Chapter 25

Chapter 25: The Digestive System

Outline

  • General Anatomy and Digestive Processes

  • Mouth Through Esophagus

  • Stomach

  • Liver, Gallbladder and Pancreas

  • Small Intestine

  • Chemical Digestion and Absorption

  • Large Intestine

General Overview of the Digestive System

  • Definition and Function of the Digestive System   - The digestive system is an organ system that processes food, extracts nutrients, and eliminates waste.   - Nutrients consumed must be broken down before they can be utilized by the body, functioning like a “disassembly line”.   - Key Terminology:     - Gastroenterology: The study of the digestive tract and its disorders.

  • Five Stages of Digestion:   1. Ingestion: Selective intake of food.   2. Digestion: Mechanical and chemical breakdown of food into usable forms.   3. Absorption: Uptake of nutrient molecules into epithelial cells of the digestive tract, into blood and lymph.   4. Compaction: Absorbing water, consolidating indigestible residue into feces.   5. Defecation: Elimination of feces.

Anatomy of the Digestive System

  • The digestive system consists of two main anatomical subdivisions:   - Digestive Tract (Alimentary Canal): A 30-foot muscular tube including:     - Mouth, Pharynx, Esophagus, Stomach, Small Intestine, and Large Intestine.     - Also referred to as the gastrointestinal (GI) tract, which mainly includes the stomach and intestines.   - Accessory Organs: These include:     - Teeth, Tongue, Salivary Glands, Liver, Gallbladder, and Pancreas.

Structural Features of the Digestive Tract

  • Open System: The digestive tract is open to the environment at both ends.   - Material in the digestive tract is considered external to the body until absorbed by epithelial cells.

  • Layers of the Digestive Tract Wall: From inner to outer surfaces:   1. Mucosa:      - Epithelium, Lamina Propria, Muscularis Mucosae.   2. Submucosa.   3. Muscularis Externa:      - Inner circular layer and outer longitudinal layer.   4. Serosa:      - Areolar tissue and mesothelium.

Variations in Layers of the GI Tract

  • Mucosa:   - Varies in epithelium type according to location (e.g., simple columnar in most, stratified squamous in mouth, esophagus, and anal canal).   - Contains mucosa-associated lymphatic tissue (MALT) abundant with lymphocytes and nodules.

  • Muscularis Externa:   - Comprises two muscle layers with valves (sphincters) regulating material passage.

  • Adventitia:   - A fibrous connective tissue layer binding organs together in some GI sections.

Digestive Processes

  • Mechanical Digestion: Physical breakdown of food into smaller parts (chewing, churning).

  • Chemical Digestion: Hydrolysis reactions breaking down dietary macromolecules into monomers:   - Polysaccharides → Monosaccharides   - Proteins → Amino Acids   - Fats → Monoglycerides and Fatty Acids   - Nucleic Acids → Nucleotides.

  • Certain nutrients (e.g., vitamins, free amino acids) are absorbed in their usable form without prior digestion.

Mouth and Oral Cavity

  • Anatomy of the Mouth: Includes structures like the tongue, teeth, and salivary glands, which are critical for digestion initiation.

  • Role of the Tongue:   - Muscular organ that manipulates food, sensitive to touch; covered in nonkeratinized stratified squamous epithelium.   - Lingual Papillae: Sites of taste buds.

  • Dentition:   - Humans have 32 adult teeth, which serve various functions in mechanical digestion:     - Incisors (cutting), Canines (puncturing), Premolars (grinding), Molars (crushing).

  • Mastication (Chewing): First step in digestion, involving various muscles and reflexes.

  • Saliva: Contains:   - 97%–99.5% water.   - Salivary amylase (digests starch).   - Lingual lipase (activated in the stomach to digest fats).   - Mucus (aids swallowing).   - Electrolytes, and antibacterial agents like lysozyme and immunoglobulin A.

Esophagus

  • A muscular tube 25-30 cm long, extending from pharynx to stomach through the esophageal hiatus in the diaphragm.

  • Lower Esophageal Sphincter: Prevents backflow of stomach contents into the esophagus, protecting its lining from acidity.

  • Mucosa: Lined with nonkeratinized stratified squamous epithelium, containing esophageal glands that secrete mucus.

