Physio Chap 64 - Secretory Functions of GI Tract
Tuesday, January 29, 2013
2:07 PM
Secretory Functions of GI Tract
 Secretory glands secrete enzymes for digestion and lubrication.
 Most are secreted only in response to food and the exact amount is secreted.
 General Principles of GI Tract Secretion
o Types of Glands
1. Single-cell mucous glands (mucous or goblet cells):
1. Function in response to irritation and secrete mucus onto epithelial surface for
lubrication.
2. Pits or Crypts (of Lieberkuhn in SI): specialized secretions
3. Deep Tubular Glands:
4. Salivary, Pancreas and Liver Glands:
1. Provide secretions for digestion or emulsification of food
o Mechanisms of Stimulation of Glands
 Effect of Food Contact
 Local epithelial stimulation activates the ENS of the gut wall to reflexively
stimulate mucous cells and the deep glands to increase secretion.
 The types of stimulation are
 Tactile stimulation
 Chemical irritation
 Distention of the gut wall.
 ANS Stimulation
 Parasympathetic
 Increases the rates of glandular secretion.
 Especially in the upper portion of the tract and some glands in the
distal portion of the large intestine.
 Secretion in the remainder of the SI and the first two thirds large intestine
is mainly in response to local neural and hormonal stimuli.
 Sympathetic
 Has dual effects
Symp stimulation slightly increases secretion.
If parasympathetic or hormonal stimulation is already in place,
symp stimulation will usually DECREASE secretion.
Mainly because of vasoconstrictive effects.
 Regulation of Glandular Secretion by Hormones
 Usually released in response to presence of food in the gut lumen.
 They then travel by blood to glands which is valuable for the increase of
gastric and pancreatic juice when food enters the stomach or duodenum.
 Hormones are polypeptides or polypeptide derivatives.
o Basic Mechanism of Secretion
 Organic Secretions
 Nutrients needed for secretion formation must arrive at gland.
 Mitochondria provide energy.
 Energy from ATP used to synthesize organic secretions in ER and golgi complex.
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Secretions transported through the ER.
In Golgi, materials are modified, added to, concentrated, and discharged into
the cytoplasm in secretory vesicles.
 Nervous signals cause cells to release secretions by first increasing cell
membrane permeability to calcium ions which then release the vesicles by
exocytosis.
 Water and Electrolyte Secretion
 The cell brings in Chloride and Sodium ions to increase the osmotic force to
bring in more water which ultimately flushes water, electrolytes, and organic
materials out of the secretory end of the glandular cell.
o Lubricating and Protective Properties of Mucus, and Importance of Mucus in the GI Tract
 Has adherent properties to make it stick tightly to food and to spread as a thin film
over surfaces.
 Has sufficient body to coat the gut wall and prevent most food contact with the
mucosa.
 Has slow resistance for slippage so particles can slide along the epithelium.
 Causes fecal particles to stick to each other to form expellable feces.
 Strongly resistant to digestion by enzymes
 Has amphoteric properties that allow it to buffer acids or alkalies; also contains
bicarbonate ions to neutralize acids.
Secretion of Saliva
o Salivary Glands and Saliva
 Principle glands are parotid, submandibular, sublingual and buccal glands.
 Parotid glands secrete mainly serous type of secretion.
 Submandibular and sublingual secrete both.
 Buccal glands secrete only mucus.
 Contains two major types of protein secretion
 Serous Secretion containing ptyalin, which is an enzyme for digesting starches.
 Mucous Secretion that contains mucin for lubricating and for surface protection.
o Secretion of Ions in Saliva
 Contains large amounts of potassium and bicarb.
 This happens due to primary and secondary secretion.
 Primary secretion releases ptyalin or mucin which stimulates secondary
secretion of potassium and bicarb in exchange for sodium and chloride,
respectively.
 Function of Saliva for Oral hygiene
 Flow of saliva helps wash away pathogenic bacteria as well as food particles.
 Saliva contains factors that destroy bacteria.
 Proteolytic enzymes attack bacteria, aid thiocyanate ions in entering the
bacteria, and digest food particles.
 Also contains protein antibodies that can destroy oral bacteria, including some
that cause dental caries.
o Nervous Regulation of Salivary Secretion
 Salivatory nuclei are located at juncture of medulla and pons and excite by both taste
and tactile stimuli.
 Especially the sour taste elicit a lot of saliva.
 Salivation can also be stimulated by higher centers of the CNS.
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The appetite area is located close to the parasympathetic centers of the anterior
hypothalamus and functions to a great extent in response to signals from taste
and smell areas.
 Blood supply to the glands affects secretion because glands require the nutrients to
make saliva.
 Activated salivary cells secrete kallikrein which splits alpha2-globululin to form
bradykinin, a strong vasodilator.
