This document discusses the secretion of various digestive substances throughout the gastrointestinal tract. It begins by describing the general principles of secretion, including the different cell types that produce mucus and enzymes. It then covers secretion in specific organs, including the stomach (gastric glands and regulation), pancreas (digestive enzymes and regulation), liver (bile production and role in fat digestion), and small intestine. The roles of nerves, hormones and other factors in stimulating or inhibiting secretion are also explained.

2. • The GIT secretory gland has to function:
1- Digestive enzymes.
2- Production mucus for lubrication and
protection.
• Most digestive secretions are formed in
response to the presence of food in the
alimentary tract.

3. GENERAL PRINCIPLES OF
ALIMENTARY TRACT SECRETION:
• Surface of the epithelium in most parts of the
gastrointestinal tract are billions of single-cell
mucous glands called simply mucous cells or
sometimes goblet cells.
• Many surface areas of the gastrointestinal tract
are lined by pits (crypts of lieberkühn).
• In the stomach and upper duodenum are large
numbers of deep tubular glands.
• Several complex glands—the salivary glands,
pancreas, and liver.

5. A) Contact of Food with the Epithelium Stimulates
Secretion—Function of Enteric Nervous Stimuli:
• local epithelial stimulation also activates the enteric
nervous system of the gut wall by:
(1) Tactile stimulation,
(2) Chemical irritation, and
(3) Distention of the gut wall.
B) Autonomic Stimulation of Secretion:
• Parasympathetic Stimulation Increases the Alimentary
Tract Glandular Secretion Rate - (glossopharyngeal and
vagus) such as: *salivary glands, *esophageal glands,
*gastric glands, *pancreas, and *Brunner’s glands in the
duodenum, *distal portion of the large intestine (pelvic).
The remainding of these by means of local neural and
hormonal stimuli.

6. • Sympathetic Stimulation Has a Dual Effect on the
Alimentary Tract Glandular Secretion Rate.
(1) sympathetic stimulation alone usually slightly
increases secretion and
(2) if parasympathetic or hormonal stimulation is
already causing copious secretion by the glands,
superimposed sympathetic stimulation usually
reduces the secretion usually by means of
vasoconstriction.
C) Regulation of Glandular Secretion by Hormones:
• The hormones are liberated from the
gastrointestinal mucosa in response to the presence
of food in the lumen of the gut.

7. BASIC MECHANISM OF SECRETION
BY GLANDULAR CELLS:
1- Nutrient material from base.
2- Mitochondria produce atp.
3- ATP is used for synthesis of
protein by the help of er and
ribosome.
4- This protein fuse with the golgi
complex for processing then
forming into a secretory vesicles.
5- This vesicle will rmain in the
cell until hormonal and neural
stimulation.
6- Ca will cause release of these
vesicles by exocytosis.
1- Secretion of Organic Substances.
2- Water and Electrolyte Secretion.

8. The importance of mucus
in the gastrointestinal tract:
• Is a thick secretion composed mainly of water, electrolytes,
and a mixture of several glycoproteins.
• Slightly different in different parts of the gastrointestinal tract.
1. Has adherent qualities.
2. Coats the wall of the gut and prevents actual contact of most
food particles with the mucosa.
3. Low resistance for slippage.
4. Fecal particles to adhere to one another to form the feces.
5. Strongly resistant to digestion by the gastrointestinal
enzymes.
6. The glycoproteins of mucus have amphoteric properties,
which means that they are capable of buffering small
amounts of either acids or alkalies.

10. • Esophageal secretions are entirely mucous and mainly
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, many compound mucous
glands can also be found.
1. The mucus secreted by the compound glands in the upper
esophagus prevents mucosal excoriation by newly
entering food.
2. The compound glands located near the esophagogastric
junction protect the esophageal wall from digestion by
acidic gastric juices that often reflux from the stomach
back into the lower esophagus.
• Despite this protection, a peptic ulcer at times can still
occur at the gastric end of the esophagus.

12. Glands of stomach:
1- Mucus-secreting cells.
2- The oxyntic or gastric (acid-forming) glands: secrete
A. Hydrochloric acid.
B. Pepsinogen.
C. Intrinsic factor.
D. Mucus.
3- The pyloric glands: secrete mainly
A. Mucus.
B. Gastrin (hormone).
• The oxyntic glands are located on the inside surfaces of
the body and fundus of the stomach.
• The pyloric glands are located in the antral portion of the
stomach.

