Introduction to Nutrition - Digestion & Absorption

Introduction to Nutrition

KIN275 Module 3

Digestion & Absorption

Instructor: Julia Totosy de Zepetnek, PhD
Institution: University of Regina
Department: Kinesiology & Health Studies

Module Learning Objectives

  1. Organization of the Digestive System

  2. Path of Ingested Food from the Oral Cavity to the Stomach

  3. Anatomy of the Small Intestine & Regulation of Accessory Organs

  4. Anatomy and Function of Large Intestine

Organization of the Digestive System

Major Subdivisions of the Digestive Tract

  • Mouth

  • Anus

  • Oral Cavity

    • Teeth, Tongue

    • Functions:

    • Mechanical processing

    • Moistening of food

    • Mixing with salivary secretions

  • Pharynx

    • Function: Muscular propulsion of materials into the esophagus

  • Esophagus

    • Function: Transport of materials to the stomach

  • Stomach

    • Functions:

    • Chemical breakdown of materials by acid and enzymes

    • Mechanical processing through muscular contractions

  • Small Intestine

    • Functions:

    • Enzymatic digestion

    • Absorption of water, organic substrates, vitamins, and ions

  • Large Intestine

    • Functions:

    • Enzymatic digestion (limited)

    • Absorption of water, organic substrates, vitamins, and ions

Accessory Organs of the Digestive System

  • Salivary Glands

    • Function: Secretion of lubricating fluid containing enzymes that break down carbohydrates

  • Liver

    • Functions:

    • Secretion of bile (crucial for lipid digestion)

    • Storage of nutrients

    • Various other vital functions

  • Gallbladder

    • Function: Storage and concentration of bile

  • Pancreas

    • Functions:

    • Exocrine cells secrete buffers and digestive enzymes

    • Endocrine cells secrete hormones

Digestive Processes: Food Disassembly

The GI tract and accessory organs work together in six key steps:

  1. Ingestion (oral cavity)

  2. Propulsion

    • Pharynx, esophagus, stomach

  3. Mechanical Breakdown (stomach)

  4. Chemical Digestion (accessory organs)

  5. Absorption (small intestine)

  6. Elimination (large intestine)

Detailed Breakdown of Each Process

1. Ingestion

  • Definition: Taking food into the digestive tract via the mouth

2. Propulsion

  • Definition: Moving food through the GI tract

    • Swallowing: voluntary action

    • Peristalsis: involuntary contraction and relaxation waves that push food between organs

  • Peristalsis: Specific contraction/relaxation cycles that squeeze food along the digestive tract

3. Mechanical Digestion

  • Purpose: Physically prepares food for chemical digestion

    • Processes involved:

    • Chewing (mastication)

    • Mixing food with saliva by the tongue

    • Churning food in the stomach

    • Segmentation: Mixing food with digestive juices; chyme is moved back and forth in the intestines

4. Chemical Digestion

  • Occurs through activated glands that secrete digestive juices into the lumen or hormones into the blood.

  • Process: Complex food molecules are broken down into chemical building blocks:

    • Polymers (e.g., carbohydrates, lipids, proteins) are converted into monomers (e.g., glucose, fatty acids, amino acids)

  • Note: Enzymatic breakdown of food begins in the mouth and is nearly complete in the small intestine.

5. Absorption

  • Definition: Passage of digested end products (along with vitamins, minerals, and water) from the GI tract lumen into the blood.

