Gas Exchange in Humans

• Lungs

  • Main organs in the respiratory system, containing the surfaces where gas exchange takes place
• Ribs and intercostal muscles

  • Intercostal muscles found between the ribs, work antagonistically in pairs to expand and contract the rib cage during breathing
  • Ribs protect the lungs and heart from physical damage
• Larynx

  • Contains the vocal cords
• Trachea

  • Connects the throat to the bronchi, C-shaped cartilage rings provide structural strength to keep it open
• Bronchi

  • Hollow tubes composed of cartilage rings that carry air from the trachea to the lungs, split into two tubes to enter the left and right lung
• Bronchioles


  • Smaller tubes which branch off from the bronchi in the lungs, leading to the alveoli
• Alveoli

  • Where gas exchange occurs, comprised of tiny air sacs with a capillary network
• Ventilation

  Moving air into and out of the lungs to allow gas exchange to occur
• Breathing in

  Internal intercostal muscles relax, external intercostal muscles contract, pulling the ribs up and out while the diaphragm flattens, pushing the abdominal muscles downwards, increasing the volume in the thorax so air enters the lungs
• Breathing out

  Volume of thorax decreases, increasing pressure so air is forced out, passive except when forcibly breathing out where internal intercostal muscles contract
• Inhaled air is made up of more oxygen than exhaled air, as oxygen is absorbed into the blood in the alveoli instead of being exhaled
• Oxygen is used in cells for respiration, and carbon dioxide is produced as a waste product
• Carbon dioxide is released from the blood at the alveoli and diffuses out into the lungs, before being exhaled, thus there is more carbon dioxide in exhaled air
• Exhaled air also contains more water vapour than inhaled air
• During physical activity

  The rate and depth of breathing increases
• When exercise is carried out

  Muscles increase the rate of respiration to produce energy for muscle contraction, requiring more oxygen and producing more carbon dioxide
• Increase in carbon dioxide in the blood

  Detected by the brain, causing the rate of breathing to speed up
• Increase in oxygen demand and carbon dioxide production during exercise


  Heart rate is also increased to pump substances around the body more quickly in the blood
• Adaptations of exchange surfaces

  • Large surface area
  • Thin surface
  • Good blood supply
  • Good ventilation with air
  • Moist
• Goblet cells

  • Found in the trachea and bronchi, adapted to secrete mucus into the respiratory tract
• Mucus movement

  Foreign pathogens and particles stick to the mucus, which is then moved upwards towards the throat by cilia, then swallowed and pathogens destroyed in the acidic conditions in the stomach