Imaging Anatomy: The Abdomen (Gastrointestinal Tract) - Study Notes

The Gastrointestinal System: Structure and Function

  • Structure

    • Composed of the gastrointestinal tract (alimentary canal) and accessory organs of digestion.

    • Main components:

    • Mouth

    • Oesophagus

    • Stomach

    • Small intestine

    • Large intestine

    • Rectum

    • Anus

    • Accessory organs: the liver, the gall bladder, and the exocrine pancreas.

  • Function

    • To provide the body with water, electrolytes and nutrients by breaking down ingested food, preparing it for uptake by the body’s cells, and eliminating waste.

    • Digestive movements are controlled by hormones and autonomic innervation (both sympathetic and parasympathetic).

    • Exceptions: chewing, swallowing and defecation involve specific reflexes and controls outside the normal peristaltic regulation.

Peritoneum and Mesenteries

  • Peritoneum

    • A large, complex serous membrane forming a closed sac within the abdominal cavity.

    • In the female, the sac is perforated by the lateral ends of the fallopian tubes.

    • The peritoneal space is a potential space between the parietal peritoneum (lining the abdominal wall) and the visceral peritoneum (enveloping abdominal organs).

    • Function: supports abdominal organs and acts as a conduit for blood vessels, lymph vessels and nerves.

  • Mesentery

    • A double layer of peritoneum that encloses the intestines and attaches them to the posterior abdominal wall.

    • Examples:

    • Small bowel mesentery (mesentery proper): suspends the jejunum and ileum.

    • Mesoappendix: peritoneum of the vermiform appendix.

    • Transverse mesocolon: peritoneum of the transverse colon.

    • Sigmoid mesocolon: peritoneum of the sigmoid colon.

  • Peritoneal classification of structures

    • Intraperitoneal: structures within the peritoneal space (e.g., stomach).

    • Retroperitoneal: structures located behind the intraperitoneal space (e.g., kidneys).

    • Subperitoneal / infraperitoneal: structures below the intraperitoneal space (e.g., bladder).

Imaging of the GIT

  • Imaging modalities

    • Predominantly fluoroscopy (real-time X-ray).

    • CT imaging also used with contrast.

  • Contrast media

    • Positive contrast: Gastrografin or Barium Sulphate (BaSO4).

    • Negative contrast: air.

  • Gastrografin (Diatrizoate Meglumine and Diatrizoate Sodium)

    • Aqueous iodinated solution; administered orally (meal/swallow) or by enema.

    • Mild laxative effect due to high osmolarity; sparingly absorbed by the GIT.

    • Risks:

    • Can cause severe reactive pulmonary oedema in adults and death in infants if aspirated due to high osmolarity.

  • Barium Sulphate (BaSO4)

    • Administered orally or by enema; not water-soluble.

    • Can cause chemical inflammation if leaked into the peritoneal cavity.

  • Undesirable properties of contrast media

    • Gastrografin: potential aspiration risk; high osmolarity.

    • BaSO4: non-solubility; potential peritoneal inflammation; unpalatable; tendency to sediment; can cause constipation.

Oral cavity (mouth)

  • The first section of the GIT; opens posteriorly into the pharynx (1) and continues into the oesophagus.

  • Functions: reservoir for chewing and mixing food with saliva; taste and odours initiate salivation and gastric juice secretion.

  • Saliva components: water, mucus, Na, bicarbonate, Cl, K; contains salivary amylase (initiates carbohydrate digestion).

  • Salivary glands (three pairs): Submandibular, Sublingual, Parotid.

Oesophagus

  • Length: approximately 25 cm.

  • Function: conducts food from the oropharynx to the stomach via peristalsis.

  • Peristalsis: coordinated sequential contraction/relaxation of outer longitudinal and inner circular muscle layers.

  • Swallowing has two phases:

    • Oropharyngeal (voluntary)

    • Oesophageal (involuntary)

  • Sphincters

    • Upper oesophageal sphincter (cricopharyngeal muscle) prevents air entry during respiration.

    • Lower oesophageal sphincter (cardiac sphincter) prevents reflux from the stomach.

Stomach

  • Function: stores food during eating, secretes digestive juices, mixes food with juices, and propels partially digested food (chyme) into the duodenum.

  • Boundaries and landmarks

    • Lower oesophageal sphincter (cardiac region).

    • Greater and lesser curvatures.

    • Pyloric sphincter (controls passage from pylorus to the duodenum).

  • Functional regions

    • Fundus (upper portion)

    • Body (middle portion)

    • Antrum (lower portion)

  • Muscle layers

    • Outer longitudinal layer

    • Middle circular layer

    • Inner oblique layer

    • Layers become thicker moving from the body toward the antrum.

  • Blood supply and drainage

    • Arterial supply from a branch of the coeliac artery.

    • Venous drainage: right/left gastric veins; splenic vein contributes to drainage patterns.

Small Intestine

  • Length and segmentation

    • Length: about 4-5 m long overall.

    • Segments: Duodenum, Jejunum, Ileum.

  • Duodenum

    • C-shaped; begins at the pylorus and joins the jejunum at the Treitz ligament.

    • Almost entirely retroperitoneal; attached to posterior abdominal wall.

    • Vascular supply: proximal to the duodenal papilla from the gastro-duodenal artery; distal portions from branches of the Superior Mesenteric Artery (SMA).

    • Venous drainage to the splenic vein or the superior mesenteric vein (SMV).

  • Jejunum

    • Located between the duodenum and ileum.

    • Begins at the Treitz ligament; division between jejunum and ileum is not anatomically distinct.

