Mammalian Lungs: Adaptations (SL IB Biology)

• Air moves in through the nose and mouth before it is carried to the lungs through the trachea
• The trachea is a tube supported by rings of cartilage which help to support its shape and ensure it stays open while allowing it to move and flex with the body
• The trachea divides to form the two bronchi (singular bronchus) with walls also strengthened with cartilage and a layer of smooth muscle that can contract or relax to change the diameter of the airways. Both trachea and bronchi are lined with ciliated epithelium to remove particles trapped in mucus that enter the airways
  • One bronchus leads to each lung
• Bronchioles branch off the two bronchi to form a network of narrow tubes
  • The walls of the bronchioles are lined with a layer of smooth muscle to alter the diameter of the bronchiole tubes
  • This helps to regulate the flow of air into the lungs by dilating when more air is needed and constricting when e.g. an allergen is present
• Groups of alveoli are found at the end of the bronchioles
• Each alveolus is surrounded by an extensive network of capillaries to provide a good blood supply for maximum gas exchange

Human Gas Exchange System Diagram

The main structures of the human gas exchange system

Adaptations of mammalian lungs for gas exchange

• Each mammalian lung is comprised of many, small alveoli
  • These provide a large surface area for gas exchange
• Alveoli are grouped around the ends of bronchioles, which spreads out to form a branched network across each lung 
  • This ensures an even distribution of alveoli throughout the lungs
• The clusters of alveoli are surrounded by an extensive capillary bed
  • This provides an increased surface area for the diffusion of oxygen and carbon dioxide between the alveoli and blood
  • Deoxygenated blood enters the capillary beds from a branch of the pulmonary artery while oxygenated blood leaves the capillary beds via a branch of the pulmonary vein
    • This maintains the concentration gradient of oxygen and carbon dioxide between the alveoli and blood
• Cells of the alveolar wall secrete a substance called surfactant which lowers the surface tension in the alveoli
  • This prevents the alveoli from collapsing and sticking together during expiration

Human Alveoli Diagram

Many, small alveoli and an extensive capillary network are examples of how the mammalian lung is adapted for gas exchange