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Stomach, Intestines and Colon - Review Answers

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STOMACH

1. Microscopic and Macroscopic Structure of the Stomach with Functional Adaptations

• Cardia - small area immediately adjacent to the esophagogastric junction
• Fundus - dome-shaped, projects superiorly above the cardia; contacts the left hemidiaphragm above and the spleen to the left
• Body (corpus) - the largest region; the main acid-secreting area
• Pyloric antrum - leads to the pyloric canal and pylorus, which controls gastric emptying into the duodenum

• The lesser curvature forms the right upper border (concave)
• The greater curvature forms the left lower border (convex)
• The stomach has anterior and posterior surfaces

• Peritoneum passes from the lesser curvature to the liver as the gastrohepatic ligament (lesser omentum)
• Hangs from the greater curvature as the greater omentum - extends to the transverse colon (gastrocolic ligament), spleen (gastrosplenic ligament), and diaphragm (gastrophrenic ligament)

• Outer longitudinal
• Middle circular (thickens at the pylorus to form the pyloric sphincter)
• Inner oblique (unique to the stomach - allows churning of food)

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2. Relations of the Stomach

• Right lobe of liver (covers the upper right portion)
• Anterior abdominal wall (left lower aspect when distended)
• Left costal margin
• Left lobe of liver

• Pancreas (body)
• Transverse colon (and transverse mesocolon)
• Left kidney and left adrenal gland (upper pole)
• Spleen
• Left hemidiaphragm
• Splenic artery
• The posterior wall of the stomach forms the anterior wall of the lesser sac (omental bursa)

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3. Blood Supply, Nerve Supply, and Lymphatic Drainage of the Stomach

• Left gastric artery (direct branch of coeliac axis) - anastomoses with
• Right gastric artery (from common hepatic artery)

• Right gastroepiploic artery (from gastroduodenal artery, itself from the hepatic artery)
• Left gastroepiploic artery (from the splenic artery) - these two form an arcade along the greater curvature (often variably incomplete)

• Vasa brevia (short gastric arteries) - arise from near the termination of the splenic artery

• Lesser curve veins (including the coronary vein) drain into the portal vein
• Greater curve veins drain into the splenic vein
• The coronary vein runs along the lesser curve toward the esophagus, then passes left-to-right to join the portal vein - becomes markedly dilated in portal hypertension, forming esophageal varices

• Parasympathetic (vagus nerve): Left vagus innervates the anterior stomach wall; right vagus innervates the posterior wall (due to 90° rotation of the stomach in development). Vagal stimulation promotes gastric acid secretion (via ECL cell histamine and direct parietal cell action) and gastric motility.
• Sympathetic: Via the coeliac plexus (T6-T10). Reduces motility and secretion; mediates pain from the stomach (referred to the epigastrium - T6-T9 dermatome).
• The Crow's foot nerve of Latarjet (terminal branch of vagus) supplies the antrum and pylorus.

• Left gastric (lesser curve) nodes drain to the coeliac nodes - the main lymphatic route
• Pyloric nodes (around pylorus and head of pancreas)
• Pancreaticosplenic nodes (upper greater curve)
• All ultimately drain into the coeliac lymph nodes, then to the cisterna chyli

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SMALL INTESTINES

1. Divisions and Relations of the Duodenum

• Extends from the pyloric orifice to the neck of the gallbladder
• Level: just to the right of vertebra L1
• Relations: anteriorly passes the bile duct, gastroduodenal artery, portal vein, and IVC
• The beginning (the "duodenal cap" or ampulla) is the commonest site of duodenal ulcers
• Only the beginning is covered by peritoneum (intraperitoneal)

• Extends from the neck of the gallbladder to the lower border of L3
• Just to the right of midline
• Anterior: crossed by the transverse colon
• Posterior: right kidney
• Medial: head of pancreas
• Contains the major duodenal papilla (common opening for bile duct + pancreatic duct - ampulla of Vater) and the minor duodenal papilla (accessory pancreatic duct)
• The foregut-midgut junction occurs just below the major duodenal papilla

• The longest section
• Crosses the inferior vena cava, aorta, and vertebral column
• Crossed anteriorly by the superior mesenteric artery (SMA) and vein - compression here causes "SMA syndrome"

