Unit One: Introduction to Physiology: The Cell and General Physiology

Unit Twelve: Gastrointestinal PhysiologyChapter 64: Secretory Functions of the Alimentary TractGuyton and Hall, Textbook of Medical Physiology, 12th editionGeneral Principles of GI Tract Secretion Anatomical Types of Glands

Mucous glands (Goblet cells)

Crypts of Lieberkuhn

Tubular glands

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 substancesGeneral Principles of GI Tract Secretion Basic Mechanism of Stimulation of the AlimentaryTract Glands

Contact of food with the epithelium stimulates secretionfunction of enteric nervous stimuli

Local epithelial stimulation also activates the ENSand include tactile stimulation, chemical irritation, and distension of the gut wall

General Principles of GI Tract Secretion Autonomic Stimulation of Secretion

Parasympathetic stimulation increases alimentarytract glandular secretion; especially true in the upper portion of the tract.

Sympathetic stimulation has a dual effect; alone it slightly increases secretion, and if parasympatheticor hormonal stimulation is already causing copiussecretion, 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 hormonesregulate the volume and character of the secretions.

The hormones are produced in response to food inthe lumen of the gut

General Principles of GI Tract Secretion Basic Mechanism of Secretion by Glandular Cells

Secretion of organic substances (Fig. 64.1)

Water and electrolyte secretion

Lubricating and protecting properties of mucous

Secretion of Saliva Saliva Contains a Serous Secretion and a MucusSecretion

Principle glands are the parotid, submandibular,and sublingual (also the tiny buccal glands)

Serous secretion contains ptyalin (amylase) and themucus contains mucin for lubrication and surfaceprotective propertiesSecretion of Saliva Secretion of Ions in Saliva

Saliva contains large quantities of potassium andbicarbonate ions

Acini secrete a primary secretion containingptyalin or mucin in a solution of ions

Ductal epithelium secretes the bicarbonate ioninto the lumenSecretion of Saliva Nervous Regulation of Salivary Secretion

Fig. 64.3 Parasympathetic nervous regulation of salivary secretionSecretion of Saliva Nervous Regulation of Salivary Secretion

Controlled mainly by parasympathetic pathwaysSalivation can be stimulated or inhibited by signalscoming from higher brain centersSalivation can occur in response to reflexes in thestomach and upper small intestinesSympathetic stimulation can increase salivation a small amountSecretion always requires adequate nutrients fromthe blood

Secretion of Saliva Esophageal Secretion

Entirely mucus and provide lubrication forswallowing

Contains both simple and compound mucus glands

Gastric Secretion Characteristics of the Gastric Secretions

Secretions from the gastric glands (oxyntic)Mucus neck cells-secrete mucusChief or peptic cells-secrete pepsinogenParietal cells- secrete HCl and intrinsic factor

Gastric Secretion

Fig. 64.4 Oxyntic gland from the body of the stomachGastric Secretion Basic Mechanism of HCl Secretion

Fig. 64.5 Schematic anatomy of the canaliculi in a parietao (oxyntic) cell

Gastric SecretionFig. 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 Secretionare AcH, Gastrin, and Histamine

AcH (from parasympathetic stimulation) excitessecretion of pepsinogen, HCl, and mucus

Both gastrin and histamine stimulate secretion ofacid by parietal cells but not the other cellsGastric Secretion Secretion of Intrinsic Factor by Parietal Cells

Essential for the absorption of vitamin B-12 in theileum

Pyloric GlandsSecretion of Mucus and Gastrin

Surface Mucus Cells- secrete large quantities of viscous mucus that coat the stomach mucosaGastric Secretion Stimulation of Gastric Acid Secretions

Parietal cells are the only cells that secrete HCl

Paritetal cells operate in close association with enterochromaffin like cells (ECL) whose primary function is to release histamine

Stimulation of acid secretion by gastrin-causes therelease of histamine from the ECL cells; release ofgastrin is in response to proteins in stomachGastric Secretion Stimulation of Gastric Acid Secretions

Regulation of pepsinogen secretion occurs in response to two signals:

Stimulation of the peptic cells by AcH from thevagus nerve or nerves of the enteric nervoussystemStimulation of peptic cell secretion in responseto acid in the stomach

Gastric Secretion Phases of Gastric Secretion

Cephalic phase

Gastric phase

Intestinal phase

Gastric Secretion Phases of Gastric Secretion

Fig. 64.7 Phases of gastric secretion and their regulationGastric Secretion Inhibition of Gastric Secretion by Other Post-Stomach Intestinal Factors

Function is to slow passage of chyme from the stomachwhen the small intestine is already filled or overactive

Presence of food initiates a reverse enterogastric reflex

Presence of acid, fat, protein breakdown products, etc.causes the release of several intestinal hormones;especially secretin which inhibits stomach secretionGastric Secretion Chemical Composition of Gastrin and Other GI Hormones

