Human Ventilation and Gas Exchange


Aerobic Respiration:         Anaerobic Respiration:
Glucose + Oxygen => Water + Carbon Dioxide + Energy         Glucose => Carbon Dioxide + Lactic Acid + (little) Energy

Gas exchange surface for humans is the lungs. Contains a branching system stemming from the trachea and ending in the alveoli.

The Alveoli:
  • Each one is folded to increase surface area
  • Each has a thin permeable surface
  • They have a diffusion gradient: Ventilation brings in air rich in oxygen into the alveoli, and takes air with increased carbon dioxide away from alveoli.
  • Blood flow/circulation maintains high carbon dioxide/low oxygen concentrations in the blood.


Note: Excess water loss from the lungs is prevented due to the limited contact with the outside air because the lungs are enclosed within the body cavity.

Maintenance of Breathing: At Rest (Nervous System)
  • Medulla controls breathing. Impulses from the inspiratory centre in the medulla cause contraction of breathing muscles.
  • Stretch receptors are stimulated by increase in size of thorax/lungs. Impulses are then sent to the expiratory centre, which inhibits the inspiratory impulses.
During Exercise
  • Chemoreceptors (in the medulla/aortic bodies/carotid bodies) are sensitive to the rise in the carbon dioxide level in blood. Impulses are then sent to the inspiratory centre.
  • This causes a more rapid rate of impulses to breathing muscles. This produces larger amounts of carbon dioxide so more oxygen is released; therefore a high rate of respiration is maintained with more haemoglobin free to act as a buffer.(A buffer is a substance, which can absorb hydrogen)


Breathing In:

  • Diaphragm contracts and flattens.
  • Intercostal muscles contract, therefore ribs move up and out.
  • The volume of the thorax increases, decreasing pressure below atmospheric pressure.
  • Oxygen flows into large air passages i.e Trachea => Bronchi => largest Bronchioles
  • Final pathway – oxygen diffuses into alveoli along the concentration gradient. In the alveoli, oxygen dissolves into a film of liquid, which then diffuses the short distance into the blood capillaries.