The document details the anatomy and physiology of gastrointestinal secretions, including those produced by the salivary glands, stomach, pancreas, liver, and intestines. It explains the composition, regulation, and functions of saliva, gastric acid, pancreatic juice, bile, and intestinal secretions, highlighting their roles in digestion and protection of the gastrointestinal tract. Additionally, it covers regulatory mechanisms influencing these secretions and their importance in digestive health.

1. Gastrointestinal secretions

3. Saliva
Glands of Salivation
• Parotid glands: secrete entirely serous type
• Submandibular glands: both serous and
mucus type
• Sublingual glands: both serous and mucus
type
• Buccal glands secrete only mucus

4. Types of secretion
• Serous type of secretions contains ptyalin, an
enzyme for the digesting starches
• Mucus secretions contain mucin for
lubrication and for surface protective
purposes

5. Composition and function of saliva
• Sodium ions are actively reabsorbed from all the salivary ducts
and potassium ions are actively secreted in exchange for sodium.
• Bicarbonate ions are secreted by ductal epithelium into lumen of
the duct
• Ptyalin (α amylase) is responsible for the digestion of
carbohydrates
• Mucus, glycoproteins
• Thiocyanate ions
• Lysozymes
• Immunoglobulins (IgA)

6. Functions of Saliva
• Preparation of food for swallowing
• Appreciation of taste
• Digestive function
• Cleansing and protective functions
• Role in speech
• Excretory function
• Regulation of body temperature
• Regulation of water balance

7. Regulation of salivation
• Salivation is mainly controlled by autonomic
nervous system. Both branches of autonomic
nervous system stimulate salivation, but
parasympathetic nervous system stimulate
much more strongly than the sympathetic
nervous system

8. Reflex regulation of salivary secretion
• Unconditioned reflex
• Conditioned reflex

9. Nervous Regulation of Salivary Secretion
• Salivary glands are controlled mainly by parasympathetic
nervous signals all the way from the superior and inferior
salivatory nuclei in the brain stem.
• The salivatory nuclei are located approximately at the
juncture of the medulla and pons and are excited by both
taste and tactile stimuli from the tongue and other areas
of the mouth and pharynx. Many taste stimuli, especially
the sour taste (caused by acids), elicit copious secretion
of saliva—often 8 to 20 times the basal rate of secretion.

10. • Esophageal Secretion
• The esophageal secretions are entirely mucous
in character and principally provide lubrication
for swallowing. The main body of the
esophagus is lined with many simple mucous
glands. At the gastric end and to a lesser
extent in the initial portion of the esophagus,
there are also many compound mucous glands.

11. Gastric Secretion
• Oxyntic glands: these secrete hydrochloric
acid, pepsinogen, intrinsic factor and mucus.
These are located on the inside surfaces of
body and fundus of the stomach
• Pyloric glands: mainly secrete mucus and
hormone gastrin. These are located in the
antral portion of the stomach

12. Secretion from oxyntic glands
A typical oxyntic gland is composed of three types
of cells
• Mucus cells which secrete mainly mucus but also
some pepsinogen
• Peptic or chief cells which secrete mainly large
quantities of pepsinogen
• Parietal or oxyntic cells which secrete
hydrochloric acid and intrinsic factor

13. • Mucous Cells
• The thick mucous present in the gastric juice is
responsible for the protection of the gastric
wall. The mucous protects the stomach wall
from irritation or mechanical injury. It prevents
the back diffusion of hydrogen ions into the
gastric mucosa. Beneath the mucous layer, a
layer rich in bicarbonate ion neutralizes
hydrogen ions.

14. • Chief Cells
• Protein digestion begins in the stomach
because of the activity of chief cells. These
cells secrete the inactive precursor protein,
pepsinogen, which is activated to the
proteolytic enzyme, pepsin, in the presence of
acid and small amounts of active pepsin.
Pepsin functions optimally at a pH of
approximately 2.

