Digestion and the Digestive System

Chapter 17: Digestion Overview

  • Lesson Outcomes
    • Describe processes of the digestive system
    • Identify organs involved in digestion
    • Understand the structure of the alimentary canal
    • Describe movements in the alimentary canal
    • Functional analysis of the mouth and salivary glands
    • Understand swallowing mechanisms
    • Know major structures of the stomach and related secretions
    • Identify components of the pancreas, liver, and gallbladder
    • Understand the structure and function of the small and large intestines
    • Describe the defecation mechanism

Overview of Digestion

  • Digestion: Mechanical and chemical breakdown of food

    • Mechanical Digestion: Reduces particle size without chemical change
    • Chemical Digestion: Changes food into simpler chemicals

  • Digestive System Components:

    • Alimentary Canal
    • Accessory Organs (e.g., pancreas, liver)

Alimentary Canal Structure

  • Characteristics:

    • Muscular tube approximately 8 meters long
    • Passes through the ventral cavity

  • Four Layers of the Wall:

    1. Mucosa: Innermost layer, a mucous membrane
    2. Submucosa: Provides nourishment and transports absorbed food
    3. Muscularis: Muscle tissue responsible for food movement
    4. Serosa: Outermost layer, secretes serous fluid to eliminate friction

Movements in the Alimentary Canal

  • Mixing Movements:

    • Rhythmic contractions in small sections (e.g., stomach)
    • Does not propel food; example: segmentation

  • Propelling Movements:

    • Moves materials in one direction (e.g., peristalsis)
    • Peristalsis: a ring contraction moving down the digestive tube

Structures of the Mouth and Their Functions

  • Mouth Components:

    • Cheeks: Lateral walls, involved in chewing
    • Lips: Surround mouth, sensory receptors for food
    • Tongue: Moves food, contains taste receptors, anchored by the lingual frenulum
    • Palate: Forms roof, separates oral cavity from nasal cavity
    • Teeth: Break food into smaller pieces, aids in mechanical digestion

  • Salivary Glands:

    • Function: Prepare food for digestion by moistening and beginning carbohydrate digestion
    • Major glands include parotid, submandibular, and sublingual
    • Saliva contains enzymes like salivary amylase for breaking down carbohydrates

Pharynx and Esophagus

  • Pharynx: Cavity behind the mouth, connects nasal and oral cavities to the esophagus.

  • Esophagus: Muscular tube (25 cm) that transports food from pharynx to stomach, does not aid in digestion.

    • Contains secretions to lubricate food.

  • Swallowing Mechanism:

    1. Voluntary Stage: Formation of bolus with saliva
    2. Involuntary Reflex: Soft palate and uvula raise, epiglottis closes, food moves towards the stomach via peristalsis

Stomach Structure and Function

  • Shape: J-shaped, pouch-like organ responsible for food mixing

  • Layers:

    • Three layers of smooth muscle (circular, longitudinal, oblique)
    • Divided into regions: cardia, fundus, body, pylorus
    • Pyloric Sphincter: Regulates release of food into small intestine

  • Gastric Secretions:

    • Pepsinogen, hydrochloric acid, gastric lipase, intrinsic factor, and mucus
    • Pepsinogen converts to pepsin for protein digestion in the acidic environment

  • Regulation: Neural (sympathetic decreases activity, parasympathetic increases) and hormonal control (gastrin increases secretion)

  • Absorption: Limited to water, certain salts, and lipid-soluble drugs

Pancreas and Its Secretions

  • Function: Endocrine and exocrine

    • Pancreatic Juice: Contains enzymes (amylase, lipase, trypsin) for digesting nutrients
    • Bicarbonate ions neutralize stomach acid

  • Regulation:

    • Secretin stimulates release of bicarbonate
    • Cholecystokinin stimulates enzyme-rich pancreatic juice release

Liver and Gallbladder

  • Liver: Largest internal organ, involved in metabolism and detoxification

    • Functions include blood filtration, storage of nutrients, and bile production

  • Bile: Aids in digestion and emulsification of fats

  • Gallbladder: Stores and concentrates bile, releases it in response to fatty meals via CCK

Small Intestine Overview

  • Structure:

    • Extends from pyloric sphincter to large intestine, divided into duodenum, jejunum, and ileum
    • Villi: Increase surface area for absorption

  • Secretions: Include enzymes (sucrase, maltase, lactase) from intestinal glands

  • Absorption Process:

    • Carbohydrates, proteins, and fats digested and absorbed by different mechanisms

Large Intestine Overview

  • Structure: Meets the small intestine at the ileocecal sphincter, includes cecum, colon, rectum, and anal canal

  • Functions: Absorbs water and electrolytes, stores feces, houses intestinal flora

  • Movement: Slower peristalsis for feces formation, with mass movements following meals

  • Feces Composition: Contains undigested materials, water, electrolytes, mucus, and bacteria