Gas exchange in humans

• Inspiration

  External intercostal muscles and diaphragm muscles contract

  Ribcage moves upwards and outwards diaphragm flattens, increases volume of thoracic cavity

  Volume in thoracic increase lung pressure decreases

  Air flows from trachea down into lungs

  Requires energy
• Expiration

  External intercostal muscles and diaphragm muscles relax

  Ribcage moves downwards and inwards diaphragm becomes curved

  Volume in thoracic cavity decreases causing air pressure to increase

  Air forced down and out of the lungs

  Passive process
• Alveoli

  O2 diffuses out the alveoli across alveolar epithelium and capillary endothelium, into haemoglobin the the blood

  CO2 diffuses into the alveoli from the blood
• Alveoli adaptations

  Alveolar epithelium thin - Short diffusion distance

  Large SA - Lots of alveoli

  Capillaries - Steep concentration gradient