Digestive System: Structure, Function, and Processes

Digestive System Overview

Introduction

The digestive system is a complex network of organs and tissues responsible for the breakdown, absorption, and excretion of food and nutrients. It consists of the muscular digestive tract and various accessory organs that aid in digestion.

• Major functions: Ingestion, mechanical processing, digestion, secretion, absorption, and excretion.

Functional Organization of the Digestive System

Components of the Digestive System

• Major Organs: Oral cavity (mouth), pharynx, esophagus, stomach, small intestine, large intestine.

• Accessory Organs: Teeth, tongue, salivary glands, liver, gallbladder, pancreas.

Organizational Structure of the Digestive Tube

• Serosa: Outermost layer, connective tissue covering.

• Muscularis: Smooth muscle layers (circular and longitudinal) responsible for movement.

• Submucosa: Contains blood vessels, nerves, and glands.

• Mucosa: Innermost epithelial layer, involved in secretion and absorption.

Processes and Control of Ingestion and Propulsion

Movement of Digestive Materials

• Peristalsis: Waves of contraction initiated by the circular muscle layer, followed by the longitudinal layer, propelling the bolus (food mass) down the tract.

• Segmentation: Mixing action with no propulsion, churning and breaking up material for better digestion.

Swallowing (Deglutition)

1. Buccal phase: Compaction of bolus and entry into pharynx; only phase under conscious control.

2. Pharyngeal phase: Elevation of larynx, bending of epiglottis, closing of glottis, and contraction of pharyngeal muscles.

3. Esophageal phase: Peristalsis pushes bolus toward stomach; travel time through esophagus is 5-6 seconds.

Mechanical and Chemical Digestion

Oral Cavity

• Functions: Sensory analysis, mechanical processing (teeth, tongue, palate), lubrication (mucus in saliva), enzymatic digestion (salivary enzymes).

• Tongue: Assists in mechanical processing, manipulation for chewing/swallowing, sensory analysis, and speech.

Salivary Glands

• Parotid: Secretes via parotid duct near upper molar.

• Sublingual: Located on the floor of the mouth.

• Submandibular: On either side of the lingual frenulum.

Saliva Functions

• Production increases during eating (about 7 mL/min).

• Composition: 99.4% water, mucins, ions, buffers, waste products, metabolites, enzymes.

• Mucins: Absorb water and form mucus.

• Salivary amylase: Begins chemical digestion of starches.

• Lingual lipase: Starts fat digestion (in small amounts).

• Antibodies and lysozyme: Help control bacterial levels.

Teeth

• Enamel: Hardest biologically manufactured substance, covers crown.

• Dentin: Makes up most of the tooth.

• Pulp cavity: Receives blood vessels and nerves.

Pharynx and Esophagus

Pharynx

• Common passageway for food, drink, and air.

• Pharyngeal muscles propel food toward the esophagus during swallowing.

Esophagus

• Muscular tube (~25 cm long), lined by stratified squamous epithelium.

• Possesses upper and lower esophageal sphincters (circular muscle bands).

Stomach

Functions

• Temporary storage of ingested food.

• Mechanical breakdown.

• Chemical digestion.

• Production of intrinsic factor (for vitamin B12 absorption).

• Chyme: Mixture of food and gastric secretions, highly acidic, partially digested.

Secretions of Gastric Glands

• Parietal cells: Secrete HCl and intrinsic factor.

• Chief cells: Produce pepsinogen (activated to pepsin by HCl).

• Goblet cells: Produce mucus.

• Endocrine cells: Produce gastrin (hormone).

Regulation of Gastric Activity

• Controlled by CNS, local reflexes, and digestive hormones.

• Three phases: Cephalic (prepares stomach), Gastric (food in stomach), Intestinal (chyme enters small intestine).

Stomach Absorption

• No nutrients absorbed in the stomach.

• Mucosa covered in alkaline mucus; epithelial cells lack transport mechanisms.

• Gastric lining is impermeable to water; digestion is incomplete.

Small Intestine

Structure and Function

• Major site of digestion and absorption (90% of nutrient absorption).

• About 6 m (20 feet) long; divided into duodenum, jejunum, and ileum.

• Ileocecal valve: Sphincter marking junction with large intestine.

Intestinal Wall

• Plicae circulares: Transverse folds increasing surface area.

• Villi: Small projections increasing surface area for absorption.

• Each villus contains a lymphatic capillary called a lacteal.

Small Intestine Secretions

• About 1.8 L intestinal juice enters daily.

• Intestinal glands secrete juice to moisten chyme, buffer acid, dissolve enzymes and products, and produce mucus and hormones.

Digestion and Absorption of Macronutrients

Carbohydrates

• Digested by salivary amylase (mouth), pancreatic amylase (small intestine).

• Absorbed as monosaccharides via facilitated diffusion and cotransport.

Lipids

• Digested by lingual lipase (mouth), pancreatic lipase (small intestine).

• Bile salts emulsify fats; absorbed as fatty acids and monoglycerides, reassembled into chylomicrons for transport via lymphatic system.

Proteins

• Digested by pepsin (stomach), pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase) in small intestine.

• Absorbed as amino acids via facilitated diffusion and cotransport.

Summary Table: Digestive System Functions

Function	Description
Ingestion	Entry of food and drink into mouth
Mechanical Processing	Crushing and mixing of food to aid movement and increase surface area
Digestion	Chemical breakdown of food to absorbable size
Secretion	Release of water, acids, enzymes, buffers
Absorption	Movement of nutrients, electrolytes, and water into body
Excretion	Elimination of waste as feces

Additional info:

• These notes integrate the structure and function of the digestive system, covering all major organs, accessory organs, and physiological processes relevant to a college-level Anatomy & Physiology course.