Gastrin, CCK, and secretin are all large polypeptides

Activity of all of these lie in the last amino acids ofthe chain

Pancreatic Secretions

Fig. 64.10 Regulation of pancreatic secretionPancreatic Secretions Pancreas

Large compound glandEnzymes secreted by the acini cellsBicarbonate secreted by small ductulesPancreatic juice produced in response to thepresence of chyme

Pancreatic Secretions Pancreatic Digestive Enzymes

For proteins: trypsin, chymotrypsin, andcarboxypeptidaseFor cbh: pancreatic amylaseFor fats: pancreatic lipase, cholesterol esterase,and phopholipase

Secretion of trypsin inhibitor prevents the digestionof the pancreas itself

Pancreatic Secretions Secretion of Bicarbonate Ions

Bicarbonate provides alkali in the pancreatic juice to neutralize the HCl coming into theduodenum from the stomachPancreatic Secretions

Fig. 64.8 Secretion of isoosmotic sodium bicarbonate solution by the pancreatic ductules and ductsPancreatic Secretions Regulation of Pancreatic Secretion-BasicStimulation That Causes Secretion

Acetylcholine-released from the parasympatheticvagus nerve endings and other cholinergic nervesassociated with the enteric nervous systemCholecystokinin-secreted by the duodenal andupper jejunal mucosa when food enters the smallintestineSecretin-secreted as in cholecystokinin when highly acidic food enters the intestined.Multiplicative effects of a, b, and cPancreatic Secretions Phases of Pancreatic Secretion

Cephalic and Gastric PhasesIntestinal PhaseSecretin stimulates secretion of bicarbonate ionsCholecystokinin production which stimulatesmore digestive enzymes by the acini (similar tovagal stimulation)Secretion of Bile by the Liver Functions of Bile

Bile salts help to emulsify the large fat moleculesof food and aid in the absorption of the digestedfat

Means of secretion of several important waste products from the blood (especially bilirubinand excesses of cholesterol)

Secretion of Bile by the Liver Physiologic Anatomy of Biliary Secretion

Bile is secreted in two stages by the liver

Initial secretion by the hepatocytes; contains bile salts, cholesterol, and other organic compounds; into the bilecanaliculiBile empties into termnal bile ducts, thenthe hepatic duct, and finally the commonbile duct into the duodenum or divertedthrough the cystic duct into the gallbladderSecretion of Bile by the Liver Physiologic Anatomy of Biliary Secretion

Bile is stored and concentrated in the gallbladder

Secretion of Bile by the LiverLiver BileGallbladder BileWater97.5 g/dl92 g/dlBile salts1.1 g/dl6 g/dlBilirubin0.04 g/dl0.3 g/dlCholesterol0.1 g/dl0.3-0.9 g/dlFatty Acids0.12 g/dl0.3-1.2 g/dlLecithin0.04 g/dl0.3 g/dlSodium Ions145 mEq/L130 mEq/LPotassium Ions5 mEq/L12 mEq/LCalcium Ions5 mEq/L23 mEq/LChloride Ions100 mEq/L25 mEq/LBicarbonate Ions28 mEq/L10 mEq/LTable 64.2 Composition of BileSecretion of Bile by the LiverFig. 64.11 Liver secretion and gallbladder emptying

Secretion of Bile by the Liver Emptying of the Gallbladder-Role of CCK

Most potent stimulus for the gallbladder toundergo rhythmic contractions when food enterthe small intestine

Main stimulus for CCK is fatty foods

Bile Salts Function in Digestion and AbsorptionHave a detergent action on fat particleswhich causes the emulsification of the fat

b.Help in the absorption by forming micellesSecretion of Bile by the LiverFig. 64.12 Formation of gallstones

Secretion of the Small Intestine Secretion of Mucus by Brunners Glands in the Duodenum

Secrete large amounts of mucus in response to:tactile or irritating stimuli on the mucosa, vagalstimulation, or secretin

Mucus protects the duodenal wall from the acidicchyme 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

Surface covered by goblet cells (mucus) and enterocytes (secrete water and electrolytes)

Mechanism of secretion of the watery fluid is unclear but involves the active secretion of chlorideions and bicarbonate ions

Secretion of the Small Intestine Secretion of Intestinal Digestive Juices by the Crypts of Lieberkuhn

Digestive enzymes in the small intestine secretion:several peptidases; sucrase, maltase, isomaltase,and lactase for splitting disaccharides into monosaccharides; and small amounts of intestinallipase

Regulation is entirely local by the enteric nervoussystemSecretion of the Small Intestine Secretion of Mucus by the Large Intestine

Many crypts but no villi in the large intestine

Epithelial cells secrete almost no enzymes butcontain many mucus cells

Rate of mucus formation is regulated by the directtactile stimulation of the epithelial cells