    • Lining specialized for absorption via villi; typically larger diameter than ileum.

    • Arterial supply from branches of the SMA.

  • Ileum

    • Follows the jejunum; ends at the ileocecal junction where the terminal ileum communicates with the cecum of the large intestine.

Large Intestine

  • Length and components

    • Length: around 1.5 m.

    • Segments: Cecum and vermiform appendix; Colon (ascending, transverse, descending, sigmoid); Rectum; Anal canal.

  • Primary function

    • Extracts water and any remaining absorbable nutrients from indigestible matter before elimination via the rectum.

  • Sphincters and valves

    • Ileocecal valve: admits chyme from ileum to the cecum.

    • O’Beirne sphincter: controls movement of wastes from the sigmoid colon to the rectum.

  • Muscular and appearance features

    • Teniae coli: longitudinal muscle layer contracted to create a gathered appearance.

    • Haustra: circular muscles form pouches between taenia coli.

  • Blood supply and drainage

    • Arterial supply: branches from the Superior Mesenteric Artery (SMA) and the Inferior Mesenteric Artery (IMA).

    • Venous drainage: Inferior Mesenteric Vein (IMV) and Superior Mesenteric Vein (SMV).

Rectum

  • The final section of the large intestine, terminating at the anus.

  • Begins at the recto-sigmoid junction and follows the shape of the sacrum; calibre similar to the sigmoid colon, dilated at the distal end to form the rectal ampulla.

  • Terminates at the anorectal canal.

  • Function: temporary storage site for faeces; defecation reflex stimulates movement into the sigmoid and rectum.

  • Defecation mechanics

    • Retrograde contraction can displace faeces out of the rectal vault until evacuation is convenient.

    • Delayed defecation can lead to constipation and hardened faeces.

The abdominal X-ray (overview of abdominal radiology)

  • Standard radiographic evaluation includes signs and shadow imaging of:

    • Psoas muscle shadow

    • Kidney shadow

    • Small bowel

    • Bladder shadow

    • Caecum

    • Ascending colon

    • Transverse colon

    • Descending colon

    • Sigmoid colon

Arterial Anatomy of the Abdomen

  • Celiac axis (1)

    • First major branch of the abdominal aorta at approximately the level of L1.

    • Supplies oxygenated blood to the liver, stomach, spleen, abdominal oesophagus, and the superior half of both the duodenum and pancreas.

  • Superior Mesenteric Artery (SMA) (2)

    • Supplies oxygenated blood to the intestine from the lower part of the duodenum to the colic flexures and the pancreas.

  • Inferior Mesenteric Artery (IMA)

    • The smallest of the three anterior branches of the abdominal aorta.

    • Supplies the large intestine from the splenic flexure to the upper part of the rectum.

Veins of the Abdomen

  • Superior Mesenteric Vein (SMV) (1)

    • Accompanies the SMA and drains the midgut into the portal venous system.

    • Should lie to the right of the SMA.

  • Inferior Mesenteric Vein (IMV) (2)

    • Drains blood from the large intestine and typically terminates at the splenic vein.

Vascular Relationships (selected examples from the vascular map)

  • Veins and arteries in the mesentery include:

    • Middle colic, Right colic, Ileocolic veins (draining into SMV)

    • Superior pancreaticoduodenal vein

    • Portal vein

    • Splenic vein, and various other intestinal venous tributaries feeding into the portal system.

    • Inferior vena cava (IVC) is located posteriorly to these venous structures.

Large vs Small Bowel: Key Differences

  • Small bowel

    • Length: about 4-5 m

    • Width: 3.5-4.5 cm

    • Segments: Duodenum, Jejunum, Ileum

    • Features: Circular folds (plicae circulares) present; Villi present; No teniae coli; No haustra; No epiploic appendages.

    • Digestion and absorption: Digestion is more complete in proximal segments; absorption via villi throughout.

  • Large bowel

    • Length: approximately 1-2 m

    • Width: 4-6 cm

    • Segments: Caecum, Colon (ascending, transverse, descending, sigmoid), Rectum, Anal canal

    • Features: Haustra present; Teniae coli present; Epiploic appendages present; No circular folds or villi; No role in digestion beyond absorption of water and electrolytes.

Rectum: Additional Details

  • Rectum begins at the recto-sigmoid junction and follows the sacrum; rectal ampulla is a dilated region near the end.

  • Termination at the anorectal canal; defecation reflex initiated by filling of the rectum.

Quick Summary of Key Anatomical Concepts

  • The GI tract is organized into layers and segments, with distinct functions in digestion, absorption, and waste elimination.

  • Peritoneal relationships (intraperitoneal, retroperitoneal, infraperitoneal) determine organ mobility and imaging appearance.

  • The enteric nervous system regulates local blood flow, motility, and secretions alongside hormonal controls.

  • Imaging uses fluoroscopy and CT with contrast; selection of contrast depends on the exam and risk considerations (e.g., Gastrografin vs BaSO4).

  • The major arterial supply is via the celiac axis, SMA, and IMA; major venous drainage occurs through the SMV and IMV into the portal system and IVC.

  • Distinctions between small and large bowel are based on length, width, wall features (villi, plicae, haustra, teniae coli), and digestive roles.

Important measurements and features:

  • Oesophagus length: 25 cm

  • Small intestine length: 4-5 m; diameter 3.5-4.5 cm

  • Large intestine length: 1-2 m; diameter 4-6 cm

  • Small bowel vs large bowel features summary: presence/absence of villi, plicae, haustra, teniae coli, and epiploic appendages as noted above.