Digestive System Overview

Introduction

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

• Digestive Tract: A continuous tube from mouth to anus, including the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.

• Accessory Organs: Salivary glands, liver, gallbladder, and pancreas contribute enzymes and other substances for digestion.

Main Subdivisions of the Digestive Tract

Regions and Their Functions

General Functions of the Digestive Tract

Key Processes

• Ingestion: Entry of food and liquids into the digestive tract via the mouth.

• Mechanical Processing: Physical breakdown of food by chewing and mixing.

• Digestion: Chemical and enzymatic breakdown of food into small molecules.

• Secretion: Release of acids, enzymes, and buffers by digestive organs and accessory glands.

• Absorption: Movement of nutrients, water, and electrolytes into the blood and lymph.

• Compaction: Dehydration and consolidation of indigestible materials into feces.

• Defecation: Elimination of feces from the body.

Muscle Tissue in the Digestive System

Types and Functions

The digestive tract contains smooth muscle tissue, which is essential for movement and mixing of contents.

• Skeletal Muscle: Found in mouth, pharynx, and upper esophagus; voluntary control.

• Cardiac Muscle: Found only in the heart; not part of the digestive tract.

• Visceral (Smooth) Muscle: Found throughout the digestive tract; involuntary control, responsible for peristalsis and segmentation.

Comparison Table: Muscle Tissue Types

Digestive Tract Structure: Surface Area Adaptations

Intestinal Features

The small intestine is specialized for absorption, with several adaptations to increase surface area:

• Plicae Circularis: Permanent folds in the mucosa, mostly in the jejunum.

• Intestinal Villi: Fingerlike projections of the mucosa.

• Microvilli (Brush Border): Extensions of epithelial cell membranes, further increasing surface area.

Together, these adaptations increase the absorptive area by approximately 600 times compared to a smooth, flat wall.

Small Intestine Regions

Duodenum

• Length: ~25 cm (10 in.)

• Receives chyme from stomach and secretions from liver, gallbladder, and pancreas.

• Main function: Neutralize acidic chyme.

Jejunum

• Length: ~2.5 m (8.2 ft)

• Major site of chemical digestion and nutrient absorption.

• Numerous plicae circulares and long villi.

Ileum

• Length: ~3.5 m (11.5 ft)

• Final segment; controls flow into the cecum of the large intestine.

• Contains aggregated lymphoid nodules (Peyer's patches).

Large Intestine

General Characteristics

• Length: ~1.5 m (4.9 ft); width: 7.5 cm (3 in.)

• Functions: Reabsorption of water, absorption of vitamins, compaction and storage of feces.

• Segments: Cecum, colon, rectum.

Gastric Wall Cells and Secretions

Cell Types and Their Functions

• Parietal Cells: Secrete intrinsic factor (for vitamin B12 absorption) and hydrochloric acid (HCl).

• Chief Cells: Secrete pepsinogen (activated to pepsin in acid; digests proteins), rennin, and gastric lipase (in infants).

• G Cells (Enteroendocrine Cells): Produce hormones that regulate digestive activity.

Hydrochloric Acid Production Equation

Parietal cells use carbonic anhydrase to produce H+ and HCO3- from CO2 and H2O:

• Hydrogen ions are secreted into the gastric gland lumen.

• Chloride ions are also secreted, forming HCl.

• Bicarbonate ions are transported into the blood, causing an "alkaline tide" after meals.

Regulation of Digestive Activities

Neural Control Mechanisms

• Short reflexes: Local control via enteric nervous system.

• Long reflexes: CNS involvement, especially via the vagus nerve.

Hormonal Regulation

• Major hormones produced by the duodenum coordinate gastric activity and digestive secretion:

• Gastrin: Stimulates gastric secretion and motility.

• Secretin: Stimulates bicarbonate secretion from pancreas.

• GIP (Gastric Inhibitory Peptide): Inhibits gastric activity, stimulates insulin release.

• CCK (Cholecystokinin): Stimulates gallbladder contraction and pancreatic enzyme secretion.

• VIP (Vasoactive Intestinal Peptide): Dilates intestinal capillaries.

• Enterocrinin: Stimulates mucus production in the intestine.

Phases of Gastric Secretion

Cephalic Phase

• Triggered by sight, smell, taste, or thought of food.

• CNS prepares stomach for food arrival via vagus nerve.

• Gastric juice secretion increases; lasts only minutes.

Gastric Phase

• Begins when food enters the stomach.

• Stimuli: Stomach distension, increased pH, presence of proteins/peptides.

• Results in increased mixing waves and secretion by parietal and chief cells.

• May last 3-4 hours.

Intestinal Phase

• Begins when chyme enters the duodenum.

• Enterogastric reflex inhibits gastric activity and motility.

• Stimulates contraction of pyloric sphincter, decreasing chyme release.

• Increases mucus production in duodenum.

Accessory Organs and Their Functions

Liver

• Produces bile for emulsification of fats.

• Stores nutrients and detoxifies substances.

Gallbladder

• Stores and concentrates bile; releases bile into duodenum in response to CCK.

Pancreas

• Produces digestive enzymes and bicarbonate.

• Enzymes include:

  • Alpha-amylase: Breaks down starches.

  • Lipase: Breaks down lipids.

  • Nucleases: Break down RNA and DNA.

  • Proteolytic enzymes: Trypsin, chymotrypsin, carboxypeptidase, elastase; break down proteins.

Summary Table: Digestive System Functions

Example: Swallowing (Deglutition)

• Buccal Phase: Voluntary; bolus formation and movement to oropharynx.

• Pharyngeal Phase: Involuntary; coordinated movement by medulla oblongata.

• Esophageal Phase: Involuntary; peristaltic waves move bolus to stomach.

Additional info:

• Digestive system works closely with the cardiovascular and urinary systems for nutrient delivery and waste removal.

• Gastric bypass surgeries alter digestive tract anatomy to promote weight loss; common procedures include Roux-en-Y and sleeve gastrectomy.