Overview of the Digestive System

• Purpose: The digestive system breaks down food into simple building blocks to be used for fuel, storage, and other functions. Nutrients travel via the bloodstream and are primarily located in the abdominal pelvic cavity.

Alimentary Canal

• Definition: The alimentary canal consists of the pathway food travels through:

  • Oral cavity

  • Pharynx

  • Esophagus

  • Stomach

  • Small intestine

  • Large intestine

• Functionality:

  • Food enters through the mouth and exits as waste at the end of the canal.

Accessory Organs

• Definition: Accessory organs aid digestion but do not come into direct contact with food. They include:

  • Teeth

  • Tongue

  • Salivary glands

  • Liver

  • Gallbladder

  • Pancreas

• Function of Accessory Organs:

  • Secrete substances that help in digestion, with the exception of teeth and tongue, which interact directly with food.

Functions of the Digestive System

• Main Goals:

  • Break down complex food into nutrients.

  • Other essential functions include:

  • Fluid, electrolyte, and acid-base homeostasis (water and electrolytes from drinks and food).

  • Vitamins and minerals, aiding in metabolic processes and waste elimination.

• **Key Processes:

  1. Ingestion:** The process of taking in food and liquids.

  2. Secretion: Release of digestive juices by digestive organs.

  3. Propulsion: Movement of food through the digestive tract via peristalsis (constriction and relaxation of muscles).

  4. Digestion:

  • Chemical Digestion: Breakdown using chemicals in saliva.

  • Mechanical Digestion: Physical breakdown of food by teeth and tongue.

  1. Absorption: Nutrients absorbed into the bloodstream.

  2. Defecation: Elimination of waste in the form of feces.

Peritoneal Membrane

• Definition of Peritoneal Membrane: Largest thoracic membrane in the body, with two layers:

  • Parietal Peritoneum: Lines the inner surface of the abdominal cavity.

  • Visceral Layer/Serosa: Covers the abdominal organs, providing protection and forming outer tissue layers.

• Cirrhosis Fluid: Found between the layers, aiding in lubricating organs, ensuring smooth movement as they function.

• Organ Classification:

  • Intraperitoneal Organs: Fully located within the peritoneal cavity.

  • Retroperitoneal Organs: Partially outside the peritoneal cavity.

Mesenteries

• Definition: Structures supporting and binding organs together, helping maintain their shape and position.

  • Greater Omentum: Extends from the base of the stomach into the pelvic area; has about 40 layers.

  • Lesser Omentum: Smaller portion extending from stomach to liver.

Blood Supply of the Digestive System

• Major Blood Vessels: Blood supply primarily from the abdominal aorta, branching to supply organs. The portal vein removes waste.

• Nerve Control:

  • Nervous system regulates functions through plexuses:

  • Superior Mesenteric Plexus

  • Splenic Plexus

  • Inferior Plexus

Histology of the Alimentary Canal

• Layers of Tissue: Four primary layers:

  1. Mucosa: Thin inner layer, contains epithelial cells that regenerate quickly, houses blood vessels, and contains mucosa-associated lymphoid tissue (MALT).

  2. Submucosa: Dense irregular connective tissue; contains blood vessels, lymphatic vessels, and submucosal glands with a nerve plexus regulating secretion.

  3. Muscularis Externa: Composed of smooth muscle; provides movement through circular and longitudinal layers.

  4. Serosa: Outermost layer of the alimentary canal composed of simple squamous epithelium and connective tissue, aiding in support and protection.

Motility of the Digestive System

• Definition of Motility: Movement of food through the digestive system primarily occurs via smooth muscle contractions, starting with skeletal muscles in swallowing.

• Key Movements:

  • Swallowing: Initiated by skeletal muscles.

  • Churning: Mixing food in the stomach.

  • Regulation of Motility: Governed by the enteric nervous system (ENS) and the autonomic nervous system.

Oral Cavity Structure and Digestive Functions

• Stages of Digestion: Ingestion, secretion, chemical and mechanical digestion, and propulsion (peristalsis).

• Tongue and Teeth:

  • The tongue mixes food and assists in swallowing; it is anchored by the lingual frenulum.

  • Teeth types (incisors for cutting, canines for tearing, molars for grinding).

• Salivary Glands: Secrete saliva to moisten food and begin carbohydrate digestion.

  • Parotid Gland: Secretes saliva near the second molar.

  • Submandibular Gland: Secretes from the oral cavity.

  • Sublingual Gland: Releases saliva under the tongue.

Composition and Functions of Saliva

• Main Components of Saliva: Water, electrolytes (sodium, chloride, potassium), enzymes (salivary amylase), and immunoglobulins.

• Functions of Saliva:

  • Moisten and lubricate food

  • Aid in chemical digestion

  • Cleanse oral cavity

  • Protect against pathogens.

Pharynx and Esophagus

• Pharynx: Consists of nasopharynx, oropharynx, and laryngopharynx, serving as a passage for food and air.

• Esophagus Functionality:

  • Made of stratified squamous non-keratinized epithelium, designed to secrete mucus for smooth transport of the bolus to the stomach, utilizing peristalsis non-stop.

• Sphincters: Regulate movement in and out of the esophagus:

  • Upper Esophageal Sphincter: Controls food entry.

  • Lower Esophageal Sphincter: Controls entry into the stomach, preventing reflux.