Gast Ran at Physio l

8/8/2019 Gast Ran at Physio l

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Gastric Anatomy & Physiology


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AnatomyIn adult life, stomach located T10 andL3 vertebral segment

Can be divided into anatomic regionsbased on external landmarks

4 regionsCardia

Fundus

Corpus (body)

Antrum


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AnatomyCardia- region justdistal to the GE

junctionFundus- portionabove and to theleft of the GE

junction


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AnatomyCorpus- region betweenfundus and antrum

Margin not distinctlyexternal, has arbitraryborders

Antrum- boundeddistally by the pylorus

Which can beappreciated by palpationof a thickened ring ofsmooth muscle


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AnatomyPosition of thestomach varies with

body habitusIn general- it is fixedat two points

Proximally at the GEjuction

Distally by theretroperitonealduodenum


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AnatomyAnterior- in contact with Lefthemi-diaphragm, left lobe andanterior segment of right lobeof the liver and the anterior

parietal surface of theabdominal wall

Posterior- Left diaphragm, Leftkidney, Left adrenal gland, andneck, tail and body of pancreas

The greater curvature is near

the transverse colon andtransverse colon mesentery

The concavity of the spleencontacts the left lateral portionof the stomach


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Vasculature


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VasculatureWell vascularized organArterial flow mainly derived from Celiac Artery

3 BranchesLeft Gastric Artery

Supplies the cardia of the stomach and distal esophagusSplenic Artery

Gives rise to 2 branches which help supply the greater curvatureof the stomach

Left GastroepiploicShort Gastric Arteries

Common Hepatic or Proper Hepatic Artery

2 major branchesRight Gastric- supples a portion of the lesser curvatureGastroduodenal artery

-Gives rise to Right Gastroepiploic artery-helps supply greater curvature in conjunctionwith Left Gastroepiploic Artery


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AnatomyVenous Drainage

Parallels arterial supply

Lymphatic drainage

Lymph from the proximal portion of the stomach drains along the lessercurvature first drains into superior gastric lymph nodes surrounding the LeftGastric Artery

Distal portion of lesser curvature drains through the suprapyloric nodes

Proximal portion of the greater curvature is supplied by the lymphaticvessels that traverse the pancreaticosplenic nodes

Antral portion of the greater curvature drains into the subpyloric and

omental nodal groupsIn general- The lymphatic drainage of the human stomach, like itsblood supply, exhibits extensive intramural ramifications and a numberof extramural communications. Therefore spread beyond is oftenbeyond region of origin at a distance from the primary lymphatic zone


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AnatomyNerve Supply

Left and RightVagus Nerves descend

parallel to the esophagus within the thoraxbefore forming a peri-esophageal plexusbetween the tracheal bifurcation and thediaphragm

From this plexus, two vagal trunkscoalesce before passing through theesophageal hiatus of the diaphragm


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AnatomyLeft (anterior) VagusNerve

Left of theesophagusBranches

Hepatic Branch

Supplies liverand Biliary

TractAnterior gastric orAnt. Nerve ofLatarget


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AnatomyRight (posterior)Vagus Nerve

Right of theesophagus

Branches

Celiac

Posterior Latarget

Innervatesposteriorgastric wall


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AnatomyParasympathetic innervation ofStomach- Vagus Nerve

90% of fiber in vagal trunk is afferent (infotransmitting from stomach to CNS)

Sympathetic innervation of Stomach-

Splanchnic NerveDerived from spinal segement T5-T10


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AnatomyMicroscopic Anatomy

Glandular portions of stomach

Lined by simple columnar epitheliumThis luminal surface is interrupted at intervals by gastricpits

Opening into these gastric pits are one or more gastricglands that have functional significance

Mucosa has three types of gastric glands

-Cardiac-Oxyntic

-Antral


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Microscopic AnatomyCardiac Glands

Location- Cardia

Contain mucous

Function- secretemucous (provides aprotective coat for liningof stomach)

Oxyntic Glands

Most distinctive featureof the stomach

Location- Fundus andCorpus

Contains many cell types


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Microscopic AnatomyParietal cells

Location- neck of gastricpitStimulated by Ach,

Histamine and GastrinSecretes HCl + IntrinsicFactor

Chief CellsLocation- base of gastricpit

Stimulus-VagalSecretes Pepsinogen

(eventually leads topepsin- digestiveenzyme)


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Microscopic AnatomyAntral Glands

Gastrin cells

Location- mucosa of distal stomachStimulus- amino acids

Secretion- Gastrin (stimulates HCl production by way ofparietal cells)

Somatostatin

Location- mucosa of distal stomach + DuodenumStimulus- HCl or low pH in duodenum

Actions- Inhibits gastric emptying, Pancreatic secretions,and gallbladder contraction


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PhysiologyThe stomach contains a number ofbiologically active peptides in nerves and

endocrine cellsEx. Gastrin, somatostatin, vasoactive intestinalpeptide (VIP), substance P, and glucagon, etc

The two peptides of greatest importance to

human disease and clinical surgery areGastrin

Somatostatin


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PhysiologyGastrin

Most important stimulus is a mealamino acids that results from proteolysis

Fat and carbohydrates are not stimuli for gastrinsecretionGastric distention that occurs from a meal will stimulatecholinergic neurons thereby releasing gastrinGastrin will then prompt Parietal cell to secrete HCl