  • Main Site: Small intestine

6. Elimination

  • Definition: Removal of indigestible substances from the body via the anus

  • Transit Time: Duration taken for food to pass through the entire gastrointestinal tract

Anatomy of the Digestive Tract

Layers of the GI Tract

  1. Mucosa

    • Mucosal epithelium

    • Lamina propria

    • Villi

    • Mucosal glands

  2. Submucosa

    • Contains blood vessels, lymphatic vessels, nerves, and connective tissue

  3. Muscularis Externa

    • Longitudinal and circular muscle layers

  4. Serosa

    • Protective outer covering of the muscularis externa

    • Note: Absent in the oral cavity, pharynx, esophagus, and rectum

Specific Structures of the GI Tract

  • Plicae Circulares: Folds that increase surface area for absorption

  • Villi and Microvilli: Projections that enhance absorption by increasing surface area

Functions of Each GI Structure

  • Stomach:

    • Storage of food (2-4 L capacity)

    • Physical degradation (mechanical breakdown) and chemical degradation (segmentation to produce chyme)

    • Secretion of intrinsic factor essential for vitamin B12 absorption in the small intestine

    • Delivery of chyme to the small intestine

Digestion Journey: From Oral Cavity to Stomach

Oral Cavity Functions

  1. Sensory Analysis: Evaluating attributes of food before swallowing

  2. Mechanical Processing: Chewing by teeth; mixing by tongue

  3. Lubrication: Mixing food with saliva and mucus

  4. Limited Digestion: Initial breakdown of carbohydrates (CHO) and lipids

Saliva Composition

  • Water: 99.4%

  • Electrolytes: (Na+, K+, Cl-) including buffers (HCO3-) to maintain pH around 7.0

  • Mucins: Contribute to the lubricating action of saliva

  • Antibodies: Help control oral bacteria

  • Enzymes:

    • Salivary Amylase: Begins starch digestion

    • Lingual Lipase: Starts lipid digestion

  • Production Rate: 1.0 - 1.5 L of saliva/day

Pharynx and Esophagus

  • Pharynx: Passageway for food, liquids, and air

  • Esophagus: Conveys food and liquid to the stomach, coordinated by 22 separate muscle groups (esophageal sphincter prevents reflux)

Stomach Anatomy and Function

  • Anatomical Regions: Cardia, Fundus, Body, Pylorus

  • Layers: Similar structure layers as the GI tract but with an extra muscle layer for strength

  • Functions:

    • Storage: ~50 mL empty; 2-4 L capacity

    • Chemical breakdown: Acid (HCl) & digestive enzymes disrupt food bonds and produce chyme

    • Secretion of pepsinogen activated by HCl to digest proteins

Gastric Secretions

  • Parietal Cells: Secrete intrinsic factor (required for vitamin B12 absorption) and HCl

    • H+ and Cl- transported independently through various mechanisms

  • Chief Cells: Secrete pepsinogen which is activated by HCl to pepsin for protein digestion

Acidic Environment of the Stomach

  • pH: Ranges from 1.5 to 2.0

  • Functions:

    1. Kill microorganisms

    2. Denatures proteins

    3. Breaks down plant cell walls and tissue

    4. Activates pepsin for protein digestion

Gastric Ulcers

  • Definition: Injured mucosal area caused by gastric juices

Summary of Digestion Process: From Mouth through Stomach

  • Mouth: Salivary amylase begins CHO digestion, little to no lipid digestion

  • Stomach: Acid neutralizes salivary amylase, halting CHO digestion, protein digestion begins as gastric acid activates proteases

Transition of Digestion to the Small Intestine

Key Hormones and Secretions in Digestion

  • Secretin: Stimulates bile secretion and pancreatic buffer release in response to acidic chyme

  • Cholecystokinin (CCK): Triggers gallbladder contraction and pancreatic juice secretion in response to lipids in chyme

Absorption in the Small Intestine

  • Nutrient Absorption: 90% of nutrient absorption occurs here, including water, vitamins, minerals, carbohydrates, lipids, and proteins

  • Structural Modifications:

    • Plicae circulares: Circular folds in the intestinal lining

    • Villi: Finger-like projections that enhance absorption

    • Microvilli: Further increase surface area

Conclusion on Digestion and Absorption Techniques

The digesting process transforms food from its initial form into usable nutrients through a complex yet efficient system that ensures maximum absorption and preparation of food, ultimately highlighting the intricate workings of the human digestive system.