Chapter 64: Secretory Functions of
the Alimentary Tract
Guyton and Hall, Textbook of Medical Physiology, 12th edition
General Principles of GI Tract Secretion
• Anatomical Types of Glands
a. Mucous glands (Goblet cells)
b. Crypts of Lieberkuhn
c. Tubular glands
d. Salivary glands, pancreas, and liver
General Principles of GI Tract Secretion
Fig. 64.1 Typical function of a glandular cell for formation and secretion
of enzymes and other secretory substances
General Principles of GI Tract Secretion
• Basic Mechanism of Stimulation of the Alimentary
Tract Glands
1. Contact of food with the epithelium stimulates
secretion—function of enteric nervous stimuli
2. Local epithelial stimulation also activates the ENS
and include tactile stimulation, chemical irritation,
and distension of the gut wall
General Principles of GI Tract Secretion
• Autonomic Stimulation of Secretion
1. Parasympathetic stimulation increases alimentary
tract glandular secretion; especially true in the
upper portion of the tract.
2. Sympathetic stimulation has a dual effect; alone it
slightly increases secretion, and if parasympathetic
or hormonal stimulation is already causing copius
secretion, the superimposed sympathetic will
reduce the secretion
General Principles of GI Tract Secretion
• Autonomic Stimulation of Secretion
3. In the stomach and intestine, several GI hormones
regulate the volume and character of the secretions.
4. The hormones are produced in response to food in
the lumen of the gut
General Principles of GI Tract Secretion
• Basic Mechanism of Secretion by Glandular Cells
1. Secretion of organic substances (Fig. 64.1)
2. Water and electrolyte secretion
3. Lubricating and protecting properties of mucous
Secretion of Saliva
• Saliva Contains a Serous Secretion and a Mucus
Secretion
1. Principle glands are the parotid, submandibular,
and sublingual (also the tiny buccal glands)
2. Serous secretion contains ptyalin (amylase) and the
mucus contains mucin for lubrication and surface
protective properties
Secretion of Saliva
• Secretion of Ions in Saliva
1. Saliva contains large quantities of potassium and
bicarbonate ions
2. Acini secrete a primary secretion containing
ptyalin or mucin in a solution of ions
3. Ductal epithelium secretes the bicarbonate ion
into the lumen
Secretion of Saliva
• Nervous Regulation of Salivary Secretion
Fig. 64.3 Parasympathetic nervous regulation of salivary secretion
Secretion of Saliva
• Nervous Regulation of Salivary Secretion
1. Controlled mainly by parasympathetic pathways
2. Salivation can be stimulated or inhibited by signals
coming from higher brain centers
3. Salivation can occur in response to reflexes in the
stomach and upper small intestines
4. Sympathetic stimulation can increase salivation a
small amount
5. Secretion always requires adequate nutrients from
the blood
Secretion of Saliva
• Esophageal Secretion
1. Entirely mucus and provide lubrication for
swallowing
2. Contains both simple and compound mucus
glands
Gastric Secretion
• Characteristics of the Gastric Secretions
1. Secretions from the gastric glands (oxyntic)
a) Mucus neck cells-secrete mucus
b) Chief or peptic cells-secrete pepsinogen
c) Parietal cells- secrete HCl and intrinsic factor
Gastric Secretion
Fig. 64.4 Oxyntic gland from the body of the stomach
Gastric Secretion
• Basic Mechanism of HCl Secretion
Fig. 64.5 Schematic anatomy of the canaliculi in a parietao
(oxyntic) cell
Gastric Secretion
Fig. 64.6 Postulated mechanism for secretion of HCl. “P” indicate active pumps
and the dashed lines epresent free diffusion and osmosis
Gastric Secretion
• Basic Factors That Stimulate Gastric Secretion
are AcH, Gastrin, and Histamine
a. AcH (from parasympathetic stimulation) excites
secretion of pepsinogen, HCl, and mucus
b. Both gastrin and histamine stimulate secretion of
acid by parietal cells but not the other cells
Gastric Secretion
• Secretion of Intrinsic Factor by Parietal Cells
a. Essential for the absorption of vitamin B-12 in the
ileum
•
Pyloric Glands—Secretion of Mucus and Gastrin
•
Surface Mucus Cells- secrete large quantities of
viscous mucus that coat the stomach mucosa
Gastric Secretion
• Stimulation of Gastric Acid Secretions
a. Parietal cells are the only cells that secrete HCl
b. Paritetal cells operate in close association with
enterochromaffin like cells (ECL) whose primary
function is to release histamine
c. Stimulation of acid secretion by gastrin-causes the
release of histamine from the ECL cells; release of
gastrin is in response to proteins in stomach
Gastric Secretion
• Stimulation of Gastric Acid Secretions
d. Regulation of pepsinogen secretion occurs in
response to two signals:
1) Stimulation of the peptic cells by AcH from the
vagus nerve or nerves of the enteric nervous
system
2) Stimulation of peptic cell secretion in response
to acid in the stomach
Gastric Secretion
• Phases of Gastric Secretion
a. Cephalic phase
b. Gastric phase
c. Intestinal phase
Gastric Secretion
• Phases of Gastric Secretion
Fig. 64.7 Phases of gastric secretion and their regulation
Gastric Secretion
• Inhibition of Gastric Secretion by Other PostStomach Intestinal Factors
a. Function is to slow passage of chyme from the stomach
when the small intestine is already filled or overactive
b. Presence of food initiates a reverse enterogastric reflex
c. Presence of acid, fat, protein breakdown products, etc.
causes the release of several intestinal hormones;
especially secretin which inhibits stomach secretion
Gastric Secretion
• Chemical Composition of Gastrin and Other GI
Hormones
a. Gastrin, CCK, and secretin are all large polypeptides
b. Activity of all of these lie in the last amino acids of
the chain
Pancreatic Secretions
Fig. 64.10 Regulation of pancreatic secretion
Pancreatic Secretions
• Pancreas
a.
b.
c.
d.
