The document summarizes the physiology of the gastrointestinal tract secretions. It describes the various glands in the GI tract, including goblet cells, tubular glands, and others. It explains the secretions of these glands, such as mucus, digestive enzymes, acid, and bicarbonate. It also discusses the stimulation and regulation of secretion, including through nervous and hormonal mechanisms. The secretions aid in digestion and protect the lining of the GI tract.

1. PHYSIOLOGY OF GIS
Keumala Hayati
Monday, April 18, 2016

2. Secretory Function of GI-Tract
Secretory
glands
Digestive
enzymes
Mucus
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3. Types of GI-tract Glands
Goblet Cells Also called : single-cell mucous glands or mucous cell.
Located on the surface of epithelium (in most part of GIT)
Specialized
Secretory
Cells
Located in the pits (invagination of the epithelium to
submucosa) of many surface areas of the GIT.
E.g : crypts of Lieberkühn in small intestine.
Tubular
Glands
Abundant in stomach and duodenum
E.g : oxyntic gland in stomach (acid- and pepsinogen-secreting
gland)
Other
glands
associated
with GIT
Salivary glands
Pancreatic glands
Liver
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4. Daily Secretion of Intestinal Juices
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5. Stimulation of GIT Glands
(basic mechanism)
• Contact of foods with epithelium
• Contact of foods in particular segments  stimulates local and
adjacent glands to secret juices
• E.g : direct contact of the surface cells by food  stimulates mucous
cells to secret mucus  mucus secretion ↑
• Enteric Nervous Stimuli
• Enteric Nervous system activation:
• Tactile
• Chemical
irritation
• distention
Stimuli
• Mucous
cells
• Deep
glands
ENS
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6. Stimulation of GIT Glands
(basic mechanism)
• Autonomic Nervous Stimuli
• Parasympathetic stimulation  ↑ GIT glandular secretion rate
Parasympathetic
stimulation
Upper portion
(innervated by
glossopharyngeal and
vagus nerves)
Salivary, esophageal,
gastric glands,
pancreas, & Brunner’s
glands in duodenum
Lower portion
(innervated by pelvic
parasympathetic
nerves)
Some glands in 1/3
distal portion of large
intestine
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7. Stimulation of GIT Glands
(basic mechanism)
• Autonomic Nervous Stimuli
• Sympathetic stimulation  dual effect
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Sympathetic
stimulation
Alone
• Slightly
increase
secretion rate
Presence of
hormonal and
parasympathetic
stimulation
• Reduce the
secretion

8. Glandular Cells Secretion
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• Organic substances
1. Diffusion of nutrients from blood vessels.
2. Formation of ATP by mitochondria.
3. ATP + substrates provided by nutrients 
to RE  synthesis of the organic
secretory substances.
Proteins secreted are formed by
ribosomes adherent to the RE.
4. Secretory materials  to Golgi complex
5. In Golgi complex, the materials :
modified  discharged into cytoplasm in
the form of secretory vesicles
6. Stored in apical-ends of the cells until
stimulated by hormonal or nervous
signals.

9. Secretion of Saliva
Saliva
Serous
secretion
Ptyalin (α-amylase)
Mucus Mucin
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Almost entirely
serous
Parotid glands
Bothd serous and
mucus
Sublingual and
submandibular
glands
Only mucus
Buccal glands

