git physiology. 2 the role of git provides the body with water, electrolytes and nutrients ...
TRANSCRIPT
GIT physiology
2
The Role of GITThe Role of GIT
Provides the body with water, electrolytes and nutrients Requires:
1. Movement of food
2. Break down the food to absorbable materials
3. Digestion of food by different juices
4. Absorption of digestive materials
5. Neural control
Organization and control of the Organization and control of the gastrointestinal systemgastrointestinal system
Digestive processes – motility, secretion, digestion & absorption
Phases – cephalic, gastric, intestinal & interdigestive period
Contacts external environment Excretes steroid metabolites Sphincters - Upper and lower
esophageal sphincters, pylorus, ileocecal valve, internal and external anal sphincters
Organs –Teeth, Salivary glands, esophagus, stomach, liver, pancreas, duodenum, ileum, jejunum, colon
Structures•Mucosa - innermost layer • epithelium- layer of specialized cells that line the lumen•Duodenum, jejunum & ileum have villi and Crypts•Colon has only Crypts•lamina propria - loose connective tissue (collagen, elastin fibrils), glands, lymphatics, lymph nodules, capillaries •muscularis mucosae•innermost layer of contractile tissue
muscularis mucosae-contractions cause mucosal folding and ridges- 600-fold increase in surface area•Submucosa•loose connective tissue (collagen, elastin fibrils), glands, nerve trunks, lymphatics, blood vessels•Muscularis externa -2 layers of smooth muscle•inner circular layer•outer longitudinal layer•Interstitial cells of Cajal
Structures
•Enteric nervous system•submucosal plexus (Meissner’s plexus)•controls secretory and sensory functions •myenteric plexus (Auerbacks’s plexus)• located between the circular and longitudinal muscle layers• coordinates muscle contractions•Serosa•outermost layer of gut• composed of squamous mesothelial cells
Structures
SecretionSecretion
Addition of fluids, enzymes, and mucus to lumen of GI tract
Secretions produced by – Salivary glands (saliva)– Gastric mucosal cells (gastric secretion)– Pancreatic exocrine cells (pancreatic secretion)– Liver (bile)
Salivary SecretionSalivary Secretion
Salivary glands produce 1 L/day of saliva
Structure of Salivary Glands – Parotid glands, submandibular glands, and sublingual
glands– Each gland delivers saliva to mouth through a duct
Major glandsMajor glands
Parotid: so-called watery serous saliva rich in amylase, proline-rich proteins– Stenson’s duct
Submandibular gland: more mucinous– Wharton’s duct
Sublingual: viscous saliva– ducts of Rivinus; duct of Bartholin
Salivary GlandsSalivary Glands
Innervation of salivary gland Innervation of salivary gland
– Sympathetic
– Parasympatethic
Composition
FunctionsFunctions
Protection– lubricant (glycoprotein)– barrier against noxious stimuli; microbial
toxins and minor traumas– washing non-adherent and acellular debris– formation of salivary pellicle
calcium-binding proteins: tooth protection; plaque
FunctionsFunctions
Buffering (phosphate ions and bicarbonate)– bacteria require specific pH conditions– plaque microorganisms produce acids from
sugars
FunctionsFunctions
Digestion– neutralizes esophageal contents– dilutes gastric chyme– forms food bolus– brakes starch
FunctionsFunctions
Antimicrobial– lysozyme hydrolyzes cell walls of some
bacteria– lactoferrin binds free iron and deprives bacteria
of this essential element– IgA agglutinates microorganisms
FunctionsFunctions
Maintenance of tooth integrity– calcium and phosphate ions
ionic exchange with tooth surface
FunctionsFunctions
Tissue repair– bleeding time of oral tissues shorter than other
tissues– resulting clot less solid than normal– remineralization
FunctionsFunctions
Taste– solubilizing of food substances that can be
sensed by receptors– trophic effect on receptors
Gastric SecretionGastric Secretion Gastric mucosal cells secrete gastric juice
– HCl and pepsinogen initiate protein digestion
– Intrinsic factor required for absorption of vitamin B12
– Mucus protects gastric mucosa from HCl
Cell Types of Gastric Mucosa– Body of stomach contains oxyntic glands
Parietal cells → HCl and Intrinsic Factor Chief cells → Pepsinogen
– Antrum of stomach contains pyloric glands G cells → Gastrin into the circulation Mucous neck cells → Mucus, HCO3-, and Pepsinogen
Gastric GlandGastric Gland
Gastric SecretionGastric Secretion
The StomachThe Stomach
EsophagusEsophagusFundusFundus
CorpusCorpus
Pyloric antrumPyloric antrumPyloric canalPyloric canal
PylorusPylorus
DuodenumDuodenum
Gastric pitGastric pit
isthmusisthmus
neckneck
basebase
Parietal cellsParietal cells
Surface mucousSurface mucouscellscells
Mucous neckMucous neckcellscells
ArgentaffinArgentaffincellcell
Chief cellsChief cells
Composition of gastric juice
Functions of HCL
Functions of gastric juice– Gastric rennin
– Intrinsic factor
– Mucus
HCl SecretionHCl Secretion Parietal cells secrete HCl which converts inactive
pepsinogen to pepsin
1. Within cell, CO2 combines with H2O to form H+ and HCO3
-
2. At apical membrane, H+ secreted into lumen of stomach via H+-K+ ATPase– Cl- follows H+ into the lumen by diffusing through Cl-
channels
3. At basolateral membrane, HCO3- absorbed into
blood via a Cl--HCO3- exchanger
– Eventually HCO3- secreted back into GI tract by pancreas
HCl SecretionHCl Secretion
Regulation of HCl SecretionRegulation of HCl Secretion ACh
– Released from vagus nerve – Binds to receptors on parietal cells– Produces H+ secretion by parietal cells– Atropine blocks muscarinic receptors on parietal cells
Histamine – Released from mastlike cells in gastric mucosa– Binds to H2 receptors on parietal cells – Produces H+ secretion by parietal cells– Cimetidine blocks H2 receptors
Gastrin – Released into circulation by G cells of stomach antrum– Binds to receptors on parietal cells– Stimulates H+ secretion
Neural regulation
Hormonal regulation
Regulation of gastric SecretionRegulation of gastric Secretion
Phases of Pancreatic SecretionPhases of Pancreatic SecretionCephalic phase: vagal – gastrin (similar to CCK) is
pancreatic secretagogue.
