Gastric Anatomy & Physiology

1. Gastric Anatomy & Physiology Medical ppt http://hastaneciyiz.blogspot.com

2. 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

3. Anatomy • Cardia- region just distal to the GE junction • Fundus- portion above and to the left of the GE junction

4. 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

5. 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

6. 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

7. Vasculature

8. 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

9. 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

10. 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

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

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

13. 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

14. 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

15. 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

16. 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)

17. 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

18. 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

19. 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

20. 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

21. Gastric Acid Secretion

22. 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 2nd messenger system which allows for stimulation of the parietal cell

23. 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

24. 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

25. Gastric Acid Secretion • Gastric Phase • Begins when food enters the stomach • The following are responsible for stimulation of acid secretion • 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 stops • Somatostatin release is increased • Entry of digestive products into the intestine begins the intestinal-phase inhibition of gastric acid secretion

26. Gastric Acid Secretion • Intestinal Phase • Also, releases HCl by way of Gastrin • Releases secretin to inhibit Gastrin which ultimately decreases Acid production

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

34. 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

35. 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

36. 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 Medical ppt http://hastaneciyiz.blogspot.com