Digestive System Review

Unit 7: Digestive System Notes

Structure of the Digestive System

• Digestive System: Comprises the digestive tract and accessory organs, working together to break down food, absorb nutrients, and eliminate waste.

• Function: Provides essential nutrients for cell maintenance, growth, and overall bodily function.

• Digestive Tract: Also known as the gastrointestinal (GI) tract or alimentary canal, is a muscular tube extending approximately 30 feet from mouth to anus, divided into several distinct organs that each play specific roles in digestion and nutrient absorption.

• Accessory Organs:

  • Salivary Glands: Three main pairs (parotid, submandibular, sublingual) that secrete saliva to aid in digestion by lubricating food, containing enzymes like amylase that begin starch digestion.

  • Liver: The largest internal organ, it produces bile, crucial for fat digestion and absorption, detoxifies various metabolites, and regulates glucose levels in the blood by storing glycogen.

  • Gallbladder: A small pouch that stores and concentrates bile between meals, releasing it into the small intestine when needed for digestion.

  • Pancreas: An exocrine gland that secretes digestive enzymes such as lipase, amylase, and proteases, and also functions as an endocrine gland, releasing hormones like insulin and glucagon to regulate metabolic processes.

Digestive Tract Composition

• Muscular Tube Characteristics:

  • Lined with a mucous membrane that protects the tract and aids in the movement of food.

  • Permanent ridges and temporary folds (plicae circulares) help increase surface area for nutrient absorption and facilitate the movement of chyme.

Layers of the Digestive Tract (from inside to outside):

1. Mucosa: The inner lining consisting of epithelial cells responsible for absorption and secretion, includes lamina propria (supporting tissue) and muscularis mucosae (smooth muscle layer).

2. Submucosa: A dense connective tissue layer containing blood vessels, lymphatic vessels, nerves, and glands that secrete various digestive fluids.

3. Muscularis: Comprised of smooth muscle tissue organized into inner circular and outer longitudinal layers, facilitating peristalsis and segmentation movements essential for digestion.

4. Serosa: The outermost layer of the digestive tract, also referred to as the visceral peritoneum, providing protection and allowing for movement of the digestive organs within the abdominal cavity.

Mesentery Details

• Mesentery: A fold of peritoneum that stabilizes certain digestive organs, preventing their tangling or displacement, while containing blood vessels, nerves, and lymphatics essential for organ function and communication.

Mucosal Features

• Mucosal Epithelium: Begins as stratified squamous epithelium in the oral cavity, transitioning to simple columnar epithelium in the stomach and intestines for optimal absorption and secretion.

• Villi: Microscopic finger-like projections in the small intestine that significantly enhance surface area for nutrient absorption, containing lacteals for fat transport.

• Circular Folds (Plicae Circulares): Permanent folds throughout the small intestine that increase surface area further, assisting in nutrient absorption and slowing the transit of chyme for efficient digestion.

Nerve Plexuses

• Parasympathetic Nerve: Activates digestive activity via the vagus nerve, enhancing glandular secretion and muscular contractions.

• Sympathetic Nerve: Inhibits digestive activity in response to stress, reducing blood flow and diverting energy to essential functions.

• Nerve Plexuses:

  • Submucosal Neural Plexus (Meissner's Plexus): Innervates mucosa and submucosa, regulating enzyme secretion and blood flow.

  • Myenteric Plexus (Auerbach's Plexus): Located in the muscularis, it coordinates peristalsis and gut motility.

Smooth Muscle in the Digestive Tract

• Composed of sheets, bundles, or sheaths of smooth muscle surrounding the digestive tissues.

• Types of Smooth Muscle:

  • Multi-unit Smooth Muscle: Innervated by individual motor units found in structures like blood vessels and the iris.

  • Visceral Smooth Muscle: Contracts as a unit due to electrical connections via gap junctions, found along the digestive tract.

• Motility:

  • Peristalsis: Rhythmic waves of contraction that propel food along the digestive tract.

  • Segmentation: Cycles of contractions that mix food with digestive juices and enhance absorption processes.

Regulation of Digestive Activities

• Local Factors: Changes in pH, physical distortion of the digestive tract, and chemical presence stimulate digestive activities.

• Neural Control Mechanisms: Governed by short and long reflexes, maintaining optimal digestive functions.

  • Short Reflex: Local actions triggered directly by chemoreceptors/stretch receptors within the GI tract.

  • Long Reflex: Involves the CNS; can impact broader areas of the digestive system.

• Hormonal Mechanisms: At least 18 hormones, including gastrin and CCK, influence various digestive processes and organ secretions.

Major Organs of the Digestive Tract

1. Oral Cavity (Mouth): Responsible for mechanical processing (chewing), moistening food with saliva, and initiating digestion via enzymatic action.

2. Pharynx: Functions as a passageway for food and air, directing food to the esophagus.

3. Esophagus: A muscular tube that transports food to the stomach through peristaltic movements.

4. Stomach: Engages in mechanical and chemical digestion, storing food temporarily and breaking down proteins with gastric juices.

