gast ran at physio l
TRANSCRIPT
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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