Esophageal Secretion
o Entirely mucous in character to provide lubrication and protection during swallowing.
o Main body has mostly simple mucous glands for lubrication during swallowing while the
distal portion has compound mucous glands for protection from acid of stomach.
Gastric Secretion
o Characteristics of Gastric Secretions
 In addition to mucous, there are two types of glands in the stomach
 Oxyntic Glands
 Secrete HCl, pepsinogen, intrinsic factor, and mucus.
 Located in the proximal 80% of stomach.
 Pyloric Glands
 Secrete mucus and gastrin.
 Located in the distal 20% of stomach.
 Secretions from Oxyntic (Gastric) Glands
 Composed of three cells
 Mucus Neck cells - secrete mucus
 Peptic (or chief) cells - secrete pepsinogen
 Parietal (or oxyntic) cells - secrete HCl and Intrinsic Factor
 Basic Mechanism of HCl secretion
 Chloride is transported from the cytoplasm of the parietal cell into the lumen of
the canaliculus, and sodium ions are transported INTO the cytoplasm from the
lumen of the canaliculus.
 Water becomes dissociated into hydrogen ions and hydroxyl ions in the cell
cytoplasm and secreted into the lumen in exchange for potassium ions.
 Water then passes into the canaliculus by osmosis.
 Carbon dioxide combines with hydroxyl ions to form bicarbonate which is
exchanged into the extracellular space for chloride ions.
 Pepsinogen is secreted and then activated by HCl.
 Intrinsic Factor is secreted along with HCl. When the acid-producing parietal cells are
destroyed the person develops not only achlorhydria (lack of acid secretion) but also
pernicious anemia because of failure of RBCs to mature.
o Pyloric Glands
 Structurally similar to oxyntic glands but have fewer peptic cells and almost no
parietal cells.
 Secrete gastrin.
o Surface Mucous Cells
 Secrete lots of alkaline, viscid mucus that coats the stomach mucosa, providing a
major shell of protection for the stomach.
o Stimulation of Gastric Acid Secretion
 Parietal Cells - only cells that secrete HCl
 Secretion is under endocrine and nervous control.
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Work closes with enterochromafin-like cells (ECL cells) which secrete histamine.
 Rate and amount of formation/secretion of HCl is directly related to
amount of histamine secreted by ECL cells.
 Stimulated most potently by gastrin.
 Also stimulated by acetylcholine released from stomach vagal nerve
endings and by hormones secreted from the ENS in the stomach
wall.
 Stimulation of Acid by Gastrin
 Secreted by Gastrin Cells: G-cells. They're black.
 Mixing of gastric juices transports gastrin quickly to ECL cells which causes the
release of histamine right into the oxyntic glands.
o Regulation of Pepsinogen Secretion
 Regulation happens with two signals
 Stimulation of peptic cells by acetylcholine released from vagus nerves or from
gastric enteric nervous plexus
 Stimulation in response to stomach acid
 Rate of secretion of pepsinogen is strongly influenced by the amount of
acid in the stomach.
o Phases of Gastric Secretion
 Cephalic Phase
 Results from sight, smell, thought, or taste of food, and the greater the appetite,
the more intense the stimulation
 Signals transmitted by dorsal motor nuclei of the vagi.
 Gastric Phase
 Food enters stomach and
 Excited long vagovagal reflexes to the brain and back
 Excites local enteric reflexes
 Excites the gastrin mechanism
 These all cause gastric secretion
 Accounts for 70% of the total gastric secretions.
 Intestinal Phase
 Presence of food in upper SI will continue to cause gastric secretion due to
release of gastrin from the duodenum.
o Inhibition of Gastric Secretion by Post-Stomach Factors
 The presence of food in the SI initiates a reverse enterogastric reflex that inhibits
stomach secretion.
 Presence of acid, fat, protein breakdown, hyperosmotic, or hypo-osmotic fluids causes
release of SI hormones.
 Secretin controls pancreatic secretion and inhibits stomach secretion.
 Also GIP, VIP, and somatostatin
 The purpose is to slow passage of chyme when the SI is already filled or overactive.
 The stomach does secrete a little juice during interdigestive periods. This may be
increased due to emotional stress and may cause peptic ulcers.
Pancreatic Secretion
o Pancreatic digestive enzymes are secreted by pancreatic acini while bicarb is secreted by
ducts and ductules.
o Pancreatic juice is secreted most in response to chyme in the upper SI.
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Insulin is secreted into the blood by the islets of Langerhans that occur in islet patches
throughout the pancreas.
Pancreatic Digestive Enzymes
 Most important pancreatic enzymes for digesting proteins are trypsin, chymotrypsin,
and carboxypolypeptidase
 These are released as inactive zymogens and are activated only after entering
the SI.
