MEPX
Chapter 7 of 14All chapters
  1. 1How the body is organised
  2. 2Cells and tissues
  3. 3The skeleton
  4. 4Muscles
  5. 5Heart and circulation
  6. 6Blood
  7. 7Lungs and breathing
  8. 8Digestion
  9. 9Kidneys and liver
  10. 10The nervous system
  11. 11The brain
  12. 12Hormones
  13. 13Skin and senses
  14. 14The immune system

Chapter 7 of 14

Lungs and breathing

Getting oxygen in and carbon dioxide out.

The exchange

Air travels down the windpipe into branching tubes that end in tiny sacs called alveoli. Oxygen crosses into the blood there and carbon dioxide crosses out, across a wall one cell thick.

  • There are around 300 million alveoli, giving a surface area like a tennis court.
  • The diaphragm does the work: it flattens to pull air in and relaxes to push it out.

What drives breathing

The urge to breathe comes from rising carbon dioxide rather than falling oxygen. That is why holding your breath becomes uncomfortable long before oxygen runs low.