15. Parietal Cells
Parietal cells
• Secrete hydrogen ions
• Activate protein digestive enzymes such as pepsinogen.
• Create a harsh environment for bacterial growth.
• Secrete intrinsic factor which binds vitamin B12 in
protein rich foods, such as meat, to prevent its
degradation in small intestine and to allow absorption
in the terminal ileum

17. Basic mechanism of hydrochloric acid secretion
• Parietal cells secrete HCl into the lumen of the stomach
and, concurrently, absorb bicarbonate ion into the blood
stream as follows
• In the parietal cells, CO2 and water are converted to
hydrogen ions and bicarbonate ions, catalyzed by carbonic
anhydrase
• Hydrogen ion is secreted into the lumen by H+
-K+
pump (H+
-
K+
ATPase). Chloride is secreted along with the hydrogen
ions, thus the secretion product of parietal cells is HCl

18. • The bicarbonate ions produced in the cells is
absorbed into the bloodstream in exchange
for chloride ions (Cl-
- HCO3
-
exchange). As
bicarbonate ion is added to the venous blood,
the pH of the blood increases (“alkaline
tide”). (Eventually this bicarbonate ion will be
secreted in pancreatic secretions to neutralize
hydrogen ions in the small intestine.)

20. Stimulation of gastric hydrogen ion secretion
• Vagal stimulation
– Increases hydrogen ion secretion by a direct
pathway and an indirect pathway.
– In the direct path, the vagus nerve innervates G
cells and stimulates gastrin secretion directly. The
neurotransmitter at these synapses is
Acetylcholine.

21. • In the indirect path, the vagus nerve innervates G
cells and stimulates gastrin secretion, which then
stimulates hydrogen ion secretion by an endocrine
action. The neurotransmitter at these synapses is
Gastrin releasing peptide.

22. Gastrin
– Is released in response to eating a meal (small
peptides, distention of the stomach, vagal
stimulation).
– Stimulates hydrogen ion secretion by interacting
with cholecystokininB (CCKB) receptor on the
parietal cells.

23. Histamine
– Is released from enterochromaffin-like (ECL) cells
in the gastric mucosa and diffuses to the nearby
parietal cells.
– Stimulates hydrogen ion secretion by activating H2
receptors on the parietal cell membrane.

24. Inhibition of gastric hydrogen ion secretion
• Negative feedback mechanisms inhibit the secretion of
hydrogen ions by parietal cells.
• The presence of food in the small intestine initiates a
reverse enterogastric reflex, transmitted through the
myenteric nervous system as well as through extrinsic
sympathetic and vagus nerves, that inhibits stomach
secretion. This reflex can be initiated by distending the
small bowel, by the presence of acid in the upper
intestine, by the presence of protein breakdown products,
or by irritation of the mucosa.

25. Somatostatin
– Inhibits gastric hydrogen ion secretion by a direct
pathway and an indirect pathway.
– In the direct pathway, somatostatin antagonizes
the stimulatory action of histamine on hydrogen
ion secretion.
– In the indirect pathway, somatostatin inhibits
release of histamine and gastrin, thus decreasing
hydrogen ion secretion indirectly.

26. Secretion and Activation of Pepsinogen
• When pepsinogen is first secreted, it has no
digestive activity. However, as soon as it comes
in contact with hydrochloric acid, it is activated
to form active pepsin.
• Pepsin functions as an active proteolytic
enzyme a highly acid medium (optimum pH
1.8 to 3.5), but above a pH of about 5 it has
almost no proteolytic activity and becomes
completely inactivated in a short time.

27. • Secretion of Intrinsic Factor. The substance intrinsic
factor, essential for absorption of vitamin B12 in the
ileum, is secreted by the parietal cells along with the
secretion of hydrochloric acid.
• When the acid-producing parietal cells of the
stomach are destroyed the person develops not only
achlorhydria (lack of stomach acid secretion) but
often also pernicious anemia because of failure of
maturation of the red blood cells in the absence of
vitamin B12 stimulation of the bone marrow.

28. Pancreas
• Pancreas is a dual organ having two functions
• Endocrine function: involves production of
hormones like insulin
• Exocrine function: involves secretion of
digestive juice- pancreatic juice

29. Pancreatic secretion
Anatomy of the exocrine part of the pancreas
• It is made up of acinar cells
• Acinar cells contain zymogen granules, which possess
digestive juices
• The ducts arising from acini join together to form
intralobular duct
• Intralobular ducts unite to form main duct of pancreas
called Wirsung’s duct
• Wirsung’s duct joins common bile duct to form ampulla
of vater which opens into the duodenum

