structure

• alveolar epithelium and capillary endothelium are very thin

  • this shortens diffusion distance from alveoli to blood
• large number of alveoli

  • increases surface area of gas exchange
• capillaries that surround the alveoli are very narrow

  • red blood cells are slowed down to squeeze through one at a time to increase the time of diffusion
• large number of capillaries around alveoli

  • circulation constantly removes oxygenated blood to maintain steed concentration gradient
• constant ventilation of air in and out of lungs

  • maintains steep concentration gradient