Stomach and Small Intestine

OBJECTIVES

  • Anatomy of the stomach and small intestine

  • Development of the stomach and midgut

  • Anatomy of the celiac artery (trunk) and SMA

  • Innervation of the stomach and small intestine

  • Clinical cases:

    • Peptic ulcer disease (gastric ulcer, duodenal ulcer)

    • Pyrosis (heartburn)

    • Congenital diaphragmatic hernia

    • Sliding hiatal hernia

    • Congenital hypertrophic pyloric stenosis

    • Gastric cancer

    • Meckel's diverticulum

REVIEW OF GIT

  • The alimentary canal has a total length of approximately 9 meters and consists of:

    • Oral cavity

    • Pharynx

    • Esophagus

    • Stomach

    • Small intestine (divided into duodenum, jejunum, and ileum)

    • Large intestine (comprising cecum, colon, rectum, and anal canal)

    • The large intestine is further subdivided into:

    • Ascending colon

    • Transverse colon

    • Descending colon

    • Sigmoid colon

  • Digestive glands include:

    • Salivary glands: are associated with ducts that open into the mouth

    • Liver and pancreas: discharge their exocrine secretions through ducts that open into the duodenum

DEVELOPMENT OF THE GUT

  • Primitive Gut Tube Development: Both the Foregut, Midgut, and Hindgut have distinct anatomical structures and arterial supplies.

FOREGUT

  • Structures:

    • Esophagus

    • Stomach

    • Duodenum (1st and upper half of 2nd parts)

    • Liver

    • Pancreas

    • Biliary apparatus

    • Gallbladder

  • Artery: Celiac Artery (CA)

MIDGUT

  • Structures:

    • Duodenum (Lower half of 2nd part, 3rd & 4th parts)

    • Jejunum

    • Ileum

    • Cecum (with Appendix)

    • Ascending Colon

    • Transverse Colon (proximal 2/3)

  • Artery: Superior Mesenteric Artery (SMA)

HINDGUT

  • Structures:

    • Transverse colon (distal 1/3)

    • Descending colon

    • Sigmoid colon

    • Rectum

    • Anal canal (above the pectinate line)

  • Artery: Inferior Mesenteric Artery (IMA)

  • Innervation:

    • Parasympathetic Innervation - Vagus Nerve (CN X):

    • Preganglionic: Dorsal Motor Nucleus (DMN) of Vagus Nerve

    • Postganglionic: Terminal Ganglia

    • Sympathetic Innervation:

    • Preganglionic: Intermediate Lateral Cell Column (IML) T5-T9 for the foregut; T10-T11 for midgut; L1-L2 for hindgut

    • Postganglionic: Celiac Ganglion for foregut; Superior Mesenteric Ganglion for midgut; Inferior Mesenteric Ganglion for hindgut

    • Sensory Innervation: Dorsal Root Ganglion (DRG) levels vary by gut region:

    • T5-T9 for Epigastrium (foregut)

    • T10-T11 for Umbilical region (midgut)

    • T12, L1-L2 for Hypogastrium (hindgut)

STOMACH

  • General Structure:

    • The stomach is the dilated section of the alimentary canal between the esophagus and the small intestine; primarily located in the left upper quadrant and partially covered by the ribs.

    • Its long axis extends downward and forward to the right, then backward and slightly upward.

Development of the Stomach

  • Develops as a fusiform dilation of the foregut by the 4th week of gestation.

  • It undergoes a 90° clockwise rotation around its longitudinal axis, positioning the left side anteriorly and the right side posteriorly.

    • The left vagus nerve innervates the anterior wall, and the right vagus nerve innervates the posterior wall.

  • Additional rotation around the anteroposterior axis leads to leftward positioning of the esophageal end and rightward positioning of the pyloric end, resulting in the development of the greater and lesser curvatures.

Main Functions of the Stomach

  • Storage of Food: Approximately 1500 mL capacity in adults.

  • Mixing of Food: Forms semifluid chyme through gastric secretions.

  • Delivery Control: Regulates the rate of chyme delivery to the small intestine to facilitate digestion and absorption.

Shape and Mobility of the Stomach

  • Fixed at both ends but highly mobile in between segments.

  • The shape varies greatly depending on the individual's body type, volume of contents, body position, and respiratory phase.

Main Parts of the Stomach

  • Cardia: Surrounds the esophagus entry point.

  • Fundus: Dome-shaped portion typically containing gas.

  • Body (Corpus): Largest section where mixing begins.

  • Antrum: Holds food before it passes into the small intestine.

  • Pylorus: Funnel-shaped section connecting to the duodenum, contains the pyloric sphincter regulating chyme exit.

  • Sulcus intermedius: Notch dividing the body from the pylorus.

  • Angular incisure: Notch in the lesser sac dividing the pylorus into antrum and canal regions.

Structure of the Stomach Wall

  • The mucous membrane is thick, vascular, and forms folds known as rugae that flatten when distended.

  • The gastric canal, a groove along the lesser curvature, allows passage from cardia to pylorus.

  • The abdominal wall contains a muscular layer with:

    • Inner oblique fibers

    • Middle circular fibers

    • Outer longitudinal fibers

  • Lower Esophageal Sphincter: At the esophagus-stomach junction, allowing relaxation during swallowing (dysfunction may lead to GERD).

  • Pyloric Sphincter: Controls chyme passage to the duodenum, a ring-like muscular structure.

Anatomical Relations

  • Anteriorly: Anterior abdominal wall, lower ribs, left diaphragm, pleura, and liver.

