids leave the
epithelial cells by
Small
facilitated diffusion, enter
intestine
the capillary blood in the
villi, and are transported
to the liver via the hepatic
portal vein.
Figure 23.32 (2 of 4)
Lipid Digestion and Fatty Acid/Glycerol Import
Fat globule
1 Large fat globules are emulsified
(physically broken up into smaller fat
droplets) by bile salts in the duodenum.
Bile salts
Fat droplets
coated with
bile salts
2 Digestion of fat by the pancreatic
enzyme lipase yields free fatty acids and
monoglycerides. These then associate
with bile salts to form micelles which
“ferry” them to the intestinal mucosa.
Micelles made up of fatty
acids, monoglycerides,
and bile salts
3 Fatty acids and monoglycerides leave
micelles and diffuse into epithelial cells.
There they are recombined and packaged
with other lipoid substances and proteins
to form chylomicrons.
4 Chylomicrons are extruded from the
Epithelial
cells of
small
intestine
Lacteal
epithelial cells by exocytosis. The
chylomicrons enter lacteals. They are
carried away from the intestine by lymph.
Figure 23.34
Summary of Lipid Digestion
Fat digestion
Foodstuff
Enzyme(s)
and source
Unemulsified
fats
Emulsification by
the detergent
action of bile
salts ducted
in from the liver
Pancreatic
lipases
Monoglycerides Glycerol
and fatty acids
and
fatty acids
Site of
action
Path of absorption
• Fatty acids and monoglycerides
enter the intestinal cells via
diffusion.
Small
intestine • Fatty acids and monoglycerides
are recombined to form
triglycerides and then
combined with other lipids and
proteins within the cells, and
the resulting chylomicrons are
Small
extruded by exocytosis.
intestine
• The chylomicrons enter the
lacteals of the villi and are
transported to the systemic
circulation via the lymph in the
thoracic duct.
• Some short-chain fatty acids
are absorbed, move into the
capillary blood in the villi by
diffusion, and are transported
to the liver via the hepatic
portal vein.
Figure 23.32 (3 of 4)
Summary of Nucleic Acid Digestion
Nucleic acid digestion
Foodstuff
Enzyme(s)
and source
Nucleic acids
Nucleotides
Pancreatic ribonuclease and
deoxyribonuclease
Brush border
enzymes
(nucleosidases
and phosphatases)
Site of
action
Path of absorption
• Units enter intestinal cells
by active transport via
Small
intestine membrane carriers.
• Units are absorbed into
capillary blood in the villi
Small
and transported to the
intestine
liver via the hepatic portal
vein.
Figure 23.32 (4 of 4)
Digestive System II: Digestive Activities
 Digestive Activities in the Mouth, Swallowing
 Digestive Activities in the Stomach
• Gastric juice
• Hormones that stimulate stomach acid and
enzyme secretion
 Contributions of the Liver in Digestion
 Digestion in the Small Intestine
• Pancreatic juice
• Hormonal stimulation of pancreas
• Methods employed to digest and absorb
carbohydrates, proteins, lipids, and nucleic
acids
 Absorption in the Large Intestine and Feces
Formation
Processes in the Large Intestine
 No digestive enzymes are
produced
 Resident bacteria digest
remaining nutrients
• Produce some vitamin K
and B
• Release methane and
hydrogen sulfide gases
 Water and vitamins K and B
are absorbed
 Remaining materials are
eliminated via feces
CH4
Vitamin K
Vitamin B
H2S
Propulsion in the Large Intestine (Colon)
 Sluggish peristalsis
 Mass movements
• Slow, powerful movements
• (Three to four times per day)
 Lack of fiber and weakening of colon
walls cause diverticula
 Presence of feces in the rectum causes
defecation reflex
• Internal anal sphincter is relaxed
• Defecation occurs with relaxation of
the voluntary external anal sphincter
Neural control of the
rectum and anal sphicters
Digestive System II: Digestive Activities
 Digestive Activities in the Mouth, Swallowing
 Digestive Activities in the Stomach
• Gastric juice
• Hormones that stimulate stomach acid and
enzyme secretion
 Contributions of the Liver in Digestion
 Digestion in the Small Intestine
• Pancreatic juice
• Hormonal stimulation of pancreas
• Methods employed to digest and absorb
carbohydrates, proteins, lipids, and nucleic
acids
 Absorption in the Large Intestine and Feces
Formation