• Ascends on or to the left of the aorta to approximately the upper border of L2
• Terminates at the duodenojejunal flexure (DJ flexure)
• The DJ flexure is supported by the suspensory muscle (ligament) of Treitz - a fold of peritoneum containing smooth muscle fibers that attaches to the right crus of diaphragm

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2. Blood Supply and Lymphatic Drainage of the Small Intestine

• Gastroduodenal artery (from common hepatic): gives the supraduodenal artery, anterior superior pancreaticoduodenal artery, and posterior superior pancreaticoduodenal artery
• Inferior pancreaticoduodenal artery (from SMA): anterior and posterior inferior pancreaticoduodenal arteries
• These form an anastomotic arcade around the head of the pancreas (clinically important - ensures duodenum maintains blood supply from both coeliac and SMA territories)

• All from the superior mesenteric artery (SMA)
• Jejunal arteries (proximal 2/5 of small bowel)
• Ileal arteries (distal 3/5 of small bowel)
• These form arterial arcades in the mesentery, then give off vasa recta (straight arteries) to the intestinal wall
• The jejunum has fewer arcades but longer vasa recta; the ileum has more multiple arcades but shorter vasa recta (important in bowel ischaemia assessment)

• All drain into the superior mesenteric vein (SMV), which joins the splenic vein to form the portal vein

• Lymph from the gut wall passes through the mesenteric lymph nodes (three tiers: intermediate, superior)
• Ultimately drains to the superior mesenteric lymph nodes and then to the coeliac lymph nodes
• The lacteals in the villi absorb fat (as chylomicrons), draining into the cisterna chyli and thoracic duct

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COLON

1. Arrangement of Muscle Layers in the Large Intestine

• Taenia libera (free band) - anteriorly placed
• Taenia mesocolica - at the mesenteric attachment
• Taenia omentalis - on the posterior surface where the greater omentum attaches

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2. Different Positions of the Appendix

1. Retrocaecal (most common - ~65%): Lies behind the cecum; may ascend to the right colic gutter. Pain and tenderness may be minimal or absent anteriorly (the "silent appendix") because the caecum (distended with gas) buffers palpation. Deep tenderness felt in the loin; psoas muscle contact causes hip flexion.
2. Pelvic/Descending (~31%): Hangs down into the pelvis over the pelvic brim. No anterior abdominal rigidity; tenderness above and right of symphysis pubis. Rectal exam shows tenderness in the rectovesical pouch or pouch of Douglas. May cause urinary frequency if in contact with bladder.
3. Postileal: Lies behind the terminal ileum. Most difficult to diagnose - pain may not shift, diarrhoea is common, marked retching occurs. Tenderness is ill-defined, may be to the right of the umbilicus.
4. Preileal: Lies in front of the terminal ileum
5. Subcaecal/Paracolic: Below the cecum or in the right paracolic gutter
6. Left-sided: Rare; seen in situs inversus or intestinal malrotation

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3. Blood Supply and Lymphatic Drainage of the Large Intestine

• Ileocolic artery - supplies cecum, appendix (via appendicular artery), and terminal ileum
• Right colic artery - supplies ascending colon
• Middle colic artery - supplies transverse colon

• Left colic artery - supplies descending colon and splenic flexure
• Sigmoid arteries - 2-4 branches to sigmoid colon
• Superior rectal artery - terminal branch, supplies upper rectum

• The marginal artery of Drummond runs along the mesenteric border of the entire colon, connecting SMA and IMA territories - provides collateral circulation
• The arc of Riolan (central anastomotic artery) - a more direct connection between middle colic and left colic arteries through the mesentery
• The splenic flexure ("watershed area") is the most vulnerable zone to ischaemia if either SMA or IMA is compromised

• Right colon drains via superior mesenteric vein to portal vein
• Left colon and sigmoid drain via inferior mesenteric vein into the splenic vein (to portal)
• Upper rectum: superior rectal vein to IMV to portal (portal system)
• Lower rectum and anal canal: middle and inferior rectal veins to internal iliac to IVC (systemic)
• This is a porto-systemic anastomosis at the rectum (relevant in portal hypertension causing haemorrhoids)

