Chapter 22: The Digestive System

Overview

• The digestive system consists of two major parts:
  • Alimentary canal (GI tract): Long tube from the mouth to the anus.
  • Accessory structures: Organs that secrete substances into the GI tract (liver, gallbladder, pancreas).

Structures of the Digestive System

• Oral Cavity (Mouth):

  • Contains tongue, teeth, and salivary glands.
  • Salivary glands: Parotid, submandibular, sublingual.

• Pharynx (Throat):

  • Shared structure between digestive and respiratory systems.

• Esophagus:

  • Long, smooth tube leading to the stomach.

• Stomach:

  • Food passes here after the esophagus.

• Small Intestine:

  • Three parts, discussed later.
  • Responsible for most absorption of nutrients.

• Large Intestine:

  • Absorbs water, forms waste.

Accessory Structures

• Salivary Glands:

  • Secrete saliva into the mouth.

• Liver:

  • Secretes bile, stored in the gallbladder.

• Gallbladder:

  • Stores bile from the liver.

• Pancreas:

  • Secretes substances into the duodenum of the small intestine.

Major Functions of the Digestive System

• Ingestion:

  • Bringing in food.

• Secretion:

  • Release of enzymes and hormones.

• Propulsion:

  • Movement of food via smooth muscle contractions (peristalsis).

• Digestion:

  • Two types:
    • Mechanical: Breaking food into smaller pieces without chemical changes.
    • Chemical: Enzymatic breakdown of macromolecules into absorbable units.

• Absorption:

  • Transfer of nutrients from the digestive tract into the bloodstream.

Key Processes

• Mechanical Digestion:

  • Physical breakdown of food (e.g., chewing).

• Chemical Digestion:

  • Breakdown of macromolecules:
    • Carbohydrates into monosaccharides.
    • Lipids (triglycerides) into free fatty acids.
    • Proteins into amino acids.

• Regulation of Motility:

  • Controlled by the autonomic nervous system (parasympathetic system).

Oral Cavity and Swallowing

• Salivary Amylase:

  • Begins carbohydrate digestion in the mouth.

• Swallowing (Deglutition):

  • Three phases:
    1. Voluntary phase: Moving food to the pharynx.
    2. Pharyngeal phase: Uvula and epiglottis prevent food from entering nasal cavity and trachea.
    3. Esophageal phase: Peristalsis moves food to the stomach.

Important Concepts

• Peristalsis:

  • Rhythmic contractions move food through the GI tract.

• Nutrient Absorption:

  • Essential for cellular function and energy production (ATP).

• Enzymes:

  • Facilitate chemical digestion.

Note on Study Tips:

• Use color coding for clarity:
  • Highlight different components of the digestive system in different colors (e.g., organs, enzymes).

This overview covers the essential aspects of the digestive system as introduced in the lecture. Further details will be explored in subsequent lectures.