Gas Exchange

Respiration Processes

Physiological respiration involves three distinct processes:


  • 1. Ventilation: Exchange of air between the atmosphere and the lungs through breathing
  • 2. Gas Exchange: Transfer of gases between the lungs and the bloodstream via passive diffusion
  • 3. Cell Respiration: Release of energy (ATP) from organic molecules, which is enhanced by oxygen and produces CO2 as waste

Gas Exchange Principles

    Gas Exchange Becomes More Challenging as Organisms Increase in Size Due to:

  • Surface-area-to-volume ratio changes
  • Increased distance from center to exterior

    Ventilation

  • Ventilation maintains concentration gradients between alveoli and blood capillaries
  • O₂: Alveoli (high) → Capillaries (lower)
  • CO₂: Capillaries (high) → Alveoli (lower)
  • Fresh air is continuously cycled into alveoli to maintain these gradients

Exchange Surfaces

    Properties of Gas Exchange Surfaces

  • Large Surface Area: More opportunity for gas diffusion
  • Permeability: Openings or pores allow for gas exchange
  • Thin Tissue Layer: Short diffusion distance
  • Moisture: Helps dissolve gases before diffusion
  • Concentration Gradient: Gases diffuse toward areas of lower concentration

    Maintaining the Concentration Gradient

  • Dense Blood Vessel Network: Provides numerous opportunities for exchange
  • Continuous Blood Flow: Transports exchanged substances away, maintaining the gradient
  • Ventilation: Ensures air/water rich in desired gases moves across the exchange surface

Pulmonary Surfactant

  • Prevents alveoli from adhering to each other by reducing surface tension
  • The surfactant is needed to reduce surface tension of the moist alveoli epidermal layer
  • Without the surfactant, smaller alveoli would collapse due to higher pressure as a result of their small radius

Breathing Mechanics

    Boyle's Law:

  • Pressure is inversely proportional to volume
  • Increased volume → decreased pressure → air flows in
  • Decreased volume → increased pressure → air flows out

    Breathing Involves Changing Pressure Within Lungs by Altering the Thoracic Cavity Volume:

  • Increased thoracic volume → decreased lung pressure → air moves in
  • Decreased thoracic volume → increased lung pressure → air moves out

    Respiratory Muscles:

  • Inspiration: Diaphragm contracts (flattens), external intercostal muscles contract (pull ribs outward)
  • Expiration: Diaphragm relaxes (curves up), internal intercostal muscles contract (pull ribs inward), abdominal muscles contract for forced exhalation
  • Demonstrates antagonistic muscle action

Ventilation and Exercise

  • Exercise changes ventilation patterns as energy demands increase
  • Chemosensors in arterial walls detect changing CO₂ levels
  • Exercise influences ventilation through:
    • Increased ventilation rate (breathing frequency)
    • Increased tidal volume (amount of air per breath)

Gas Exchange Part 2

  • All organisms must exchange gases with the environment in order to survive
  • Oxygen is needed for aerobic respiration (ATP production)
  • Carbon dioxide is a waste product that must be removed (alters pH of blood)
  • Most unicellular organisms perform gas exchange via diffusion
  • Multicellular organisms tend to have specialised ventilation systems to enable gas exchange
    • These systems continually cycle fresh air and maintain concentration gradients

Respiratory Surfaces

    SMART

  • Surface area - large enough to support exchange
  • Moist - gases diffuse easier when dissolved in solution
  • Absorptive - surfaces must be permeable to gases
  • Rich blood system - connected to dense capillary network
  • Thin tissue layer - only lied with single cell layer

These adaptations are important as gas exchange is a passive process and does not require ATP or membrane proteins.

Gas Exchange in the Lungs

Gas exchange in mammals involves specialised respiratory structures that are called lungs.


    Path of Air

  • Air enters via the nose or the mouth
  • The air travels down the trachea
  • The trachea splits into two bronchi
  • Each bronchus forms bronchioles
  • The bronchioles terminate in alveoli
  • Alveoli exchange respiratory gases

    Alveoli:

  • Where gas exchange occurs in the lungs
  • Surrounded by capillaries
  • Alveoli are composed of two types of cells called pneumocytes
  • Type I: Flattened cells that are responsible for gas exchange
  • Type II: Granular cells that secrete a pulmonary surfactant