10A Posterior Abdominal Region
Lecture 10A: Posterior Abdominal Region
Learning Objectives
Describe components and attachments of abdominal viscera:
Intra-abdominal esophagus
Stomach
Small intestine
Large intestine
Appendix
Describe the courses of vessels, lymphatics, and nerves supplying:
Small intestine
Large intestine
Appendix
Rectum
Anus
Identify pertinent enteric histological components that contribute to the digestive process.
Discuss anatomic pathology and consequences of:
Gastric and duodenal ulcers
Pyloric stenosis
Atresia
Malrotation
Volvulus
Meckel’s diverticulum
Intussusception
Appendicitis
Diverticula
Polyps
Discuss structures, attachments, and functions:
Liver
Gallbladder
Pancreas
Spleen
Identify pancreatic enzymes and their contribution to digestive function.
Describe the courses of vessels, lymphatics, and nerves supplying:
Liver
Gallbladder
Pancreas
Spleen
Discuss anatomic pathology and consequences of:
Cirrhosis
Ascites
Caput medusae
Cholelithiasis
Intra-abdominal abscesses
Cholecystitis
Pancreatitis
Splenomegaly
Discuss normal anatomical path of food, bile, and pancreatic juice through digestive organs.
Identify which abdominal organs are intraperitoneal or retroperitoneal.
Fetal Intestinal Rotation
Midgut elongation and rotation:
Begins by the end of Week 5
The connection between the midgut and yolk sac narrows to a thin vitelline stalk.
This stalk serves as a lead point for the midgut's development.
Physiologic herniation:
The midgut grows ventrally into the base of the umbilicus.
Undergoes a 90° counterclockwise rotation around the axial vitelline artery, which becomes the superior mesenteric artery (SMA).
Formation of bowel loops:
The pre-arterial portion of the midgut lies to the right of the SMA axis.
The midgut rapidly elongates, forming multiple intestinal loops.
Bowel loops return abruptly to the abdominal cavity through the umbilical ring.
Return to the abdominal cavity (Week 10):
Undergoes an additional 180° counterclockwise rotation for a total of 270°.
The returning gut pushes the colon to the periphery, facilitating the fusion of visceral peritoneum with parietal peritoneum, forming secondarily retroperitoneal structures (e.g. parts of the colon).
Final Positioning:
After gut returns, the umbilical ring shrinks, completing abdominal wall closure.
Conditions Related to Abdominal Rotation
Mesenteric occlusion:
Can result in gangrene in less than 1 hour.
Malrotation:
A congenital condition resulting from improper intestine formation during pregnancy, causing abnormal positioning in the abdomen.
Can lead to bowel obstruction or twisting.
Volvulus:
A complication of malrotation, occurs when intestines twist, cutting off their blood supply.
Can occur independently of malrotation.
Meckel's Diverticulum
Vitelline duct (omphalomesenteric duct):
A remnant of the yolk sac.
If development becomes arrested, Meckel's Diverticulum is a common outcome.
_Develops into:Persistent vitelline duct (from ileum to umbilicus)
Vitelline duct cyst
Vitelline sinus
Meckel diverticulum
Vitelline band_
Rule of 2's:
2% of the population have a Meckel's diverticulum.
Half of the symptomatic cases present before the age of 2; others in the first 2 decades of life.
In adult patients, only about 2% of diverticula become symptomatic.
More common in males (2x) than females.
Usually located within 2 feet of the ileocecal valve.
Typically around 2 inches in length.
Half contain heterotopic mucosa (usually gastric, occasionally pancreatic).
Complications of Meckel's Diverticulum:
Ileal perforation
Bleeding
Intestinal obstruction
Perforation
Diverticulitis
Carcinoma of the diverticulum
Intussusception
Definition:
“Telescoping” of one segment of the bowel into another.
Triad of symptoms:
Vomiting
Abdominal pain
Passage of blood via rectum
Complications:
Obstruction
Ischemia/Necrosis
Sepsis
Internal bleeding
Lead points:
Lymphoid hyperplasia, Meckel’s diverticulum, lymphoma, leukemia, cystic fibrosis, postoperative complications, inflammatory bowel disease, polyps, or after recent rotavirus immunization.
Treatments for Intestinal Pathologies
Barium enema
Surgery
Large Intestine
Anatomy:
Extends from distal ileum to anus.
Larger diameter compared to the small bowel.
Function:
Absorbs water and electrolytes (salts).
Forms and stores feces.
Houses microbiota to aid in fermentation of undigested materials.
Regions and Functions of the Large Intestine
Cecum:
First part of the large intestine located in the right lower quadrant.
Receives chyme from the ileum via the ileocecal valve, which acts as a sphincter to slow chyme flow and prevent backflow.
Vermiform appendix: attached and contains lymphoid tissue.
Ascending Colon:
Runs upward along the right abdominal wall to the liver.
Absorbs most remaining water and key nutrients, solidifying undigested material into stool.
Transverse Colon:
Runs horizontally across the abdomen.
Descending Colon:
Runs down the left side of the abdominal wall, stores feces for eventual defecation.
Sigmoid Colon:
S-shaped portion leading to the rectum, contracts to increase pressure, moving stool to the rectum.
Rectum:
Straight muscular tube storing feces, with stretch receptors initiating the defecation reflex.
Anal Canal:
Terminal segment with internal (involuntary) and external (voluntary) anal sphincters.
