Digestive System
Digestive Tract and the Digestive System
Overview
Consists of major organs: Oral Cavity, Esophagus, Stomach, Small and Large Intestines, Anus.
Associated Glands: Salivary Glands, Liver, Gallbladder, Pancreas.
Other Names: Gastrointestinal Tract, Alimentary Canal.
Primary Function: To obtain molecules from ingested food.
Functions of the Gastrointestinal Tract (GIT)
Ingestion
Introduction of food and liquids into the oral cavity.
Mastication
Chewing that divides solid food into digestible pieces.
Motility
Muscular movements that propel materials through the tract.
Secretion
Release of mucus, digestive enzymes, acidic and alkaline fluids, and bile.
Hormone Release
Hormones that control motility and secretion locally.
Chemical Digestion
Breakdown of macromolecules into smaller units.
Absorption
Uptake of small molecules and water into blood and lymph.
Elimination
Removal of undigested, unabsorbed materials.
General Structure of the Gastrointestinal Tract
The GIT is a hollow tube with variable lumen diameter, comprising four main layers:
1. Mucosa (Mucous Membrane)
Epithelial Lining: Protects and secretes.
Lamina Propria: Loose connective tissue with blood vessels and lymphatics.
Muscularis Mucosae: Smooth muscle layer separating mucosa from submucosa.
2. Submucosa
Denser connective tissue with larger blood and lymph vessels.
Contains Submucosal (Meissner) Plexus: Autonomic nerve network.
3. Muscularis (Muscularis Externa)
Thick smooth muscle layer with two or more sublayers.
Contains Myenteric (Auerbach) Nerve Plexus: Autonomic nerve controlling muscle contractions.
4. Serosa
Thin layer of loose connective tissue covered by simple squamous epithelium, protecting the GIT.
Oral Cavity
Lined by Stratified Squamous Epithelium:
Keratinized: Resistant to abrasions (gingiva, hard palate).
Non-keratinized: Lining mucosa over soft palate, cheeks, pharynx; retains nuclei upon shedding.
Lips/Labia
Structure:
Core of striated muscles for mobility.
Surfaces:
Internal surface: thick non-keratinized lining with minor salivary glands.
Vermillion Zone: thin keratinized epithelium with sensory innervation, no sweat or salivary glands.
Outer surface: thin skin with epidermal and dermal layers.
Tongue
Composed of striated skeletal muscle and covered by thick keratinized epithelium.
Functions:
Manipulates ingested material during chewing and swallowing.
Contains numerous papillae on dorsal surface.
Lingual Papillae Types
Filiform Papillae: Elongated, keratinized, aiding in food movement.
Fungiform Papillae: Mushroom-shaped, contain taste buds, lightly keratinized.
Foliate Papillae: Rudimentary in humans, located on the sides of the tongue.
Vallate Papillae: Largest, located in front of the sulcus terminalis.
Taste Buds
Ovoid structures with 50-100 cells per taste bud, functioning in taste perception.
Types of tastants:
Salty, Sour, Sweet, Bitter, Umami.
Teeth
Adults have 32 permanent teeth divided into incisors, canines, premolars, and molars.
Dentin: Calcified tissue containing hydroxyapatite and collagen.
Enamel: Hardest tissue, primarily composed of calcium hydroxyapatite.
Components of Periodontium:
Cementum, Periodontal Ligament, Alveolar Bone.
Alimentary Canal Components
Esophagus: Muscular tube, primary function is transport to stomach.
Stomach: Dilated digestive segment for mixing and initial breakdown of food into chyme.
Divided into four major regions: Cardia, Pylorus, Fundus, Body.
Gastric Mucosa and Pits
Contains gastric pits that secrete mucous and gastric juice; protects from acid.
Gastric glands consist of various cell types, including parietal and chief cells.
Small Intestine
Final site for digestion and nutrient absorption, comprised of three segments: Duodenum, Jejunum, Ileum.
Villi: Mucosal outgrowths that increase surface area for absorption.
Cells in Small Intestine
Enterocytes: Absorptive cells with a brush border to maximize absorption.
Goblet Cells: Secrete mucins for lubrication.
Paneth Cells: Release antimicrobial peptides and enzymes.
Muscularis and Submucosa of the Small Intestine
Muscularis: Two layers allowing peristalsis; myenteric plexus coordinates contractions.
Submucosa: Contains large blood vessels and Brunner’s glands for alkalizing chyme.
Large Intestine Functions
Absorbs water and electrolytes; forms feces.
Composed of: Cecum, Colon (Ascending, Transverse, Descending, Sigmoid), Rectum.
Features of Large Intestine
No villi; larger diameter than small intestine, puckered into haustra.
Contains goblet cells and absorptive colonocytes.
Important role in MALT (Mucosa-Associated Lymphoid Tissue).
Anal Canal
Transitional zone from rectum to anal canal with stratified squamous epithelium.
Anal Columns: Longitudinal folds associated with sphincters for defecation.
Summary of GIT Features
Distinguishing features: Mucosa, Submucosa, Muscularis, and Serosa vary across the gastrointestinal regions.