Recall

• What happens to SA: vol ratio as size increases?

  It decreases (SA increases but volume increases faster so ratio decreases )
• What strategies are adoptees to overcome a lower SA:vol ratio?

  evolution of specific body shapes ( flatworm short diffusion path ) , specialised exchange surfaces ( human lungs ) and body shapes ( artic fox small ears and round head reduces SA:vol ratio to decrease heat loss )
• describe structure of ventilation system in humans

  Trachea ( windpipe ) branches into two bronchi , these branch into many bronchioles ending in air sacs (alveoli) for gas exchange

  intercostal and diaphragm muscles contract to move air in and out
• Describe inspiration in ventilation

  External intercostals contract to move ribs up and out , diaphragm contracts to flatten and move down

  this increases volume and decreases pressure so air flows in - active process
• Describe expiration in ventilation at rest

  External intercostals relax to move the ribs in and down , diaphragm relaxes to a dome shape and moves up

  this decreases volume and pressure so air flows out - passive process
• Describe expiration in ventilation during exercise

  Internal intercostals contract antagonistically to the external intercostals ( which are relaxing along with diaphragm ) to move ribs in and down

  this decreases volume and increases pressure so air flows out - active
• Adaptations in the lungs to improve exchange :

  • large surface area due to millions of tiny alveoli and fine network of blood capillaries
  • most lining of alveoli to dissolve gases
  • thin walls ( alveoli epithelium and blood carpillaries endothelium each one cell thick ) short diffusion path
  • ventilation and blood circulation maintain diffusion gradient
• Elastic fibres


  When breathing in, lungs inflate and widen due to elastic fibres stretching - fibres recoil to push air out during exhalation
• Goblet cells

  Secrete mucus in the trachea and bronchi to trap particles
• Ciliated epithelium

  • Found in trachea
  • Bronchi
  • Bronchioles
• Cartilage

  Rings support airways and prevent collapse when breathing
• Ciliated epithelium


  Cilia waft dust and bacteria caught up in mucus up to throat to be swallowed (prevents lung infection)
• Smooth muscle

  Controls diameter of trachea, bronchi and bronchioles - allow expansion and increased airflow at exercise