Gas Exchange


  • Distinguish between ventilation, gas exchange and cell respiration
  • Explain the need for a ventilation system
  • Describe the features of alveoli that adapt them to gas exchange
  • Explain the mechanism of ventilation of the lungs in terms of volume and pressure changes caused by the internal and external intercoastal muscles, the diaphragm and abdominal muscles

Respiratory System Diagram

Hand drawn diagram of the ventilation system

Why do we need a ventilation system?

  • The ventilation system allows for oxygen into the lungs and the excretion of CO2.
  • It ensures that the concentration of the respiratory gases within the lungs encourages the diffusion of each gas in a direction that is beneficial to the body.
    • There has to be more CO2 in the blood than the alveoli and more oxygen in the alveoli than blood.

Ventilation, Gas Exchange and Cell Respiration


    • is the flow of air in and out of alveoli in 2 stages - respiration and expiration
      • Breathing in (Inspiration) - increases concentration of oxygen between alveoli and blood
      • Breathing out (Expiration) - removes CO2
    • Depends on muscles - intercostal muscles and diaphragm

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Gas Exchange

    • Its the diffusion of gases in the alveoli
    • 2 locations where the gas exchange occurs
      • Lungs - oxygen moves from the air of the lung into the bloodstream. The carbon dioxide moves in the opposite direction
      • Capillary Bed - opposite gas exchange occurs: oxygen diffuses out of the bloodstream and into a body cell. CO2 diffuses out of the body cell into the capillary bed.
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Cell Respiration

    • There are two types of cell respiration: Aerobic and Anaerobic
      • Aerobic - uses oxygen in the mitochondria and produces carbon dioxide
      • Anaerobic - does not use oxygen to produce carbon dioxide

Alveoli and Gas Exchange

spherical shape of alveoli
provides large surface area for respiratory gases to diffuse through
flattened single cell-thick
prevents respiratory gases from diffusing through more cell layers
moist inner lining
allows for efficient diffusion
associated capillary bed nearby
respiratory gases do not have to diffuse far to reach single thick capillaries
Screen Shot 2012-11-04 at 9.55.23 PM.png

The fact that there are also millions of alveoli within the lung contributes to the increase in overall surface area for gas exchange.

Mechanism of Ventilation

  • Inspiration
    • Diaphragm contracts and flattens downwards
    • Abdominal muscles relax
    • external intercostals muscles contract
    • ribs goes up
    • volume increases
    • pressure in lungs decreases
    • air enters
  • Respiration
    • diaphragm relaxes and moves upwards
    • abdominal muscles contract
    • external intercostals muscles relax
    • ribs fall
    • volume decreases
    • pressure increases
    • air flows out