THE DIGESTIVE SYSTEM

Chapter 25: The Digestive System Anatomy & Physiology

25.1 Digestive Function

• Digestive System: Organ system processing food, extracting nutrients, and eliminating waste.

• Stages of Digestion:

  • Ingestion: Selective intake of food.

  • Digestion: Mechanical and chemical breakdown of food into usable form.

  • Absorption: Uptake of nutrients into epithelial cells of the digestive tract, then into blood and lymph.

  • Compaction: Absorbing water and consolidating indigestible residue into feces.

  • Defecation: Elimination of feces.

25.2 Mechanical and Chemical Digestion

Mechanical Digestion

• Physical breakdown into smaller particles by:

  • Teeth: Cutting and grinding action.

  • Stomach & Intestines: Churning action.

Chemical Digestion

• Hydrolysis reactions propelled by digestive enzymes:

  • Carbohydrates → Monosaccharides.

  • Proteins → Amino acids.

  • Fats → Monoglycerides & Fatty acids.

  • Nucleic acids → Nucleotides.

• Nutrients existing in usable form include vitamins, amino acids, minerals, cholesterol, and water.

25.3 General Anatomy of the Digestive System

• Subdivision: Digestive tract (30 ft muscular tube: mouth to anus) and accessory organs (e.g., teeth, tongue, salivary glands, liver, gallbladder, pancreas).

• Layers of Digestive Tract Wall:

  • Mucosa: Lines lumen; includes epithelium, lamina propria, muscularis mucosae.

    • Simple columnar in most of the digestive tract

    • Stratified Squamous form the mouth through esophagus, and in lower anal canal

  • Submucosa: Thicker layer containing blood vessels, glands, and nerves.

  • Muscularis Externa: Two layers of muscle; regulates passage of material via sphincters.

  • Serosa: Outer layer; provides supportive structure.

25.4 Enteric Plexus

• Nervous network inside digestive organs regulating motility, secretion, and blood flow.

• Composed of:

  • Submucosal plexus: Controls glandular secretions and muscular movements.

  • Myenteric plexus: Controls peristalsis.

25.5 Relationship to the Peritoneum

• Mesentery: Connective tissue sheet anchoring the intestines, prevents twisting, and provides passage for vessels and nerves.

• Organ Relationships:

  • Intraperitoneal: Organs encased by mesentery (e.g., stomach, liver).

  • Retroperitoneal: Organs against posterior body wall (e.g., duodenum, pancreas).

25.6 Regulation of the Digestive Tract

• Control Mechanisms:

  • Neural Controls:

    • Short reflexes (myenteric) via myenteric plexus for local control.

    • Long reflexes (vagovagal) involving vagus nerve for systemic responses.

  • Hormonal Controls: Hormones like gastrin and secretin affect distant digestive tract parts.

  • Paracrine Secretions: Local chemical messengers that affect neighboring cells.

25.7 The Mouth

• Functions: Ingestion, taste, chewing (mastication), chemical digestion, swallowing.

• Structure: Enclosed by cheeks, lips, tongue; lined by stratified squamous epithelium for protection.

• Tongue: Manipulates food, senses taste; has intrinsic extit{ (subtle movements)} and extrinsic muscles (stronger movements).

• Salivary Glands: Secrete saliva for moisture, digestion initiation, and cleansing teeth. Includes intrinsic (minor) glands and extrinsic (major) glands.

  The Tongue

  • Functions:

    • Ingestion: The tongue is essential for handling food during the intake process.

    • Taste: It is equipped with taste buds that allow for the perception of different flavors.

    • Chewing (Mastication): The tongue aids in moving food around in the mouth to facilitate chewing.

    • Chemical Digestion: It plays a role in mixing food with saliva, starting the digestive process.

    • Swallowing: The tongue is crucial for pushing food towards the pharynx during swallowing.

  • Structure:

    • The tongue is enclosed by cheeks and lips, and lined by stratified squamous epithelium for protection.

    • It consists of both intrinsic and extrinsic muscles:

      • Intrinsic Muscles: Allow for subtle adjustments in shape and size for precise movements.

      • Extrinsic Muscles: These provide stronger movements for manipulation and positioning of food.

