Exchange of gases in the lungs

• alveoli in gas exchange 

  • About 300 mil of alveoli per human lung, each alveoli is lined with epithelial cells only 0.05-0.3um thick
  • Around each alveolus is a network of pulmonary capillaries narrow so that red blood cells are flattened against the thin capillary wall ( one cell thick ) in order to squeeze through.
• PART ONE Diffusion between alveoli and blood very rapid because 

  • Distance between alveolar air and red blood cell is reduced and red blood cell are flattened against the capillary wall
  • Walls of alveoli and capillary and very thin allowing a short diffusion pathway
  • Breathing movements constantly ventilate lungs and actions of the heart constantly circulating blood around the alveoli ensuring a steep concentration gradient of gases to be exchanged is maintained.
• PART TWO diffusion between alveoli and blood very rapid because:

  • Red blood cell slowed as they pass through pulmonary capillary allowing more time for diffusion
  • Blood flow through the pulmonary capillaries maintains a concentration gradient
  • Alveoli and pulmonary ca[illaries have a very large total surface area.