Digestive

Learning Objectives

• Describe the anatomy and organization of the digestive system.

• Explain the processes of digestion, absorption, and motility.

• Identify the roles of digestive secretions and hormones.

• Relate physiological mechanisms to common digestive disorders.

Overview of the Digestive System

Major Functions

• Ingestion: The act of taking in food.

• Propulsion: The movement of food through the digestive tract, including:   - Swallowing: Voluntary action that pushes food into the pharynx.   - Peristalsis: Involuntary waves of muscle contraction that move food through the digestive tract.

• Mechanical Digestion: Physical breakdown of food into smaller pieces.

• Chemical Digestion: Enzymatic breakdown of food into nutrients.

• Absorption: The process where nutrients from digested food are absorbed into the blood or lymph.

• Defecation: The elimination of indigestible substances from the body as feces.

Overview of the Digestive System Structure

Two Divisions

Alimentary Canal

• Pathway: Mouth → Pharynx → Esophagus → Stomach → Small Intestine → Large Intestine → Anus.

Accessory Organs

• Include salivary glands, liver, gallbladder, and pancreas, that aid in digestion but are not part of the alimentary canal.

Oral Cavity & Swallowing

Functions

• Ingestion: Taking in food and liquid.

• Mastication: The process of chewing food.

• Lubrication: Saliva helps to moisten food for easier swallowing.

• Carbohydrate Digestion: Initiated by the enzyme salivary amylase.

Secretions

• Salivary Amylase: Enzyme from salivary glands that begins carbohydrate digestion.

Swallowing Phases

1. Oral Phase: Voluntary control where food is pushed to the back of the mouth.

2. Pharyngeal Phase: Involuntary and primarily controlled by the medulla, where food passes through the pharynx.

3. Esophageal Phase: Involuntary peristaltic waves move food down the esophagus to the stomach.

Salivary Reflex

Mechanism of Salivary Secretion

• Stimuli: Pressure receptors and chemoreceptors in the mouth activate salivation.

• Cerebral Cortex: Thinking about, seeing, or smelling food enhances saliva production via autonomic nervous system.

Glandular Secretions

• Sublingual Gland: Increased salivary secretion.

• Parotid Gland: Major gland that contributes to saliva production.

• Submandibular Gland: Also contributes to salivary secretion.

The Stomach

Functions of the Stomach

• Temporary Storage of Food: Food can be stored before digestion.

• Mechanical Mixing: Blends food with gastric juices to form chyme.

• Initiates Protein Digestion: Begins enzymatic breakdown of proteins.

Relevant Structures

• Gastroesophageal Sphincter: Regulates the entry of food from the esophagus into the stomach.

• Pyloric Sphincter: Controls the passage of chyme from the stomach to the duodenum.

• Fundus: Upper part of the stomach where food can be stored.

• Body: The main region of the stomach.

• Antrum: Bottom portion of the stomach that’s involved in grinding solid foods.

Histology of Stomach

Regulation of Gastric Secretion

Phases of Regulation

1. Cephalic Phase: Triggered by sight or smell of food leading to vagal stimulation and increased digestive activity.

2. Gastric Phase: Initiated by food entering the stomach which stimulates gastrin release.

3. Intestinal Phase: When chyme enters the duodenum, it stimulates the release of inhibitory hormones, like secretin and CCK (cholecystokinin).

Hormonal Control

• Gastrin: Increases acid secretion in the stomach.

• Secretin: Decreases gastric activity and stimulates bicarbonate secretion from the pancreas.

• CCK: Stimulates the release of bile from the gallbladder and pancreatic enzymes.

Control Mechanisms of the Digestive System

Nervous Reflexes

• Intrinsic Nerve Plexus (Enteric Nervous System): Controls muscle contractions and secretion in GI tract.

• Short Reflexes: Respond within the digestive system.

• Long Reflexes: Involve the autonomic nervous system, affecting gastric motility and secretion across multiple segments.

Motility of the GI Tract

1. Peristalsis: Propulsive movement pushing contents forward.

2. Segmentation: Mixing contractions to enhance digestion and absorption rates.

3. Tonic Contractions: Sphincters retain contents between segments and control passage.

The Small Intestine

Sections & Functions

• Sections: Divided into the duodenum, jejunum, and ileum.

• Major Site of Digestion and Absorption: Food is digested and nutrients absorbed into the bloodstream.

Structural Adaptations

• Circular Folds (Plicae Circulares): Increase surface area for absorption.

• Villi: Projections that contain capillaries and lacteals for nutrient transport.

• Microvilli: Form the brush border to further increase surface area.

Secretions

• Intestinal Juice: Composed of mucus and digestive enzymes, aiding in digestion.

• Pancreatic Enzymes and Bile: Assist in the digestion of macromolecules.

Pancreas and Digestive Enzymes

Exocrine Function

• Secretes: Digestive enzymes (amylase, lipase, trypsin, chymotrypsin, nucleases) and bicarbonate.   - Bicarbonate: Neutralizes gastric acid in the duodenum.

Regulation of Secretions

• Secretin Release: Triggered by acid in the duodenal lumen, stimulating bicarbonate addition.

• CCK Release: Stimulated by the presence of fats and proteins, promoting enzyme secretions.

Liver and Bile

Functions of the Liver

• Bile Production: Emulsifies fats for easier digestion.

• Detoxification: Removes toxins from the blood.

• Nutrient Metabolism: Processes glucose, lipids, and amino acids.

• Storage: Stores glycogen and fat-soluble vitamins.

Bile Composition & Pathway

• Components: Comprised of water, bile salts, cholesterol, bilirubin, and electrolytes.

• Pathway: Liver → bile ducts → gallbladder → duodenum via common bile duct.

Gallbladder

Function

• Stores and Concentrates Bile: Regulates bile secretion into the small intestine.

Hormonal Regulation

• CCK: Induces contraction of gallbladder, releasing bile into the duodenum.

Clinical Connection

• Gallstones: Result from cholesterol precipitation in the bile, blocking bile flow, causing jaundice and pain.

Nutrient Absorption

Nutrient Types and Mechanisms

The Large Intestine

Main Functions

• Absorb Water & Electrolytes: From indigestible food matter.

• Ferment Undigested Carbohydrates: Gut microbiota aid in fermentation.

• Produce Vitamins: Such as vitamin K and B12 through microbial metabolism.

• Form and Expel Feces: Moved through slow peristalsis and mass movements.

Anatomy

• Pathway: Cecum → Colon → Rectum → Anal Canal.

Motility

• Slow Peristalsis: Allows for absorption.

• Mass Movements: Rapid contractions that move content towards the rectum for elimination.

Summary of Major Functions of the Digestive System

Common Digestive Disorders

Additional Activity

Trace a bite of food through the digestive system, naming each organ in sequence. After this exercise, share with a classmate and fill in:

• Major enzymes involved

• Type of digestion occurring (mechanical or chemical) in each organ.