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

Digestive System Overview

Introduction

The digestive system is responsible for breaking down the food we eat and converting it into nutrients that cells can use for energy, growth, and repair. It consists of two main components: the gastrointestinal (GI) tract and accessory organs.

• GI Tract (Alimentary Canal): 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

• Ingestion: Taking food and liquids into the mouth.

• Mixing and Propulsion: Movement of food through the GI tract by 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 molecules.

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

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

GI Tract Structure and Histology

Layers of the GI Tract Wall

The wall of the GI tract is composed of four basic layers, which are similar throughout but may vary in thickness and composition depending on the region.

1. Mucosa:

   • Innermost lining, in direct contact with the lumen.

   • Consists of epithelium, connective tissue, and a thin layer of smooth muscle.

   • Functions: protection, secretion, absorption.

2. Submucosa:

   • Areolar connective tissue binding mucosa to muscularis.

   • Contains blood vessels, lymphatics, and nerves (enteric nervous system).

3. Muscularis:

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

   • Responsible for peristalsis and mixing.

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

4. Serosa/Adventitia:

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

   • Secretes slippery fluid for lubrication.

Abdominal Cavity Peritoneum

Peritoneum and Peritoneal Folds

• Parietal Peritoneum: Lines the inside of the abdominal cavity.

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

• Peritoneal Cavity: Space between layers, contains serous fluid.

• Mesentery: Double layer of visceral peritoneum anchoring the small intestine to the posterior abdominal wall.

• Mesocolon: Anchors the large intestine/colon.

• Lesser Omentum: Connects stomach and duodenum to the liver.

• Greater Omentum: Hangs in front of abdominal organs, attaches to the greater curvature of the stomach and transverse colon; contains adipose tissue and immune cells.

GI Tract Organs

Mouth/Oral Cavity

• Ingestion: Food enters the mouth 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 gland: Anterior to ear, superficial to masseter.

  • Submandibular gland: Underneath mandible.

  • Sublingual gland: Underneath tongue.

• Saliva:

  • 1-1.5 liters/day, slightly acidic.

  • 99% water, 1% electrolytes and proteins.

  • Contains mucin and salivary amylase.

Esophagus

• Collapsible tube lined with stratified squamous epithelium.

• Located anterior to vertebral column, posterior to heart and trachea.

• Pierces diaphragm at esophageal hiatus to reach stomach.

• Transports bolus via peristalsis (coordinated contraction of circular and longitudinal muscles).

• Lower esophageal (cardiac) sphincter controls entry to stomach and prevents reflux.

Stomach

• J-shaped organ below diaphragm, left of liver.

• Can hold up to 6.4 liters of food.

• Regions: cardia, fundus, body, pylorus.

• Pyloric sphincter controls passage to duodenum.

• Wall has four layers; muscularis has three muscle layers (longitudinal, circular, oblique).

Stomach Mucosa

• Contains gastric glands with specialized cells:

  • Surface mucous cell: Secretes mucus for protection.

  • Mucous neck cell: Secretes mucus.

  • Parietal cell: Secretes HCl and intrinsic factor (for vitamin B12 absorption).

  • Chief cell: Secretes pepsinogen (inactive form of pepsin).

  • G cell: Secretes gastrin (hormone).

• Pepsin: Protease activated by acidic environment, breaks down proteins.

Stomach Functions

• Temporary storage of food.

• Secretes gastric juice (forms chyme when mixed with food).

• Mechanical and chemical digestion of proteins and lipids.

• Moves chyme into duodenum.

• Absorbs water, alcohol, and certain drugs (e.g., aspirin).

Small Intestine

• About 3 meters long, 2.5 cm diameter.

• Extends from pyloric sphincter to ileocecal sphincter (between ileum and cecum).

• Mucosa highly folded to increase surface area for absorption.

• Attached to posterior abdominal wall via mesentery.

• Three parts: duodenum, jejunum, ileum.

Digestive Processes: Mechanical and Chemical Digestion

Mechanical Digestion

• Physical breakdown of food into smaller pieces.

• Examples:

  • Chewing in mouth.

  • Churning in stomach.

  • Emulsification of fats.

Chemical Digestion

• Enzymatic breakdown of macromolecules into absorbable units.

• Enzymes break bonds in carbohydrates, proteins, and lipids.

• Examples:

  • Salivary amylase breaks down starch to sugars.

  • Pepsin breaks down proteins to peptides.

  • Lipase breaks down fats to fatty acids and glycerol.

Absorption and Defecation

Absorption

• End products of digestion move from GI tract lumen into epithelial cells, then into interstitial fluid and blood.

• Proteins: Absorbed as amino acids.

• Lipids: Absorbed as fatty acids and glycerol.

• Carbohydrates: Absorbed as monosaccharides (e.g., glucose, fructose, galactose).

Defecation

• Elimination of waste products, indigestible food, bacteria, and cells from the GI tract.

• Material is expelled through the rectum and anus as feces.

Accessory Organs

Teeth and Tongue

• Teeth: Responsible for mechanical breakdown of food.

• Tongue: Assists in mixing food and swallowing.

Liver, Gallbladder, and Pancreas

• Liver: Produces bile, processes nutrients, detoxifies substances.

• Gallbladder: Stores and concentrates bile.

• Pancreas: Produces digestive enzymes and bicarbonate, releases them into the duodenum.

Summary Table: Layers of the GI Tract Wall

Layer	Main Components	Function
Mucosa	Epithelium, connective tissue, smooth muscle	Protection, secretion, absorption
Submucosa	Areolar connective tissue, blood/lymph vessels, nerves	Support, transport, innervation
Muscularis	Circular and longitudinal muscle (plus oblique in stomach)	Movement (peristalsis, mixing)
Serosa/Adventitia	Areolar connective tissue, epithelium	Protection, lubrication

Summary Table: Digestive Processes and Products

Process	Location	Enzymes/Actions	End Products
Mechanical Digestion	Mouth, stomach	Chewing, churning, emulsification	Smaller food particles
Chemical Digestion	Mouth, stomach, small intestine	Amylase, pepsin, lipase, etc.	Amino acids, monosaccharides, fatty acids, glycerol
Absorption	Small intestine	Transport across epithelium	Nutrients in blood/lymph
Defecation	Large intestine, rectum	Elimination	Feces

Key Equations and Terms

• Peristalsis: Coordinated contraction and relaxation of muscularis layers to propel food.

• Chyme: Semi-fluid mass of partly digested food and gastric juice.

• Pepsinogen → Pepsin: $\text{Pepsinogen} + \text{HCl} \rightarrow \text{Pepsin}$

• Emulsification: Breakdown of large fat globules into smaller droplets, increasing surface area for enzyme action.

Additional info:

• The greater omentum contains immune cells that help with wound healing and tissue repair.

• The lesser omentum connects the stomach and duodenum to the liver, while the greater omentum attaches to the greater curvature of the stomach and transverse colon.

• Accessory organs are essential for digestion but are not part of the GI tract tube itself.