Gastrointestinal Anatomy and Physiology

Anatomy of the Stomach

• The stomach is located in the epigastrium, left hypochondrium, and umbilical regions.
• Common shape: J-shaped
• Less common shape: Steer-horn
• The cardiac orifice is where the esophagus meets the stomach, typically in contact with the left lobe of the liver. It's more fixed.
• The pyloric orifice is at the junction with the duodenum, in contact with the quadrate lobe of the liver and is more mobile than the cardiac orifice. It has a true sphincter with a thickening of the circular muscle layer, an anterior groove, and pre-pyloric vein.
• The lesser curvature (right border) is attached to the lesser omentum, with right and left gastric vessels running along it, and has an angular notch.
• The greater curvature (left border) is attached to the gastro-splenic ligament, greater omentum, and right and left gastro-epiploic vessels.
• The fundus is the dome-shaped portion above the horizontal line joining the cardiac orifice to the greater curvature.
• The body is situated between the horizontal line that limits the fundus and the vertical line from the angular notch to the greater curvature.
• The pylorus is the part to the right of the previous vertical line, subdivided into the pyloric antrum (dilated part next to the body), pyloric canal (distal narrow part), and the pyloric sphincter (the end of the pylorus).
• The anterior relations of the stomach include the left lobe of the liver, diaphragm, left costal margin, and anterior abdominal wall.
• The postero-inferior surface (stomach bed) is related to the transverse colon, transverse mesocolon, body of the pancreas, splenic artery, left kidney, left suprarenal gland, left crus of diaphragm, and spleen.

Ligaments of the Stomach (Omenta)

• Greater omentum
• Lesser omentum
• Gastro-splenic ligament
• Gastro-phrenic ligament

Arterial Supply of the Stomach

• The blood supply is derived from the celiac trunk and its branches.
• Left gastric artery: Arises from the celiac trunk, runs along the lesser omentum and is the largest artery of the stomach.
• Right gastric artery: Arises from the hepatic artery, runs along the lesser curvature and anastomoses with the left gastric artery.
• Left gastro-epiploic artery: Arises from the splenic artery, runs along the greater curvature and anastomoses with the right gastric artery.
• Right gastro-epiploic artery: Arises from the gastro-duodenal artery (branch of hepatic artery), runs along the greater curvature, and anastomoses with the left gastro-epiploic artery.
• Short gastric arteries: Arise from the splenic artery and supply the fundus.

Venous Drainage of the Stomach

• Venous blood drains into the portal circulation.
• Lt. gastric vein
• Rt. gastric vein
• Lt gastroepiploic vein
• Short gastric veins
• Rt. gastro epiploic vein

Lymphatic Drainage of the Stomach

• Lymphatics pass to the following lymph nodes:
• Paracardiac LNs (around cardiac end)
• Lt. gastric LNs (along Lt. gastric vessels)
• Pyloric LNs (around the pyloric end)
• Pancreatico splenic LNs (accompany splenic a.)
• Rt. gastroepiploic LNs (along Rt. gastroepiploic vessels).

Nerve Supply (Autonomic Fibers) of the Stomach

• Sympathetic fibers arise from the celiac ganglion around coeliac trunk. Preganglionic fibers originate from thoracic ganglia 5, 6, 7, & 8.
• Parasympathetic fibers are derived from the anterior and posterior gastric nerves which are continuations of the left and right vagus nerves, respectively.

Gastrointestinal Nerve Reflexes

• Local reflexes are integrated within the enteric nervous system.
• Ganglionic reflexes originate in the gut and travel to paravertebral sympathetic ganglia, then return to the GI tract.
• The enterogastric reflex inhibits stomach motility and secretion, starting in the small intestine.
• The colo-ileal reflex inhibits emptying of ileal contents into the colon, starting in the colon.
• Central reflexes have pathways to the spinal cord or brain stem.
• Vagal reflexes travel from the stomach and duodenum to the brain stem, through vagus nerves, to control gastric motor and secretory activity.
• Defecation reflex travels from colon and rectum to the spinal cord and back again to produce colon, rectal, and abdominal contractions.

