GIT physiology

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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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