Digestive System: Structure, Function, and Physiology (Chapter 22)

Digestive System Overview

Introduction

The digestive system is responsible for the breakdown of food, absorption of nutrients, and elimination of waste. It consists of a series of organs forming the alimentary canal and accessory organs that aid in digestion.

Pathway of Food and Fluids

Main Organs in Sequence

• Mouth (Oral Cavity): Entry point for food and fluids; mechanical breakdown begins here.

• Pharynx: Passageway for food from mouth to esophagus.

• Esophagus: Muscular tube transporting food to the stomach via peristalsis.

• Stomach: Mixes and digests food using acids and enzymes.

• Small Intestines: Major site for enzymatic digestion and nutrient absorption.

• Large Intestines: Absorbs water and forms feces.

• Rectum: Stores feces before elimination.

• Anus: Terminal opening for waste elimination.

Processes of the Digestive System

Key Steps

• Ingestion: Food enters the digestive tract via the mouth.

• Propulsion: Movement of food through the tract, including swallowing (deglutition) and peristalsis.

• Mechanical Breakdown: Physical breakdown of food by chewing (mastication), mixing, and churning.

• Chemical Breakdown: Enzymatic breakdown of food into small organic fragments for absorption. Carbohydrates, lipids, and proteins are reduced to their monomers (e.g., glucose, fatty acids, amino acids).

• Absorption: Movement of nutrients, electrolytes, vitamins, and water across the epithelium into blood or lymph.

• Defecation/Egestion: Elimination of indigestible material as feces.

Groups of Digestive System Organs

Classification

• Digestive Tract Organs (Alimentary Canal/GI Tract): Mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus. Responsible for digestion and absorption.

• Accessory Digestive Organs: Teeth, tongue, salivary glands, liver, gallbladder, pancreas. Aid in the process of digestion and absorption.

Digestive Tract Organs: Structure and Function

Wall Structure of the Alimentary Canal

• Mucosa: Innermost layer; secretes mucus, absorbs nutrients, and may secrete hormones.

• Submucosa: Contains glands, blood vessels, and nerves (submucosal nerve plexus).

• Muscularis externa: Responsible for peristalsis and segmentation; contains intrinsic nerve plexuses (myenteric plexus).

• Serosa: Outermost connective tissue layer.

Intrinsic Nerve Plexuses

• Enteric Nervous System (ENS): "Brain of the gut"; controls digestive tract motility and secretion.

• Myenteric Plexus: Controls gastrointestinal motility.

• Submucosal Plexus: Regulates glandular secretions and blood flow.

Accessory Digestive Organs

Functions

• Teeth: Chew food, aiding mechanical breakdown.

• Tongue: Tastes, pushes food for swallowing, and assists in mixing food with saliva.

• Salivary Glands: Secrete saliva containing enzymes (e.g., salivary amylase) for carbohydrate digestion.

• Liver: Produces bile for emulsification of fats.

• Gallbladder: Stores and concentrates bile, releasing it into the small intestine.

• Pancreas: Produces digestive enzymes and bicarbonate; releases pancreatic juice into the small intestine.

Digestive Processes in Specific Organs

Mouth (Oral Cavity)

• Teeth: Mechanical breakdown by chewing.

• Tongue: Squashing, compaction, and mixing of food.

• Salivary Glands: Secrete saliva for enzymatic breakdown of carbohydrates.

Pharynx and Esophagus

• Pharynx: Passageway for food from mouth to esophagus.

• Esophagus: Propels food to stomach via peristalsis.

Stomach

• Mechanical breakdown: Mixing, churning, and grinding of food.

• Chemical breakdown: Secretion of gastric juices (e.g., pepsin) for protein digestion.

• Formation of chyme: Semi-liquid mixture of partially digested food.

Small Intestine

• Duodenum: Receives chyme, bile, and pancreatic juice; site of most chemical digestion.

• Jejunum and Ileum: Major sites for nutrient absorption.

Large Intestine

• Absorption: Water and electrolytes are absorbed.

• Formation of feces: Indigestible material is compacted for elimination.

Digestive Enzymes and Chemical Digestion

Enzyme Action

• Enzymes: Biological catalysts that speed up chemical reactions in digestion.

• Substrate: The molecule upon which an enzyme acts.

• Enzyme-Substrate Complex: Temporary association during the reaction.

General Reaction:

$\text{Enzyme (E)} + \text{Substrate (S)} \rightarrow \text{ES Complex} \rightarrow \text{Enzyme (E)} + \text{Product (P)}$

Major Digestive Enzymes

• Salivary Amylase: Begins carbohydrate digestion in the mouth.

• Pepsin: Initiates protein digestion in the stomach.

• Pancreatic Enzymes: Digest carbohydrates, proteins, and fats in the small intestine.

Bile and Fat Digestion

Emulsification and Absorption

• Bile salts: Produced by the liver, stored in the gallbladder; emulsify fat globules into smaller droplets.

• Pancreatic lipase: Breaks down triglycerides into monoglycerides and free fatty acids.

Process:

• Fat globules + bile salts → emulsified droplets

• Emulsified droplets + pancreatic lipase → monoglycerides + free fatty acids

Summary Table: Digestive Tract Organs and Functions

Organ	Main Function
Mouth	Mechanical breakdown, carbohydrate digestion
Pharynx	Passageway for food
Esophagus	Propulsion via peristalsis
Stomach	Mixing, protein digestion, formation of chyme
Small Intestine	Chemical digestion, nutrient absorption
Large Intestine	Water absorption, feces formation
Rectum	Storage of feces
Anus	Elimination of waste

Additional Info

• Deglutition: The act of swallowing, involving voluntary and involuntary muscle actions.

• Peristalsis: Rhythmic contractions of the muscularis externa that propel food through the digestive tract.

• Segmentation: Contractions that mix food and increase contact with digestive enzymes.

• Defecation: The process of eliminating indigestible material from the body.