Gastric Anatomy & Physiology

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AnatomyIn adult life, stomach located T10 and L3 vertebral segmentCan be divided into anatomic regions based on external landmarks

4 regionsCardiaFundusCorpus (body)Antrum

AnatomyCardia- region just distal to the GE junctionFundus- portion above and to the left of the GE junction

AnatomyCorpus- region between fundus and antrum

Margin not distinctly external, has arbitrary borders

Antrum- bounded distally by the pylorus

Which can be appreciated by palpation of a thickened ring of smooth muscle

Anatomy Position of the

stomach varies with body habitusIn general- it is fixed at two points

Proximally at the GE juctionDistally by the retroperitoneal duodenum

AnatomyAnterior- in contact with Left hemi-diaphragm, left lobe and anterior segment of right lobe of the liver and the anterior parietal surface of the abdominal wallPosterior- Left diaphragm, Left kidney, Left adrenal gland, and neck, tail and body of pancreasThe greater curvature is near the transverse colon and transverse colon mesenteryThe concavity of the spleen contacts the left lateral portion of the stomach

Vasculature

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 curvature of the stomach

Left GastroepiploicShort Gastric Arteries

Common Hepatic or Proper Hepatic Artery2 major branches

Right Gastric- supples a portion of the lesser curvatureGastroduodenal artery

-Gives rise to Right Gastroepiploic artery

-helps supply greater curvature in conjunction with Left Gastroepiploic Artery

AnatomyVenous Drainage

Parallels arterial supply

Lymphatic drainageLymph from the proximal portion of the stomach drains along the lesser curvature first drains into superior gastric lymph nodes surrounding the Left Gastric ArteryDistal portion of lesser curvature drains through the suprapyloric nodesProximal portion of the greater curvature is supplied by the lymphatic vessels that traverse the pancreaticosplenic nodesAntral portion of the greater curvature drains into the subpyloric and omental nodal groups

In general- The lymphatic drainage of the human stomach, like its blood supply, exhibits extensive intramural ramifications and a number of extramural communications. Therefore spread beyond is often beyond region of origin at a distance from the primary lymphatic zone

AnatomyNerve Supply

Left and Right Vagus Nerves descend parallel to the esophagus within the thorax before forming a peri-esophageal plexus between the tracheal bifurcation and the diaphragmFrom this plexus, two vagal trunks coalesce before passing through the esophageal hiatus of the diaphragm

AnatomyLeft (anterior) Vagus Nerve

Left of the esophagus

BranchesHepatic Branch

Supplies liver and Biliary Tract

Anterior gastric or Ant. Nerve of Latarget

AnatomyRight (posterior) Vagus Nerve

Right of the esophagus

BranchesCeliacPosterior Latarget

Innervates posterior gastric wall

AnatomyParasympathetic innervation of Stomach- Vagus Nerve

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

Sympathetic innervation of Stomach- Splanchnic Nerve

Derived from spinal segement T5-T10

AnatomyMicroscopic Anatomy

Glandular portions of stomachLined by simple columnar epitheliumThis luminal surface is interrupted at intervals by gastric pits

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

Mucosa has three types of gastric glands -Cardiac

-Oxyntic-Antral

Microscopic AnatomyCardiac Glands

Location- Cardia Contain mucous

Function- secrete mucous (provides a protective coat for lining of stomach)

Oxyntic GlandsMost distinctive feature of the stomachLocation- Fundus and CorpusContains many cell types

Microscopic AnatomyParietal cells

Location- neck of gastric pitStimulated by Ach, Histamine and GastrinSecretes HCl + Intrinsic Factor

Chief CellsLocation- base of gastric pitStimulus- VagalSecretes Pepsinogen (eventually leads to pepsin- digestive enzyme)

Microscopic AnatomyAntral Glands

Gastrin cellsLocation- mucosa of distal stomachStimulus- amino acidsSecretion- Gastrin (stimulates HCl production by way of parietal cells)

SomatostatinLocation- mucosa of distal stomach + DuodenumStimulus- HCl or low pH in duodenumActions- Inhibits gastric emptying, Pancreatic secretions, and gallbladder contraction

PhysiologyThe stomach contains a number of biologically active peptides in nerves and endocrine cells

Ex. Gastrin, somatostatin, vasoactive intestinal peptide (VIP), substance P, and glucagon, etcThe two peptides of greatest importance to human disease and clinical surgery are

GastrinSomatostatin

PhysiologyGastrin

Most important stimulus is a mealamino acids that results from proteolysis Fat and carbohydrates are not stimuli for gastrin secretionGastric distention that occurs from a meal will stimulate cholinergic neurons thereby releasing gastrinGastrin will then prompt Parietal cell to secrete HCl

