In mammals, gas exchange involves a specialised structure called the lungs (responsible for pulmonary ventilation)

• Air enters the respiratory system through the nose or mouth and passes through the pharynx to the trachea
  

  • The trachea is ciliated and lined with mucus in order to trap and expel any foreign particulate matter

• The air travels down the trachea until it divides into two bronchi (singular: bronchus) which connect to the lungs
  

  • The right lung is composed of three lobes, while the left lung is only comprised of two (smaller due to position of heart)
    

• Inside each lung, the bronchi divide into many smaller airways called bronchioles, greatly increasing surface area
  

  • The bronchioles contain smooth muscle innervated by the autonomic nervous system to allow for the regulation of air flow
    

• Each bronchiole terminates with a cluster of air sacs called alveoli, where gas exchange with the bloodstream occurs
  

  • Certain alveolar cells secrete pulmonary surfactant – this reduces surface tension and prevents alveoli from collapsing
    

  • The alveoli are connected to a dense network of capillary beds – this allows for the optimised exchange of gases with the blood