30. Pancreatic secretion
• Contains high concentration of HCO3 whose
purpose is to neutralize the digestive enzymes
reaching the duodenum
• Contains enzymes for digesting all 3 major
types of food: proteins, carbohydrates and fats

31. Enzymatic components
• The more important of enzymes are
• Trypsin which is activated from trypsinogen in
the presence of enzyme called enterokinase.
Enterokinase is secreted by intestinal mucosa
when chyme comes on contact with mucosa
• Chymotrypsinogen is activated by trypsin to
form chymotrypsin

32. • Procarboxyploypeptidase is also activated in
the presence of trypsin to form
carboxypolypeptidase. It splits some peptides
into aminoacids.
• Pancreatic lipase hydrolyzes neutral fats into
fatty acids and monoglyceride
• Cholesterol esterase causes hydrolysis of
cholesterol ester and phospholipase splits fatty
acids from phospholipids.

33. • Pancreatic amylase hydrolyzes starches,
glycogen and most other carbohydrates
except cellulose to form disaccharides and a
few trisaccharides.
• Trypsin inhibitor is secreted into the acini of
pancreas and it prevents the activation of
trypsin both inside the secretory cells and in
the acini and ducts of the pancreas

34. Secretion of Bicarbonate ions
• Carbon dioxide diffuses to the interior of the
cell from the blood and combines with the
water in the presence of carbonic anhydrase to
form carbonic acid. This carbonic acid in turn
dissociates into bicarbonate ions and hydrogen
ions. The bicarbonate ions are actively
transported in exchange for the chloride ions
and enters into the lumen of the duct

36. Regulation of pancreatic secretion
• Stimuli of pancreatic secretion are
• Acetylcholine
• Choleccystokinin secreted when food enters small
intestine
• Secretin is secreted in response to acidic food
• Acetylcholine and cholecystokinin cause production of
large quantities of digestive enzymes whereas secretin
stimulates secretion of large quantities of water and
bicarbonate

37. Phases of pancreatic secretion
• Cephalic phase
• Gastric phase
• Intestinal phase

38. Secretions of bile by liver
• Normally 600-1200 ml /day bile is secreted by
the liver. Bile contains bile salts, phospholipids,
cholesterol, and bile pigments.
• Secretion of bile Bile is secreted in two stages
• Initial portion is secreted by liver hepatocytes.
This secretion contains large amounts of bile
acids, cholesterol and other organic
constituents. It is secreted into minute bile
canaliculi.

40. Storage of bile
• Most of the bile from liver enters the gallbladder
where it is stored. It is released from gallbladder
into the duodenum whenever required. The
maximum volume of the gallbladder is 30 -60 ml.
A large amount of water and electrolytes (except
calcium and potassium) are absorbed resulting in
high concentration of bile salts, bile pigments,
cholesterol, fatty acids and lecithin

41. Composition and function of bile
• The most abundant substances secreted in the
bile are bile salts, which account for about one
half of the total solutes also in the bile. Also
secreted or excreted in large concentrations
are bilirubin, cholesterol, lecithin, and the
usual electrolytes of plasma.

43. • Bile salts
• The liver cells synthesize about 6 grams of bile salts
daily. The precursor of the bile salts is cholesterol,
which is either present in the diet or synthesized in
the liver cells during the course of fat metabolism.
• Bile salts are amphipathic molecules and are
emulsifier. They are potassium or sodium salts of
bile acids, which are conjugated with glycine and to
lesser amount with taurine

44. • The precursor of the bile salts is cholesterol,
which is either present in the diet or
synthesized in the liver cells during the course
of fat metabolism.
• The cholesterol is first converted to cholic acid
or chenodeoxycholic acid in about equal
quantities.

45. These acids in turn combine principally with
glycine and to a lesser extent with taurine to
form glyco- and tauro conjugated bile acids.
The salts of these acids, mainly sodium salts,
are then secreted in the bile.
Due to bacterial action in the intestine the
primary bile acids are converted into secondary
bile acids which are transported back to the
liver through enterohepatic circulation.

46. • Role of Secretin in Helping to Control Bile
Secretion.
• In addition to the strong stimulating effect of
bile acids to cause bile secretion, the hormone
secretin that also stimulates pancreatic
secretion increases bile secretion, sometimes
more than doubling its secretion for several
hours after a meal.