15. Secretions from the Oxyntic
(Gastric) Glands:
(1) mucous neck cells, which
secrete mainly mucus.
(2) peptic (or chief) cells,
which secrete large
quantities of
pepsinogen.
(3) parietal (or oxyntic) cells,
which secrete
hydrochloric acid and
intrinsic factor.

17. • The Basic Factors That Stimulate Gastric
Secretion Are Acetylcholine, Gastrin, and
Histamine:
• Acetylcholine - pepsinogen by peptic cells.
- hydrochloric acid by parietal cells.
- mucus by mucous cells.
• Gastrin and Histamine –strongly act parietal cells.
- little effect of other cells.

18. Secretion and Activation of Pepsinogen:
1. Secreted by the peptic and mucous cells of the
gastric glands.
2. Has no digestive activity.
3. Contact with HCL acid, it is activated to form
active pepsin.
4. Pepsin functions as an active proteolytic
enzyme in a highly acidic medium (optimum ph
1.8 to 3.5).
5. Hydrochloric acid is as necessary as pepsin for
protein digestion in the stomach.

19. Secretion of Intrinsic Factor by Parietal Cells:
A. secreted by the parietal cells along with the
secretion of hydrochloric acid.
B. intrinsic factor, which is essential for absorption
of vitamin B12 in the ileum.
C. When the acid-producing parietal cells of the
stomach are destroyed
D. The person develop achlorhydria and pernicious
anemia.
E. (pernicious anemia) failure of maturation of the
red blood cells in the absence of vitamin B12

21. Secretion from pyloric glands:
Structurally similar to the oxyntic glands.
Contain few peptic cells.
No parietal cells.
Contain mostly mucous cells, identical with the
mucous neck cells of the oxyntic glands.
Secrete the hormone Gastrin, which plays a key
role in controlling gastric secretion.
• The stomach has a SURFACE MUCOUS CELLS:
which is a thick, alkaline, viscid mucus.

22. STIMULATION OF GASTRIC ACID
SECRETION
• Secretion of this acid is under continuous
control by both endocrine and nervous
signals.
• Enterochromaffin-like cells (ecl cells), the
primary function of which is to secrete
histamine.
• Pyloric gland (gastin or g cells) – gastrin
ecl cells – histamine partial cells - Hcl

24. :
• Cephalic phase, a gastric phase, and an intestinal
phase.
A) cephalic phase: secretion of gastric juice by the
stimuli arising from head region (cephalus).
1. Unconditioned reflex.
2. Conditioned reflex.
B) gastric phase: secretion of gastric juice when food
enters the stomach.
C) intestinal phase: secretion of gastric juice when
chyme enters the intestine.

26. PANCREATIC
SECRETION

27. Introduction:
• internal structure is similar to that of the
salivary glands.
• Pancreatic acini – produce pancreatic
digestive enzyme.
• Duct’s from the acini – produce large quantity
of sodium bicarbonate.
• Pancrease also produce an insulin but it’s not
part of panceatic enzyme.

30. PANCREATIC DIGESTIVE
ENZYMES:
• Enzymes for digesting (carbohydrate, protein, lipid).
• It also contains large quantities of bicarbonate ions, which
play an important role in neutralizing the acidity of the
chyme emptied from the stomach into the duodenum.
• Trypsin, chymotrypsin, and carboxypolypeptidase are
digestive enzyme for protien.
• Trypsin and chymotrypsin not relsease of individual amino
acid.
• But carboxypolypeptidase may do.
• Pancreatic amylase (execpt cellulose) release of
disaccharides and a few trisaccharides from starch,
glycogen, carbohydrate.

31. • The main enzymes for fat digestion are:
(1) pancreatic lipase, which is capable of hydrolyzing
neutral fat into fatty acids and monoglycerides;
(2) Cholesterol esterase, which causes hydrolysis of
cholesterol esters;
(3) phospholipase, which splits fatty acids from
phospholipids.
• trypsinogen, chymotrypsinogen, and
procarboxypolypeptidase are inactive form of
pancreatic digestive enzyme.
• In the intestinal mucosa an enzyme called
enterokinase causes trypsinogen to be trypsin
intern causes autocatalytic.
• Trypsin also change the chymotrypsinogen, and
procarboxypolypeptidase to become active from.

