Gas exchange in humans

• Gas exchange system


  Responsible for getting oxygen into the blood and removing carbon dioxide
• Features of gas exchange surfaces in humans

  • Large surface area
  • Thin surface
  • Good blood supply
  • Good ventilation with air
• Large surface area provides more space for diffusion to take place, thereby speeding up the rate of gas exchange
• Thin surface reduces the distance across which the gases must diffuse, thereby allowing a faster diffusion rate
• Good blood supply ensures that the concentration gradients are always maintained
• Good ventilation with air ensures that the concentration gradients of carbon dioxide and oxygen are maintained
• Parts of the breathing system

  • Larynx
  • Trachea
  • Lungs
  • Diaphragm
  • Bronchi
  • Bronchioles
  • Alveoli
  • Capillaries
  • Ribs
  • Intercostal muscles
• Function of cartilage in the trachea


  Strengthen and support the trachea, keep it open during breathing, and prevent it from collapsing
• Ventilation (Inspiration)


  1. External intercostal muscles contract, internal intercostal muscles relax

  2. Diaphragm contracts and moves downwards

  3. Volume of thorax increases

  4. Pressure inside lungs decreases

  5. Air moves into lungs
• Ventilation (Expiration)


  1. External intercostal muscles relax, internal intercostal muscles contract

  2. Diaphragm relaxes and moves upwards

  3. Volume of thorax decreases

  4. Pressure inside lungs increases

  5. Air moves out of lungs
• Volume increases


  Pressure decreases, air moves into lungs
• Volume decreases


  Pressure increases, air moves out of lungs
• Lime water test can be used to investigate the differences in composition between inspired and expired air
• Differences in composition between inspired and expired air


  • More oxygen in inspired air (21%) than expired air (16%)
  • Less carbon dioxide in inspired air (0.04%) than expired air (4%)
  • Less water vapor in inspired air than expired air
• Oxygen diffuses from alveoli into blood, carbon dioxide diffuses from blood into alveoli
• Exercise


  Increases rate and depth of breathing
• Exercise causes body cells to respire faster, producing more carbon dioxide, which is detected by the brain to increase rate and depth of breathing
• Respiratory tract protection


  • Lined with mucus produced by goblet cells
  • Lined with ciliated epithelial cells that move mucus towards throat
• Mucus traps pathogens and particles, cilia move it towards throat where it can be swallowed, protecting lungs