Digestive System II: Anatomy & Physiology of the Abdominal Cavity and Associated Organs

Abdominal Cavity

Boundaries and Contents

The abdominal cavity houses the major organs of digestion and is lined by the peritoneal cavity. It is suspended and supported by two bony rings: the thoracic skeleton and the pelvic girdle.

• Boundaries: Thoracic diaphragm (superior), pelvic inlet (inferior, formed by pelvic diaphragm/levator ani).

• Contents: Abdominal organs and peritoneal cavity.

Digestive System Overview

Alimentary Canal and Accessory Organs

The digestive system consists of the alimentary canal and accessory digestive organs, each with specialized functions in digestion and absorption.

• Alimentary canal: Mouth, pharynx, esophagus, stomach, small intestine, large intestine.

• Accessory organs: Teeth, tongue, gallbladder, salivary glands, liver, pancreas.

• Supporting structures: Peritoneum, mesenteries, organization of abdominal cavity.

Large Intestine

Structure and Function

The large intestine is responsible for absorbing water and electrolytes from digested residue, which contains few nutrients. It also propels feces toward the rectum through mass peristaltic movements.

• Main functions: Absorption of water/electrolytes, propulsion of feces.

• Components: Cecum, appendix, colon, rectum, anal canal.

• Digestion: Small amount by bacteria.

Ileocecal Junction

The ileocecal junction marks the transition from the small intestine (ileum) to the large intestine (cecum).

• Ileocecal valve: Controls flow between ileum and cecum.

• Cecum: Blind pouch at the beginning of the large intestine.

Appendix

The vermiform appendix is a narrow, tube-like structure attached to the cecum. It contains lymphoid tissue and serves as a safe haven for beneficial gut bacteria.

• Location: Various positions relative to the cecum.

• Function: Immune role, bacterial reservoir.

Appendicitis

Appendicitis is the inflammation of the appendix, often due to blockage and infection.

• Symptoms: Pain in lower right abdomen, periumbilical region.

• Pathology: Blocked secretions, swelling, possible rupture.

• Treatment: Appendectomy (surgical removal).

Colon

The colon is divided into several segments, each with distinct anatomical features.

• Segments: Cecum, ascending colon (right side), hepatic flexure, transverse colon, splenic flexure, descending colon (left side), sigmoid colon.

Special Features of the Large Intestine

• Diameter: Greater than small intestine.

• Taenia coli: Three strips of thickened longitudinal muscle.

• Haustra: Sacculations or pouches formed by taenia coli.

• Epiploic appendages: Fat-filled pouches hanging from the colon.

Rectum

The rectum is the fixed, terminal part of the large intestine, specialized for strong contractions during defecation.

• Features: Retroperitoneal/subperitoneal, no taenia coli or epiploic appendages.

• Folds: Superior, middle, inferior transverse rectal folds; lateral flexures.

Anal Canal

The anal canal is the final segment of the digestive tract, transitioning from involuntary to voluntary control.

• Epithelium: Simple columnar (upper), stratified squamous (lower), skin (distal).

• Anal sinus: Releases mucus when compressed by feces.

• Pectinate line: Divides upper involuntary and lower voluntary regions.

• Sphincters: Internal (smooth muscle, involuntary), external (skeletal muscle, voluntary).

Defecation Reflex

The defecation reflex coordinates the process of expelling feces from the body.

• Visceral sensory fibers: Transmit signals to spinal cord.

• Visceral motor fibers: From sacral spinal cord, stimulate contraction of sigmoid colon and rectum.

• Cerebral cortex: Inhibits voluntary motor neurons, allowing relaxation of external anal sphincter.

• Outcome: Defecation can be consciously controlled or becomes unavoidable.

Microscopic Anatomy of the Large Intestine

The large intestine has specialized histological features for its absorptive and protective functions.

• Mucosa: Simple columnar epithelium (becomes stratified squamous at anal canal), no villi, many goblet cells in intestinal crypts, absorptive colonocytes.

• Muscularis externa: Taenia coli.

• Serosa: Epiploic appendages.

Liver

Gross Anatomy and Functions

The liver is the largest gland in the body, performing over 500 functions, including metabolism, detoxification, and bile production.

• Weight: ~1500g (2.5% of adult body weight).

• Digestive function: Bile production.

• Regenerative capacity: High.

Lobes and Ligaments

• Lobes: Right, left, caudate, quadrate.

• Ligaments: Falci form (anterior wall), round ligament (umbilicus), coronary ligament (diaphragm).

Fissures and Porta Hepatis

• Fissures: H-shaped, right sagittal (groove for IVC, fossa for gall bladder), left sagittal (round ligament anteriorly, ligamentum venosum posteriorly).

• Porta hepatis: Entry/exit for hepatic artery proper, hepatic portal vein, bile duct.

Microscopic Anatomy

• Liver lobule: Functional unit.

• Central vein: Drains blood from lobule.

• Hepatocytes: Functional cells; manufacture blood proteins, produce bile salts, detoxify poisons, store sugars/vitamins, process protein/fat, regenerate.