Once Gastric distention diminishes, VIP-containingneurons are activated causing stimulation ofsomatostatin, thus attenuating Gastrin secretionOverall, a lumen pH >3.0 will potentiate gastrinrelease, whereas a pH


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PhysiologySomatostain

Like Gastrin, plays an integral role in gastricphysiology

Also, used for important therapeutic applicationsin treatment of digestive diseases

Main stimulus is a low or acidic (


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Gastric Acid Secretion


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Gastric Acid SecretionBasolateral membrane of the parietal cellcontains specific receptors for the three major

stimulants of acid productionHistamine

Gastrin

Acetylcholine

Each stimulant has its own 2nd messengersystem which allows for stimulation of theparietal cell


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Gastric Acid SecretionHumans normally secrete 2 to 5 mEq/h of HClin the fasting state, constituting basal acid

secretionBoth Vagal tone and ambient Histamine secretionare presumed to regulate basal acid secretion

Gastrin is not thought to play a role in basal acid

secretionTherefore, a Vagotomy or use of H2 blockers (ex.Cimetidine) will decrease basal acid production


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Gastric Acid SecretionStimulated acid secretion begins with

Cephalic phase

Thought, sight or smell of food stimulates acidsecretion

Mediated by Vagal stimulation

Vagal dischargeDirects the cholinergic mechanism for stimulation

Can be inhibited by Atropine (anticholinergic)Inhibits release of somatostatin

Vagal effects inhibit tonic inhibition that isprovided by somatostatin


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Gastric Acid SecretionGastric Phase

Begins when food enters the stomachThe following are responsible for stimulation of acid

secretionPresence of partially hydrolyzed food constituents

Gastric distention

Gastrin is the most important mediator of this phase

Ends when Antral muscosa is exposed to acidWhen luminal pH is


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Gastric Acid SecretionIntestinal Phase

Also, releases HCl by way of Gastrin

Releases secretin to inhibit Gastrin whichultimately decreases Acid production


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Other FactorsPepsin

Secreted from gastric chief cells

Contributes to the overall coordination ofthe digestive process

Main function is to initiate proteindigestion, usually is incomplete

Partially hydrolyzed protein by pepsin areimportant signals for release of

Gastrin

Cholecystokinin


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Other FactorsIntrinsic Factor (IF)

Located in the parietal cells (oxyntic gland)

Main function is to absorb cobalamin(Vitamin B12) form ileal mucosa and thentransported to the liver

Secretion of IF is similar to acid secretion

stimulatedAch

Histamine

Gastrin


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Other FactorsBicarbonate

Secreted from the gastricmucosa

Theory is that bicarbonate issecreted to maintain aneutral pH at the mucosalsurface, even if acidic inlumen

Cholinergic agonist, vagalnerve stimulation have beenshown to increase gastricbicarbonate production


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Gastric MotilityTo understand gastric motility the stomach is dividedinto two functional terms as two different regionswhich have distinctive smooth muscle

Proximal 1/33 layers of smooth muscle

Outer longitudinalMiddle CircularInner Oblique

Distal 2/3Only a distinctive outer longitudinal layer

Gastric smooth muscle ends at the pylorus, a septumof connective tissue marks the change from pylorusto the duodenum


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Gastric MotilityProximal 1/3

Have prolonged and tonic gastric contractions

No action potentials or pacesetterThus no peristalsis

Distal 2/3In general, gastric smooth muscle exhibitmyoelectric activity based on a highly regular

pattern, called slow wavesSlow waves set a maximum rate at which contrations canoccur (3 contractions/min); they do not causecontractions


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Gastric MotilityContraction occur when action potentialare phase locked with a crest of a slow

wave patternWhen an action potential is combined witha pacesetter potential (partially depolarizedsmooth muscle cells) a ring of smooth

muscle cell contraction moves withperistalsis


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Coordination of ContractionReceptive Relaxation

Vagally mediated relaxation of fundus (proximalstomach) when degluttination occurs

Allows the proximal stomach to act as a storagesite for ingested food in the immediatepostprandial period

Meal is accepted without a significant increase in intra-gastric pressure

Soon proximal contractile activity increaseseventually leading to compressive movement ofgastric content form fundus to antrum


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Coordination of ContractionFood enters antrum

Food peristaltically propelled toward the pylorus

Pylorus closes before the antral contractionThis coordinated closing allows for small bolus of liquidand food particles to pass, while the main bulk of thegastric content undergoes retropulsion back into theantrum

Next, there is a churning action in the antrum thatmixes the ingested food particle, gastric acid andpepsin

Solid food particles >1mm will not pass through thepylorus


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Coordination of ContractionOverall, Liquids are empty more quickly than solid

Liquids empty exponentially

Solids endure this lag period or antral contraction (empties

linearly)In general

Proximal stomach is the dominant force in determining liquidemptying based on the gastroduodenal pressure gradientgenerated by proximal gastric contractions

Distal stomach is postulated as controlling emptying of solidsthrough its grinding and peristaltic actions

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