Digestive System Lectures

Learning Objectives

• State the key structure and function of the entire gastrointestinal tract.

• Define in detail the layers of the gut wall, including the tissue and cells found in each.

• Define the mucosa.

• Relate the structure of each layer to the overall function of the gastrointestinal tract.

Overview of the Gastrointestinal Tract

• The digestive system is a long tube from the mouth to the anus.

• Organs such as the esophagus, stomach, small and large intestine are essential for digestion.

• Accessory organs include the salivary glands, tongue, pancreas, liver, and gallbladder.

Key Functions of the Digestive System

• Ingestion: Taking in food.

• Digestion: Breaking down food into nutrients.

• Absorption: Pulling nutrients into the bloodstream.

• Excretion: Removing waste products.

The Journey of Food Through the Digestive System (Pizza Example)

• Oral Cavity: Food enters the mouth where teeth masticate (chew) it into smaller fragments, initiating mechanical digestion.

• The tongue tastes and manipulates the food.

• The hard palate provides a surface for the tongue to break food against.

• The soft palate and uvula prevent food from entering the nasal cavity, directing it into the esophagus.

• Salivary Glands: Sublingual, submandibular, and parotid glands secrete saliva to lubricate food.

• Saliva helps compact food into a bolus.

• Saliva contains amylase, which begins chemical digestion by breaking down carbohydrates into smaller sugars.

• Pharynx and Esophagus: Upon swallowing, the epiglottis seals off the airway, directing the bolus into the esophagus.

• Swallowing is a voluntary action that begins propulsion which includes peristalsis.

• Peristalsis involves involuntary waves of muscle contraction and relaxation, moving food through the GI tract.

• Stomach: Mechanical and chemical digestion continues, breaking down food into specific nutrients.

• Mechanical Digestion: A physical process that reduces food size to increase surface area and mobility.

• Chemical Digestion: Involves digestive secretions (enzymes, water, acids, salts) breaking down complex food molecules into chemical building blocks.

• This process starts in the mouth and continues in the stomach, but mainly occurs in the small intestine.

• Absorption: Nutrients enter the bloodstream primarily in the small intestine, facilitated by specific structures.

• Defecation/Excretion: Undigested materials are removed from the body as feces.

Functions of the GI Tract

Layers of the Gastrointestinal Tract Wall

• The GI tract has different tissue and cell layers that facilitate digestive organ function.

• The GI tract is essentially a tube characterized by the following layers from outermost to innermost:

  • Adventitia/Serosa

  • Muscularis Externa

  • Submucosa

  • Mucosa

Layer Breakdown

• Adventitia/Serosa: Outermost layer of the GI tract.

  • Adventitia: Thick fibrous connective tissue.

  • Serosa: Slippery serous membrane.

• Muscularis Externa: Layer of smooth muscle.

• Submucosa: Dense layer of tissue containing blood vessels, lymphatics, and nerves.

• Mucosa: Innermost layer lining the lumen; comprised of three layers.

  • Muscle layer.

  • Connective tissue.

  • Epithelial lining cells.

Detailed Look at Each Layer

Serosa vs. Adventitia

• Serosa: Thin layer of connective tissue covered with a simple squamous epithelium.

  • Present in the lower esophagus and ends just before the rectum.

  • Covers organs within cavities not exposed to the outside (e.g., stomach, small and large intestine).

  • Secretes a lubricating fluid to reduce friction between organs during movement.

• Adventitia: Outermost layer of organs not associated with the peritoneal cavity.

  • Located in the oral cavity, pharynx, upper esophagus, and rectum.

  • Comprised of fibrous connective tissue that binds structures together.

Muscularis Externa

• Muscle type (smooth or skeletal) depends on location.

• Skeletal muscle: Mouth, pharynx, upper esophagus, and anus; allows voluntary control of swallowing and defecation.

• Smooth muscle: Remainder of esophagus, stomach, and small and large intestines; involuntary control.

