Digestive System Learning Objectives

Digestive System Components

• The digestive system consists of:
  • Alimentary canal (mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus)
  • Accessory organs (salivary glands, liver, pancreas, gallbladder)

Functions of the Digestive System

• General functions:
  • Ingestion
  • Gastrin stimulates parietal cells to secrete HCl and ECL cells to secrete histamine, which stimulates HCl secretion
  • Gastrin also increases gastric motility and promotes the growth of gastric mucosa

Intestinal Phase

• Enterogastric reflex:
  • Triggered by the presence of chyme in the duodenum, especially if it's acidic, fatty, or hyperosmotic
  • Stretch receptors and chemoreceptors in the duodenum initiate this reflex
  • Inhibits gastric secretion and motility, and tightens the pyloric sphincter
  • Slows gastric emptying, preventing overloading of the duodenum
• Hormonal mechanisms:
  • Cholecystokinin (CCK) is secreted by the duodenum in response to fats and proteins in chyme
    • Inhibits gastric secretion and motility, and stimulates the secretion of pancreatic enzymes and bile
  • Secretin is secreted by the duodenum in response to acidic chyme
    • Stimulates the secretion of pancreatic bicarbonate to neutralize the acid
  • Gastric inhibitory peptide (GIP) is secreted by the duodenum in response to glucose and fat
    • Inhibits gastric secretion and motility

Absorption Mechanisms

• Water:
  • Absorbed by osmosis, following the absorption of solutes like carbohydrates and amino acids
  • Glycerol from lipids and certain amino acids can be converted into glucose (gluconeogenesis)
  • Acetyl-CoA from lipids and certain amino acids can be used to produce ketone bodies
  • The liver is the central hub for regulating the metabolism of all three macronutrients
• Carbohydrates, lipids, and proteins:
  • Absorbed through different mechanisms in the small intestine

Absorptive vs. Post-Absorptive Metabolism

• Absorptive Metabolism:
  • Occurs after a meal when nutrients are being absorbed from the digestive tract
  • Characterized by high blood glucose, amino acids, and lipids
  • Insulin is the dominant hormone, promoting uptake and storage of nutrients
  • Glycogenesis, lipogenesis, and protein synthesis are favored
  • The body is in an anabolic state, building and storing energy reserves
• Post-Absorptive Metabolism:
  • Occurs between meals or during fasting when no nutrients are being absorbed
  • Characterized by low blood glucose, amino acids, and lipids
  • Glucagon is the dominant hormone, promoting the breakdown and release of stored nutrients
  • Glycogenolysis, lipolysis, and protein breakdown are favored
  • The body is in a catabolic state, breaking down energy reserves for fuel

Mouth Structure

• Tongue:
  • Attached to the floor of the mouth by the lingual frenulum
  • Extrinsic tongue muscles:
    • Hyoglossus: attaches the tongue to the hyoid bone
    • Genioglossus: runs from the chin to the tongue, supporting and moving the tongue
    • Styloglossus: originates from the styloid process of the temporal bone and inserts into the tongue
  • Papillae:
    • Small, bump-like projections on the surface of the tongue that contain taste buds and help with the manipulation of food
    • Four types of papillae: vallate, foliate, fungiform, and filiform

Salivary Glands

• Exocrine glands that produce and secrete saliva into the oral cavity
• Three main pairs of salivary glands:
  • Parotid salivary glands:
    • Located anterior to the ears and deep to the skin
    • Secrete saliva through ducts that open near the upper second molars
  • Submandibular salivary glands:
    • Located beneath the mandible
    • Ducts open under the tongue, near the lingual frenulum
  • Sublingual salivary glands:
    • Smallest salivary glands
    • Located beneath the tongue

Tooth Structure

• Pulp cavity:
  • Hollow space within the center of the tooth that contains the pulp, which consists of nerves, blood vessels, and connective tissue
  • Extends from the crown to the root
• Root canal:
  • Portion of the pulp cavity that extends through the root of the tooth
  • Contains the nerve and blood supply for the tooth
• Apical foramen:
  • Opening at the tip of the root through which the nerves and blood vessels enter and exit the pulp cavity

Teeth

• Permanent teeth (also known as Adult Teeth):
  • Second set of teeth that replace the primary (deciduous) teeth
  • 32 permanent teeth in total, with 16 in each jaw
  • Types of permanent teeth:
    • Incisors (lateral & central): four front teeth in each jaw, used for cutting and biting
    • Cuspids (canines): pointed teeth next to the incisors, used for tearing and gripping food
    • Premolars (bicuspids): teeth behind the cuspids, used for grinding and crushing food
    • Molars: large, flat teeth at the back of the mouth, used for grinding and chewing food

Dental Formula

• Represents the number and types of teeth in each quadrant of the mouth
• For permanent teeth, the dental formula for one quadrant is: 2:1:2:3 (2 incisors: 1 canine: 2 premolars: 3 molars)