5. Small Intestine: The primary site for enzymatic digestion and nutrient absorption, consisting of three segments - duodenum, jejunum, and ileum.

6. Large Intestine: Absorbs remaining water, compacts waste into feces, and stores it before elimination. Includes cecum, colon, and rectum.

Functions of the Digestive Tract

• Ingestion: The entry of food into the oral cavity where it is first processed.

• Mechanical Digestion: The physical breakdown of food via chewing and muscular contractions.

• Chemical Digestion: The enzymatic breakdown of complex food molecules into simpler forms.

• Secretion: The release of digestive fluids (water, acids, enzymes, buffers) into the GI tract.

• Absorption: The movement of digested nutrients into the bloodstream for distribution throughout the body.

• Defecation: The elimination of indigestible substances and waste products from the body.

Oral Cavity Anatomy

• Lining: Composed of stratified squamous epithelium, keratinized in areas like the gums and tongue surface, adapted to withstand abrasion.

• Structures:

  • Teeth: Different types (incisors, canines, premolars, molars) serve roles in cutting, tearing, and grinding food.

  • Tongue: A muscular organ aiding in food manipulation, tasting, and swallowing, covered with taste buds.

  • Gums: Provide support and protection for teeth and contribute to overall oral hygiene.

Pharynx and Esophagus Functionality

• Pharynx: Functions as a passageway for both food and air, divided into three sections - nasopharynx (for air), oropharynx, and laryngopharynx (for food).

• Esophagus:

  • Segments: Includes the upper esophageal sphincter (prevents air entry) and lower esophageal sphincter (prevents backflow of stomach contents).

  • Movement: Engages peristalsis, a series of coordinated waves to transport food to the stomach efficiently.

Stomach Overview

• Regions:

  • Fundus: Stores undigested food and gases released during digestion.

  • Body: The main site for food mixing and initial digestion.

  • Pylorus: The narrow end controlling the release of chyme into the small intestine, regulated by the pyloric sphincter.

• Functions: Temporarily stores food, chemically breaks down proteins and fats, and secretes alkaline mucus to protect the stomach lining from gastric acid.

Small Intestine Structure and Function

• Segments:

  • Duodenum: A short segment where chyme is mixed with bile and pancreatic juice for digestion.

  • Jejunum: Primarily responsible for nutrient absorption, with extensive villi and microvilli.

  • Ileum: Absorbs remaining nutrients and bile acids, concluding at the ileocecal valve where it meets the cecum of the large intestine.

• Adaptations: Circular folds, villi, and microvilli greatly increase the absorptive surface area, allowing efficient nutrient absorption.

Hormonal Regulation of Digestion

• Key Hormones:

  • Gastrin: Stimulates gastric acid secretion and enhances gastric motility, released in response to food consumption.

  • Secretin: Triggers bicarbonate production by the pancreas and reduces gastric activity to prevent acidity in the small intestine.

  • CCK (Cholecystokinin): Stimulates bile release from the gallbladder and pancreatic enzyme secretion, particularly for fat digestion.

  • GIP (Gastric Inhibitory Peptide): Inhibits gastric activity and promotes insulin release to manage blood sugar following meals.

Large Intestine Functions

• Structure: Divided into the cecum, various sections of the colon (ascending, transverse, descending, sigmoid), and the rectum.

• Main Functions: Reabsorbs water and electrolytes, compacts waste into feces for elimination, and absorbs vital vitamins produced by gut flora (e.g., Vitamin K).

Accessory Digestive Organs

1. Salivary Glands: Produce saliva containing enzymes that initiate digestion, maintain oral health, and lubricate food.

2. Liver: Plays a central role in metabolism, detoxification, and producing bile for digestion, also involved in cholesterol management and nutrient processing.

3. Gallbladder: Concentrates bile produced by the liver, releasing it into the small intestine to emulsify fats during digestion.

4. Pancreas: Produces various digestive enzymes and hormones, essential for breaking down carbohydrates, fats, and proteins, as well as regulating blood sugar levels.

Clinical Considerations in Digestive Disorders

• Periodontal Disease: A prevalent condition caused by plaque buildup, leading to inflammation of gums and potential tooth loss, requiring early intervention for management.

• Hepatitis: Inflammation of the liver, which can be caused by viral infections, excessive alcohol consumption, or autoimmune diseases, potentially leading to liver failure.

• Peptic Ulcers: Erosions in the lining of the stomach or duodenum, often caused by H. pylori infection or prolonged use of NSAIDs, may present with abdominal pain and require specific treatments.

• Gallstones: Solid particles that form in the gallbladder, potentially leading to cholecystitis, pain, and digestive issues; management options include dietary changes or surgical interventions.

• Constipation and Diarrhea: Common gastrointestinal issues related to the absorption of water in the large intestine, needing management according to underlying causes and symptoms.