Ch_17_-_Digestive_System__2025_

The Digestive System

Chapter 17 Overview

• This chapter provides an in-depth look at the digestive system, articulating its components, functions, and processes.

General Characteristics of the Digestive System

Introduction

• The digestive system is a long tube comprised of specialized structures such as the stomach and intestines.

• The inner portion (lumen) is continuously exposed to the outside environment, populated by various microbes including bacteria.

Functions of Food in Digestion

• Food entering the digestive tube is essential for maintaining and regulating bodily functions.

• It is broke down into molecular building blocks:

  • Sugars (Carbohydrates)

  • Amino Acids (Proteins)

  • Nucleotides (Nucleic Acids)

  • Fatty Acids and Glycerol (Lipids)

• These molecules are absorbed through the tube walls and utilized for constructing proteins and structures necessary for the body's functionality.

Digestion

• Mechanical Digestion: Physical breakdown of food into smaller pieces without altering its chemical structure.

• Chemical Digestion: Breakdown of food into simpler chemical forms by changing its molecular structure.

Components of the Digestive System

Digestive Organs

• Major organs include:

  • Mouth

  • Pharynx

  • Esophagus

  • Stomach

  • Small Intestine

  • Large Intestine

  • Rectum

  • Anus

Accessory Organs

• Organs that aid digestion but are not part of the alimentary canal:

  • Salivary Glands

  • Liver

  • Gallbladder

  • Pancreas

Alimentary Canal and Accessory Structures

• Alimentary Canal: A long tube extending from the mouth to the anus, facilitating food passage.

• Accessory Organs: Secrete enzymes and liquids to assist in digestion, though food does not pass through them directly.

Structure of the Alimentary Canal

Length and Composition

• The alimentary canal is approximately 8 meters long.

• Divided into:

  • Mucosa: Innermost layer containing epithelial cells.

  • Submucosa: Connective tissue with blood vessels and nerves.

  • Muscularis: Muscle tissue composed of circular and longitudinal muscle layers.

  • Serosa: Outermost layer preventing friction between organs.

Movements of Digestion

• Mixing Movements: For example, segmentation which moves materials back and forth without directional propulsion.

• Propelling Movements: Such as peristalsis, moving food along the digestive tract through coordinated muscle contractions.

Nervous System Control of Digestion

Autonomic Control

• Parasympathetic Impulses: Enhance digestion by stimulating secretion and motility.

• Sympathetic Impulses: Inhibit digestive activities, reducing secretion and motility.

Enteric Nervous System

• The submucosal plexus controls secretion.

• The myenteric plexus manages gastrointestinal motility.

The Mouth

Function and Anatomy

• The first segment of the alimentary canal where food ingestion and mastication occur.

• It is crucial for speech and sensory reception, surrounded by the lips, cheeks, tongue, and palate.

Anatomy Components

• Structures:

  • Tongue: Assists in food manipulation and contains taste buds.

  • Teeth: Most durable structures in the body aiding in mechanical breakdown of food.

  • Palate: Comprising hard and soft sections, facilitates proper food routing towards the pharynx.

Salivary Glands

Structure and Function

• Salivary glands produce saliva, aiding in food moistening and taste enhancement, containing enzymes crucial for digestion.

• Three main pairs: parotid, submandibular, and sublingual glands.

The Pharynx and Esophagus

Pharynx

• Divided into three sections: nasopharynx, oropharynx, and laryngopharynx, serving as a passageway to the esophagus without digestion.

Esophagus

• A muscular tube connecting the pharynx to the stomach.

• Contains mucous glands that lubricate the bolus of food for smooth transport.

The Stomach

Structure and Functions

• J-shaped organ with multiple functions:

  • Mixing food with gastric juice.

  • Initiating protein digestion.

  • Absorbing limited nutrients.

Gastric Secretions

• Gastric glands produce secretion including:

  • Chief Cells: Secrete pepsinogen, later activated to pepsin.

  • Parietal Cells: Produce hydrochloric acid and intrinsic factor.

  • Mucous Cells: Secrete protective mucous.

The Pancreas

Endocrine and Exocrine Functions

• The pancreas serves dual roles:

  • Endocrine: Producing hormones like insulin.

  • Exocrine: Secreting pancreatic juice which contains enzymes essential for digesting carbohydrates, proteins, and fats.

Structure

• The pancreatic duct connects to the duodenum, crucial for digestive enzyme transport.

The Liver and Gall Bladder

Liver Functions

• The liver, the largest internal organ, performs vital functions including nutrient metabolism and detoxification.

• It produces bile, which is stored and concentrated in the gall bladder.

Gall Bladder Role

• Stores bile, releasing it into the duodenum to aid in fat digestion.

The Small Intestine

Structure and Function

• A long tubular organ where most nutrient absorption occurs, divided into three sections:

  • Duodenum: Shortest segment, fixed in place.

  • Jejunum: Middle section, thicker and more active.

  • Ileum: Last section, equipped with immune tissue.

The Large Intestine

Anatomy and Function

• Larger in diameter, responsible for absorbing water and electrolytes while forming and storing feces.

• Lacks villi, features haustra for effective waste storage.

Digestive Process Overview

• Digestion involves preliminary breakdown by salivary enzymes, gastric acids, and pancreatic juices, with nutrient absorption conducted primarily in the small intestine.

Summary of Major Digestive Enzymes

• Enzymatic activities span the digestive process, healing nutrient breakdown and absorption throughout various segments of the digestive system.