Large compound gland
Enzymes secreted by the acini cells
Bicarbonate secreted by small ductules
Pancreatic juice produced in response to the
presence of chyme
Pancreatic Secretions
• Pancreatic Digestive Enzymes
a. For proteins: trypsin, chymotrypsin, and
carboxypeptidase
b. For cbh: pancreatic amylase
c. For fats: pancreatic lipase, cholesterol esterase,
and phopholipase
Secretion of trypsin inhibitor prevents the digestion
of the pancreas itself
Pancreatic Secretions
• Secretion of Bicarbonate Ions
a. Bicarbonate provides alkali in the pancreatic
juice to neutralize the HCl coming into the
duodenum from the stomach
Pancreatic Secretions
Fig. 64.8 Secretion of isoosmotic sodium bicarbonate solution by the pancreatic ductules and ducts
Pancreatic Secretions
• Regulation of Pancreatic Secretion-Basic
Stimulation That Causes Secretion
a. Acetylcholine-released from the parasympathetic
vagus nerve endings and other cholinergic nerves
associated with the enteric nervous system
b. Cholecystokinin-secreted by the duodenal and
upper jejunal mucosa when food enters the small
intestine
c. Secretin-secreted as in cholecystokinin when
highly acidic food enters the intestine
d. Multiplicative effects of “a, b, and c”
Pancreatic Secretions
• Phases of Pancreatic Secretion
a.
b.
c.
d.
Cephalic and Gastric Phases
Intestinal Phase
Secretin stimulates secretion of bicarbonate ions
Cholecystokinin production which stimulates
more digestive enzymes by the acini (similar to
vagal stimulation)
Secretion of Bile by the Liver
• Functions of Bile
a. Bile salts help to emulsify the large fat molecules
of food and aid in the absorption of the digested
fat
b. Means of secretion of several important waste
products from the blood (especially bilirubin
and excesses of cholesterol)
Secretion of Bile by the Liver
• Physiologic Anatomy of Biliary Secretion
a. Bile is secreted in two stages by the liver
1) Initial secretion by the hepatocytes;
contains bile salts, cholesterol, and other
organic compounds; into the bile
canaliculi
2) Bile empties into termnal bile ducts, then
the hepatic duct, and finally the common
bile duct into the duodenum or diverted
through the cystic duct into the gallbladder
Secretion of Bile by the Liver
• Physiologic Anatomy of Biliary Secretion
b. Bile is stored and concentrated in the gallbladder
Secretion of Bile by the Liver
Table 64.2 Composition of Bile
Liver Bile
Gallbladder Bile
97.5 g/dl
92 g/dl
Bile salts
1.1 g/dl
6 g/dl
Bilirubin
0.04 g/dl
0.3 g/dl
Cholesterol
0.1 g/dl
0.3-0.9 g/dl
Fatty Acids
0.12 g/dl
0.3-1.2 g/dl
Lecithin
0.04 g/dl
0.3 g/dl
145 mEq/L
130 mEq/L
Potassium Ions
5 mEq/L
12 mEq/L
Calcium Ions
5 mEq/L
23 mEq/L
Chloride Ions
100 mEq/L
25 mEq/L
Bicarbonate Ions
28 mEq/L
10 mEq/L
Water
Sodium Ions
Secretion of Bile by the Liver
Fig. 64.11 Liver secretion and gallbladder emptying
Secretion of Bile by the Liver
• Emptying of the Gallbladder-Role of CCK
a. Most potent stimulus for the gallbladder to
undergo rhythmic contractions when food enter
the small intestine
b. Main stimulus for CCK is fatty foods
•
Bile Salts Function in Digestion and Absorption
a. Have a detergent action on fat particles
which causes the emulsification of the fat
b. Help in the absorption by forming micelles
Secretion of Bile by the Liver
Fig. 64.12 Formation of gallstones
Secretion of the Small Intestine
• Secretion of Mucus by Brunner’s Glands in the
Duodenum
a. Secrete large amounts of mucus in response to:
tactile or irritating stimuli on the mucosa, vagal
stimulation, or secretin
b. Mucus protects the duodenal wall from the acidic
chyme coming from the stomach
c. Can be inhibited by sympathetic stimulation
Secretion of the Small Intestine
• Secretion of Intestinal Digestive Juices by the
Crypts of Lieberkuhn
a. Surface covered by goblet cells (mucus) and
enterocytes (secrete water and electrolytes)
b. Mechanism of secretion of the watery fluid is
unclear but involves the active secretion of chloride
ions and bicarbonate ions
Secretion of the Small Intestine
• Secretion of Intestinal Digestive Juices by the
Crypts of Lieberkuhn
c. Digestive enzymes in the small intestine secretion:
several peptidases; sucrase, maltase, isomaltase,
and lactase for splitting disaccharides into
monosaccharides; and small amounts of intestinal
lipase
d. Regulation is entirely local by the enteric nervous
system
Secretion of the Small Intestine
• Secretion of Mucus by the Large Intestine
a. Many crypts but no villi in the large intestine
b. Epithelial cells secrete almost no enzymes but
contain many mucus cells
c. Rate of mucus formation is regulated by the direct
tactile stimulation of the epithelial cells