3.6

Introduction to GI Physiology

• Lecture Information:

  • Course: PSIO 202 University of Arizona

  • Instructor: Tim Maley, PhD

  • Contact: maleytim@arizona.edu

  • Office: Gittings 113

  • Office Hours: Tuesday/Thursday 10-11am

Learning Objectives

• Understand the key functions and processes in the GI tract:

  1. Define and list main functions of the GI tract.

  2. Identify and describe characteristics of each tissue layer in the GI tract, including their functions.

Overview of GI Processes

• Four general processes throughout the GI tract:

  1. Motility:

  • Involves movements within the GI tube.

  • Mediated by smooth muscle, which is autorhythmic.

  1. Secretion:

  • Addition of substances to the GI tube such as enzymes, water, and chemicals.

  1. Digestion:

  • Breakdown of substances—both mechanical (chewing) and chemical (enzyme action).

  1. Absorption:

  • Uptake of digested nutrients from the GI tube into the body.

Regulation of Digestion

• Three major regulatory mechanisms:

  1. Local Factors:

  • Includes pH, physical distortion, and chemical stimuli (nutrients).

  1. Neural Control:

  • Comprises short (in GI tract) and long (CNS) reflex pathways.

  1. Hormonal Control:

  • Involves at least 18 hormones secreted by enteroendocrine cells.

Anatomy of the Digestive System

• Key Organs & Structures:

  • Components include mouth, salivary glands, pharynx, esophagus, stomach, intestines, liver, and gallbladder.

  • Notable sphincters:

  • Lower esophageal, pyloric, and ileocecal.

Digestive System – Mass Balance

• In & Out:

  • Everything entering the digestive system must eventually be excreted.

  • Average intake: 2.0 liters/day of food and drink, with secretions of 7.0 liters/day (enzymes, ions, water, acid, mucus, bile).

  • Reabsorption: 8.9L/day, with only 0.1L/day excreted as feces.

Layers of the GI Tract

• Four Main Layers:

  1. Mucosa:

  • Features muscularis mucosae, lamina propria, and epithelial layer.

  • Varies from simple to stratified epithelium, including blood and lymph vessels.

  1. Submucosa:

  • Loose connective tissue with larger blood and lymphatic vessels and the submucosal plexus (nerve network).

  1. Muscularis Externa:

  • Contains circular and longitudinal smooth muscle layers; controlled by myenteric plexus.

  1. Serosa:

  • Composed of areolar connective tissue and simple squamous epithelium; secretes a lubricating fluid.

Digestive System Regulation

• Regulatory Mechanisms:

  • Involves neural, endocrine, and local signals.

  • Short Reflexes:

  • Integrated in the GI tract (enteric nervous system).

  • Long Reflexes:

  • Integrated via the CNS responding to external stimuli (sight, smell, taste).

  • Effectors:

  • Smooth muscle and glands that alter motility and secretion.

Enteric Nervous System

• Dual plexus structure (myenteric and submucosal) integrating local control.

• Acts autonomously or coordinated with the CNS.

• Responds to stimuli via different receptors: mechanoreceptors, chemoreceptors, osmoreceptors.

Hormonal Regulation of the Digestive System

• Key hormones affect digestive activities:

  • Secretion of acid and enzymes: Influences both motility and secretory activity.

  • Hormonal signaling helps to modulate digestion in response to internal and external cues.

Introduction to GI Physiology

• Lecture Information:

  • Course: PSIO 202, University of Arizona

  • Instructor: Tim Maley, PhD

  • Contact: maleytim@arizona.edu

  • Office: Gittings 113

  • Office Hours: Tuesday/Thursday 10-11am

Learning Objectives

• Understand the key functions and processes in the GI tract:

  1. Define and list the main functions of the GI tract, which include digestion, absorption, secretion, and motility; these functions are essential for nutrient processing and waste elimination.

  2. Identify and describe characteristics of each tissue layer in the GI tract, including the mucosa, submucosa, muscularis externa, and serosa, along with their specific roles and interactions in maintaining gastrointestinal health and function.

Overview of GI Processes

• Four general processes throughout the GI tract:

  1. Motility:

     • Involves coordinated movements within the GI tube necessary for moving food and mixing it with digestive secretions. Different types of movements include peristalsis and segmentation, mediated by smooth muscle, which is autorhythmic due to pacemaker cells.

  2. Secretion:

     • Addition of substances to the GI tube such as digestive enzymes, bile, hydrochloric acid, and water, which facilitate the breakdown of food and absorption of nutrients. Each section of the GI tract secretes specific substances to aid in processing food.

