Gas exchange in humans

• Features of Gas Exchange Surfaces

  • Large surface area to allow faster diffusion of gases across the surface
  • Thin walls to ensure diffusion distances remain short
  • Good ventilation with air so that diffusion gradients can be maintained
  • Good blood supply to maintain a high concentration gradient so diffusion occurs faster
• Alveolus


  The gas exchange surface in humans
• Structures in the human breathing system

  • Nose
  • Pharynx
  • Larynx
  • Trachea
  • Bronchi
  • Bronchioles
  • Alveoli
• Investigating the Differences in Inspired & Expired Air

  1. Air drawn through boiling tube A

  2. Air blown into boiling tube B

  3. Lime water becomes cloudy when carbon dioxide is bubbled through it
• The lime water in boiling tube A will remain clear, but the limewater in boiling tube B will become cloudy
• This shows us that the percentage of carbon dioxide in exhaled air is higher than in inhaled air
• Composition of Air


  • Oxygen (20-21% in air, 16% in exhaled air)
  • Carbon dioxide (0.04% in air, 4% in exhaled air)
  • Water vapour (more in exhaled air)
  • Temperature (higher in exhaled air)
• Investigating the Effects of Physical Activity on Breathing

  1. Count breaths per minute at rest

  2. Measure average chest expansion over 5 breaths at rest

  3. Exercise for at least 3 minutes

  4. Immediately after exercising, count breaths per minute and measure average chest expansion over 5 breaths
• Following exercise, the number of breaths per minute will have increased and the chest expansion will also have increased
• External intercostal muscles


  Found on the outside of the ribcage
• Internal intercostal muscles


  Found on the inside of the rib cage
• Function of cartilage in the trachea

  To support the airways and keep them open during breathing
• Volume & Pressure Changes in the Lungs

  1. Diaphragm contracts, increasing volume of chest cavity, decreasing air pressure, drawing air in

  2. Diaphragm relaxes, decreasing volume of chest cavity, increasing air pressure, forcing air out

  3. External intercostal muscles contract, increasing volume of chest cavity, decreasing air pressure, drawing air in

  4. External intercostal muscles relax, decreasing volume of chest cavity, increasing air pressure, forcing air out

  5. Internal intercostal muscles contract, decreasing volume of chest cavity more, forcing air out more forcefully and quickly
• Reason for Differences Between Inspired & Expired Air

  • Oxygen absorbed from air (20-21% to 16%)
  • Carbon dioxide added to air (0.04% to 4%)
  • Water vapour added to air
  • Temperature of air increases
• Explaining the Link Between Physical Activity & Breathing

  1. Muscles respiring more aerobically, needing more oxygen and producing more carbon dioxide

  2. Lactic acid builds up, lowering pH, needing to be removed by combining with oxygen (repaying oxygen debt)

  3. Increased carbon dioxide levels detected by chemoreceptors in brain

  4. Brain sends nerve impulses to increase rate and depth of breathing
• Protecting the Breathing System

  • Passages lined with ciliated epithelial cells
  • Cilia beat and push mucus up towards nose and throat
  • Mucus traps particles, pathogens, and dust