Digestive System Notes

Digestive System

Main and Accessory Organs

• Main Organs:
  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Small Intestine
  • Large Intestine
  • Rectum
  • Anus
• Accessory Organs: (connected to the alimentary canal by ducts)
  • Teeth
  • Tongue
  • Gallbladder
  • Salivary Glands
  • Liver
  • Pancreas

Main Functions of the Digestive System

1. Ingestion: Occurs in the mouth.
2. Propulsion (Peristalsis): Movement of food through the digestive tract.
3. Digestion: Breakdown of food into absorbable components.
   • Mechanical Digestion: Prepares food for chemical digestion (e.g., chewing, churning in the stomach).
   • Chemical Digestion: Complex molecules are broken down into simpler components by enzymes.
4. Absorption: Transport of digested nutrients into the blood and lymph.
5. Defecation: Elimination of indigestible substances as feces.

Peritoneum and Its Subdivisions

• Peritoneum: A serous membrane.
  • Visceral Peritoneum: Surrounds digestive organs.
  • Parietal Peritoneum: Lines the body wall.
• Peritoneal Cavity: A potential space between the visceral and parietal peritoneum.
• Mesentery: A double layer of peritoneum.
  • Holds organs in place (e.g., loops of the small intestine).
  • Sites of fat storage.
  • Provides a route for circulatory vessels and nerves.
• Mesocolon: Specifically associated with the large intestine.

Peristalsis and Segmentation

• Peristalsis: Waves of contraction and relaxation of musculature in organ walls that propel food forward.
• Segmentation: Rhythmic local constrictions of the intestine that mix food with digestive juices and enhance nutrient absorption.

Layers of the Alimentary Canal Wall

The alimentary canal, from the esophagus to the anus, consists of four layers:

1. Mucosa: Innermost layer.
   • Consists of:
     • Epithelium (for absorption and secretion of mucus)
     • Lamina Propria
     • Muscularis Mucosae (thin layer of smooth muscle that produces local movements of the mucosa)
2. Submucosa: External to the mucosa.
   • Contains blood vessels, lymphatic vessels, and nerve fibers.
3. Muscularis Externa: External to the submucosa (responsible for peristalsis).
   • Two layers:
     • Circular Layer (inner layer that squeezes the gut tube)
     • Longitudinal Layer (outer layer that shortens the gut tube)
4. Serosa: Outermost layer.
   • Same as the visceral peritoneum.
   • Made up of simple squamous epithelium.

Oral Cavity Borders

• Lips (anteriorly)
• Cheeks (laterally)
• Palate (superiorly)
  • Hard Palate
  • Soft Palate
  • Uvula (end of soft palate)
• Tongue (inferiorly)
• Fauces of Oropharynx (posteriorly)

Functions of the Tongue

• Sense of taste.
• Food manipulation during chewing, repositioning food between teeth.
• Mixing food with saliva to form a bolus.
• Swallowing: moving the bolus into the pharynx.
• Lingual Frenulum: Secures the tongue to the floor of the mouth.

Deciduous and Permanent Teeth

• Deciduous (Baby) Teeth: 20
  • First appear at 6 months.
• Permanent Teeth: 32
  • Erupt by the end of adolescence.
• Dental Formula: 2I, 1C, 2P, 3M (divided in quarters; 8 x 4 = 32)
  • 2 Incisors
  • 1 Canine
  • 2 Premolars
  • 3 Molars

Parts and Layers of a Tooth

• Parts:
  1. Crown (exposed surface)
  2. Neck (where regions meet at the gum line)
  3. Root (in tooth socket)
• Layers:
  1. Enamel (outer layer, hardest tissue in the body)
  2. Dentin (underlies enamel)
  3. Pulp Cavity (center of tooth)

Major Salivary Glands

Saliva is a complex mixture of water, ions, mucus, and enzymes that:

• Moistens the mouth.
• Dissolves food chemicals for tasting.
• Wets food and binds it into a bolus.
• Parotid Glands:
  • Parotid duct runs parallel to the zygomatic arch near the angle of the jaw.
• Submandibular Glands:
  • Lie along the medial surface of the mandible (under the mandible).
• Sublingual Glands:
  • Under the tongue.
  • Lie in the floor of the oral cavity.
  • Contain primarily mucus cells.

