Gastric Anatomy & Physiology

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Anatomy In adult life, stomach located T10 and L3 vertebral segment Can be divided into anatomic regions based on external landmarks

4 regions Cardia Fundus Corpus (body) Antrum

Anatomy

Cardia- region just distal to the GE junction Fundus- portion above and to the left of the GE junction

Anatomy

Corpus- 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 habitus In general- it is fixed at two points Proximally at the GE juction Distally by the retroperitoneal duodenum

Anatomy

Anterior- 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 wall Posterior- Left diaphragm, Left kidney, Left adrenal gland, and neck, tail and body of pancreas The greater curvature is near the transverse colon and transverse colon mesentery The concavity of the spleen contacts the left lateral portion of the stomach

Vasculature

Vasculature

Well vascularized organ Arterial flow mainly derived from Celiac Artery 3 Branches Left Gastric Artery Supplies the cardia of the stomach and distal esophagus Splenic Artery Gives rise to 2 branches which help supply the greater curvature of the stomach Left Gastroepiploic Short Gastric Arteries Common Hepatic or Proper Hepatic Artery 2 major branches Right Gastric- supples a portion of the lesser curvature Gastroduodenal artery -Gives rise to Right Gastroepiploic artery -helps supply greater curvature in conjunction with Left Gastroepiploic Artery

Anatomy

Venous Drainage Parallels arterial supply Lymphatic drainage Lymph from the proximal portion of the stomach drains along the lesser curvature first drains into superior gastric lymph nodes surrounding the Left Gastric Artery Distal portion of lesser curvature drains through the suprapyloric nodes Proximal portion of the greater curvature is supplied by the lymphatic vessels that traverse the pancreaticosplenic nodes Antral 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

Anatomy Nerve 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 diaphragm From this plexus, two vagal trunks coalesce before passing through the esophageal hiatus of the diaphragm

Anatomy

Left (anterior) Vagus Nerve Left of the esophagus Branches Hepatic Branch Supplies liver and Biliary Tract Anterior gastric or Ant. Nerve of Latarget

Anatomy

Right (posterior) Vagus Nerve Right of the esophagus Branches Celiac Posterior Latarget Innervates posterior gastric wall

Anatomy Parasympathetic 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

Anatomy

Microscopic Anatomy Glandular portions of stomach Lined by simple columnar epithelium This 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 Anatomy

Cardiac Glands Location- Cardia Contain mucous Function- secrete mucous (provides a protective coat for lining of stomach) Oxyntic Glands Most distinctive feature of the stomach Location- Fundus and Corpus Contains many cell types

Microscopic Anatomy

Parietal cells Location- neck of gastric pit Stimulated by Ach, Histamine and Gastrin Secretes HCl + Intrinsic Factor Chief Cells Location- base of gastric pit Stimulus- Vagal Secretes Pepsinogen (eventually leads to pepsin- digestive enzyme)

Microscopic Anatomy

Antral Glands Gastrin cells Location- mucosa of distal stomach Stimulus- amino acids Secretion- Gastrin (stimulates HCl production by way of parietal cells) Somatostatin Location- mucosa of distal stomach + Duodenum Stimulus- HCl or low pH in duodenum Actions- Inhibits gastric emptying, Pancreatic secretions, and gallbladder contraction

Physiology

The stomach contains a number of biologically active peptides in nerves and endocrine cells Ex. Gastrin, somatostatin, vasoactive intestinal peptide (VIP), substance P, and glucagon, etc The two peptides of greatest importance to human disease and clinical surgery are Gastrin Somatostatin

Physiology

Gastrin Most important stimulus is a meal amino acids that results from proteolysis Fat and carbohydrates are not stimuli for gastrin secretion Gastric distention that occurs from a meal will stimulate cholinergic neurons thereby releasing gastrin Gastrin will then prompt Parietal cell to secrete HCl Once Gastric distention diminishes, VIP-containing neurons are activated causing stimulation of somatostatin, thus attenuating Gastrin secretion Overall, a lumen pH >3.0 will potentiate gastrin release, whereas a pH <3.0 will inhibit its release

Physiology

Somatostain Like Gastrin, plays an integral role in gastric physiology Also, used for important therapeutic applications in treatment of digestive diseases Main stimulus is a low or acidic (<3.0)luminal pH Many peptides have shown to release somatostatin Ex. Secretin, Cholecystokinin and gastrin In 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 Secretion

Basolateral membrane of the parietal cell contains specific receptors for the three major stimulants of acid production Histamine Gastrin Acetylcholine Each stimulant has its own 2 system which allows for stimulation of the parietal cell nd messenger

Gastric Acid Secretion

Humans 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 secretion Gastrin is not thought to play a role in basal acid secretion Therefore, a Vagotomy or use of H2 blockers (ex. Cimetidine) will decrease basal acid production

Gastric Acid Secretion Stimulated acid secretion begins with

Cephalic phase Thought, sight or smell of food stimulates acid secretion Mediated 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 Secretion

Gastric Phase Begins when food enters the stomach The following are responsible for stimulation of acid secretion stops Presence of partially hydrolyzed food constituents Gastric distention Gastrin is the most important mediator of this phase Ends when Antral muscosa is exposed to acid When luminal pH is <2.0 in the antrum, gastrin release Somatostatin release is increased Entry of digestive products into the intestine begins the intestinal-phase inhibition of gastric acid secretion

Gastric Acid Secretion

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

Other Factors Pepsin

Secreted from gastric chief cells Contributes to the overall coordination of the digestive process Main function is to initiate protein digestion, usually is incomplete Partially hydrolyzed protein by pepsin are important signals for release of Gastrin Cholecystokinin

Other Factors Intrinsic 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 liver Secretion of IF is similar to acid secretion stimulated Ach Histamine Gastrin

Other Factors

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

Gastric Motility

To understand gastric motility the stomach is divided into two functional terms as two different regions which have distinctive smooth muscle Proximal 1/3 3 layers of smooth muscle Outer longitudinal Middle Circular Inner Oblique Distal 2/3 Only 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 Motility

Proximal 1/3 Have prolonged and tonic gastric contractions No action potentials or pacesetter Thus no peristalsis Distal 2/3 In 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 Motility

Contraction occur when action potential are phase locked with a crest of a slow wave pattern When 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 Relaxation Vagally 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 antrum Food peristaltically propelled toward the pylorus Pylorus 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 exponentially Solids endure this “lag period” or antral contraction (empties linearly) In general Proximal stomach is the dominant force in determining liquid emptying based on the gastroduodenal pressure gradient generated by proximal gastric contractions Distal stomach is postulated as controlling emptying of solids through its grinding and peristaltic actions

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