Digestive System II: Anatomy & Physiology of the Abdominal Cavity, Large Intestine, Liver, Gallbladder, Pancreas, and Peritoneum

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, and pancreas

• Peritoneum and mesenteries: Structures that organize and support abdominal organs

Abdominal Cavity

Boundaries and Contents

The abdominal cavity contains the major digestive organs and is defined by specific anatomical boundaries.

• Contains: Abdominal organs and peritoneal cavity

• Supported by: Thoracic skeleton and pelvic girdle

• Boundaries:

  • Superior: Thoracic diaphragm

  • Inferior: Pelvic inlet (pelvic diaphragm/levator ani)

Large Intestine

Structure and Function

The large intestine is responsible for water and electrolyte absorption, fecal formation, and limited digestion by bacteria.

• Functions:

  • Absorbs water and electrolytes

  • Mass peristaltic movements propel feces toward the rectum

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

• Digested residue: Contains few nutrients; small amount of bacterial digestion

Microscopic Anatomy

• Mucosa: Simple columnar epithelium (becomes stratified squamous at anal canal), many goblet cells, no villi

• Muscularis externa: Taenia coli (longitudinal muscle bands)

• Serosa: Epiploic appendages (fat-filled pouches)

• Absorptive cells: Colonocytes, absorb water and electrolytes

Ileocecal Junction

Transition from Small to Large Intestine

The ileocecal junction marks the entry of the small intestine into the large intestine.

• Ileocecal valve: Junction between ileum and cecum, regulates flow

• Ileocecal papilla: Mucosal fold at the junction

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

Appendix

Structure and Function

The vermiform appendix is a narrow tube attached to the cecum, with immune and microbial functions.

• Contains lymphoid tissue: Contributes to immune defense

• Safe haven for beneficial gut bacteria

Appendicitis

• Inflammation: Appendix becomes inflamed or filled with pus

• Symptoms: Pain in lower right abdomen (parietal peritoneum irritation), periumbilical region (visceral pain)

• Pathophysiology: Blocked secretions cause swelling; rupture can occur

• Treatment: Appendectomy (surgical removal)

Colon

Regions and Flexures

The colon is divided into several regions, each with specific anatomical landmarks.

• Cecum

• Ascending colon: Right side of posterior abdominal wall

• Hepatic flexure: Bend between ascending and transverse colon

• Transverse colon

• Splenic flexure: Bend between transverse and descending colon

• Descending colon: Left side of posterior abdominal wall

• Sigmoid colon

Special Features

• Greater diameter 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 colon

Rectum and Anal Canal

Rectum

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

• Retroperitoneal and subperitoneal

• No taenia coli or epiploic appendages

• Rectal folds: Superior, middle, and inferior transverse folds

• Flexures: Superior, intermediate, and inferior lateral flexures

Anal Canal

The anal canal is the final segment of the digestive tract, with specialized structures for fecal passage and continence.

• Begins: At the portion of the rectum passing the levator ani

• Anal sinus: Releases mucus when compressed by feces

• Pectinate line: Divides upper involuntary and lower voluntary regions

• Epithelium:

  • Simple columnar (upper)

  • Stratified squamous (lower)

  • Skin (distal)

• Internal anal sphincter: Smooth muscle, involuntary

• External anal sphincter: Skeletal muscle, voluntary

Defecation Reflex

Defecation is controlled by a reflex involving sensory and motor pathways, as well as voluntary control.

• Visceral sensory nerve fibers: Transmit signal to spinal cord

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

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

• Outcomes:

  • Defecation can be consciously initiated

  • If denied, rectum relaxes

  • Eventually unavoidable

Liver

Gross Anatomy and Functions

The liver is the largest gland in the body, with extensive metabolic and digestive functions.

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

• Functions:

  • Over 500 metabolic functions

  • Bile production

  • Regeneration

Lobes and Ligaments

• Lobes: Right, left, caudate, quadrate

• Ligaments:

  • Falci form ligament (anterior body wall)

  • Round ligament (ligamentum teres hepatis, to umbilicus)

  • Coronary ligament (to diaphragm)

Fissures and Porta Hepatis

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

• Porta hepatis: Entry/exit for vessels, nerves, and ducts (hepatic artery proper, hepatic portal vein, bile duct)

Microscopic Anatomy

• Liver lobule: Functional unit

• Central vein: Drains blood from lobule

• Hepatocytes: Functional cells

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

• Stellate macrophages: Destroy bacteria in sinusoids

Hepatocyte Functions

• Manufacture blood proteins

• Produce bile salts

• Detoxify poisons (e.g., alcohol)

• Store sugars and vitamins

• Process protein and fat

• Regeneration capacity

Cirrhosis of the Liver

• Cellular damage and scarring

• Primary site for detoxification

• Causes: Chronic alcoholism (alcoholic cirrhosis), hepatitis B and C

Gallbladder

Structure and Function

The gallbladder stores and concentrates bile produced by the liver, releasing it into the duodenum.