• 4 tiers of nodes following arterial supply:
  1. Epicolic nodes - on the bowel wall
  2. Paracolic nodes - along the marginal artery
  3. Intermediate nodes - along named colic arteries
  4. Principal nodes - at the origin of SMA (right colon) or IMA (left colon)
• From right colon: to SMA nodes then coeliac nodes
• From left colon: to IMA nodes then para-aortic nodes then cisterna chyli

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4. Features Differentiating the Colon from the Small Intestine

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5. Anatomical and Functional Differences: Right vs Left Colon

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EMBRYOLOGY

1. Development of the Gastrointestinal Tract

• Includes: pharynx, esophagus, stomach, duodenum (proximal to major duodenal papilla), liver, biliary system, pancreas, spleen
• The stomach develops as a dilation of the foregut in week 4-5
• It undergoes a 90° clockwise rotation (viewed from below) around its longitudinal axis:
  • The left side becomes the anterior surface; right becomes posterior
  • This explains why the left vagus innervates the anterior stomach and right vagus innervates the posterior
  • The dorsal mesentery (dorsal mesogastrium) swings to the left, forming the lesser sac (omental bursa)
  • The ventral mesogastrium becomes the lesser omentum and falciform ligament

• Includes: duodenum (distal to papilla), jejunum, ileum, cecum, appendix, ascending colon, proximal 2/3 of transverse colon
• The midgut loop herniates into the extraembryonic coelom (umbilical cord) at week 6 (physiological umbilical herniation) because the liver/kidneys are too large
• The loop rotates 270° counterclockwise around the SMA as its axis during weeks 6-12
• At week 10, the gut returns to the abdominal cavity, completing the rotation
• The cecum ends up in the right iliac fossa; the ascending colon becomes retroperitoneal

• Includes: distal 1/3 transverse colon, descending colon, sigmoid, rectum, upper anal canal
• The hindgut terminates in the cloaca, which is divided by the urorectal septum into the:
  • Anterior urogenital sinus
  • Posterior anorectal canal

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2. Anatomical/Embryological Basis for Congenital Anomalies

• Duodenal atresia: Failure of the solid cord stage to recanalize. During weeks 5-6, the duodenal lumen is completely obliterated by rapid epithelial proliferation. Normally recanalization occurs by week 8-9. Failure produces a complete obstruction. Associated with Down syndrome (trisomy 21) in ~30% of cases. Produces the classic "double bubble" sign on X-ray (air in stomach and duodenum with none distally).
• Jejunoileal atresia: Caused by intrauterine vascular accidents (mesenteric ischaemia) - a late event, explaining why no chromosomal association exists. An apple-peel deformity (type IIIb) involves loss of a large segment.
• Stenosis: Same mechanisms but incomplete, producing partial obstruction.

• During weeks 6-10, the midgut normally herniates into the extraembryonic coelom (physiological umbilical herniation) then returns to the abdominal cavity at week 10 as the abdomen enlarges
• Failure of the gut to return to the abdomen produces omphalocele
• More precisely: failure of the lateral body folds to close (lateral fold defect) allows large quantities of bowel, and sometimes liver, to remain outside
• The sac is intact (distinguishing it from gastroschisis where there is no sac and bowel lies free)
• Associated with chromosomal abnormalities (trisomy 13, 18, 21), Beckwith-Wiedemann syndrome, and cardiac defects

• During development, the midgut is connected to the yolk sac via the vitello-intestinal (omphalomesenteric) duct
• Normally this duct obliterates completely by week 7
• Failure of obliteration results in Meckel's diverticulum - a persistence of the proximal portion of the vitello-intestinal duct
• It is a true diverticulum (contains all layers of the intestinal wall)
• Located on the antimesenteric border of the ileum, approximately 60 cm (2 feet) from the ileocaecal valve
• Rule of 2s: 2% of population; 2 feet from ileocaecal valve; 2 cm long; 2:1 male:female; presents in first 2 years of life; contains 2 types of ectopic mucosa (gastric most common, also pancreatic)
• Ectopic gastric mucosa secretes acid - can cause peptic ulceration and painless rectal bleeding
• May act as a lead point for intussusception or undergo diverticulitis (mimics appendicitis)