• Inner circular muscle layer: Arranged in circular rings.

  • Contracts and constricts the tract behind the food, preventing backward movement.

• Longitudinal layer: Arranged along the length of the GI tract.

  • Relaxes and lengthens, pulling food forward.

• Peristalsis: Coordinated wave-like muscle contractions that squeeze the bolus in one direction.

• Sphincters: Thickened circular muscle layers that control passage between GI tract segments (e.g., esophageal sphincter).

• Myenteric Plexus: Nerve network between circular and longitudinal muscle layers.

  • Coordinates muscle contractions and relaxation, controlling gut motility and smooth muscle relaxation upon activation.

Submucosa

• Layer of loose connective tissue, blood vessels, lymphatics, and nerves.

• Submucosal Plexus: Nerve network within the submucosa.

• Responsible for controlling blood vessel size and secretion of digestive juices.

Mucosa

• Inner lining of the intestine; mucous membrane lining cavities open to the exterior of the body.

• Comprised of three layers:

  • Epithelium.

  • Lamina Propria.

  • Muscularis Mucosa.

Muscularis Mucosa

• Outermost layer of the mucosa.

• Thin layer of smooth muscle that contracts to break down food.

• Causes folds to form within the mucosal layer.

• Increases local movement and absorption by exposing new surfaces.

Lamina Propria

• Middle layer of the mucosa.

• Thin layer of loose connective tissue with blood and lymph vessels.

• Essential for nutrient absorption into the bloodstream.

• Provides support for the epithelium.

  • Binds to the muscularis mucosa below.

• Mucosa-associated lymphatic tissue (MALT) provides defense against infection.

Epithelial Layer

• Innermost/top layer of the mucosa.

• Cells in direct contact with ingested food.

• Epithelium type varies by location.

  • Stratified squamous epithelium (mouth, esophagus, anus): Multilayered for protection against abrasion (e.g., from food like Doritos).

  • Simple columnar epithelium (remainder of tract): Secretes enzymes and absorbs nutrients.

• Key cell types:

  • Goblet cells.

  • Enteroendocrine cells.

• Renewal rate: Fast (5-7 days) to maintain function.

Structural Differences Based on Organ Location

• Protection from abrasion (stratified squamous epithelium in the esophagus).

• Stomach: Muscularis externa has an additional oblique muscle layer for vigorous mixing and mechanical disruption.

• Small Intestine: Epithelial structure allows efficient nutrient absorption through increased surface area.

Control of the Gastrointestinal Tract

• GI tract walls contain sensors (mechanoreceptors, chemoreceptors) that regulate digestive functions.

• Receptors detect mechanical and chemical stimuli, such as stomach expansion due to food presence.

• Stimulation of receptors provokes reflexes that further the digestion process.

• Nerve Plexuses:

  • Intrinsic Nerve Plexus: Responds to stimuli within the GI tract.

  • Extrinsic Nerve Plexus: Involves central and autonomic nervous systems; responds to stimuli outside the digestive system.

  • Enteric Nervous System: Governs GI tract function via both intrinsic and extrinsic nerve plexuses.

• Hormones:

  • Released to assist with digestion.

  • Control effectors like muscles, glands, and blood vessels.

  • Gastrin: Secreted in the stomach in response to food presence; stimulates gastric acid secretion to break down food into chyme.

• Neural and hormonal controls regulate GI tract effectors within those different layers of the GI described.

Summary

• The digestive system ingests, digests food, absorbs nutrients, and excretes waste.

• The lecture described the layers of the gut wall, setting the stage for the next lecture on how specific structural changes dictate and maintain overall function.

Revision Questions

• State the key structure and function of the entire gastrointestinal tract.

• Define in detail the layers of the gut wall, including the tissue and cells found in each.

• Define the mucosa.

• Relate the structure of each layer to the overall function of the gastrointestinal tract.