33. • Secretion of Trypsin Inhibitor Prevents Digestion
of the Pancreas.
• REGULATION OF PANCREATIC SECRETION:

35. Phases of Pancreatic Secretion:
• Secretion of pancreatic juice is regulated by both nervous and hormonal factors.
A) Cephalic phase:
• Vagal nerve endings secrete acetylcholine, which stimulates pancreatic secretion.
B) Gastric Phase:
• When gastrin is transported to pancreas through blood, it stimulates the
pancreatic secretion, so the pancreatic juice secreted during gastric phase is rich
in enzymes.
C) Intestinal Phase:
• Hormones Stimulating Pancreatic Secretion
i. Secretin
ii. Cholecystokinin
• Hormones Inhibiting Pancreatic Secretion
i. Pancreatic polypeptide (PP) secreted by PP cells in islets of Langerhans of pancreas
ii. Somatostatin secreted by D cells in islets of Langerhans of pancreas
iii. Peptide YY secreted by intestinal mucosa
iv. Peptides like ghrelin and leptin

37. BILE SECRETION
BY THE LIVER

39. Introduction:
• One of the many functions of the liver is to
secrete bile, normally between 600 and 1000
ml/day.
A- Bile acids help fat digestion and absorption by
means of:
(1) emulsify the large fat particles of the food into
many minute particles.
(2) aid in absorption of the digested fat end
products through the intestinal mucosal
membrane.
B- excretion of bilirubin and excess cholestrol.

40. PHYSIOLOGIC ANATOMY OF BILIARY
SECRETION:
• Bile is secreted in two stages by the liver:
1- the hepatocytes (large amounts of bile acids,
cholesterol,and other organic constituents).
2- directly into the duodenum or stored in
gallbladder.

43. Storing and Concentrating Bile in the
Gallbladder:
• Bile secreted when it’s needs until that it’s stored in
gallbladder.
• The maximum volume that the gallbladder can hold
is only 30 to 60 milliliters.
• Every 12 hr the bile secretion is 450ml contains
water, Na, Cl, other electrolyte but the epithelial
mucosa of gallbladder is absorbed continuously of
these until it become concentrated.
• The concentrated bile contain bile salts, cholesterol,
lecithin, and billirubin.
• Bile is normally concentrated up to 5 fold or as much
as 15 – 20 folds

44. Cholecystokinin Stimulates
Gallbladder Emptying:
1. The gallbladder begins to empty after meals
comes to intestinal.
2. Gallbladder emptying is rhythmical contractions
of the gallbladder wall.
3. Simultaneous relaxation of the sphincter of oddi,
4. Acetylcholine-secreting nerve fibers from both
the vagi and the intestinal enteric nervous
system are less effect on gallbladder contraction.

48. • 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.
• These glands secrete large amounts of alkaline
mucus in response to:
(1) tactile or irritating stimulion the duodenal mucosa;
(2) vagal stimulation, which causes increased brunner
gland secretion concurrently with increase in
stomach secretion; and
(3) gastrointestinal hormones, especially secretin.
• Brunner’s glands are inhibited by sympathetic
stimulation. (ulcer)

49. SECRETION OF INTESTINAL DIGESTIVE
JUICES BY THE CRYPTS OF LIEBERKÜHN:
• Lie between the intestinal villi.
• 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.
(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 the end products
of digestion.

50. Mechanism of Secretion of the
Watery Fluid:
• In the presence of active secretion of Cl and
bicarbonate causes electrical attractiveness of
Na ion, so these ions together cause osmotic
movement of water.

51. Digestive Enzymes in the Small
Intestinal Secretion:
(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.
• The life cycle of an intestinal epithelial cell is about 5 days.
• The epithelial cells deep in the crypts of Lieberkühn
continually undergo mitosis.
• As the villus cells age, they are finally shed into the
intestinal secretions.
REGULATION OF SMALL INTESTINE SECRETION—LOCAL STIMULI
(enteric Nervous system) – tactile stimuli.

52. SECRETION BY
THE LARGE
INTESTINE

54. • Has many crypts of lieberkühn;
• Unlike the small intestine, does not have villi.
• The epithelial cells not secrete digestive
enzymes.
• They secrete mucus.
• Mucus contains moderate amounts of
bicarbonate ions.

55. • Mucus in the large intestine protects the
intestinal wall against excoriation, but in
addition, it provides an adherent medium for
holding fecal matter together.
• 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 a marked
increase in mucus secretion.