• Location: Visceral surface of liver (gallbladder fossa)

• Parts: Fundus, body, neck

• Cystic artery: Usually a branch of right hepatic artery

• Function: Stores and concentrates bile, expels bile into bile duct

• Note: Gallbladder does not produce bile

Bile Duct

• Bile canaliculi: Hepatocytes secrete bile into these channels

• Bile duct: Unites with main pancreatic duct at the Ampulla of Vater before entering the duodenum via the major duodenal papilla

Gallstones

• Biliary colic: Intense, spasmodic pain

• Cholecystitis: Inflammation of the gallbladder

• Cholecystectomy: Surgical removal of gallbladder

Pancreas

Structure and Secretions

The pancreas has both exocrine and endocrine functions, crucial for digestion and metabolic regulation.

• Parts: Head, neck, body, tail

• Exocrine: Produces most digestive enzymes for the small intestine

• Endocrine: Produces insulin and glucagon to regulate blood sugar

Pancreatic Ducts

• 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: Enzymes are activated in the duodenum

Abdominal Organs and Embryological Gut

Abdominal Organs

• Mostly gut organs, plus spleen and kidneys

Embryological Gut 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 part of transverse colon, descending colon, sigmoid colon, rectum

Peritoneum and Mesenteries

Peritoneum

The peritoneum is a serous membrane that lines the abdominal cavity and covers abdominal organs.

• Simple squamous epithelium

• Parietal peritoneum: Lines body wall

• Visceral peritoneum: Covers organs

• Peritoneal cavity: Space lined by peritoneum, surrounds GI organs (no organs within the cavity itself)

Mesentery

• Double layer of peritoneum

• Extends from body wall to organ, holds organs in place

• Stores fat

• Provides route for vessels and nerves

• Lost if organ is pushed against posterior body wall during development

Types of Mesenteries and Peritoneal Structures

• Dorsal mesentery

• Ventral mesentery

• Omentum: Greater and lesser omentum

• Peritoneal ligaments and folds

Greater Omentum

• Attaches greater curvature of stomach to transverse colon

• Includes gastrocolic ligament

• Functions in wound healing and immune defense

Lesser Omentum

• Attaches liver to lesser curvature of stomach

• Connects liver, stomach, and duodenum

• Includes hepatoduodenal and gastrohepatic ligaments

• Encloses the portal triad

Peritoneal Ligaments

• Double folds of peritoneum attaching organs to each other or body wall

• Examples: Falciform ligament (liver to anterior abdominal wall)

Intraperitoneal, Retroperitoneal, and Secondarily Retroperitoneal Organs

Type	Description	Examples
Intraperitoneal	Completely surrounded 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

• Referred to as "The Mesentery"

Processes of Digestion and Histology of Alimentary Canal

Major Processes

• Ingestion: Taking in food

• Propulsion: Movement of food (swallowing, peristalsis)

• Mechanical breakdown: Physical breakdown (chewing, churning)

• Digestion: Chemical breakdown of food

• Absorption: Uptake of nutrients

• Defecation: Elimination of indigestible substances

Histology of Alimentary Canal

• Mucosa: Epithelium varies by region (stratified squamous in mouth/esophagus, simple columnar in stomach/intestines)

• Cells:

  • Enteroendocrine cells: Secrete hormones

  • Goblet cells: Secrete mucus

  • Absorptive cells: Take in nutrients, water, electrolytes

Summary Table: Digestive Tract Regions and Functions

Organ	Major Functions
Mouth & Accessory Organs	Ingestion, propulsion, mechanical breakdown
Stomach	Mechanical breakdown, propulsion, digestion, absorption
Small Intestine	Mechanical breakdown, propulsion, digestion, absorption
Large Intestine	Propulsion, absorption, defecation

Additional info: The notes cover topics from Ch. 23 (Digestive System) and Ch. 1 (Human Body Orientation), Ch. 4 (Tissue), and Ch. 25 (Urinary System, in reference to abdominal organs), and are highly relevant for Anatomy & Physiology college students.