Gas exchange in humans

• Topics covered in the lesson


  • Structures of the respiratory system
  • Features of gas exchange surfaces
  • Differences between inspired and expired air
  • Effects of physical activity on rate and depth of breathing
  • Role of respiratory muscles in breathing
  • Effects of physical activity on rate and depth of breathing
  • Ways the respiratory system protects itself from pathogens and particles
• Role of the breathing system


  Move air into and out of the lungs so that oxygen needed for respiration can diffuse into the bloodstream and carbon dioxide can diffuse outwards
• Main structures of the respiratory system


  • Lungs
  • Diaphragm
  • Intercostal muscles
  • Mouth/nasal passages
  • Larynx
  • Trachea
  • Bronchi
  • Bronchials
  • Alveoli
• Alveoli


  • Vast number and specialized shape maximizes the area of the exchange surface
  • Excellent blood supply as each alveolus is surrounded by a network of capillaries
  • Walls of the alveoli and the capillaries that surround them are only one epithelial cell thick
  • Very well ventilated by pulmonary ventilation
• Breathing in


  1. Air passes through mouth/nasal passages

  2. Through larynx

  3. Into trachea

  4. Trachea divides into bronchi

  5. Bronchi divide into bronchials

  6. Air reaches alveoli
• Lime water in tube A

  Remains clear
• Lime water in tube B

  Turns milky
• Expired air contains more carbon dioxide and less oxygen than inspired air
• Expired air is saturated with vapor while the vapor content of inspired air varies
• Investigating effects of physical activity on breathing


  1. Subject breathes in and out of spirometer

  2. Depth of breathing indicated by amplitude of trace

  3. Breathing rate indicated by frequency of trace

  4. Both breathing rate and tidal volume increase during exercise
• Respiratory muscles


  • Diaphragm and muscular sheet below lungs
  • Intercostal muscles that move the rib cage
• Breathing in


  1. Diaphragm contracts and flattens

  2. External intercostals contract to expand ribcage

  3. Increases volume of thorax and lungs

  4. Decreases air pressure within

  5. Air rushes in
• Breathing out forcibly


  1. Diaphragm relaxes and domes upwards

  2. Internal intercostal muscles contract to pull ribcage downwards and inwards

  3. Decreases volume of thorax and lungs

  4. Increases air pressure within

  5. Air forced outwards
• Inspired air contains around 21% oxygen and 0.04% carbon dioxide
• Some oxygen is absorbed into the bloodstream while carbon dioxide diffuses into the alveoli
• Expired air contains less oxygen and significantly more carbon dioxide than atmospheric air
• Expired air is saturated with water vapor
• Physical activity increases


  Breathing rate and tidal volume increase
• Carbon dioxide accumulates in muscle cells during exercise


  Diffuses into blood plasma more rapidly
• Brain detects rising carbon dioxide in blood

  Stimulates breathing muscles to contract more rapidly and forcefully
• Goblet cells


  Secrete sticky mucus that traps pathogens and dust
• Ciliated cells


  Carry the mucus up towards the top of the trachea to be swallowed
• Alveoli are extremely delicate and vulnerable to physical damage and infection