  3. Digestion:

     • Breakdown of substances into smaller, absorbable components through mechanical (e.g., chewing) and chemical (e.g., enzymatic action) processes. Enzymes specific to carbohydrates, proteins, and fats are secreted at various points along the GI tract.

  4. Absorption:

     • Uptake of digested nutrients from the GI tube into the bloodstream or lymphatic system, primarily occurring in the small intestine. Nutrients like glucose, amino acids, fatty acids, vitamins, and minerals are absorbed through the intestinal walls and transported to the body for use.

Regulation of Digestion

• Three major regulatory mechanisms:

  1. Local Factors:

     • Includes factors such as pH levels, physical distortion from food presence, and chemical stimuli like nutrients, which can localize and facilitate digestive processes.

  2. Neural Control:

     • Comprises short (in GI tract) reflex pathways, which involve local autonomic responses, and long (CNS) reflex pathways that integrate signals from the central nervous system in response to external stimuli (e.g., the sight or smell of food).

  3. Hormonal Control:

     • Involves at least 18 hormones secreted by enteroendocrine cells that regulate various digestive processes like secretion and motility, modulating the digestive response based on nutrient content and overall body needs.

Anatomy of the Digestive System

• Key Organs & Structures:

  • Components include the mouth (which initiates digestion), salivary glands (which secrete saliva containing enzymes), pharynx, esophagus, stomach (the primary site for enzymatic breakdown and acid secretion), intestines (small and large), liver (which produces bile for fat emulsification), gallbladder (which stores bile), and pancreas (which provides additional enzymes and bicarbonate).

  • Notable sphincters include:

    • Lower esophageal sphincter (prevents acid reflux),

    • Pyloric sphincter (controls passage from stomach to small intestine),

    • Ileocecal valve (regulates flow from small to large intestine).

Digestive System – Mass Balance

• In & Out:

  • Everything entering the digestive system must eventually be excreted. The average intake is about 2.0 liters/day of food and drink, complemented by secretions totaling 7.0 liters/day (which include enzymes, ions, water, acid, and mucus). The intestines reabsorb approximately 8.9 liters/day, with only about 0.1 liters/day excreted as feces, illustrating the efficiency of the system.

Layers of the GI Tract

• Four Main Layers:

  1. Mucosa:

     • Features the muscularis mucosae (which facilitates local movement), lamina propria (containing blood and lymph vessels, and immune cells), and epithelial layer (specialized for secretion and absorption). The type of epithelium varies from simple columnar in the intestines to stratified squamous in the esophagus.

  2. Submucosa:

     • Composed of loose connective tissue, it houses larger blood and lymphatic vessels, and the submucosal plexus (which contains nerves that regulate secretions).

  3. Muscularis Externa:

     • Contains circular and longitudinal smooth muscle layers, which are responsible for peristalsis and segmentation; controlled primarily by the myenteric plexus.

  4. Serosa:

     • Made up of areolar connective tissue and simple squamous epithelium, secreting a lubricating fluid to allow for smooth movement against surrounding organs.

Digestive System Regulation

• Regulatory Mechanisms:

  • Involves a complex interplay of neural, endocrine, and local signals that collaboratively control the digestive processes.

  • Short Reflexes:

    • Integrated in the GI tract by the enteric nervous system, allowing for rapid responses to local stimuli.

  • Long Reflexes:

    • Integrated via the CNS, allowing the body to react to external stimuli such as the sight, smell, or taste of food, thus preparing the digestive system for incoming food.

  • Effectors:

    • Include smooth muscle and glands that can alter motility, secretion, and overall digestive activity in response to signals.

Enteric Nervous System

• Dual plexus structure (myenteric and submucosal) integrates local control within the digestive system.

• Functions autonomously or in coordination with the CNS, allowing rapid adjustments during digestion and responding to stimuli via different receptors such as mechanoreceptors (detect mechanical changes), chemoreceptors (monitor chemical composition), and osmoreceptors (sense osmotic pressure changes).

Hormonal Regulation of the Digestive System

• Key hormones released (such as gastrin, secretin, and cholecystokinin) affect various digestive activities:

  • Secretion of acid and enzymes: Hormones influence both motility and secretory activity at different points throughout the GI tract; for example, gastrin stimulates acid production in the stomach, while secretin increases bicarbonate secretion from the pancreas.

  • Hormonal signaling plays a crucial role in modulating digestion and metabolic processes in response to internal and external cues.