  • Posteriorly: Lies against the lesser sac (omental bursa), diaphragm, left suprarenal gland, left kidney, splenic artery, pancreas, spleen, and transverse colon.

Surface Anatomy of the Stomach

  • Cardiac orifice located at the T11 vertebral level.

  • Pyloric orifice at the L1 vertebral level.

  • The Prepyloric vein of Mayo is a surgical landmark marking the pyloroduodenal junction, draining into the right gastric vein.

STOMACH BLOOD SUPPLY

Arterial Supply

  • Left Gastric Artery: From celiac artery, supplies the lower esophagus and upper part of the lesser curvature.

  • Right Gastric Artery: From hepatic artery proper, supplies the lower right part of the lesser curvature.

  • Short Gastric Arteries: From splenic artery, supply the fundus.

  • Left Gastroepiploic Artery: From splenic artery, supplies upper part of the greater curvature.

  • Right Gastroepiploic Artery: From gastroduodenal branch, supplies lower part of the greater curvature.

Venous Drainage

  • Left and right gastric veins drain into the portal vein.

  • Short gastric veins and left gastroepiploic veins join the splenic vein.

  • Right gastroepiploic vein drains into the superior mesenteric vein.

Lymphatic Drainage

  • Gastric lymph nodes drain into celiac nodes, ultimately reaching the thoracic duct and left venous angle.

  • Virchow Metastasis: Cancer spread into left supraclavicular lymph nodes (Troisier's sign); a sign of GI cancer, referring to a hard, palpable left supraclavicular lymph node.

NERVE SUPPLY

Parasympathetic Innervation

  • Supplied by the Vagus Nerve (CN X), contributing to stomach musculature activity and inhibitory actions on pyloric sphincter and gastric glands.

    • Preganglionic neurons: Located in the dorsal motor nucleus of vagus nerve in the brainstem.

    • Postganglionic: Found in terminal ganglia within the stomach wall.

Sympathetic Innervation

  • Preganglionic fibers come from T5-T9 (IML) and influence the celiac plexus (where postganglionic neurons reside).

    • Influences stomach musculature, gastric acid secretion, and pyloric sphincter action.

Sensory Innervation

  • Sensory fibers from DRG T5-T9 carry pain sensations from the stomach to the central nervous system.

CLINICAL CONDITIONS

Peptic Ulcer Disease

  • Gastric ulcer refers to peptic ulcers forming in the stomach; duodenal ulcers in the duodenum. Most occur due to infection by Helicobacter Pylori, which damages the gastric mucosa.

Gastroscopy

  • Procedure where a gastroscope is introduced via nasal or oral route to examine the gastrointestinal tract.

Gastric Ulcer and Vagotomy

  • Vagotomy: The operation to reduce acid secretion by removing parts of the vagus nerve associated with the gastric secretory region.

Other Conditions

  • Pyrosis: Also known as heartburn or acid reflux, often associated with conditions like GERD or hiatal hernia.

  • Congenital Diaphragmatic Hernia: High mortality in infants due to lung hypoplasia

  • Sliding Hiatal Hernia: Occurs with age, affecting vagal trunks.

  • Congenital Hypertrophic Pyloric Stenosis: Characterized by thickening of pyloric sphincter leading to obstructive symptoms.

  • Gastric Cancer: Primarily adenocarcinoma, treated with total gastrectomy due to lymphatic spread possibility.

SMALL INTESTINE

  • Overview: The longest part of the alimentary canal, extending from the pylorus to the ileocecal junction, mainly involved in digestion and absorption, divided into duodenum, jejunum, and ileum.

DUODENUM

  • C-shaped structure consisting of four parts, primarily retroperitoneal except for the first half of the first part.

Mucous Membrane and Papillae

  • Major duodenal papilla formed by the convergence of the bile duct and main pancreatic duct with the sphincter of Oddi controlling flow into the duodenum.

Anatomy of the Duodenum

1st or Superior Part

  • Anterior relations include the liver and gallbladder; posterior relations involve lesser sac and greater vessels.

2nd or Descending Part

  • Relations include right kidney, liver, and coils of the small intestine.

3rd or Horizontal Part

  • Functionally connects intestinal messentery to surrounding major blood vessels.

4th or Ascending Part

  • Fixed by the suspensory ligament of Treitz, terminating at the duodenojejunal flexure.

ARTERIAL SUPPLY

  • Superior half: Supplied by the superior pancreaticoduodenal artery (foregut); lower half: supplied by the inferior pancreaticoduodenal artery (midgut).

VEINS

  • Superior and inferior pancreaticoduodenal veins correspondingly drain into the superior mesenteric vein.

LYMPHATIC DRAINAGE

  • Ascending via pancreaticoduodenal nodes to the gastroduodenal nodes, and later to celiac nodes.

MECKEL'S DIVERTICULUM

  • A congenital anomaly of a persistent vitellointestinal duct that can become inflamed. Typically asymptomatic, potential for complications includes diverticulitis, bleeding, and perforation. It occurs about 2 feet (61 cm) from the ileocecal junction and usually measures about 2 inches (5 cm) in length.

REFERENCES

  • Gray's Anatomy for Students by Richard L. Drake, Wayne Vogl, Adam W. M. Mitchell; 2nd edition.

  • Clinically Oriented Anatomy by Keith L. Moore, Arthur F. Dalley, A. M. R. Agur; 7th edition.

  • Langman's Medical Embryology by Thomas W Sadler; 12th edition.

  • Netter's Clinical Anatomy by John T. Hansen, David R. Lambert, Frank H. Netter.