Cardiac Sphincter

• Acts to close off the lumen and prevent regurgitation of acidic stomach juices into the esophagus.
• Located between the stomach and esophagus.

Main Parts of the Stomach

The stomach is a storage tank where food is churned into a paste called chyme. Food remains in the stomach for about 4 hours.

• Cardiac Region (Cardia)
  • Cardiac Sphincter
• Fundus
• Body
• Pyloric Part
  • Pyloric Antrum
  • Pyloric Canal
  • Pyloric Sphincter (valve for stomach and duodenum)
• Rugae of Mucosa (mucosal folds)
• Greater Curvature
• Lesser Curvature

Muscularis Externa of the Stomach

The stomach's muscularis externa contains three layers:

1. Longitudinal Layer
2. Circular Layer
3. Oblique Layer

Small Intestine

Main Parts

1. Duodenum: A small C-shaped loop.
   • Most chemical digestion occurs here.
   • Receives enzymes and bile.
   • The main pancreatic duct and common bile duct enter the duodenum.
     • Sphincters control the entry of bile and pancreatic juices.
2. Jejunum:
   • Most nutrient absorption takes place here.
3. Ileum:
   • Absorbs most vitamins and minerals.

Main Functions

• Longest portion of the alimentary canal.
• Completion of digestion.
  • Bile from the liver/gallbladder.
  • Enzymes from the pancreas.
• Site of most enzymatic digestion and absorption.

Modifications for Absorption

• Circular Folds: Transverse ridges of mucosa and submucosa.
• Villi: Fingerlike projections of the mucosa, covered with simple columnar epithelium.
• Microvilli: Further increase the surface area for absorption.

Large Intestine

Main Parts

1. Cecum
   • Opening of the appendix.
2. Ascending Colon
3. Transverse Colon
4. Descending Colon
5. Sigmoid Colon
6. Rectum
7. Anus

Special Features

• Teniae Coli: Thickening of the longitudinal muscularis externa, forming bands.
• Haustra: "Bumps" or puckering created by the teniae coli.
• Epiploic Appendages: Fat-filled pouches of visceral peritoneum.
• Right Colic (Hepatic) Flexure: Between the ascending and transverse colon.
• Left Colic (Splenic) Flexure: Under the spleen.
• Ileocecal Valve: Prevents backflow from the large intestine into the small intestine.
• Anal Canal:
  • External Anal Sphincter

Main Functions

• Absorbs water and electrolytes.
• Digested residue contains few nutrients.
• Small amount of digestion by bacteria.
• Mass peristaltic movements force feces toward the rectum.

Liver

Main Parts

Located in the RUQ under the diaphragm, extending to the LUQ.

• Liver Capsule: Connective tissue sheet covering the liver.
  • Falciform Ligament: Anterior thickened part of the capsule; separates the larger right lobe from the left lobe.
  • Round Ligament: Posterior thickened part of the capsule that separates the left lobe from the quadrate lobe.
  • Ligamentum Venosum: Separates the caudate lobe from the left lobe.

Porta Hepatis

Allows passage of:

1. Hepatic Portal Vein (brings blood toward the liver)
2. Hepatic Artery Proper
3. Common Hepatic Duct (bile duct; brings bile out of the liver)

Main Functions (Digestive System)

• Bile Production
  • Emulsification of fats (lipids)

Gallbladder

Bile Storage and Delivery

• Bile is stored in the gallbladder.
• The gallbladder contracts and secretes bile to the duodenum via the ducts.

Pancreas

Functions

• Exocrine Function:
  • Acinar cells make, store, and secrete enzymes that digest food in the small intestine.
  • Enzymes: Amylase, Lipase (lipids)
• Endocrine Function:
  • Produces insulin and glucagon.

Main Pancreatic Duct

• Extends the length of the pancreas.
• Joins the bile duct to form the hepatopancreatic ampulla, which empties into the duodenum.

Peptic Ulcer Cause

• Caused by Helicobacter pylori.