Electrical Activity of GIT Smooth Muscle

• Two basic types of electrical waves: - Slow waves: Undulating changes in resting membrane potential, frequency varies with digestive tract section (small intestine 10-20/min, stomach 3-8/min). Intrinsic to smooth muscle. - Spikes potentials: True action potentials, occuring when resting membrane potential becomes more positive than ~-40mV. Involve calcium and sodium ions.

Factors Affecting Gastric Acid Secretion

• Stimulatory factors: - Increased parasympathetic discharge (acetylcholine) - Histamine - Gastrin - Severe emotional stress - Hypoglycemia (through stimulating the hypothalamus and vagus nucleus)
• Inhibitory factors: - Marked drop in pH (high acidity) which causes direct parietal cell inhibition - Decreased gastrin hormone secretion - Entero-gastric reflex - Secretin, cholecystokinin (CCK) and somatostatin - Prostaglandins have anti-histaminic effects, inhibiting gastrin acid secretion.

Phases of Gastric Secretion

• Cephalic phase: Stimulation of acid secretion by seeing, smelling, tasting, or thinking of food. Unconditioned reflex (food in the mouth). Neurogenic signals from cerebral cortex, transmitted to stomach via vagus nerve. Accounts for ~20% of HCl secretion.

• Gastric phase: Food entering the stomach stimulates acid secretion through local enteric reflexes, long excitatory vagal reflexes, and the secretion of gastrin. Accounts for ~70% of HCl secretion.

• Intestinal phase: After stomach emptying, a small amount of HCl is still secreted due to gastrin, but this effect is inhibited by the enterogastric reflex (in response to the pH, presence of lipids, or the distension of the small intestine). Accounts for ~10% of acid secretion.

Peptic Ulcer Disease (PUD)

• An erosion of stomach and upper small intestine mucosa, often extending to deeper layers.
• Caused by an imbalance between gastric secretion rate and mucosal protection.
• Factors affecting PUD include bacterial infection with Helicobacter pylori, effects of NSAIDs (Nonsteroidal anti-inflammatory drugs), overproduction of HCl or pepsin, and lowered mucus or bicarbonate secretion.

Zollinger-Ellison Syndrome

• A rare condition caused by gastrinomas (tumors) in the duodenum or pancreas.
• Excessive gastrin secretion leads to increased HCl secretion by parietal cells.
• Insufficient neutralization of the excess acid can lead to gastric and duodenal ulcers.

Changes at Gastro-esophageal Junction

• Changes at the junction affect the mucosa (epithelium, corium, muscularis), submucosa, musculosa, and serosa/adventitia.

• The esophageal epithelium changes to the simple columnar secretory epithelium.

• The submucosa's esophageal glands gradually end at the stomach side.

• The muscularis thickens with an oblique layer (inner oblique, middle circular and outer longitudinal).

• The adventitia (esophageal) changes to serosa (stomach).

Histology of the Stomach

• Mucosa

• Epithelium: Simple columnar epithelium, secretes thick, insoluble, alkaline mucus for protection from auto-digestion
• Lamina Propria (Corium): Dense areolar connective tissue
• Glands: Different types of glands with different secretory products (mucous neck, chief, parietal, etc.). The glands have unique features in different parts.

• Submucosa

• Loose connective tissue containing blood vessels, lymphatics, and Meissner's plexus (submucosal plexus).

• Muscularis Externa

• Three layers of smooth muscle: inner oblique, middle circular, and outer longitudinal.
• Myenteric plexus (Auerbach)

• Serosa

• Loose connective tissue covered by mesothelium.

Test your knowledge on various aspects of gastrointestinal anatomy and physiology. This quiz covers topics such as gastric function, cell types in the gastric glands, and the role of different fibers and vessels in the stomach's anatomy. Perfect for students studying gastroenterology or related medical fields.