  Saliva and Salivary Glands

  Saliva: A clear, watery fluid produced in the mouth, comprised of about 99% water, with the remaining 1% consisting of electrolytes, enzymes, antimicrobial substances, and mucus.

  • Functions of Saliva:

    • Moistening: Keeps the oral cavity moist, aiding in speech and swallowing.

    • Digestion: Begins the digestion of carbohydrates through the enzyme amylase, which breaks down starch into maltose and dextrin.

    • Taste: Dissolves food particles, enhancing taste perception by allowing taste receptors to engage with food.

    • Protection: Contains antimicrobial compounds like lysozyme and immunoglobulin A, which help prevent infections and maintain oral health.

  Salivary Glands: Glands that secrete saliva into the oral cavity. They are divided into major and minor glands.

  • Major Salivary Glands:

    1. Parotid Gland: Largest; mainly secretes serous fluid rich in amylase.

    2. Submandibular Gland: Produces both serous fluid and mucus; responsible for the majority of saliva production.

    3. Sublingual Gland: Smallest; primarily secretes mucus.

  • Minor Salivary Glands: Numerous, small glands located throughout the oral mucosa, providing continuous saliva secretion for lubrication.

  Regulation of Saliva Production: Stimulated by the autonomic nervous system, with increased production during eating due to the sight, smell, or taste of food; inhibited during stress or dehydration.

25.8 The Pharynx and Esophagus

• Pharynx: Muscular funnel connecting mouth to esophagus; regulates swallowing.

• Esophagus: Tube connecting pharynx to stomach; has structural adaptations for protection during food transit.

Microscopic Anatomy of the Digestive System

Microscopic anatomy studies the histological structures of the digestive system at a cellular level.

Key Components:

• Mucosa: The innermost layer of the digestive tract, composed of three sub-layers:

  • Epithelium: Varies throughout the tract; functions include secretion and absorption.

  • Lamina Propria: Loose connective tissue that contains blood vessels, lymphatic vessels, and nerves.

  • Muscularis Mucosae: Thin layer of smooth muscle that helps in the movement of the mucosal layer, enhancing absorption and secretion.

• Submucosa: A thicker layer consisting of dense connective tissue containing larger blood vessels and nerves, providing support and connecting layers.

• Muscularis Externa: Composed of two layers of smooth muscle (inner circular and outer longitudinal) that facilitate peristalsis and segmentation for digestion and propulsion of food.

• Serosa: The outer layer covering the digestive organs, composed of connective tissue and mesothelium, providing protection and reducing friction with surrounding organs.

Specialized Structures:

• Villi and Microvilli: Located in the small intestine, these structures increase the surface area for absorption. Villi are finger-like projections, while microvilli are microscopic folds on the epithelial cells.

• Glands: Submucosal and mucosal glands secrete digestive enzymes, hormones, and mucous, playing a crucial role in digestion and maintaining intestinal health.

25.9 The Stomach

• Function: Food storage, mechanical breakdown, and protein digestion yielding chyme.

• Structure: J-shaped with four regions (cardiac, fundic, body, pyloric) and special layers in the mucosa for gastric secretion.

• Secretions: Gastric juice produced contains enzymes and acids vital for digestion.

• Protection: Mucous coat, tight junctions, and rapid cell replacement protect the stomach lining from gastric juices.

• Clinical Notes: Issues such as ulcers and gastritis can arise from disruptions in protective mechanisms.

25.10 The Small Intestine

• Function: Major site for digestion and nutrient absorption, longest part of the digestive system.

• Regions: Duodenum (initial portion with neutralization), jejunum (primary absorption), ileum (final absorption).

• Internal Structure: Contains folds, villi, and microvilli to maximize absorptive surface area.

25.11 The Large Intestine

• Structure: Measures 1.5 m long, involving the cecum, colon, rectum, and anal canal.

• Functions: Water absorption, feces formation; excess water recovery leads to constipation.

• Microbiome: Over 1000 species residing aid in digestion and synthesizing vitamins; gas production results from bacterial activity.

25.12 Colorectal Cancer

• Risk Factors: Age, genetic predisposition, lifestyle factors (diet, smoking).

• Symptoms: Diarrhea, constipation, blood in stool; can lead to severe complications if untreated.