The Digestive System: Structure, Function, and Processes

The Digestive System

Introduction

The digestive system is a complex series of organs and tissues responsible for breaking down food, absorbing nutrients, and eliminating waste. It consists of the gastrointestinal (GI) tract and accessory organs, each with specialized functions and histological features.

Basic Processes of Digestion

Overview of the Six Basic Processes

• Ingestion: Intake of food into the GI tract, typically through the mouth.

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

• Mixing and Propulsion: Alternating contraction and relaxation of smooth muscle to mix and move food (peristalsis and segmentation).

• Digestion: Mechanical (physical breakdown) and chemical (enzymatic hydrolysis) processes reduce food to absorbable molecules.

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

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

Major Components and Accessory Structures of the GI Tract

GI Tract Organs

• Mouth

• Pharynx

• Esophagus

• Stomach

• Small Intestine (duodenum, jejunum, ileum)

• Large Intestine (colon, rectum, anus)

Accessory Structures

• Teeth and Tongue: Mastication and manipulation of food

• Salivary Glands: Secrete saliva containing digestive enzymes

• Liver: Produces bile for fat emulsification

• Pancreas: Secretes digestive enzymes and bicarbonate

Histological Layers of the GI Tract

Four Main Layers

• Mucosa: Innermost layer; includes epithelium, lamina propria (areolar connective tissue), and muscularis mucosae (smooth muscle).

• Submucosa: Areolar connective tissue; contains blood vessels, lymphatics, and nerves.

• Muscularis: Smooth muscle (circular and longitudinal layers); responsible for peristalsis and segmentation. Skeletal muscle is present in mouth, pharynx, and upper esophagus.

• Serosa (or Adventitia): Outermost layer; serous membrane (visceral peritoneum) or connective tissue (adventitia in esophagus).

Key Terms and Definitions

• Gastro-: Pertaining to the stomach

• Entero-: Pertaining to the intestine

• Hepatic: Pertaining to the liver

• Renal: Pertaining to the kidney(s)

• GI: Gastrointestinal; stomach and intestines

• SI: Small intestine

• LI: Large intestine

• Mechanical Digestion: Physical reduction of food size

• Chemical Digestion: Enzymatic hydrolysis of chemical bonds in food

Biological Macromolecules in Digestion

Types and Their Absorbable Forms

• Carbohydrates: Digested to monosaccharides (e.g., glucose)

• Proteins: Digested to amino acids

• Lipids: Digested to fatty acids and glycerol

• Nucleic Acids: Digested to nucleotides

Enzymatic Pathways of Digestion

General Schema

Each macromolecule follows a specific enzymatic pathway, where enzymes catalyze the breakdown of large molecules into absorbable units. The product of one reaction becomes the substrate for the next.

• Carbohydrates: Polysaccharides → Disaccharides → Monosaccharides

• Proteins: Polypeptides → Dipeptides → Amino acids

• Lipids: Triglycerides → Glycerol + Fatty acids

• Nucleic Acids: DNA/RNA → Nucleotides

Digestive Enzymes: Key Facts

Critical Information

• Name of enzyme

• Production site

• Target site

• Substrate (substance acted upon)

• Intermediate product(s)

• End product(s)

Example: Amylase (produced in mouth and pancreas) acts on starch to produce maltose.

Mechanical vs. Chemical Digestion

Comparison

• Mechanical Digestion: Includes mastication (chewing), segmentation (mixing), and peristalsis (propulsion).

• Chemical Digestion: Involves enzymes and hormones breaking chemical bonds in food.

Example: Salivary amylase begins chemical digestion of starch in the mouth; mastication is mechanical digestion.

Peritoneum and Peritoneal Cavity

Structure and Function

• Parietal Peritoneum: Lines the wall of the abdominopelvic cavity.

• Visceral Peritoneum: Covers organs within the cavity.

• Peritoneal Cavity: Fluid-filled space between parietal and visceral layers.

• Large Folded Sheets: Mesenteries, omenta, and ligaments bind organs and contain blood vessels, nerves, and fat.

Timeline of Food in the GI Tract

Approximate Transit Times

• Mouth: 15–30 seconds

• Esophagus: 2–4 seconds

• Stomach: 2–5 hours

• Small Intestine: 3–5 hours

• Large Intestine: 10–12 hours (up to 72 hours)

Note: Gastric emptying is slowed by high fat content in the duodenum.

Summary Table: Digestive Enzymes

Enzyme	Produced by	Site of Action	Substrate	End Product
Salivary Amylase	Salivary glands	Mouth	Starch	Maltose
Pancreatic Amylase	Pancreas	Small Intestine	Starch	Maltose
Maltase	Duodenal mucosa	Small Intestine	Maltose	Glucose
Pepsin	Stomach (chief cells)	Stomach	Proteins	Polypeptides
Trypsin	Pancreas	Small Intestine	Polypeptides	Dipeptides
Lipase	Pancreas	Small Intestine	Triglycerides	Glycerol + Fatty acids
DNAse/RNAse	Pancreas	Small Intestine	DNA/RNA	Nucleotides

Additional info:

• Digestive hormones such as gastrin, secretin, and cholecystokinin (CCK) regulate enzyme secretion and organ motility.

• Absorption mechanisms include active transport, facilitated diffusion, and simple diffusion, depending on the nutrient.

• The GI tract's histology and function vary by region, reflecting specialized roles in digestion and absorption.