Structures Overview
Ileocecal Valve
Cecum
Appendix
Ascending Colon
Hepatic/Left colic flexure
Transverse Colon
Splenic/Right Colic Flexure
Descending Colon
Sigmoid Colon
Rectum
Taenia coli
Appendix epiploica/omental appendices
Haustrum
Internal Anal Sphincter
Appendix
A narrow, hollow, blind-ended tube connected to the cecum.
Location where all three taenia coli meet.
Contains a large amount of lymphoid tissue, serves as a bacterial reservoir.
Histological Components of the Large Intestine
Taeniae coli:
Three longitudinal smooth muscle bands along the colon.
Haustra:
Sacculations formed by the contraction of taeniae coli, causing a tumbling movement of stool for water absorption.
Epiploic appendages:
Small fat-filled pouches along the colon, providing cushioning and reducing friction.
Mucosa:
Lacks villi (unlike the small intestine) but has numerous goblet cells for mucus secretion.
Blood Supply to the Large Intestine
Superior Mesenteric Artery (SMA):
Branches:
Inferior pancreaticoduodenal artery
Middle colic artery
Jejunal arteries
Ileal arteries
Right colic artery
Ileocolic artery
Anterior cecal artery
Posterior cecal artery
Appendicular artery
Areas supplied:
Lower duodenum
Head of pancreas
Transverse colon
Jejunum
Ileum
Ascending colon
Cecum, appendix, terminal ileum
Inferior Mesenteric Artery (IMA):
Branches:
Left colic artery
Sigmoid arteries (2-3)
Superior rectal artery
Areas supplied:
Descending colon
Sigmoid colon
Superior part of the rectum
Marginal artery
Formed by anastomoses of various branches and supplies the colon.
Rectum & Anal Arterial Supply
Inferior mesenteric artery:
Supplies superior rectal artery.
Internal iliac artery:
Supplies middle rectal artery.
Internal pudendal (internal iliac artery):
Supplies inferior rectal artery.
Venous Drainage of the Large Intestine
From the Superior Mesenteric Vein:
Drains regions like the cecum, appendix, ascending colon, and transverse colon.
Includes:
Middle colic vein
Right colic vein
Ileocolic vein
Appendicular vein
Marginal vein
From the Inferior Mesenteric Vein:
Drains regions such as the descending colon, sigmoid colon, and rectum.
Includes:
Left colic vein
Sigmoid veins
Superior rectal vein
Marginal vein
Lymphatic Drainage of the Large Intestine
Follows arterial blood supply and categorized as:
Epicolic (Appendicular)
Paracolic
Intermediate colic (Ileocolic)
Superior/Inferior Mesenteric
Pre-Aortic (Lateral Aortic/Celiac)
Innervation of Abdominal Viscera
Extrinsic Innervation
Receives motor impulses from the CNS (visceral efferent).
Sympathetic innervation derived from:
Lesser & least splanchnic nerves (to splenic flexure)
Lumbar & sacral splanchnic nerves (below)
Parasympathetic innervation arises from:
Vagus nerve
Pelvic splanchnic nerves (S2-4)
Sends sensory impulses to CNS (visceral afferent).
Intrinsic Innervation
Regulates digestive tract activities through the Enteric Nervous System.
Components:
Myenteric (Auerbach) Plexus: Controls peristalsis and muscle tone.
Submucosal (Meissner) Plexus: Controls glandular secretion, local blood flow, and mucosal activity.
Modulated by parasympathetic (vagus and pelvic splanchnic) and sympathetic inputs.
Pathologies Related to Intestinal Health
Appendicitis
Small lumen prone to obstruction.
Obstruction increases luminal pressure and vascular compromise, leading to exudate with bacterial transudation, gangrene, and possible perforation.
Diverticulum
An out-pouching of bowel can be true (containing all 4 layers, congenital) or false (without muscularis externa, acquired).
Diverticulitis
Complications may include abscess formation, bleeding from vessel erosion, perforation from obstruction, and bowel obstruction from inflammatory stricture.
Diverticulitis indicates the presence of inflammation.
Polyps and Cancer
Adenomatous polyps: Progressive stages from hyperplasia to carcinoma.
Hyperplasia: Increased cell size/layers, minimal risk.
Metaplasia: Replacement by new epithelial type.
Dysplasia: Pre-cancerous growth, could be low-grade/high-grade.
Carcinoma-in-situ: Confined to originating tissue.
Invasive cancer: Crosses basement membrane, spreads to different tissue.
Metastasis: Cancer cells implant elsewhere from the original tumor.
Diagnostic Techniques
Imaging modalities include MRI, CT, PET Scan, Colonoscopy, Colon Capsule Endoscopy, and Barium Enema.
Biliary System
Gallbladder
Functions:
Receives, concentrates, and stores bile.
Regions: Fundus, Body, Neck.
Arterial Supply: Cystic artery.
Bile aids in fat emulsification and neutralization of stomach acid, primarily functioning by:
Bile salts breaking down fats, increasing surface area for pancreatic lipase.
Formation of micelles for absorption of lipids and fat-soluble vitamins.
Excretion of bilirubin and heavy metals.
Neutralizing gastric acid upon entering duodenum.
Pathologies of Biliary System
Cholelithiasis (gallstones): Can be composed of cholesterol (most common) or calcium bilirubinate.
Predisposing factors: obesity, gender, and certain dietary practices.
Complications:
Choledocholithiasis (stones in the main bile duct), biliary colic, and cholecystitis (inflammation).
Symptoms: Abdominal pain, jaundice, and pale stools due to obstruction.
Conclusion
Abdominal and digestive health is crucial in understanding both anatomical and pathological processes affecting various organs.
Knowledge of anatomy, blood supply, innervation, and associated diseases is essential for diagnosis and treatment.