human gas exchange system

• breathing = movement of air into & out of the lungs

  ventilation = scientific word for breathing

  respiration = chemical reaction to release energy in the form of ATP

  gas exchange = diffusion of oxygen from the air in the alveoli into the blood & of CO2 from the blood into the air in the alveoli
• structure of human gas exchange system:

  • alveoli
  • bronchioles - lead to alveoli
  • bronchi - branch into bronchioles
  • trachea - have tough cartilage rings, helping to support it
  • lungs
• ventilation involves:

  • diaphragm
  • antagonistic interaction between external & internal intercostal muscles
• inspiration:

  • external intercostal muscles contract (pull ribs up & out)
  • internal intercostal muscles relax
  • diaphragm contracts (pulls down)
  • air pressure in lungs initially drops, but as air moves in rises above atmospheric pressure
  • lung volume increases (to decrease pressure)
  • air moves into the lungs (from atmospheric pressure to lower pressure)
• expiration:

  • external intercostal muscles relax
  • internal intercostal muscles contract (pull ribs down & in)
  • diaphragm relaxes (pulls up)
  • air pressure in lungs initially greater that atmospheric pressure, but drops as air moves out
  • lung volume decreases (to increase pressure)
  • air moves out of lungs (from high pressure to atmospheric pressure)
• when external intercostal muscles contract, the ribcage is pulled up & out, increasing the volume in the thorax, leading to inspiration
• when internal intercostal muscles contract, the ribcage is pulled down & in, decreasing the volume in the thorax, leading to expiration
• gas exchange in the alveoli:

  • very short diffusion distance as alveoli & capillaries just made up of a single layer of cells
  • CO2 diffuses out of deoxygenated blood in the capillaries into the alveoli & oxygen diffuses across the epithelium into the blood
• alveolar epithelium:

  • around 300 million tiny air sacs in each lung, creating a very large surface area for gas exchange
  • epithelial walls are very thin, minimising diffusion distance
  • each alveoli surrounded by network of capillaries to remove exchanged gases, therefore maintaining a concentration gradient