Digestive System: Structure and Function (ANAT 100 Study Notes)

Digestive System Overview

Introduction

The digestive system is responsible for breaking down the food we eat into nutrients that can be absorbed and used by the body's cells. It consists of the gastrointestinal (GI) tract and accessory organs, each with specialized roles in digestion and absorption.

• Gastrointestinal (GI) Tract: A continuous tube from mouth to anus, including the mouth, pharynx, esophagus, stomach, small intestine, large intestine, anal canal, and anus.

• Accessory Organs: Structures that aid digestion but are not part of the tube, such as teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Functions of the Digestive System

Major Functions

• Ingestion: Taking food and liquids into the mouth.

• Mixing and Propulsion: Movement of food through the GI tract via muscular contractions (peristalsis).

• Secretion: Release of water, acid, buffers, and digestive enzymes into the tract (about 7 liters/day).

• Digestion:

  • Mechanical Digestion: Physical breakdown of food (chewing, churning, emulsification).

  • Chemical Digestion: Enzymatic breakdown of macromolecules into smaller units (e.g., proteins to amino acids).

• Absorption: Movement of digested nutrients from the GI tract into the blood or lymph.

• Defecation: Elimination of indigestible substances and waste as feces.

Wall of the GI Tract

Layers of the GI Tract

The GI tract wall is composed of four main layers, each with distinct functions:

• Mucosa:

  • Inner lining; epithelium in direct contact with lumen contents.

  • Contains connective tissue and a thin layer of smooth muscle.

  • Functions in protection, secretion, and absorption.

• Submucosa:

  • Areolar connective tissue binding mucosa to muscularis.

  • Contains blood vessels, lymphatics, and enteric nervous system neurons.

• Muscularis:

  • Thick muscle layer; typically two layers (circular and longitudinal), three in the stomach (adds oblique).

  • Skeletal muscle in mouth, pharynx, and upper esophagus; smooth muscle elsewhere.

• Serosa/Adventitia:

  • Outer layer; serosa below diaphragm (visceral peritoneum), adventitia above.

  • Secretes slippery fluid for lubrication.

Abdominal Cavity Peritoneum

Peritoneum Structure

• Parietal Peritoneum: Lines the abdominal cavity.

• Visceral Peritoneum (Serosa): Covers organs within the cavity.

• Peritoneal Cavity: Space between layers containing serous fluid.

Peritoneal Folds

• Mesentery: Double layer attaching small intestine to posterior abdominal wall.

• Mesocolon: Double layer attaching large intestine/colon to posterior wall.

• Lesser Omentum: Connects stomach and duodenum to liver.

• Greater Omentum: Hangs in front of abdominal organs, contains adipose tissue and immune cells, aids in protection and healing.

GI Tract Organs

Mouth/Oral Cavity

• Ingestion: Food enters and is mixed with saliva to form a bolus.

• Mechanical Digestion: Chewing (mastication) breaks food into smaller pieces.

• Chemical Digestion: Saliva contains salivary amylase to begin carbohydrate breakdown.

Salivary Glands

• Three pairs: Parotid, submandibular, sublingual.

• Saliva: 1-1.5 liters/day, slightly acidic, contains mucin and salivary amylase.

Esophagus

• Collapsible tube lined with stratified squamous epithelium.

• Transports bolus via peristalsis (coordinated muscle contractions).

• Lower esophageal/cardiac sphincter regulates entry into stomach and prevents reflux.

Stomach

• J-shaped organ below diaphragm, can hold up to 6.4 liters.

• Regions: cardia, fundus, body, pylorus.

• Pyloric sphincter controls passage to duodenum.

• Muscularis has three layers: longitudinal, circular, oblique (for enhanced mechanical digestion).

• Mucosa contains specialized cells:

  • Surface mucous cells (protective mucus)

  • Parietal cells (HCl, intrinsic factor)

  • Chief cells (pepsinogen)

  • G cells (gastrin hormone)

• Functions: storage, secretion of gastric juice, mechanical and chemical digestion, absorption of water, drugs, alcohol.

Small Intestine

• About 3 meters long, 2.5 cm diameter.

• Extends from pyloric sphincter to ileocecal sphincter.

• Regions: duodenum, jejunum, ileum.

• Mucosa highly folded (villi, microvilli) for increased absorption.

• Major function: absorption of nutrients.

Digestive Processes: Key Comparisons

Mechanical vs. Chemical Digestion

• Mechanical Digestion: Physical breakdown (chewing, churning, emulsification).

• Chemical Digestion: Enzymatic breakdown of macromolecules.

  • Carbohydrates → monosaccharides (e.g., glucose)

  • Proteins → amino acids

  • Lipids → fatty acids and glycerol

Absorption Comparison Table

Region	Main Absorbed Substances
Stomach	Water, alcohol, some drugs
Small Intestine	Nutrients (amino acids, monosaccharides, fatty acids, vitamins, minerals)
Large Intestine	Water, electrolytes, some vitamins

Accessory Organs

Liver and Gallbladder

• Liver: Produces bile, metabolizes nutrients, detoxifies substances, stores vitamins.

• Gallbladder: Stores and concentrates bile, releases it into duodenum.

• Bile: Contains bile salts (cholesterol byproducts), pigments (from hemoglobin breakdown), aids in fat emulsification.

Pancreas

• Endocrine and exocrine gland.

• Produces pancreatic juice containing digestive enzymes (amylase, lipase, proteases) and bicarbonate.

• Pancreatic juice drains into duodenum via the pancreatic duct, joining bile from the liver/gallbladder.

Integration and Nutrient Transport

Hepatic Portal System

• Blood from the GI tract is drained to the liver via the hepatic portal vein.

• Allows the liver to process absorbed nutrients before they enter systemic circulation.

Summary Table: GI Tract Layers

Layer	Main Components	Function
Mucosa	Epithelium, connective tissue, smooth muscle	Protection, secretion, absorption
Submucosa	Areolar connective tissue, blood/lymph vessels, nerves	Support, transport, neural regulation
Muscularis	Smooth/skeletal muscle	Movement (peristalsis, mixing)
Serosa/Adventitia	Connective tissue, epithelium	Protection, lubrication

Key Terms and Definitions

• Peristalsis: Wave-like muscular contractions moving food through the GI tract.

• Chyme: Semi-fluid mass of partially digested food in the stomach.

• Emulsification: Breakdown of large fat globules into smaller droplets.

• Villi/Microvilli: Finger-like projections in the small intestine increasing surface area for absorption.

• Sphincter: Circular muscle regulating passage between GI tract regions.

Example: Digestion of a Sandwich

• Chewing in the mouth (mechanical), salivary amylase begins carbohydrate digestion.

• Swallowed bolus moves down esophagus via peristalsis.

• Stomach churns and mixes food, secretes gastric juice, begins protein and fat digestion.

• Chyme enters small intestine, where pancreatic enzymes and bile further digest nutrients; absorption occurs.

• Remaining material enters large intestine, water and electrolytes absorbed, waste formed as feces.

Additional info: These notes expand on the original slides by providing definitions, examples, and tables for comparison, suitable for college-level study.