Chapter 17: The Digestive System - Comprehensive Study Guide

Overview of the Digestive System

• Primary Function: The digestive system breaks down food into smaller components into forms that the body can utilize.

• Molecular Breakdown Examples:

  • Proteins are broken down into amino acids.

  • Complex carbohydrates are broken down into monosaccharides.

• Gastroenterology: The scientific study of the digestive tract and its associated disorders.

Stages of Digestion

• Ingestion: The intake of food into the oral cavity.

• Digestion: The mechanical and chemical breakdown of food into a form usable by the body.

• Absorption: The uptake of nutrient molecules into the epithelial cells of the digestive tract, and subsequently into the blood or lymph.

• Compaction: The absorption of water and the consolidation of indigestible residue into feces.

• Defecation: The elimination of feces from the body.

Types of Digestion

• Mechanical Digestion: The physical breakdown of food into smaller particles.

  • Examples: Cutting and grinding action of teeth; churning of the stomach and small intestine.

  • Purpose: To expose a greater surface area of food to the action of digestive enzymes.

• Chemical Digestion: A series of catabolic reactions that break covalent bonds through enzymatic hydrolysis (adding water).

  • Examples: Breaking polysaccharides into monosaccharides; breaking proteins into amino acids.

  • Production Sites: Digestive enzymes are produced in the salivary glands, stomach, pancreas, and small intestine.

  • Nutrients requiring no digestion: Vitamins, minerals, cholesterol, and water are already present in usable forms.

• Catabolism Example (Maltase): The enzyme Maltase breaks down the disaccharide Maltose by adding $H_2O$ to produce two Glucose monosaccharides.

General Anatomy and Accessory Organs

• Digestive Tract (Alimentary Canal/GI Tract):

  • A continuous tube extending from the mouth to the anus.

  • It is open to the external environment at both ends.

  • Components: Mouth, pharynx, esophagus, stomach, small intestine, and large intestine.

• Accessory Organs:

  • Organs that assist in digestion but are not part of the primary tube.

  • Components: Teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

The Mouth (Oral or Buccal Cavity)

• Functions: Ingestion, taste, chewing, and both mechanical and chemical digestion.

• Mastication: The mechanical breakdown of solid particles and mixing them with saliva.

• Lining: Lined with stratified squamous epithelium.

• The Tongue:

  • Manipulates food between the teeth.

  • Covered with nonkeratinized stratified squamous epithelium.

  • Lingual Papillae: Bumps on the surface that contain taste buds.

  • Anatomy: Consists of a root, body, epiglottis, lingual tonsils, and palatine tonsils (lymphatic masses).

Teeth and Dentition

• Function: To break food into smaller pieces to facilitate swallowing and increase surface area for enzyme action.

• Types of Teeth (Midline to Rear):

  • Incisors ($n=4$ per jaw): Used to bite off food.

  • Canines/Cuspids ($n=2$ per jaw): Used to puncture and shred food.

  • Premolars/Bicuspids ($n=4$ per jaw) and Molars ($n=6$ per jaw): Used to crush and grind food.

• Sets of Teeth:

  • Deciduous Teeth: Also known as baby teeth; total of $20$.

  • Permanent Teeth: Also known as adult teeth; total of $32$.

• Tooth Structure:

  • Crown: The portion visible above the gingiva (gum); covered by nonliving enamel.

  • Root: The portion embedded in the jaw bone tooth socket, below the gum; covered by cement.

  • Dentin: Hard, living connective tissue making up most of the tooth.

  • Pulp: Located in the pulp cavity (crown) and root canal (root); contains loose connective tissue, blood vessels, lymphatic vessels, and nerves.

Saliva and Salivary Glands

• Saliva Properties: A watery solution that moistens the mouth, cleanses teeth, inhibits bacterial growth, and dissolves molecules for taste.

• Key Salivary Enzymes:

  • Salivary Amylase: Initiates the digestion of starch.

  • Lingual Lipase: Initiates the digestion of fats.

• Major Salivary Glands:

  • Parotid Glands: Largest glands; produce salivary amylase.

  • Submandibular Glands: Produce a mixture of mucins and serous fluid.

  • Sublingual Glands: Produce mainly mucins.

Histological Layers of the Digestive Tract

• 1. Mucosa (Mucous Membrane):

  • Lines the lumen.

  • Epithelium: Simple columnar (stomach to large intestine) or stratified squamous (oral cavity to esophagus and lower anal canal).

  • Lamina Propria: Contains Mucosa-Associated Lymphatic Tissue (MALT).

  • Muscularis Mucosae: Thin layer of smooth muscle.

• 2. Submucosa:

  • Loose connective tissue containing blood vessels, lymphatic vessels, and nerves.

• 3. Muscularis Externa:

  • Smooth muscle responsible for motility (mixing and propulsion).

  • Structure: Inner circular layer and outer longitudinal layer.

• 4. Serosa:

  • The outermost protective layer.

Neural Regulation and Motility

• Enteric Nervous System (ENS):

  • A branching network of neurons in the submucosa and muscularis externa.

  • Functions: Coordinates muscle activity for mixing and propulsion; regulates secretions to maintain optimal $pH$.

  • Autonomy: Can function independently of the Central Nervous System (CNS).

• Movement Types:

  • Mixing Movements: Rhythmic contractions in small sections that do not move material in a single direction (e.g., segmentation in small intestine, churning in stomach).

  • Propelling Movements: Moves material in one direction. Peristalsis is a wave-like ring of contraction that progresses down the tube.

Mesenteries

• Definition: Connective tissue sheets that suspend the stomach and intestines from the abdominal wall.

• Functions: Hold viscera in place; provide pathways for blood vessels, nerves, lymph nodes, and adipose tissue.

