a. Explain how oxygen from atmospheric air reaches the capilaries surrounding the alveoli in the lungs of a mammal  
  • Muscular contraction forces diaphragm down;
  • intercostal muscles contract moving ribs up and out;
  • increases thorax volume
  • pressure in thorax falls below atmospheric
  • air flows in through trachea and bronchi
  • oxygen diffuses into alveoli (they are too delicate for mass flow)
  • oxygen dissolves into moisture film on alveoli wall
  • oxygen diffuses (the short distance) into the capilary
  Any 6 bullet points (6)