Gastric phase: gastrin-vagovagal reflex.
Intestinal phase: most important presence of chyme in
duodenum, acid stimulates secretin bicarbonate rich,
peptides amino acids specially tryptophan and
phenylalanine and fatty acids CCK.
Neural effect by vagus.
Pancreatic SecretionPancreatic Secretion Exocrine pancreas secretes ~1 L/day into duodenum
– Fluid consists of HCO3- and enzymes
HCO3- neutralizes H+ delivered to duodenum from stomach
Enzymatic portion digests carbohydrates, proteins, and lipids into absorbable molecules
Structure of Pancreatic Exocrine Glands – Comprises ~90% of pancreas
Rest of pancreatic tissue is endocrine pancreas and blood vessels– Acinar Cells
Line blind end of branching duct system Secrete enzymatic portion
– Ductal Cells Line the ducts Secrete aqueous HCO3
- component
HCO3HCO3-- Secretion Secretion
Apical membrane of ductal cells contains a Cl--HCO3
- exchanger Basolateral membrane contains Na+-K+ ATPase and a
Na+-H+ exchanger
1. CO2 and H2O combine in cells to form H+ and HCO3-
2. HCO3- is secreted into pancreatic juice by Cl--HCO3
- exchanger
3. H+ is transported into blood by Na+-H+ exchanger Absorption of H+ causes acidification of pancreatic venous blood
Composition
Digestive enzymesPeptidases
Trypsin and chymotrypsin
Nucleases
Lipases
Amylases
Regulation of Pancreatic Regulation of Pancreatic SecretionSecretion
Acinar cells (enzymatic secretion)– Receptors for CCK and muscarinic receptors for ACh– CCK is most important stimulant
I cells secrete CCK in presence of amino acids and fatty acids in intestinal lumen
– ACh also stimulates enzyme secretion Ductal cells (aqueous secretion of HCO3
-)– Receptors for CCK, ACh, and secretin– Secretin (from S cells of duodenum) is major stimulant
Secreted in response to H+ in intestine
– Effects of secretin are potentiated by both CCK and ACh
Bile SecretionBile Secretion
Bile Secretion and RecyclingBile Secretion and Recycling
1. Produced and secreted by liver
2. Stored in gallbladder
3. Ejected into small intestine when gallbladder contracts
4. After lipids absorbed, bile salts are recirculated to liver via enterohepatic circulation
– Absorption of bile salts from ileum into portal circulation– Delivery back to liver
5. Extraction of bile salts from the portal blood by hepatocytes
Bile Secretion and Recycling Bile Secretion and Recycling
Composition of BileComposition of Bile
A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes
Bile salts are cholesterol derivatives that:– Emulsify fat– Facilitate fat and cholesterol absorption– Help solubilize cholesterol
Enterohepatic circulation recycles bile salts The chief bile pigment is bilirubin, a waste
product of heme
Functions of bileFunctions of bile
Necessary for digestion and absorption of lipids in small intestine
Mixture of bile salts, bile pigments, and cholesterol
Bile salts emulsify lipids to prepare them for digestion
Solubilize products of lipid digestion in packets called micelles
Entrohepatic circulation of bile salts
Regulation of bile secretion– Bile independent fraction of biliary secretion– Bile dependent fraction of biliary secretion
Release of bile from gallbladder
Gallbladder
Intestinal SecretionIntestinal Secretion
Duodenal epithelial cell: Brunner’s gland
HCO3 and mucus.
Intestinal crypt: isotonic Na+ Cl- solution.
Surface cells of villi are absorptive and crypt
site for secretion.
Ileum and proximal colon: mucus and alkaline.
Schematic diagram of intestinal villi and crypt
Functional organization of villus
Pathway traversed by absorbed solute
Regulation of secretionRegulation of secretion
Neural
Hormonal
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