• Lesser Omentum: Extends from the stomach to the liver.

• Greater Omentum: Hangs from the inferior margin of the stomach and loosely covers the small intestine.

The Swallowing Process (Deglutition)

• Oral Phase: Under voluntary control; the tongue forms a food bolus and pushes it into the laryngopharynx.

• Pharyngeal Phase: Involuntary; the palate, tongue, vocal cords, and epiglottis block airways. Pharyngeal constrictors drive the bolus downward; breathing is briefly suspended.

• Esophageal Phase: Involuntary; peristalsis drives the bolus toward the stomach. Relaxation of the lower esophageal sphincter allows entry.

The Esophagus

• Anatomy: A muscular tube passing through the mediastinum and the esophageal hiatus of the diaphragm.

• Cardial Orifice: The opening where the esophagus meets the stomach.

• Lower Esophageal Sphincter (Cardiac Sphincter): Prevents stomach contents from regurgitating into the esophagus (preventing heartburn).

The Stomach: Anatomy and Secretions

• General Feature: A muscular sac in the upper left abdominal cavity; functions as a storage organ and mechanical/chemical digester.

• Chyme: A paste of semidigested food produced by the stomach.

• Regions of the Stomach:

  1. Cardial Part: Within $3\,cm$ of the cardiac orifice.

  2. Fundus: Superior dome against the diaphragm.

  3. Body: The largest distal portion.

  4. Pyloric Part: Region approaching the small intestine; includes the pyloric sphincter.

• Gastric Secretions (Gastric Juice):

  • Pepsinogen: Inactive form of pepsin; secreted by Chief cells.

  • Gastric Lipase: Fat-splitting enzyme; secreted by Chief cells.

  • Hydrochloric Acid ($HCl$): Produced by Parietal cells; activates pepsinogen into pepsin.

  • Intrinsic Factor: Produced by Parietal cells; essential for Vitamin $B_{12}$ absorption.

  • Mucus: Secreted by Mucous cells; protects the lining.

  • Pepsin: Active protein-digesting enzyme formed in the presence of $HCl$.

  • Gastrin: Hormone secreted by Enteroendocrine cells (G cells) in gastric pits.

Protection of the Stomach

• The stomach is protected from its own acidic environment via:

  1. Alkaline Mucus: Coated in thick mucus that neutralizes $HCl$.

  2. Tight Junctions: Prevent gastric juice from seeping between epithelial cells.

  3. Cell Replacement: Epithelial cells live only $3\,to\,6$ days and are rapidly replaced.

• Failure of these mechanisms can result in a peptic ulcer.

The Intestinal Phase and Regulatory Hormones

• Triggered when chyme enters the duodenum.

• Hormonal Control:

  • Secretin: Released by duodenal enteroendocrine cells in response to acidic chyme; stimulates pancreatic duct cells to secrete Bicarbonate ions to buffer acid.

  • Cholecystokinin (CCK): Released in response to fats; stimulates the gallbladder to contract and the pancreas to release enzymes; suppresses gastric activity.

The Pancreas and Pancreatic Juice

• Pancreatic Juice Components:

  • Pancreatic Amylase: Digests starch and glycogen.

  • Pancreatic Lipase: Breaks down triglycerides.

  • Nucleases: Digest nucleic acids.

  • Bicarbonate Ions: Make the juice alkaline ($pH$ buffer).

• Proteolytic Enzymes:

  • Trypsin: Digests proteins; released as inactive trypsinogen (activated by enterokinase).

  • Chymotrypsin: Digests proteins; activated by trypsin.

  • Carboxypeptidase: Digests proteins; activated by trypsin.

The Liver and Gallbladder

• Liver: The body's largest gland (approx. $3\,lbs$). Only digestive function is the secretion of Bile to emulsify fats.

  • Anatomy: Includes Right, Left, Caudate, and Quadrate lobes; Falciform and Round ligaments.

  • Hepatocytes: Liver cells that secrete bile into channels.

• Bile Flow: Hepatocytes $\rightarrow$ Common Hepatic Duct + Cystic Duct (from gallbladder) $\rightarrow$ Bile Duct $\rightarrow$ Duodenum.

• Gallbladder: Stores and concentrates bile.

• Bile Composition: Contains minerals, cholesterol, phospholipids, bile acids, and pigments.

  • Bilirubin: The principal pigment derived from hemoglobin decomposition.

The Small Intestine

• Primary Function: Site of nearly all chemical digestion and nutrient absorption.

• Divisions:

  1. Duodenum

  2. Jejunum

  3. Ileum

• Adaptations for Surface Area:

  • Circular Folds: Internal ridges.

  • Villi: Finger-shaped projections covered with Enterocytes (absorptive cells) and Goblet cells (mucus).

  • Microvilli: Form the Brush Border on absorptive cells; contain enzymes like lipase and peptidase for final digestion.

Nutrient Absorption in the Small Intestine

• Capillaries: Absorb most hydrophilic nutrients into the blood.

• Lacteals: Specialized lymphatic capillaries that absorb most triglycerides (fats).

• Triglyceride Absorption Process:

  1. Fatty acids result from triglyceride digestion.

  2. Fatty acids enter epithelial cells and are resynthesized into triglycerides in the smooth endoplasmic reticulum.

  3. Triglycerides are encased in protein to form Chylomicrons.

  4. Chylomicrons leave the cell and enter the lacteal for transport via lymph.

The Large Intestine

• Anatomy:

  • Cecum: Pouch in the lower right quadrant where the large intestine begins.

  • Appendix: A $2\,to\,7\,cm$ tube attached to the cecum, rich in lymphocytes.

  • Taeniae Coli: Three bands of longitudinal muscle that organize the colon into pouches (haustra).

• Functions: Focuses on water absorption and compaction.