Finishing
Digestion #1
STOMACH
Rumen: softens food.
Reticulum: more softening into cud.
Omasum: digestion.
Abomasum: more digestion!
1
Rumen
2
Reticulum
Intestine
Esophagus
4
Abomasum
3
Omasum
• A J-shaped organ with walls made of three smooth muscle layers,
epithelial tissue and connective tissue.
• Entrance through the lower esophageal sphincter, exit through the
pyloric sphincter.
• The stomach is a muscular bag
which stores food and secretes
gastric juice, which converts the
bolus into acidic chyme.
• Can expand when full or
contract when empty, resulting in
rugae, or wrinkles forming.
Esophagus
5 µm
Lower
esophageal
sphincter
Stomach
Pyloric
sphincter
Rugae
Interior surface
of stomach
Duodenum
(Small intestine)
• The left & right gastric
arteries supply the
stomach with fresh
blood.
• Stomach is drained by
the left & right gastric
veins which link to the
hepatic portal vein.
A mucosa layer of epithelial tissue containing gastric glands in gastric
pits. These glands secrete acid, hormones and enzymes.
Three muscle layers (oblique, circular & longitudinal layers) aid in
churning of food.
An external serosa layer of connective tissue for strength & flexibility.
• Gastric pits in the inner epithelium lead to gastric
glands.
• Endocrine cells: secrete the hormone gastrin, which turns on the
whole stomach (seen later).
• Parietal cells: secrete HCI, an
acid capable of denaturing
proteins, killing germs and
breaking down plant cell walls.
• Chief cells: secrete
pepsin, (or pepsinogen),
the main protease
(protein-digesting
enzyme) found in the
stomach. Also secrete
some gastric lipase,
which begins lipid
digestion.
• Mucus cells: secrete protective
mucus.
• The stomach wall is lined by exocrine glands that house the
various types of cells responsible for secreting gastric juice
(HCI acid, pepsin), and mucus.
Interior surface
of stomach
Epithelium
3
Pepsinogen
2
1
H+
Cl–
Chief cells
secrete
pepsinogen
Parietal cells
secrete HCI
acid.
1 Pepsinogen and HCl
are secreted.
HCl
Gastric gland
Mucus cells
secrete
protective
mucus
Pepsin
2 HCl converts
pepsinogen to pepsin.
3 Pepsin activates
more pepsinogen.
Chief cell
Parietal cell
G cells
Secrete Gastrin
PROTEIN STRUCTURE
• The amino acid sequence
makes up a protein’s
primary structure.
• Successive folding results in
the formation of the
secondary & tertiary
structures.
• If two or more tertiary
structure proteins bind
together a quaternary
structure protein forms.
• HCI will destroy the 2ry, 3ry &4ry structures, unfolding
the protein.
• Proteases will digest the 1ry structure into small
peptides or individual amino acids.
Control of the stomach
• Three phases are involved:
–
Cephalic phase prepares the stomach to receive food.
–
Gastric phase begins when food enters the stomach.
–
Intestinal phase controls the emptying of the stomach into the
duodenum.
• Sight, smell, taste or thoughts of food stimulate the CNS. The
vagus nerve stimulates endocrine, mucus, chief and parietal cells.
• Gastrin is released into the blood...
• Gastrin stimulates the release of more HCI and pepsin.
• Low, acidic pH and stretching of the stomach wall stimulate “mixing
waves” of contracting smooth muscle.
• In response to acidic pH, duodenum secretes secretin, which inhibits
G-cells & parietal cells. Also stimulates pancreas to produce buffers.
• Duodenum also secretes the hormone cholecystokinin (CKK), which
also inhibits the G-cells and encourages the liver and pancreas to
release enzymes and bile.
Liver
Gallbladder
Bile
Stomach
Secretin
and CCK
–
Gastrin
+
CCK
+
Pancreas
Duodenum of
small intestine
Secretin
+
Key
CCK
+
+
–
Stimulation
Inhibition
• Three hormones are involved in digestion.
–
Gastrin is secreted mainly by the G cells in the gastric glands in
response to the sight, scent or thought of food. Preps the
stomach by stimulating chief & parietal cells to release gastric
juices).
–
Secretin is secreted by the epithelium of the duodenum in
response to acidic chyme. Stimulates the pancreas to release
pancreatic juice (which contains bicarbonate, an acid
neutralizer). It also acts on G cells inhibiting gastrin production.
–
Cholecystokinin (CCK) is also secreted by the epithelium of
the duodenum in response to acidic chyme. It stimulates the
gall bladder to release bile salts and stimulates the pancreas
to produce pancreatic enzymes. Also acts on G cells inhibiting
gastrin production.