Stomach

  • Functions as a food storage organ, performing mechanical and chemical breakdown:   - Internal volume varies from 50 mL (empty) to 1.0–1.5 L (after a meal) to up to 4 L when extremely full.

  • Gross Anatomy:   - J-shaped with lesser and greater curvatures, divided into four regions:     - Cardiac, Fundic, Body, Pyloric.

  • Innervation and Circulation:   - Receives parasympathetic fibers from the vagus and sympathetic fibers from celiac ganglia.   - Supplied by branches of the celiac trunk with blood drained through the hepatic portal system for liver filtration.

Microscopic Anatomy

  • Mucosa: Covered by simple columnar epithelium containing mucin, forming gastric mucus.

  • Gastric Pits: Lead to gastric glands producing secretions involved in digestion.

  • Cells of Gastric Glands:   - Mucous Cells: Secretes mucus.   - Parietal Cells: Secretes HCl and intrinsic factor (crucial for vitamin B12 absorption).   - Chief Cells: Secretes pepsinogen.   - Enteroendocrine Cells: Release hormones regulating digestion.

Gastric Secretions and Motility

  • Gastric Juice: Secreted up to 2-3 liters per day, containing water, HCl, and pepsin—activates digestion.

  • Functions of HCl:   - Activates pepsin.   - Degrades connective tissue in food.   - Converts ferric ions to ferrous ions.   - Acts as a defense against pathogens.

  • Gastric Motility:   - Includes peristaltic contractions for churning food, mixing it with gastric juice.   - Regulated by pacemaker cells and influenced by food intake.

  • Regulation of Gastric Function:   - Divided into three phases:     - Cephalic Phase: Brain controls stomach response to food cues.     - Gastric Phase: Stomach regulates itself after food ingestion.     - Intestinal Phase: Small intestine moderates gastric activity based on chyme presence.

Liver, Gallbladder, and Pancreas

  • Liver: The largest gland (around 1.4 kg) with multiple functions, primarily secreting bile to aid in digestion.

  • Hepatocytes: Cells that absorb nutrients post-meal, filter blood, secrete proteins and hormones, and manage glucose levels.

  • Gallbladder: Stores and concentrates bile; connected to the bile duct that also transports bile from the liver to the small intestine.

  • Pancreas: A spongy gland producing digestive enzymes and releasing them via the pancreatic duct into the duodenum:   - Endocrine Role: Insulin and glucagon secretion.   - Exocrine Role: Produces pancreatic juice containing enzymes like lipase and amylase.

Small Intestine

  • Major site for chemical digestion and nutrient absorption, 2.7 to 4.5 m long based on muscle tone.

Gross Anatomy

  • Divided into:   1. Duodenum: Receives chyme and digestive juices; pH neutralization occurs here.   2. Jejunum: Major site of nutrient absorption; thicker walls and rich blood supply.   3. Ileum: Absorbs remaining nutrients; contains Peyer patches for immunity.

Microscopic Anatomy

  • Lined with simple columnar epithelium, modified for absorption:   - Villi: Projections increasing surface area for absorption.   - Microvilli: Further increase surface area and harbor enzymes for digestion.

Digestion and Absorption

  • Carbohydrates:   - Digestible carbohydrates like starch are converted into glucose by the enzyme amylase, primarily in the small intestine.

  • Proteins:   - Proteins are broken down into amino acids by proteases in the stomach and small intestine.

  • Lipids:   - Lipid digestion is facilitated by emulsification via bile acids, allowing lipases to act effectively.

  • Nucleic Acids and Minerals:   - Nucleic acids are hydrolyzed to nucleotides and absorbed; minerals are absorbed as needed along the intestine.

Large Intestine

  • Function: Receives indigestible residue, absorbs water and salts, and produces feces.

  • Components: Includes cecum, colon (ascending, transverse, descending, sigmoid), and anal canal.

Microscopic Anatomy

  • Lined with simple columnar epithelium and abundant goblet cells for mucus secretion, with a higher density of lymphatic tissue for immunity.

Bacterial Flora

  • Contains diverse bacteria essential for synthesizing certain vitamins and fermentation of organic materials, producing gas (flatus).

Absorption and Motility

  • Takes 12 to 24 hours for the large intestine to process contents, mainly through haustral contractions and mass movements triggered by gastric or intestinal activity.