Digestive System Lecture 2

Digestive System Overview

• The digestive system is a muscular tube that extends through the body, composed of several parts: oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.
• Also known as the alimentary tract or gastro-intestinal (GI) tract due to the importance of the stomach and intestine in digestion.
• Function: to change complex organic nutrient molecules into simple organic and inorganic molecules for absorption into the blood or lymph.

Stages of Digestion

• Ingestion: introduction of food and liquid into the oral cavity.
• Mastication: chewing, which divides solid food into digestible pieces.
• Motility: muscular movements of materials through the tract.
• Secretion: lubricating and protective mucus, digestive enzymes, acidic and alkaline fluids, and bile.
• Hormone release: local control of motility and secretion.
• Chemical digestion: enzymatic degradation of large macromolecules into smaller molecules and their subunits.
• Absorption: of small molecules and water into the blood and lymph.
• Elimination: indigestible, unabsorbed components of food.

Oral Cavity

• Consists of teeth, tongue, and openings of salivary glands.
• Two parts: vestibule of the mouth (outside teeth and gums) and mouth cavity proper (inside teeth and gums).
• Teeth: 20 primary teeth in children, 32 permanent teeth in adults; incisors (cutting) and molars (grinding) teeth.

Tongue

• Mass of striated muscle covered by mucosa, allowing high mobility.
• Functions: chewing, speech, taste, and swallowing.
• Two surfaces: smooth ventral surface and irregular dorsal surface with papillae and lingual tonsils.

Salivary Glands

• Exocrine glands producing saliva with digestive, lubricating, and protective functions.
• Collectively produce 1-1.5 L of saliva per day, composed of 99.4% water, ions, and enzymes.
• Two types: minor salivary glands (10% of saliva) and major salivary glands (three pairs: parotid, submandibular, and sublingual).

Pharynx and Esophagus

• Pharynx: connects oral cavity to esophagus; divided into oropharynx, laryngopharynx, and nasopharynx.
• Esophagus: muscular tube, 25 cm long, carrying food into the stomach; no additional digestive processes occur here.

Stomach

• Muscular sac, 50 ml in empty state, expanding to 1.5 L after a meal.
• Located in the upper left quadrant of the abdominal cavity.
• Rugae (folds) in the mucosa allow for expansion without tearing.
• Two ends guarded by muscular rings or sphincters: lower esophagus sphincter (LES) and pyloric sphincter.
• Gastric juice: mixture of hydrochloric acid (HCl) and pepsin, produced by special cells in the stomach lining.

Small Intestine

• Longest part of the digestive tract, 6m long, but narrow in diameter (2.5 cm).
• Divided into three parts: duodenum (first 25 cm), jejunum (2.5 m), and ileum (3.5 m).
• Functions: breaks down food, absorbs nutrients, and allows food to pass from the stomach into the small intestine.
• Villi (tiny finger-like projections) increase the surface area for absorption.

Large Intestine

• About 6.5 cm in diameter and 1.5 m long.
• Outer longitudinal muscle fibers form three separate bands on the surface.
• Functions: absorption of water, electrolytes, and vitamins, and storage of feces.