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Gas Exchange

Subject: Biology
Topic: 8
Cambridge Code: 0610 / 0970 / 5090

Gas Exchange Principles​

Gas exchange - Movement of O₂ and CO₂ between organism and environment

Requirements for Efficient Exchange​

  1. Large surface area - More area for diffusion
  2. Thin barrier - Short diffusion distance
  3. Good blood supply - Transport gases quickly
  4. Moist surface - Gases dissolve in water

Mammalian Respiratory System​

Nasal Cavity​

  • Warms and moistens air
  • Ciliated epithelium traps dust
  • Olfactory receptors

Trachea​

  • Windpipe from larynx to lungs
  • Cartilage rings - Support, allow flexibility
  • Cilia and mucus - Trap particles

Bronchi​

  • Trachea divides into left and right bronchi
  • Enter lungs
  • Further divide into smaller bronchioles

Lungs​

  • Left lung: 2 lobes (smaller, heart accommodation)
  • Right lung: 3 lobes
  • Protected by rib cage
  • Diaphragm and intercostal muscles control ventilation

Alveoli​

Alveoli - Air sacs where gas exchange occurs

Structure:

  • Tiny sacs (0.1-0.2 mm)
  • Walls: Single epithelial cell layer
  • Surrounded by capillaries
  • ~300 million in human lungs

Function:

  • O₂ diffuses from alveolus into blood
  • CO₂ diffuses from blood into alveolus

Surface area: ~70 m² (tennis court sized)

Breathing Mechanism​

Inspiration (Inhalation)​

Diaphragm contracts:

  • Moves downward
  • Increases thoracic volume
  • Decreases pressure (negative pressure)
  • Air enters lungs

External intercostal muscles contract:

  • Ribs move up and out
  • Increases volume further

Expiration (Exhalation)​

Diaphragm relaxes:

  • Moves upward
  • Decreases thoracic volume
  • Increases pressure
  • Air pushed out

Internal intercostal muscles contract:

  • Ribs move down and in
  • Further volume decrease

Gas Exchange at Alveoli​

Oxygen Movement​

  • Concentration: Higher in alveolus than blood
  • Method: Diffusion
  • Transport: Binds to hemoglobin in RBCs
  • Delivery: Carried to body tissues

Carbon Dioxide Movement​

  • Concentration: Higher in blood than alveolus
  • Method: Diffusion
  • Source: From cellular respiration
  • Removed: Exhaled

Adaptation of Alveoli​

Large surface area:

  • 300 million alveoli
  • Total ~70 m²
  • Enables rapid gas exchange

Thin walls:

  • Single cell layer
  • Short diffusion distance (~0.5 micrometers)
  • Rapid diffusion

Good blood supply:

  • Dense capillary network
  • Quick removal and delivery of gases

Moist surface:

  • Gases dissolve in fluid
  • Allow diffusion

Fish Gill Structure​

Gas exchange surface: Gills

Gill Structure​

  • Gill filaments: Thin structures with large surface area
  • Lamellae: Thin folds on filaments
  • Small diameter: Short diffusion distance
  • Capillaries: Carry blood

Counter-Current Flow​

  • Water: Flows in one direction
  • Blood: Flows opposite direction
  • Advantage: Maintains concentration gradient
  • Efficiency: ~80% extraction possible

Breathing Frequencies​

Animals - Vary by size and metabolic rate

  • Mouse: ~160 breaths/minute
  • Human: ~12-20 breaths/minute (at rest)
  • Whale: ~1 breath/few minutes

Larger animals: Slower rate (less surface-to-volume ratio needs)

Ventilation Control​

Neural Control​

Breathing center: Medulla oblongata

  • Controls rate and depth
  • Responds to CO₂, O₂, pH

Chemoreceptors:

  • Detect blood CO₂, pH, O₂
  • Send signals to medulla
  • Adjust breathing

Reflex Actions​

  • Coughing, sneezing
  • Yawning
  • Breath-holding (voluntary, limited)

Key Points​

  1. Gas exchange by diffusion across moist surface
  2. Alveoli maximize surface area
  3. Diaphragm and intercostal muscles control breathing
  4. O₂ in, CO₂ out at lungs
  5. Opposite at tissues (O₂ out, CO₂ in)
  6. Efficient systems have large surface area
  7. Counter-current flow in fish gills

Practice Questions​

  1. Describe alveoli structure
  2. Explain breathing mechanism
  3. Draw and label respiratory system
  4. Compare mammalian and fish gas exchange
  5. Explain counter-current flow advantage
  6. Describe gas exchange at tissues

Revision Tips​

  • Know all respiratory structures
  • Learn breathing mechanism clearly
  • Understand diffusion direction
  • Know alveoli adaptations
  • Compare gas exchange surfaces
  • Practice drawing diagrams