10. Secretion of Saliva
• Stage 1 : primary secretion  involved acini
• Stage 2 : ions secretion  involved salivary duct
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11. Esophageal Secretion
• Entirely mucus
• provide lubrication for swallowing
• Compound mucous glands at the gastric-end of
esophagus
• Protection from digestion by acidic gastric juices that often reflux
from the stomach
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12. Gastric Secretion
Tubular Glands of
Stomach
Oxyntic
(acid-forming)
glands
HCl
Pepsinogen
Mucus
Pyloric gland
Mucus
Gastrin
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13. Gastric Secretion
• Oxyntic glands :
• mucous neck cells  secrete
mainly mucus
• peptic (chief) cells  secrete
pepsinogen
• parietal (or oxyntic) cells  secrete
HCl and intrinsic factor.
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14. Gastric Secretion
• Basic Mechanism of HCl secretion
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15. Gastric Secretion
• Acetylcholine :
• ↑ secretion of pepsinogen by peptic cells
• HCl by parietal cells
• mucus by mucous cells
• Gastrin and histamine strongly stimulate secretion of acid
by parietal cells but have little effect on the other cells
• HCl + pepsinogen  acvtive pepsin
• Intrinsic factor (secreted by parietal cells) important for
absorption of vit-B12
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16. Gastric Secretion
• Pyloric glands
• Contain mostly mucous cells
• Secrete :
• Mucus (thick, alkaline, viscid mucus)  Lubrication and protection from
digestion by gastric acids
• Gastrin  hormone controlling gastric secretion
• Response in stimuli associated with ingestion of meal  secretion of gastrin
by G cells of the antrum of the stomach.
• 2 forms : large, G-34 (34 amino acid). Small, G-17 (17 amino acid)
• Food containing protein (antral-end of stomach)  stimulate gastrin cells in
pyloric glands  secretion of gastrin  to blood  transported to ECL
(enterochromaffin-like cells)  secretion of histamine into deep oxyntic cells
 secretion of HCl
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17. Gastric Secretion
• Regulation of Pepsinogen secretion
• acetylcholine released from the vagus nerves or from the gastric
enteric nervous plexus
• acid in the stomach.
• Acid : elicits additional enteric nervous reflexes that
support the original nervous signals to the peptic cells.
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18. Pancreatic Secretion
• Pancreatic Digestive Enzymes
• Protein digestion :
• Trypsin and chymotrypsin  split proteins into peptides (but do not
cause release of individual amino acids.
• Carboxypolypeptidase  splits some peptides into individual amino
acids.
• Carbohydrate digestion :
• Pancreatic amylase  hydrolyzes starches, glycogen, and most other
carbohydrates (except cellulose) to form disaccharides and a few
trisaccharides
• Fat digestion :
• Pancreatic lipase  hydrolyzing neutral fat into fatty acids and
monoglycerides
• Cholesterol esterase  causes hydrolysis of cholesterol esters
• Phospholipase  splits fatty acids from phospholipids
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19. Pancreatic Secretion
• Bicarbonate
secretion by
pancreatic ductules
and ducts
• HCl +NaHCO3 
NaCl +H2CO3
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20. Pancreatic Secretion
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21. Bile Secretion by the Liver
• Bile acids
• Fat digestion :
• Emulsify large fat
• Fat absorption through intestinal muosal membrane
• Excretion of bilirubin
• Bile secretion
• Initial secretion
• Bile flow (stimulated by secretin)
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22. Bile Secretion by the Liver
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23. Bile Secretion by the Liver
• Composition of bile
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24. Bile Secretion by the Liver
• Bile  stored in gallbladder
• Factors secreting bile :
• Cholecystokinin
• Acethylcholine-secreting nerve fibers (vagus nerves)
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25. Secretion of Small Intestine
• Brunner’s Glands  secrete large amount of alkaline
mucus
• Stimulation :
• Tactile
• Vagal
• GI hormones (secretin)
• Inhibited by sympathetic stimulation
• Function : protection
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26. Secretion of Small Intestine
• Crypts of Lieberkühn and the villi are
covered by an epithelium composed
of two types of cells :
• goblet cells, which secrete mucus
• Function : lubricates and protects the
intestinal surfaces,
• enterocytes, which, in the crypts, secrete
large quantities of water and electrolytes
and, over the surfaces of adjacent villi,
reabsorb the water and electrolytes along
with the end products of digestion.
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27. Secretion of Small Intestine
• Secretion of water and electrolytes :
• active secretion of chloride ions into the crypts and
• active secretion of bicarbonate ions
Causes electrical drag of positively charged sodium ions
through the membrane and into the secreted fluid
Cause osmotic movement of water
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28. Secretion of Small Intestine
• The enterocytes of the mucosa contain digestive enzymes
that digest specific food substances while they are being
absorbed through the epithelium;
• Peptidase : small peptides  amino acids
• Sucrase, Maltase, Isomaltase, Lactase
• Intestinal lipase : neutral fat  glycerol and fatty acids
• Local stimulation : tactile or iritative stimuli from chyme in
duodenum  enteric nervous reflexes  secretion of
intestinal digestive juices
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29. Secretion of Mucus by Large Intestine
• Crypts of Lieberkühn (do not have villi)
 secrete only mucus
• Regulated by : direct, tactile stimulation of the epithelial
cell lining  local nervous reflexes to mucous cells
• Parasympathetic stimulation (pelvic nerves)  ↑ mucus
secretion (1/3 distal part)
• Function of mucus :
• protects the intestinal wall against excoriation
• adherent medium for holding fecal matter together
• protects the intestinal wall from the bacterial activity inside the
feces
• the mucus plus the alkalinity of the secretion (a pH of 8.0 caused
by large amounts of sodium bicarbonate) provides a barrier to keep
acids formed in the feces from attacking the intestinal wall
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30. Reference
• Hall JE, Guyton AC. Textbook of Medical Physiology. 13th
ed. Philadelphia: Elsevier; 2016.
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31. THANK YOU