 Trypsin is most important and activates all the others, including itself.
 Trypsin inhibitor is released along with Trypsin so that the pancreas isn't "eaten"
by its own enzymes.
 If the pancreatic ducts are blocked, trypsin inhibitor might be
overwhelmed and the enzymes digest the entire pancreas, leading to
acute pancreatitis. Bad News Bears.
 Pancreatic Amylase digests carbs.
 Fat Digestion is done by
 Pancreatic Lipase - hydrolyzation of neutral fat into fatty acids and
monoglycerides
 Cholesterol Esterase - hydrolysis of cholesterol esters
 Phospholipase - splits fatty acids from phospholipids
Secretion of Bicarb - Make sure you understand this picture!
Regulation of Pancreatic Secretion
 Basic Stimuli
 Acetylcholine released from parasympathetic vagus
 Cholecystokinin released by duodenum and upper jejunum when food enters SI
 Secretin released by duodenum and jejunum when highly acidic food enters SI
 Acetylcholine and CCK stimulate the acinar cells
 Secretin stimulates water and bicarb secretion to wash the enzymes down into the SI
and neutralize the chyme.
 When all of these are released together, the effects are multiplicative.
 Phases of Pancreatic Secretion
 Cephalic and Gastric
 This uses the same signals from brain as the stomach and enzymes in both
are stimulated and secreted but they do not leave the pancreas.
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Intestinal Phase
 Secretin is released and the enzymes are washed out of the pancreas and
into the SI.
 CCK
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Released from I cells in response to partial protein digestion in chyme entering
the SI.
 Like Secretin it is released into the blood and induces 70-80% of total pancreatic
secretion.
Secretion of Bile by the Liver and Functions of the Biliary Tree
o Bile serves two functions
 Has important role in fat digestion and absorption by
 Helping to emulsify the large fat molecules so lipase can get to the surface
 Aid in absorption of fat end products through the intestinal membrane
 Serves as a means for excretion of bilirubin and excess cholesterol.
o Physiologic Anatomy of Biliary Secretion
 Secreted in two stages
 Initially secreted by hepatocytes and contains bile acids, cholesterol and is
secreted into canaliculi
 Flows out to reach the hepatic duct and common bile duct to duodenum or to
the gall bladder.
 Secretin also stimulates the addition of watery bicarb to the bile during its flow.
 Storing and Concentrating Bile in the Gallbladder
 Water, sodium, chloride, and other electrolytes are absorbed through the
gallbladder mucosa to concentrate the bile.
 Most happens by active transport of sodium through the epithelium.
 Emptying the Gallbladder
 Most potent stimulus is CCK which is released in response to fatty foods in the
duodenum.
 Also stimulated in part by acetylcholine-secreting nerve fibers.
o Function of Bile Salts in Fat Digestion and Absorption
 Two important actions
 Detergent action on fat particles to decrease surface tension and break up the
fat globules - called emulsifying or detergent function
 Help in the absorption of fatty acids, monoglycerides, cholesterol, and other
lipids.
 Do this by forming micelles.
 About 94% of bile salts are reabsorbed into the blood from the small intestine.
o Gallstone Formation
 Causes
 Too much water absorption from bile
 Too much absorption of bile acids from bile
 Too much cholesterol in bile
 Inflammation of the epithelium
Secretions of the Small Intestine
o Mucus Secretion by Brunner's Glands of Duodenum
 Secrete alkaline mucus in response to
1. Tactile or irritating stimuli
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2. Vagal stimulation which causes increased Brunner's secretion and increased
stomach secretion.
3. Gastrointestinal Hormones, especially secretin
 Inhibited by sympathetic stimulation
o Secretions of Digestive Juices by Crypts of Lieberkuhn
 Lie between intestinal villi and are covered with
 Goblets cells that secrete mucus to lubricate and protect
 Enterocytes that secrete water and electrolytes and then absorb these with the
digestive products.
 Mechanism of secretion of watery fluid
 Process believed to have two ways
 Active secretion of chloride into crypts
 Active secretion of bicarb
 Both of these cause electrical drag and then osmotic pull to secrete water.
 Enzymes
 Peptidases for splitting small peptidases
 Sucrase, maltase, isomaltase, and lactase for splitting dissacharides
 Intestinal lipase for splitting neutral fats
o Regulation of Small Intestine Secretion
 Local enteric nervous reflexes initiated by tactile or irritative stimuli from chyme Most Important
Secretions of Large Intestine
o Like the small intestine, it has lots of crypts of Lieberkuhn but has no villi.
o Epithelial cells have mostly mucous cells.
 Rate of secretion regulated mainly by direct, tactile stimulation of the cells and by
local nervous reflexes.
o Stimulation of pelvic nerves from spinal cord carries parasympathetic innervation to distal
one half large intestine.