Gas exchange in humans

• Gas exchange surfaces


  • Large surface area to allow faster diffusion
  • Thin walls to ensure diffusion is easier
  • Good ventilation with air to maintain diffusion gradient
  • Good blood supply to maintain high concentration gradient for faster diffusion
• Intercostal muscles


  Muscles that can only pull, not push, so there are two sets to move the ribs up and down
• Trachea


  • Has rings of cartilage around it to keep it open while breathing
  • If not present, the sides will collapse inward when air pressure drops
• Volume and pressure changes in breathing

  1. Diaphragm contracts, increasing volume and decreasing air pressure, leading to air being inhaled

  2. External intercostal muscles contract, pulling ribs up and out, increasing volume and decreasing pressure, leading to air being inhaled

  3. Internal intercostal muscles contract, pulling ribs down and in, decreasing volume and increasing pressure, leading to air being exhaled
• During physical activity

  Rate of gas exchange increases
• Exercise increases the frequency and depth of breathing
• Passages down to the lungs are lined with ciliated epithelial cells
• Ciliated epithelial cells

  Have tiny hairs that beat to push mucus up the passage towards the nose and throat where it's removed
• Mucus

  Made by goblet cells, traps particles, pathogens and dust to prevent them from getting to the lungs
• Lactic acid is produced during anaerobic respiration


  It must be removed because it may decrease pH and denature enzymes
• The longer it takes to return to normal breathing, the more lactic acid is produced and the greater the oxygen debt to be repaid
• Boiling tube A will remain clear, boiling tube B will be cloudy