47. • Bile pigments
• Bile pigments are the excretory products in the bile.
Bilirubin and biliverdin are the two bile pigments and
bilirubin is the major bile pigment in human being.
• Bilirubin: A major bile pigment, bilirubin is a lipid
soluble metabolite of haemoglobin. Transported to
the liver attached to the protein, it is then
conjugated and excreted as water soluble
glucuronides. These give a golden color to bile.

48. • Biliverdin: Heme splits into iron and pigment
biliverdin which then reduces to bilirubin.
• Stercobilin: It is produced from metabolism of
bilirubin by intestinal bacteria. It gives brown
color to the stool.

49. • Phospholipids (mainly lecithin)
• It is insoluble in water but are solubilized by
bile salt micelles
• Cholesterol
• It is present in small amount. It is insoluble in
water and must be solubilized by bile salt
micelles before it can be secreted in the bile.

50. Control of bile secretion and gall bladder
contraction
• Secretin causes secretion of bicarbonate ions and
fluid into bile canalicular ducts
• Secretion of bile salts by hepatocytes is directly
proportional to hepatic portal vein concentration
of bile salts
• Cholecystokinin causes gallbladder contraction
and sphincter of Oddi relaxation

51. Functions of bile salts
• Emulsification of fats
• Absorption of fats
• Choleretic action
• Cholagogue action
• Laxative action
• Prevention of gallstone formation

52. Small intestine secretions
Secretion of mucus by Brunner’s glands
• A large number of compound mucous glands,
called Brunner’s glands, is located in the wall
of the first few centimeters of the duodenum,
mainly between the pylorus of the stomach
and the papilla of Vater where pancreatic
secretion and bile empty into the duodenum.

53. • These glands secrete large amounts of alkaline
mucus in response to
• (1) tactile or irritating stimuli on the duodenal
mucosa;
• (2) vagal stimulation, which causes increased
Brunner’s glands secretion concurrently with
increase in stomach secretion; and
• (3) gastrointestinal hormones, especially secretin.

54. • The function of the mucus secreted by
Brunner’s glands is to protect the duodenal
wall from digestion by the highly acid gastric
juice emptying from the stomach.
• Mucus also contains large amount of
bicarbonate ions which neutralize the acid
entering from the stomach.

55. Secretion of Intestinal Digestive
Juices by the Crypts of Lieberkühn

56. • Over the entire surface of the small intestine are
located small pits called crypts of Lieberkühn
• These crypts lie between the intestinal villi. The
surfaces of both the crypts and the villi are
covered by an epithelium composed of two types
of cells:
• (1) a moderate number of goblet cells, which
secrete mucus that lubricates and protects the
• intestinal surfaces

57. • (2) a large number of 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 end products of
digestion.

58. • Digestive Enzymes in the Small Intestinal
Secretion
• The enterocytes of the mucosa, especially
those that cover the villi, do contain digestive
enzymes that digest specific food substances
while they are being absorbed through the
epithelium.

59. • These enzymes are the following:
• (1) several peptidases for splitting small peptides
into amino acids
• (2) four enzymes—sucrase, maltase isomaltase,
and lactase—for splitting disaccharides into
monosaccharides
• (3) small amounts of intestinal lipase for splitting
neutral fats into glycerol and fatty acids.

60. • Regulating small intestine secretion are
controlled by local enteric nervous reflexes,
especially reflexes initiated by tactile or
irritative stimuli from the chyme in the
intestines.

61. Secretions of the Large Intestine
• Mucus Secretion. The mucosa of the large
intestine, like that of the small intestine, has
many crypts of Lieberkühn; however, unlike
the small intestine, there are no villi. The
epithelial cells contain almost no enzymes.
Instead, they consist mainly of mucous cells
that secrete only mucus.

62. • Stimulation of the pelvic nerves from the
spinal cord, which carry parasympathetic
innervation to the distal one half to two thirds
of the large intestine, also can cause marked
increase in mucus secretion. This occurs along
with increase in peristaltic motility of the
colon

63. Functions of mucus
• Mucus in the large intestine protects the
intestinal wall against excoriation,
• It provides an adherent medium for holding
fecal matter together.
• It protects the intestinal wall from the great
amount of bacterial activity that takes place
inside the feces

64. • The mucus plus the alkalinity of the secretion
(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.