Digestive System Overview

Introduction

The digestive system is responsible for the breakdown of food, absorption of nutrients, and elimination of waste. It consists of the gastrointestinal (GI) tract and accessory organs that aid in digestion and absorption.

Pathway of Food Through the GI Tract

Tracing the Digestive Pathway

• Mouth → Pharynx → Esophagus → Stomach → Small Intestine (duodenum, jejunum, ileum) → Large Intestine (cecum, colon, rectum) → Anus

• Sphincters and valves regulate passage: upper/lower esophageal sphincters, pyloric sphincter, ileocecal valve, internal/external anal sphincters.

Example: Food enters the mouth, is propelled by swallowing through the pharynx and esophagus, mixed and digested in the stomach, nutrients absorbed in the small intestine, and waste eliminated via the large intestine and anus.

Physical Properties and Functions of Digestive Organs

Major Organs of the GI Tract

• Mouth: Mechanical breakdown, mixing with saliva.

• Pharynx: Passageway for food and air.

• Esophagus: Transports food to stomach via peristalsis.

• Stomach: Churns food, secretes acid and enzymes, initiates protein digestion.

• Small Intestine: Main site of digestion and absorption; divided into duodenum, jejunum, ileum.

• Large Intestine: Absorbs water, forms feces; includes cecum, colon, rectum, anus.

Accessory Organs

• Salivary Glands: Secrete saliva containing enzymes (amylase) for carbohydrate digestion.

• Liver: Produces bile for fat emulsification.

• Gallbladder: Stores and concentrates bile.

• Pancreas: Secretes digestive enzymes and bicarbonate into the duodenum.

Layers of the GI Tract

Histological Organization

• Mucosa: Innermost layer; secretion, absorption, protection.

• Submucosa: Connective tissue with blood vessels, nerves, lymphatics.

• Muscularis externa: Smooth muscle for peristalsis (usually inner circular and outer longitudinal layers; stomach has an additional oblique layer).

• Serosa: Outermost layer; visceral peritoneum.

Structures of the Mouth, Pharynx, and Teeth

Oral Cavity and Associated Structures

• Tongue: Manipulates food, contains taste buds.

• Oropharynx: Passageway for food from mouth to esophagus.

• Salivary Glands: Parotid (serous), submandibular (mixed), sublingual (mucous).

• Teeth: Incisors, canines, premolars, molars; structure includes crown, neck, root, enamel, dentin, pulp cavity.

Stomach Anatomy

Regions and Features

• Cardiac/lower esophageal sphincter: Prevents reflux.

• Cardia, Fundus, Body, Pylorus: Regions of the stomach.

• Pyloric sphincter: Controls passage to duodenum.

• Greater & Lesser Curvatures: Outer and inner stomach borders.

• Rugae: Folds allowing stomach expansion.

• Muscularis externa: Three layers: longitudinal, circular, oblique.

Small Intestine Anatomy

Regions and Microscopic Features

• Duodenum, Jejunum, Ileum: Sequential regions for digestion and absorption.

• Villi: Fingerlike projections increasing surface area.

• Absorptive cells: Enterocytes with microvilli (brush border) for nutrient absorption.

• Goblet cells: Secrete mucus for protection.

• Paneth cells: Secrete antimicrobial substances.

• Intestinal crypts (Crypts of Lieberkühn): Glandular structures at base of villi.

• Lacteal: Lymphatic capillary for fat absorption.

• Blood capillary: Absorbs nutrients into bloodstream.

Large Intestine Anatomy

Regions and Features

• Ileocecal valve: Controls entry from ileum to cecum.

• Colon: Ascending, transverse, descending, sigmoid regions.

• Appendix: Lymphoid tissue attached to cecum.

• Taenia coli: Longitudinal muscle bands.

• Haustra: Pouches formed by taenia coli.

• Rectum and Anus: Terminal regions for feces storage and elimination.

• Hepatic (right) and Splenic (left) flexures: Bends in the colon.

Accessory Organs: Pancreas, Liver, Gallbladder

Pancreas

• Body, tail, head: Anatomical regions.

• Pancreatic duct: Main duct for enzyme delivery to duodenum.

• Accessory duct: Secondary duct (if present).

• Hepatopancreatic sphincter (of Oddi): Controls flow of bile and pancreatic juice into duodenum.

Liver and Gallbladder

• Liver lobes: Right, left, caudate, quadrate.

• Falciform ligament: Separates right and left lobes.

• Round ligament (ligamentum teres): Remnant of fetal umbilical vein.

• Hepatic ducts (right & left): Drain bile from liver lobes.

• Common hepatic duct: Formed by right and left hepatic ducts.

• Gallbladder: Stores and concentrates bile.

• Cystic duct: Connects gallbladder to common bile duct.

• Common bile duct: Delivers bile to duodenum.

Liver Lobule Structure

• Lobule: Functional unit of liver.

• Hepatocytes: Liver cells performing metabolic functions.

• Portal triad: Branches of hepatic artery, portal vein, bile duct.

• Canaliculus: Channels for bile secretion.

• Sinusoid: Capillary channels for blood flow.

• Kupffer cells: Macrophages in sinusoids.

• Central vein: Drains blood from lobule.

Histology of Digestive Organs

Salivary Glands

• Parotid: Serous acinar cells (watery, enzyme-rich saliva).

• Submandibular: Mixed serous and mucous acinar cells.

• Sublingual: Mostly mucous acinar cells (viscous saliva).

• Serous vs. Mucous: Serous cells secrete watery, enzyme-rich fluid; mucous cells secrete viscous, lubricating mucus.

Stomach

• Gastric pit: Surface opening to gastric glands.

• Gastric gland: Contains secretory cells.

• Parietal cells: Secrete hydrochloric acid (HCl) and intrinsic factor.

• Chief cells: Secrete pepsinogen (inactive enzyme).

• Surface epithelium (mucous cells): Secrete protective mucus.

• Mucous neck cells: Secrete acidic mucus.

Small Intestine

• Villus: Increases absorptive surface area.

• Lumen: Central cavity for chyme passage.

• Intestinal crypt: Glandular invaginations.

• Brush border: Microvilli-covered surface for enzyme activity.

• Brunner’s glands (duodenum): In submucosa; secrete alkaline mucus.

• Peyer’s patches (ileum): Lymphoid nodules in submucosa.

• Mucosa, submucosa, muscularis, serosa: Standard GI tract layers.

Large Intestine

• Goblet cells: Abundant, secrete mucus for lubrication.

• Intestinal crypt: Glandular structures.

• Mucosa, submucosa, muscularis, serosa: Standard layers.

• Lumen: Central cavity.

Pancreas

• Islet of Langerhans (pancreatic islet): Endocrine cells.

• Alpha cells: Secrete glucagon (raises blood glucose).

• Beta cells: Secrete insulin (lowers blood glucose).

• Pancreatic acinar cells: Secrete digestive enzymes (exocrine function).

Liver

• Lobule, central vein, sinusoids, hepatocytes: See above.

Cirrhosis

• Cirrhosis: Chronic liver disease characterized by replacement of healthy tissue with scar tissue, impairing liver function.

Blood Glucose and Diabetes

Normal and Abnormal Blood Glucose Ranges

• Best diagnostic tool: Fasting blood glucose and A1C are preferred for diagnosis.

• A1C blood test: Measures average blood glucose over past 2-3 months.

Additional info: The A1C test is especially useful for monitoring long-term glucose control in diabetic patients. Cirrhosis can result from chronic alcohol use, hepatitis, or other liver insults, and leads to impaired metabolism and detoxification.