Once Gastric distention diminishes, VIP-containing neurons are activated causing stimulation of somatostatin, thus attenuating Gastrin secretionOverall, a lumen pH >3.0 will potentiate gastrin release, whereas a pH <3.0 will inhibit its release

PhysiologySomatostain

Like Gastrin, plays an integral role in gastric physiologyAlso, used for important therapeutic applications in treatment of digestive diseases

Main stimulus is a low or acidic (<3.0)luminal pHMany peptides have shown to release somatostatin

Ex. Secretin, Cholecystokinin and gastrinIn contrast, stimulation of Vagal nerves along with cholinergic neurons inhibit somatostatin

Overall, the most important gastric function of somatostatin is to regulate acid secretion and gastrin release

Gastric Acid Secretion

Gastric Acid SecretionBasolateral membrane of the parietal cell contains specific receptors for the three major stimulants of acid production

HistamineGastrinAcetylcholine

Each stimulant has its own 2nd messenger system which allows for stimulation of the parietal cell

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

Both Vagal tone and ambient Histamine secretion are presumed to regulate basal acid secretionGastrin 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

Gastric Acid SecretionStimulated acid secretion begins with

Cephalic phaseThought, sight or smell of food stimulates acid secretionMediated by Vagal stimulation

Vagal discharge Directs the cholinergic mechanism for stimulation

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

Vagal effects inhibit tonic inhibition that is provided by somatostatin

Gastric Acid SecretionGastric Phase

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

Presence of partially hydrolyzed food constituentsGastric distention

Gastrin is the most important mediator of this phase

Ends when Antral muscosa is exposed to acidWhen luminal pH is <2.0 in the antrum, gastrin release stopsSomatostatin release is increased

Entry of digestive products into the intestine begins the intestinal-phase inhibition of gastric acid secretion

Gastric Acid SecretionIntestinal Phase

Also, releases HCl by way of GastrinReleases secretin to inhibit Gastrin which ultimately decreases Acid production

Other FactorsPepsin

Secreted from gastric chief cellsContributes to the overall coordination of the digestive processMain function is to initiate protein digestion, usually is incomplete

Partially hydrolyzed protein by pepsin are important signals for release of

GastrinCholecystokinin

Other FactorsIntrinsic Factor (IF)

Located in the parietal cells (oxyntic gland)Main function is to absorb cobalamin (Vitamin B12) form ileal mucosa and then transported to the liverSecretion of IF is similar to acid secretion

stimulated Ach HistamineGastrin

Other FactorsBicarbonate

Secreted from the gastric mucosaTheory is that bicarbonate is secreted to maintain a neutral pH at the mucosal surface, even if acidic in lumenCholinergic agonist, vagal nerve stimulation have been shown to increase gastric bicarbonate production

Gastric MotilityTo understand gastric motility the stomach is divided into two functional terms as two different regions which 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 septum of connective tissue marks the change from pylorus to the duodenum

Gastric MotilityProximal 1/3

Have prolonged and tonic gastric contractionsNo action potentials or pacesetter

Thus no peristalsis

Distal 2/3In general, gastric smooth muscle exhibit myoelectric activity based on a highly regular pattern, called slow waves

Slow waves set a maximum rate at which contrations can occur (3 contractions/min); they do not cause contractions

Gastric MotilityContraction occur when action potential are phase locked with a crest of a slow wave patternWhen an action potential is combined with a pacesetter potential (partially depolarized smooth muscle cells) a ring of smooth muscle cell contraction moves with peristalsis

Coordination of Contraction

Receptive RelaxationVagally mediated relaxation of fundus (proximal stomach) when degluttination occurs Allows the proximal stomach to act as a storage site for ingested food in the immediate postprandial period

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

Soon proximal contractile activity increases eventually leading to compressive movement of gastric content form fundus to antrum

Coordination of Contraction

Food enters antrumFood peristaltically propelled toward the pylorusPylorus closes before the antral contraction

This coordinated closing allows for small bolus of liquid and food particles to pass, while the main bulk of the gastric content undergoes retropulsion back into the antrum

Next, there is a churning action in the antrum that mixes the ingested food particle, gastric acid and pepsin

Solid food particles >1mm will not pass through the pylorus

Coordination of Contraction

Overall, Liquids are empty more quickly than solid

Liquids empty exponentiallySolids endure this “lag period” or antral contraction (empties linearly)

In generalProximal stomach is the dominant force in determining liquid emptying based on the gastroduodenal pressure gradient generated by proximal gastric contractionsDistal stomach is postulated as controlling emptying of solids through its grinding and peristaltic actions

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