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CLINICAL ANATOMY

1. Anatomical Basis for Clinical Scenarios

• A large gallstone (usually >2.5 cm) in the gallbladder causes pressure necrosis of the wall
• This creates a cholecystoduodenal fistula (between the gallbladder and the duodenum - most common) or occasionally a cholecystocolic or cholecystogastric fistula
• The gallstone passes through the fistula into the bowel
• It migrates distally; smaller stones pass spontaneously but a large stone impacts at the narrowest part of the small bowel - the terminal ileum (just proximal to the ileocaecal valve), causing obstruction
• On X-ray: Rigler's triad - (i) small bowel obstruction, (ii) air in the biliary tree (pneumobilia), (iii) ectopic radio-opaque gallstone
• The duodenum is the most common fistula site because of its close anatomical proximity to the gallbladder fundus and the right lobe of the liver

• The posterior wall of the first part of the duodenum is in contact with the pancreas and with the posterior abdominal wall structures
• A posterior penetrating ulcer erodes through the duodenal wall into the pancreatic head
• Pain is referred to the back (T8-L1 dermatomal distribution) because the pancreas and retroperitoneal structures share the same sympathetic innervation
• A subphrenic abscess from perforation can also cause ipsilateral shoulder tip pain (phrenic nerve, C3-C5)

• The gastroduodenal artery (GDA) passes posterior to the first part of the duodenum
• The GDA is a large branch of the common hepatic artery and carries high-pressure flow
• A posterior duodenal ulcer can erode directly into the gastroduodenal artery, causing massive arterial haemorrhage (haematemesis, melaena, haemodynamic shock)
• This is why posterior wall duodenal ulcers are far more dangerous than anterior wall ulcers (which, if they perforate, cause peritonitis but not typically direct arterial bleeding)

1. Luminal obstruction (faecolith, lymphoid hyperplasia) raises intraluminal pressure
2. Raised intraluminal pressure compresses the appendicular artery - a single end-artery with no collaterals
3. Venous obstruction occurs first (veins being lower-pressure), leading to mucosal and mural ischaemia
4. Progressive bacterial invasion of the ischaemic wall
5. Arterial thrombosis follows, completely cutting off blood supply
6. Transmural gangrene results - the entire wall becomes necrotic
7. Perforation follows, leading to peritonitis or abscess

• The lack of collateral circulation means that even partial obstruction of the appendicular artery produces rapid ischaemia - this is unique to the appendix compared to other parts of the colon which have the marginal artery as collateral supply

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2. Sites of Referred Pain from GI Tract, Anatomical Basis, and Hollow vs Solid Organ Pain

• Visceral afferent fibers travel with sympathetic nerves back to the spinal cord
• They converge on the same dorsal horn neurons as somatic afferents from the body wall (dermatome)
• The brain misinterprets the visceral pain as arising from the body surface at that dermatomal level
• This is convergence theory (Ruch) of referred pain

• Early: Visceral pain via T10 afferents (SMA territory/midgut) - felt as periumbilical cramping
• Late: Once peritoneum is inflamed, somatic pain fibers are recruited - pain localises to the right iliac fossa (McBurney's point) with tenderness, guarding, and rebound

• Contain a lumen; undergo muscular contraction
• Pain arises from distension, spasm, or obstruction of the wall
• Character: Colicky (cramping, comes in waves corresponding to peristaltic waves), poorly localised
• Example: Intestinal obstruction - intermittent colicky central pain; biliary colic - right hypochondrial waves
• No peritonism unless perforation or peritonitis occurs
• Associated with: nausea, vomiting, pallor, sweating (visceral response)

• No lumen; enclosed in a capsule
• Pain arises from distension of the capsule (rapid swelling), infarction, or rupture
• Character: Constant, dull, deep ache - more persistent, less colicky; poorly localised initially
• Examples: Hepatic congestion from heart failure (right hypochondrial ache); splenic infarct; renal mass
• Liver parenchyma has no pain receptors; pain from liver pathology is from capsule distension (Glisson's capsule) or diaphragmatic irritation
• Pancreatic pain: constant, boring epigastric pain radiating to the back (retroperitoneal organ - no capsule)
• If a solid organ ruptures (e.g., splenic rupture), blood irritates the peritoneum - converting to somatic (sharp, localised) pain

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