Digestive Anat Lect Notes

Anatomy Lecture Notes Section 5: The Digestive System

Components of the Digestive System

• Gastrointestinal (G.I.) Tract: Also known as the alimentary canal, it is a long muscular tube starting from the oral cavity to the anus.

  • Pathway: Oral cavity ➔ Pharynx ➔ Esophagus ➔ Stomach ➔ Small intestine ➔ Large intestine ➔ Rectum ➔ Anus

• Accessory Structures and Organs: Include salivary glands, mucous glands, tongue, teeth, liver, gallbladder, and pancreas, all essential for digestion.

Functions of the G.I. Tract

• Peristalsis: Food is propelled through the G.I. tract by muscle contractions in the walls of the tract.

• Nutrient Breakdown: The primary purpose is to break down nutrients from food (from large polymers to smaller monomers) for absorption into the body.

Basic Digestive Processes

1. Ingestion: Taking food/drink into the mouth.

2. Propulsion: Movement through the alimentary canal; includes tongue and cheek movement along with swallowing and peristaltic movements.

3. Mechanical Digestion: Physical breakdown of food (e.g., chewing, churning) is necessary before chemical digestion.

4. Chemical Digestion: Enzymatic breakdown of food into simple building blocks by enzymes.

5. Absorption: Nutrients are transported from the G.I. tract’s lumen across the epithelial lining into blood and lymph vessels.

6. Defecation: Elimination of indigestible material and waste (feces).

Overview of Digestion Locations

• Mechanical Digestion: Mainly in the mouth and stomach.

• Chemical Digestion: Begins in the stomach and continues significantly in the small intestine for proteins, fats, and carbohydrates.

• Large Intestine: Reabsorbs excess water and electrolytes; undigested material is excreted as feces.

Alimentary Canal

• Definition: A hollow, specialized tube from the oral cavity to the anus.

Oral Cavity

• Structure: Formed by lips, cheeks, tongue, palates, and throat; allows entry of food, liquid, and air.

• Organs in Mouth: Teeth, tongue, and ducts of extrinsic salivary glands, aiding in ingestion and digestion.

• Lips: Flexible structures forming the mouth's opening; covered with keratinized stratified squamous epithelium.

Teeth and Tongue

• Teeth: Facilitates mastication (chewing); consists of soft gums and hard enamel-covered surfaces.

  • Types of Teeth: Incisors, canines, premolars, and molars, totaling 32 permanent teeth.

  • Function: Biting and grinding food; shapes the mouth and face; important for speech.

• Tongue: Made of muscles and epithelium; aids in tasting, manipulating food, and speech production.

Salivary Glands

• Functions: Produce saliva, containing enzymes (lingual amylase) and antibacterial agents (lysozyme).

  • Intrinsic Glands: Small glands inside the oral cavity; assist salivation.

  • Extrinsic Glands: Larger glands (e.g., parotid, submandibular, sublingual) that secrete saliva into the mouth.

Pharynx and Esophagus

Pharynx

• Description: Funnel-shaped tube connecting the nasal cavity and mouth to the esophagus and larynx.

• Regions: Oropharynx (from mouth to esophagus) and laryngopharynx (connects to the larynx).

Esophagus

• Function: Muscular tube transporting food from the mouth to the stomach.

• Segments: Composed of three muscle types - proximal (skeletal), middle (both), and distal (smooth).

  • Lower Esophageal Sphincter: Prevents regurgitation into the esophagus.

Stomach

• Description: J-shaped organ between the esophagus and small intestine; divided into four regions: cardia, fundus, body, and pylorus.

• Functions:

  • Short-term storage of food.

  • Mechanical and chemical digestion of proteins.

  • Initial processing and mixing of ingested food into chyme.

• Components: Gastric glands secrete enzymes and acids; rugae allow expansion.

Small Intestine

• Description: Long tube (20 feet) with three regions: duodenum, jejunum, and ileum.

• Functions: Majority of digestion and nutrient absorption occurs here;

  • Duodenum: Mixes pancreatic juices and bile; crucial for digestion.

  • Jejunum: The main site of nutrient absorption.

  • Ileum: Final segment; continues absorption and ends at the cecum.

• Surface Modifications: Plicae circulares, villi, and microvilli enhance surface area for absorption.

Large Intestine

• Overview: Extends around the small intestine; includes cecum, colon, and rectum (approximately 4.5 feet long).

• Functions:

  1. Reabsorb water and compact feces.

  2. Absorb vitamins and electrolytes.

  3. Store fecal matter.

• Cecum: First part, receives contents from the ileum; vermiform appendix attached.

• Colon Regions: Ascending, transverse, descending, and sigmoid.

Peritoneum and Mesenteries

• Layers of Peritoneum:

  • Visceral: Covers organs.

  • Parietal: Lines abdominal cavity.

• Mesenteries: Double layers of peritoneal membrane aiding in suspending digestive tract portions, includes:

  • Greater omentum: Protective drape over intestines.

  • Lesser omentum: Connects liver and stomach.

  • Mesentery Proper: Attaches to the small intestine.

  • Mesocolon: Supports the colon.

Liver

• Location: Right upper quadrant of the abdomen; divided into lobes (right, left, caudate, quadrate).

• Functions: Mechanical filter, detoxification, and bile production; processes nutrients absorbed from intestines.

• Bile Function: Emulsifies fats to enhance digestion by increasing surface area for enzymes.

Gallbladder

• Description: Pear-shaped organ storing bile; releases bile into the intestines when triggered by lipid intake.

Pancreas

• Location: Behind the stomach; has both exocrine and endocrine functions.

• Exocrine Functions: Produces pancreatic juices containing digestive enzymes for carbohydrate, lipid, protein, and nucleic acid breakdown.

• Endocrine Functions: Produces insulin and glucagon, regulated by Islets of Langerhans.