• Portal triad: Bile duct, portal venule, portal arteriole.

• Stellate macrophages: Destroy bacteria in sinusoids.

Cirrhosis of the Liver

Cirrhosis is characterized by cellular damage and scarring, most commonly due to chronic alcoholism or hepatitis B/C.

• Primary site for detoxification.

• Alcoholic cirrhosis: Most frequent cause.

Gall Bladder

Structure and Function

The gall bladder stores and concentrates bile produced by the liver, releasing it into the duodenum to aid fat digestion.

• Location: Visceral surface of liver (gallbladder fossa).

• Parts: Fundus, body, neck.

• Cystic artery: Usually a branch of right hepatic artery.

• Function: Stores/concentrates bile, expels bile into bile duct.

• Note: Gall bladder does not produce bile.

Bile Duct

Bile produced by hepatocytes travels through bile canaliculi to small bile ducts, then to the bile duct, which unites with the main pancreatic duct at the ampulla of Vater before entering the duodenum.

• Major duodenal papilla: Entry point into duodenum.

Gallstones

Gallstones are solid deposits that can block the bile duct, causing pain and inflammation.

• Biliary colic: Intense, spasmodic pain.

• Cholecystitis: Inflammation of the gallbladder.

• Cholecystectomy: Surgical removal of gallbladder.

Pancreas

Structure and Functions

The pancreas has both exocrine and endocrine functions, contributing to digestion and blood sugar regulation.

• Parts: Head, neck, body, tail.

• Exocrine: Produces digestive enzymes for small intestine.

• Endocrine: Produces insulin and glucagon to regulate blood sugar.

Pancreatic Duct

• Main pancreatic duct: Joins bile duct to form hepatopancreatic ampulla (ampulla of Vater), empties into duodenum.

• Accessory pancreatic duct: May enter duodenum alone.

Microscopic Anatomy

• Acinar cells: Make, store, and secrete pancreatic enzymes (22+ kinds).

• Enzyme activation: Occurs in the duodenum.

Abdominal Organs and Embryological Gut

Abdominal Organs

• Includes: Gut organs, spleen, kidneys.

Embryological Gut

The gut is divided into foregut, midgut, and hindgut, each giving rise to specific derivatives.

Region	Derivatives
Foregut	Esophagus, stomach, pancreas, duodenum, liver, bile ducts
Midgut	Small intestine distal to bile duct, cecum, appendix, ascending colon, most of transverse colon
Hindgut	Distal transverse colon, descending colon, sigmoid colon, rectum

Peritoneum and Mesenteries

Peritoneum

The peritoneum is a serous membrane lining the abdominal cavity and covering abdominal organs.

• Layers: Parietal (lining body wall), visceral (covering organs).

• Epithelium: Simple squamous.

Mesentery

Mesenteries are double layers of peritoneum that suspend and support abdominal organs.

• Functions: Hold organs in place, store fat, provide routes for vessels/nerves.

• Types: Dorsal and ventral mesentery.

• Loss: If organ is pushed against posterior body wall during development.

Peritoneal Cavity

• Lined by peritoneum.

• Surrounds GI organs, but no organs within the cavity itself.

Mesenteries and Omenta

• Greater omentum: Attaches greater curvature of stomach to transverse colon; functions in wound healing and immune defense.

• Lesser omentum: Attaches liver to lesser curvature of stomach and duodenum; encloses portal triad.

• Peritoneal ligaments: Double folds attaching organs to each other or body wall.

Peritoneal Organ Classification

Type	Description	Examples
Intraperitoneal	Completely covered by peritoneum	Stomach, first part of duodenum, jejunum, ileum, cecum, appendix, transverse colon, sigmoid colon, liver, tail of pancreas, spleen
Retroperitoneal	Never had a mesentery	Kidneys, ureters, suprarenal glands, inferior 2/3 of rectum
Secondarily Retroperitoneal	Lost mesentery during development	Parts of duodenum, head/neck/uncinate process of pancreas, ascending colon, descending colon

Mesentery Proper

• Mesentery of small intestine: Sometimes referred to as "The Mesentery".

Processes of Digestion (Summary Table)

Process	Description
Ingestion	Taking in food via mouth
Propulsion	Movement of food through GI tract (swallowing, peristalsis)
Mechanical Breakdown	Physical breakdown (chewing, churning, segmentation)
Digestion	Chemical breakdown of food by enzymes
Absorption	Transport of nutrients from lumen to blood/lymph
Defecation	Elimination of indigestible substances

Histology of the Alimentary Canal (Summary Table)

Region	Epithelium	Specialized Cells
Mouth, pharynx, esophagus	Stratified squamous	Stem cells, mucous cells
Stomach	Simple columnar	Surface mucous cells, parietal cells, chief cells, enteroendocrine cells
Small intestine	Simple columnar	Enterocytes, goblet cells, enteroendocrine cells, Paneth cells
Large intestine	Simple columnar	Colonocytes, goblet cells

Additional info: Tables and histological details inferred from standard A&P textbooks to provide complete context for exam preparation.